Patent Application
20100149676
The current invention refers to methods of developing multilevel three-dimensional (ML3D) recording systems which could be implemented in the form of a plug-in drive to replace conventional magnetic disk drive in computer and other applications. In accordance with an exemplary embodiment, using three (instead of two) dimensions allows recording, storing, and retrieving information with substantially higher areal densities and data rates compared to any state-of-the-art 2D memory technology. For example, with the proposed technology, the entire library of Congress could be recorded in a device the size of a quarter. The demand for more data has no limits and continues to exponentially grow. Ultra-high-density and super-fast memory is finding use in a wide spread of applications ranging from the Internet and broadband communications to rapidly growing mobile device applications.
During the last couple of years, for the first time, researchers witness that the recorded data in conventional longitudinal magnetic media become highly unstable as the areal density increases beyond approximately 100 Gbit/in2. To underscore the significance of the modern situation in the industry, it could be mentioned that for the first time the industry switched to a different technology perpendicular recording which is merely an incremental solution that promises only a factor of 2 to 3 increase in information density. Most of the other known alternative technologies, e.g. heat-assisted magnetic recording (HAMR) and patterned media, are of 2-D nature and promise to defer the superparamagnetic limit only somewhat beyond one terabit/in2, not to mention that implementation of each of them has too many open questions. It is clear that to defer the superparamagnetic limit substantially beyond the one terabit/in2 mark, at some point it will be necessary to start to stack recording layers in a third (vertical) dimension. The vertical stacking underlies the concept of 3-D magnetic memory—the primary subject of this invention. It can be appreciated that the invention has several forms and/or technology generations including both media and transducer development.
In accordance with an exemplary embodiment, a multilayered three-dimensional (3D) recording system, comprises: a three-dimensional (3D) media, the three-dimensional media includes a plurality of magnetic sublayers, wherein each magnetic sublayer is adapted for writing data to; and a recording head having a trailing edge, and wherein the trailing edge has a higher permeability than the recording head.
In accordance with another exemplary embodiment, a method of writing information into a three-dimensional media comprising: providing a three-dimensional media having a number of magnetic sublayers; and writing data onto the three-dimensional media with a recording head by continuously varying a drive current during at least one scan along a track to vary the depth of penetration in the magnetic sublayers of the three-dimensional media, and wherein reaching a sublayer in the three-dimensional media is obtained by exceeding the coercivity value in the sublayer, which is sufficient to reverse the magnetization required for recording information.
In accordance with a further exemplary embodiment, a method of reading information from a three-dimensional media comprises: providing a three-dimensional media having a number of magnetic sublayers with information thereon; and reading a stray field emanating from a bit location on a surface of the three-dimensional media.
In accordance with another exemplary embodiment, a method of reading magnetic information from a three-dimensional media comprises: providing a three-dimensional media having a number of magnetic sublayers with data thereon; and optically reading the data using a magneto-optical Kerr effect, wherein a polarized light passes through a region in the magnetic sublayers changes its polarization depending on the magnetization of the region.
a)-(d) are schematics illustrating: (a) a recording across the thickness via continuous variation of the recording field, wherein the boundary curve is the field profile when H=Hc, where Hc is the coercivity; (b) a perpendicular recording system, including FIB images of FIB-fabricated; (c) a diasingle pole writer with a 80-nm trackwidth; and (d) a MFM nanoprobe for reading a component of the stray magnetic field.
a)-6(e) represent (a) an SEM image of electron beam lithography (EBL or E-Beam lithography) fabricated nanostructures with dimensions ranging from 30-500 nm; (b) and (c) represent SEM images of the various nanostructures after deposition of 2 decoupled magnetic layers with various dimensions and similar dimensions, respectively; (d) represents an AFM (bottom left)/MFM image which clearly shows 3 distinguishable signal levels both visually and on the sectional analysis; and (e) shows an atomic force microscope (AFM) (topography)/magnetic force microscope (MFM) image of the similar dimensions nanostructures which also show at least 3 signal levels.
a)-(d) illustrate schematics (not to scale) showing: (a) a Co/Pd stack in the conventional sense as used in perpendicular recording: Pd interlayers are approximately 0.5 nm thick; (b) a 3-D (3-dimensional) implementation of multilayers: with Co (top) and Co/Pd in the conventional sense (bottom) as “single” layers in a stack, wherein the micromagnetic simulations illustrating the effect of the interlayer spacing on the ability of the magnetization to follow the recording field profile; (c) wherein the interlayer spacing is approximately less than (<) 1 nm (strong exchange coupling); and (d) wherein the interlayer spacing is approximately greater than (>) approximately 1 nm (weak exchange coupling).
a)-(c) illustrate: (a) a schematic to illustrate the concept of creating a coercivity (or Curie temperature) gradient to improve the writing process, and Kerr signals for a 7-layer stack of Co/Pt multilayers with a composition of Pt(5 nm)/[Co(0.45 nm)/Pt(0.55 nm)]×7/Pt(2.5 nm)/Pd(2.5 nm) with coercivity of approximately 300 Oe (
a)-(b) are schematics illustrating 3D recording systems with a single pole head (a—left) and a ring head (b—right).
a)-(b) are diagram illustrating: (a) sequential recording of different levels in a 3-D patterned media; and (b) The recording field profiles in the first, second, and third layers at a given value of the drive current in the recording transducer above the cell.
a)-(c) illustrate: (a) the “charge” representation of different digital values in the bit cell consisting of many magnetic layers; (b) the stray field versus the digital level in the 3-d multi-level mode; and (c) SNR versus the number of layers.
a)-(b) is an illustration of the effect of using a laminated single pole head to record into a 3D media wherein (a) and (b) show head structures and field profiles without and with an additional trailing layer made of a material with higher magnetic permeability, respectively and the dark (red) color indicates the region where the recording field exceeds the coercivity of the media, and the relatively permeability of the trailing layer, μ1, substantially exceeds the permeability of the rest of the pole, μ2.
a)-(b) are diagrams showing: (a) a bi-layer trailing pole structure; and (b) reluctance model to describe the flow of magnetic flux through a laminated pole.
a)-(b) are schematics of 3D systems with 3D media with (a) a continuous soft underlayer (SUL) and (b) a patterned SUL.
a)-(c) are recording field profiles for three systems: (a) without a SUL, (b) with a continuous SUL; and (c) a patterned SUL, respectively, and wherein a darker red color indicates a stronger recording field.
a)-(b) are cross-section diagrams (not to scale) illustrating the process of recording into 3D media through the multi-scan process with the information recorded into different layers during different scans of a track, and wherein (a) recording into the bottom layer (Layer N) with the recording field exceeding the coercivity in Layer N during recording scan 1; and (b) recording into the next to the bottom layer (Layer N−1) with the recording field exceeding the coercivity in Layer N−1 while weaker than the coercivity in Layer N during recording scan 2.
a)-(c) are schematics showing the magnetization directions in the three layers of a 3-layer ML3D media during: (a) a full hysteresis loop to choose levels 1 to 6; (b) a minor loop to choose level 7; and (c) a minor loop to choose level 8.
a)-(b) are illustrations of the principle of differential reading with (a) a trivial differential reader including a magnetoresistive element (GMR, TMR, etc) and an independent “soft” layer; and (b) a magnetoresistive element integrated with the trailing soft pole of the recording head, and wherein in the latter case, the trailing pole is used as the “soft” shield necessary to enable the differential mechanism.
a)-(b) are schematics of (a) a differential reader and (b) a differential pair incorporated by shields integrated in a 3D system.
a)-(b) are schematics to illustrate optical reading of information from 3D media in (a) a reflection mode and (b) a transmission mode.
a)-(b) are schematics to illustrate optical reading of information from 3D media in accordance with another embodiment.
a)-(b) are (a) an illustration of 8 signal levels resulting from a 3-layer 3D media (not to scale); and (b) an experiment clearly indicated 8 signal levels coming from a 3-level 3D media.
In accordance with an exemplary embodiment, the concept of multilevel (ML) 3D magnetic recording relies on using more than two signal levels to code recorded information. This is in contrast with conventional recording schemes in which they use binary coding methods, meaning that the signal recorded into or read back from magnetic media has only two states: presence or absence of the magnetization reversal in a bit transition. For example,
ML magnetic recording refers to the use of multiple signal values to encode data onto a magnetic disk. By using more than two levels, more information can be put in the minimum feature size.
Before exploring the feasibility of ML magnetic recording, it may be helpful to learn from the experience of other industries with regard to ML data coding. For example, in 2004, one of the electronics giants LSI Logic acquired Calimetrics—the first company which focused on the development of a multilevel optical recording system for CD or DVD technologies. Though Calimetrics ceased to exist mostly because of the suddenly raised popularity of writable DVD technology, it was successful in introducing a multilevel signal standard in the world of information related technologies, especially overseas. Today, it is becoming normal for new technologies to start to prosper first overseas where it is often easier to launch new standards because of the lack of a well-established traditional technology infrastructure. Although the ML technology by Calimetrics offered only 8 signal levels and was not rewritable, it still appeared successful especially in its tremendous impact on modern DVD technologies.
However, it can be appreciated that differences between ML 3D magnetic recording and ML optical technology exist, which include (a) the ML technology developed by Calimetrics competed in the market of DVD/CD technologies while the analyzed ML magnetic technology, at least at this early stage, is targeting the market of hard drives; (b) the Calimetrics technology was optics-based while the exemplary embodiment technology is magnetics-based. Consequently, the magnetic technology would be rewritable, which is in contrast with the read-only Calimetrics technology; and (c) the optical technology used only 8 signal levels while the ML magnetic technology has potential for over 1000 levels.
Traditionally, scaling laws have been followed to advance magnetic data storage industry to a next level/generation. Scaling implies that with each next generation, all dimensions of a recording system 200 have been respectively reduced, as schematically illustrated in
It can be appreciated that perpendicular recording may defer the fundamental superparamagnetic limit for several more years, and which could bring the areal densities in demonstration data storage systems hopefully to one terabit-per-square-inch.
Unlike many other alternatives to the conventional recording (such as, for example, heat-assisted magnetic recording (HAMR) and patterned media), perpendicular recording has been quickly adopted by the industry because of its similarity to the conventional technology. For example, during the last two years, most leading companies in the industry have transitioned to perpendicular recording. Perpendicular recording is expected to defer the superparamagnetic limit to approximately 1 Terabit/in2. Alternatively, with multilevel 3D recording, in accordance with an exemplary embodiment, areal densities above 10 Terabit/in2 are desirable.
The concept of three-dimensional (3D) magnetic recording was proposed about ten years ago. The technology is different from the traditional approach to deal with one surface (of the recording media) only. Instead, a third vertical dimension is used to store information not only on the surface but also in the volume of the recording media 400. In other words, the information is packed also across the thickness, as shown in
In accordance with an exemplary embodiment, several advantages of a ML3D systems include a) the technology merits due to 3D magnetics and multilevel signal, b) simplicity, and c) potential as discussed below in more detail. As mentioned above, this is the only magnetic technology which relies on the use of a third dimension to record, retrieve and store information in a device. In other words, the information will be recorded not only on a surface but also across the thickness or in the volume of the recording media. As a result, substantially more information could be recorded per unit area and thus the fundamental data density limit could be extended substantially further. In addition, the exploitation of multi-level signal will substantially loosen fabrication requirement. In other words, unlike conventional magnetic and silicon technologies, this effort won't be limited by the many problems arising during fabrication of sub-100-nm devices. In this context, “multilevel” implies the ability to record more information per unit surface area.
Another advantage, is the relative simplicity of multilevel 3D recording, which is due to the fact that this technology has some similarity with perpendicular recording. Similar to perpendicular recording, the information, at least at the initial stage, will be recorded perpendicular to the disk. However, other media configurations can not be disregarded. For example, longitudinally oriented media or media with the magnetization tilted at some arbitrary angle may have some advantages perpendicular media don't have. As the complexity of multilevel 3D recording system increases with each next generation, it can be appreciated that some benefit can be obtained to eventually switch to one of the non-perpendicular modes.
In addition, because of the two obvious technology merits, 3D magnetics and multilevel signal configuration, respectively, the technology has long-term future potential compared to any other alternative data storage technology. While 2D technologies are in the end of their roadmaps, the described ML3D technology is just in the beginning of its roadmap. In accordance with an exemplary embodiment, a varying recording field 510 can be used to sequentially record across the media thickness, as illustrated in
Logic operations such as AND, OR, NOT, etc. can also be performed using the ML3D system. In this system, the multilevel signals can be attributed to particular logic operations. For example, considering the most trivial case of having a two-stack magnetic media for which each stack has a distinguishable coercivity value, the first string of bits can be recorded on the layer exhibiting the higher coercivity value with a relatively large magnetic field followed by the recording of the second string of bits (directly above the first string of bits) using the lower coercivity field value. The read-back signal is thus proportional to the superposition of the signals emanating from both magnetic layers and the interpretation of this signal may be read in a particular fashion in order to perform the different logic operations on the two strings of bits.
For example, the layers can be deposited via E-Beam lithography based pre-patterned media as shown in
Alternatively, in accordance with another exemplary embodiment, the magnetic layers (or sublayers) can be deposited via regular sputtering systems. Continuous and patterned versions of 3D media could be used. For illustration purposes, a focused ion beam (FIB) can be used to further pattern 3D media within the plane of the disk. A straightforward MFM experiment can be performed to demonstrate the presence of more than one signal levels. In accordance with an exemplary embodiment, a continuous CoCrPt-based 3-layer 3D media can be sputter-deposited and then patterned via FIB into a square periodic arrays with a linear period of 80 nm. Then, the media can be demagnetized using an alternating decaying external magnetic field.
It can be appreciated that this experiment illustrates that 3D media can be partially polarized across the thickness thus resulting in a multilevel signal configuration. On the contrary, it is known that in conventional recording the magnetization remains in one of the two saturated states and no partial polarization is feasible. To further perfect the concept of generating a multilevel signal, various mechanisms have been developed and explored. In accordance with an exemplary embodiment, one of the general tasks is to maximize the number of signal levels while maintaining signal-to-noise ratio (SNR) between the signal values in each pair of adjacent levels above approximately 5 dB. In accordance with an embodiment, the 5 dB value is dictated by the requirements of the novel data coding methods, as described below. In addition, it can be appreciated that in accordance with an exemplary embodiment, it would be desirable to develop a media in which each magnetic grain could be recorded independently (without a chain clustering effect). For example, a conventional CoCr-based composition (as used in conventional longitudinal recording media) can be used for this purpose as well and the media could be made even thicker to fully maximize the effective number of individual Co grains participating in the process.
In accordance with an alternative exemplary embodiment, a conventional Co/Pd (or Co/Pt) multilayer structures with strong surface-induce perpendicular anisotropy can be used. The adequately high perpendicular anisotropy is one of the key properties pursued in the design of a perpendicular recording system. Similarly, the “3-D magnetic media” can be fabricated as a stack of magnetic layers (or sublayers) separated from each other by thin non-magnetic interlayers 410, as shown in
In accordance with an exemplary embodiment, one of the purposes of some of the interlayers, Pd (Pt), is to break the quantum “exchange” coupling between the adjacent stacks of strongly coupled multilayers (within each stack) and thus to independently record information into the separate stacks. In accordance with an exemplary embodiment, each stack can consist of one or more pairs of strongly coupled Co and Pd (or Pt). Effectively, each stack acts as an individual magnetic layer in the sense that the magnetization is always in the same direction in each Co layer within the stack. The interface provides the perpendicular anisotropy. The more interfaces (or pairs) that are used the stronger the signal. However, it can be appreciated that the quality of the interface can suffer with the increase of the number of layers (or sublayers). In addition, it would be desirable to minimize the net thickness of 3D media with maximizing the number of effective stacks.
In accordance with an exemplary embodiment, assuming perpendicular magnetic anisotropy, the magnetization in the adjacent stacks can be directed in opposite directions because of the lack of adequate exchange coupling. The lack of the exchange coupling is due to the sufficiently thick quantum-mechanical exchange breaking interlayer (greater than (>) approximately 1-2 nm). Otherwise, due to the exchange coupling, all the layers (or sublayers) in a bit cell would have magnetization directed in one direction. The latter occurs in the multilayers as used in perpendicular recording. In the traditional case, the interlayer separation is chosen to be less than (<) approximately 1 nm to maximize the exchange coupling between the adjacent layers and thus increase the perpendicular anisotropy due the strong surface-induced anisotropy. This difference between Co/Pd (Pt) multilayers, as used in the conventional perpendicular mode and in the proposed 3-D implementation, is described below in more detail.
It can be appreciated that for the described 3-D implementation; it is not desirable to increase the thickness of Pd (Pt) non-magnetic layers to break the exchange coupling between the adjacent Co layers. In accordance with an exemplary embodiment, if the Pd layers are not adequately thin, the surface effects would be substantially reduced and thus the magnetic anisotropy would be determined by the interplay of intrinsic crystalline and shape anisotropy rather than by the surface-induced anisotropy. However, it can be appreciated that with an adequately high quality process, this may not be a problem. For example, in accordance with an exemplary embodiment, an interface between Co and Pd (Pt) may be sufficient to induce adequately strong perpendicular anisotropy. Alternatively, one interface may not be enough for certain applications.
In accordance with an exemplary embodiment, Co/Pd (Pt) multilayers 700 can be used instead of the individual Co layers. In this embodiment, adjacent “single” magnetic layers (Co/Pd multilayers in the conventional sense) are separated by a relatively thick (greater than (>) approximately 1 nm) Pd interlayer to break the exchange coupling between the “single” layers 720, as shown in
In accordance with an exemplary embodiment, to simulate the grain interactions in 3-D media, a micromagnetic code can be used. In this simulation, a 3-D media is represented as a stack of alternating magnetic and non-magnetic layers with each layer consisting of interacting grains (in the form of a simplified square mesh). The short-range exchange coupling between adjacent grains and the long-range magnetostatic coupling between grains are described through the Landau-Lifshits-Gilbert (LLG) formalism. Each square grain had a 3-nm side and a thickness equal to the respective layer's thickness. In accordance with an exemplary embodiment, as the magnetic layers, conventional Co/Pd-multilayers were modeled with an anisotropy constant, K, of 5×105 erg/cc, saturation magnetization of 250 emu/cc, and coercivity field of 8 kOe. The relative exchange coupling constant, he, was modeled to be anisotropic: with the in-plane and perpendicular-to-the-plane exchange constants equal to 0.1 and 0.5, respectively.
c) shows a micromagnetically simulated magnetization pattern 730 after applying the recording field from a single pole head to a stack of multilayers with an interlayer separation (Pd layer thickness) of less than one nanometer. The field from the single pole head was also modeled micromagnetically. In accordance with an exemplary embodiment, the stack of multilayers were made of a high moment FeAlN amorphous alloy with an anisotropy field, Hk, of 15 Oe, saturation magnetization, 4πMs, of 20 kG, and an exchange constant, A, of 10−6 erg/cm. Before applying the recording field, the media was saturated in the opposite direction. In accordance with an embodiment, it was observed that the magnetization pattern in the media did not follow the curved profile of the recording field, which can be explained by the relatively strong exchange coupling between the adjacent magnetic layers. Because of the strong coupling, the magnetization appears uniform across the thickness. In accordance with an exemplary embodiment, this is exactly the case in the conventional implementation of Co/Pd multilayers in perpendicular magnetic recording. It can be appreciated that in the two plane directions, the inter-granular exchange interaction was modeled to be substantially weaker so that individual bit cells could be defined in the plane. In contrast, in the proposed 3-D implementation, the interlayer thickness was chosen sufficiently thick to break the exchange coupling so that the magnetization pattern could follow the profile of the recording field and thus each grain within the disk plane and across the thickness could be accessed independently 740, as illustrated in
In accordance with an exemplary embodiment, if there are approximately one thousand “individual” layers and the cross-section of each cell is approximately 80×80 nm2, this type of 3-D media could store the amount of information in the 100 Tbit/in2 density range and even more in the patterned form of 3-D media, as explained below. In addition, it can be appreciated that the effective areal density is limited not by the maximum capacity of the recording media, but rather by the mechanisms of data recording and retrieval.
It can be appreciated that due to the 3-D approach, the bit cell cross-section does not have to be ultra-small to achieve relatively high densities. In fact, no ultra-sophisticated nanofabrication tools may be necessary to achieve areal densities exceeding what is projected to be achieved within the next decade with 2-D recording systems (e.g., heat-assisted magnetic recording (HAMR)) using E-beam or focused ion beam (FIB). In the case of ML3D, the effective density can be achieved just via the deposition of a stack of layers. For example, an arrangement with a bit cell with a 160×160 nm2 cross-section and a stack of 20 “independent” layers would be equivalent to an effective areal density of approximately 500 Gigabit/in2. For example, to achieve a 160×160 nm2 bit cell cross-section, UV-optical lithography is more than sufficient. However, as more advanced fabrication methods, such as E-beam and FIB-based and imprint lithography are used, the areal densities can increase beyond 1 Terabit/in2.
In accordance with a previous embodiment, it was shown that one approach to increase by a factor of approximately, the effective recording and sensitivity field can be through using a patterned soft underlayer (PSUL) or even patterned soft interlayers (PSUI). It can be appreciated that one of the practical advantages due to the use of a PSUL can include (a) SNR drastically increases as a result of patterning a SUL; (b) patterning of SUL increases both the recording and sensitivity field gradients, which can be critical for maximizing the areal density during writing and reading, respectively; and (c) the recording and sensitivity fields remain well localized across the entire thickness to generate a multitude of signal levels.
As previously mentioned, multilevel 3D magnetic recording can improve the data capacity of any of the proposed alternate technologies. In the case of HAMR, a localized light spot in the order of tens of nanometer is focused in the near field regime in order to effectively though instantaneously reduce the coercivity of highly anisotropic material, such as, for example, FePt-L10 material. Since the stability of each recorded bit is directly proportional to the effective anisotropy of the material and the effective volume of each grain, it is possible to substantially scale down the grain size and thus increase the areal density in a HAMR system. Using multilevel 3D recording can also increase the areal density by fabricating media 800 in which isolated magnetic layers exhibit a range of Curie temperatures, resulting in Curie temperature gradient across the thickness 830 of the 3D media (
Another way to effectively increase the number of individual layers would be through creating a gradient of the coercivity across the thickness of 3D media. In accordance with an exemplary embodiment, one mechanism to vary the coercivity in a Co/Pd-based 3D media 800 is through variation of the thicknesses 830 of Co and Pd layers in each Co/Pd pair across the media thickness, as shown in
To illustrate the concept of varying coercivity, in accordance with an exemplary embodiment, Co/Pd multilayer-based 3D media was fabricated with two distinct stacks of two different values of coercivity, 300 and 3500 Oe, respectively. As shown in
In accordance with an exemplary embodiment, at least over six layers (or over 26=64 signal levels) can be independently accessed in the above described manner. As previously mentioned, using a different type of media and such signal enhancing mechanisms as the integration with a patterned soft underlayer even soft interlayers and others might further increase signal to noise ratio (SNR) in 3D recording systems.
In accordance with an embodiment, several implementations of a 3-D memory device can be obtained. In accordance with an exemplary embodiment, E-Beam lithography can be used to fabricate a prototype with sub-100-nm dimensions. Alternatively, in accordance with another embodiment, a focused ion beam (FIB) can be used to fabricate a prototype with sub-100-nm dimensions. One of the proposed mechanisms to access data takes advantage of a E-Beam lithography and/or FIB-developed methods is to control strong magnetic fields using a “soft” magnetic underlayer (SUL) under the 3-D recording medium. During the write process, the use of a SUL allows to substantially increase the recording field across the entire thickness of the 3-D medium. During the readback process, the magnetic “softness” of the SUL strongly influences the sensitivity of each read element and thus will be used as a mechanism to identify a uni-field plane (2-D layer).
In accordance with an exemplary embodiment, to minimize the inter-symbol interference and improve stability, the recording medium can be patterned in all three dimensions. It can be appreciated that in an exemplary embodiment, the integration side of multilevel 3D (ML3D) recording can be addressed. Unlike earlier work, which focused on certain 3D media configurations, in accordance with an exemplary embodiment, the invention relates to the integration side of multilevel 3D (ML3D) recording, and the designing a ML3D recording systems including a recording head, a 3D media, and a data channel, as a whole.
As described in the previous section to enable multilevel 3D recording, it was proposed to use a certain 3D media configuration 1000, as illustrated in
In such an implementation, the recording process is produced by continuously varying the drive current I. By gradually increasing the current from zero to some large value, the information can be gradually recorded in individual layers across the 3D media starting with the top (closest to the recording head) and finishing the bottom layer, as described above and illustrated in
a) illustrates a micromagnetically simulated example of how each layer in a bit cell stack can be recorded in a patterned 3-D media in ML3D. In accordance with an exemplary embodiment, initially, a bit cell is assumed to be saturated. This means that the entire bit cell stack is magnetized in one direction (blue color). According to a trivial encoding scheme, this state may, for example, reflect a digital state “10”. Then, the recording field would be increased in the opposite direction to reverse the magnetization (red color) in the top layer. This state could reflect a digital “9”. When the field is further increased to the value overcoming the coercivity of the second (counting from the top) layer, the magnetization in the second layer is reversed. This state would reflect a digital “8”, and so on. The simulated recording field profiles in the first, second, and third layers, at an arbitrary current value in the recording transducer above the cell are shown in
ML3D Recording Work during Reading
In accordance with an embodiment, directly reading the stray magnetic field from a location above the media can accomplish the goal of the most readback operations. Similar to conventional magnetic recording, a giant magnetoresistive (GMR) or tunneling magnetoresistive (TMR) or another sensor can be used to read the magnetic field emanating from the media in the vicinity of each bit cell. In other words, this reading scheme reflects the ML3D recording mode, as described above (see
As a result, the signal levels read back from different information states are not clearly distinguished from each other if the number of levels exceeds approximately 5, as shown in
In accordance with an exemplary embodiment, to increase the strength and the trailing gradient of the recording field, a high-permeability layer 1410 can be added to the trailing edge 1420 of the pole of the head 1430, as shown in
In accordance with an exemplary embodiment, a recording head is disclosed having different values of relative magnetic permeability (not magnetic moment) between the two magnetic layers. Particularly, as explained below, the permeability of the trailing layer (Layer 1) should be substantially higher than the permeability of the leading layer (Layer 2). For example, as described below, varying the permeability of a soft magnetic material, e.g. Permalloy (Ni/Fe 81/19), during the deposition process can be obtained by just slightly changing the deposition conditions. For example, if sputtering is used to deposit Permalloy films, by slightly changing chamber pressure or deposition temperature or cooling condition, one can vary the relatively permeability from 200 to over 10,000. In accordance with an alternative embodiment, to change the saturation magnetization one might need to use a totally different material as the trailing layer. It can be appreciated that in general, over 100 distinct layers can be used, with each magnetic layer (or sublayer) in the recording media approximately 1-2 nm thick. In addition, the thickness of the trailing layer can vary from approximately 10 nm to 200 nm depending on the targeted areal density and the number of layers to be accessed during writing.
The mechanism of accessing different layers across the thickness of 3D media 1500 can be obtained by gradually increasing the current through the drive coil 1510 the field reaches deeper layers in the recording media 1500, as illustrated in
To illustrate the concept, in accordance with an exemplary embodiment, a single pole head illustration was used. It can be appreciated that the concept is valid also for the ring head case. Again, the single pole head is best used with a recording media with a soft underlayer. The ring head works well both with and without a soft underlayer. It can be appreciated that the information along a track is recorded by the trailing edge of the recording field because the trailing edge is the last part of the head “seen” by any location along the track in the media as the disk spins under the head. The dark color (dark red color) in the figures indicates the regions in which the recording field exceeds the coercivity in the media. Also, it should be noted that for the simplicity of the description, it was assumed that the layers (or sublayers) of the recording media have uniform properties, i.e. have the same coercivity value. However, it can be appreciated that in accordance with another exemplary embodiment to further enhance this effect, a gradient of the coercivity across the thickness, i.e. variation through different layers (or sublayers), can be created as described above.
In accordance with an exemplary embodiment, the dependence of the recording field on the drive current for the above described four-layer 3D recording system with a laminated pole structure is shown in
It can be appreciated that in accordance with an exemplary embodiment, the concept can be extended to 3D media with over 100 layers.
In accordance with an exemplary embodiment, the three-dimensional (3D) media and recording head parameters preferably are within the following ranges:
a. The two magnetic layers (i.e., recording head and trailing edge) of the recording head in the head pole are preferably made of one or more soft magnetic materials with different values of the relative permeability. To accomplish the change of the permeability, one can either use different materials with different permeability values or the same material type with different values of permeability. The latter is definitely a substantially more cost-effective solution. For example, one can use the popular soft material Permalloy (Ni/Fe-81/19). It is a very straightforward task to vary the permeability in layers during sputter deposition process via variation of sputtering conditions, e.g. pressure, temperature, cooling condition, etc. Via such straightforward methods, the value of the relative permeability could be controllably changed from few hundreds to over tens of thousands. Optionally, other soft materials could be used: high moment Fe/Co compositions, high moment alloys such as FeAlN, FeTaN, and others.
b. The thickness of the trailing (i.e., trailing edge) and leading layers (recording head), i.e., Layers 1 and 2, respectively, in accordance with an exemplary embodiment are preferably in the ranges of approximately 50-200 nm and approximately 1-10 pin, respectively, depending on the number of the recording layers in 3D media and the targeted surface density. For example, for a 50-layer 3D media (with the thickness of each pair of magnetic and separation layers of approximately 2 nm), wherein the relative permeability of the trailing and leading layers to be approximately 2,000 and 100, respectively, the saturation magnetization of 2.4 T for both layers, the trailing and leading layers are preferably approximately 200 nm and 5 μm thick to achieve the areal density above 10 Terabit/in2.
c. In accordance with an alternative embodiment, the materials of the trailing (i.e., trailing edge) and leading layers (recording head) can be made of different materials. For example, the higher moment material for the trailing layer will provide the higher achievable saturation recording field. Approximately, the saturation (meaning maximum) recording field scales with the saturation magnetization of the utilized material. The alternative materials for the trailing layer could be high moment Fe/Co compositions, high moment alloys such as FeAlN, FeTaN, and others.
d. The thickness of the trailing pole or trailing edge (Layer 1) in accordance with an exemplary embodiment is preferably in the range from 50 nm to 200 nm to minimize the skew angle sensitivity. The skew angle is preferably avoided especially with the use of single pole head.
e. The thickness of each magnetic layer in the 3D recording media is preferably in the range of 1 to 5 nm.
f. In accordance with an exemplary embodiment, the magnetic recording layers in the 3D media can be made of any magnetic recording material. Layers can be all of the same composition, or different compositions can be used for each layer. In accordance with an exemplary embodiment, the recording layers can comprise stacks of coupled Co/Pd or Co/Pt multilayers, with one or more coupled pair in each layer. Co and Pd (or Pt) thickness in each coupled layer can range from approximately 0.1 to 40 nm. In addition, it can be appreciated that the thicknesses of the Co and Pd (or Pt) does not have to be identical and/or coincide. For example, in accordance with an exemplary embodiment, the magnetic layers in the 3D media (where the information can be separately recorded) can be made of Co/Pd or Co/Pt multilayers, with one or more pairs in each layer, with Co and Pd (or Pt) layers ranging from 0.1 to 2.0 nm.
In accordance with a further embodiment, the magnetic recording layers can further comprise cobalt and chromium based materials, for example, such as that described in: Magnetic properties and read/write characteristics of Co—Cr—(Pt,Ta)/(Cr—Ti,Cr) thin film media, Shiroishi et al, Journal of applied physics: Annual conference on magnetism and magnetic materials, 73: 5569 (1993); Magnetic Properties and Recording Characteristics of Co—Cr—Ta—Pt Thin Film Media, Toshio et al., Japanese Journal of Applied Physics, Volume 32, Issue 9R, pp. 3823 (1993); Preparation of Co—Cr—Pt alloy film with high perpendicular coercivity and large negative nucleation field, Keitoku et al., Journal of Magnetism and Magnetic Materials, Volume 235, Number 1, October 2001, pp. 34-39; Perpendicular Co—Cr magnetic recording media prepared by sputtering using ECR microwave plasma, Yamamoto et al., Magnetics, IEEE Transactions on, Volume 32, Issue 5, September 1996 Page(s):3825-3827; Magnetic energy distribution in polycrystalline sputtered CoCr magnetic thin films, Al-Sharab et al., EPJ. Applied 2008, vol. 42, no 2, pp. 125-128; Zero magnetostriction Fe—Co—Cr magnetic recording media, U.S. Pat. No. 4,396,575; and Magnetic thin film material for magnetic recording, U.S. Pat. No. 5,560,786 (Co—Cr—P—Pt or Co—Cr—P—Ni), each of which is incorporated herein by reference in its entirety.
In accordance with an exemplary embodiment, the magnetic recording layers may further comprise one or more stacked CoCrPt/Pd or CoCrPt multilayers.
In accordance with another exemplary embodiment, the magnetic recording layers can comprise Fe—Pt materials in the L10 phase as described in Monodispersed and highly ordered L10 FePt nanoparticles prepared in the gas phase, Qui et al, Applied Physics Letters, Volume 88, Issue 19, id. 192505 (2006); Magnetic and structural properties of L10-FePt from a viewpoint of high-density recording media, Okamota et al., Nippon Oyo Jiki Gakkai Kenkyukai Shiryo, 139: 95-103 (2005); Growth of L10 ordered FePt alloy films at reduced temperatures, Martins et al., physica status solidi, 201: 837 (2004); L10 ordered FePt based double-layered perpendicular recording media with (002) oriented FeCo films as a soft magnetic underlayer, Hu et al., Thin Solid Films, 8: 2067-2070 (2008); and Crystallography of ordering studies of the L10 phase transformation of FePt thin film with Ag top layer, Zhao et al., J. Appl. Phys. 95: 7154-6 (2006), each of which is incorporated herein by reference in its entirety.
g. The separation layers in the 3D media can be made of non-magnetic materials such as Pd, Pt, Ti, Ta, Cu, Au, Ag, MgO, ITO, or/and other materials.
h. The separation layers in the 3D media could be made also of other magnetic materials to break the exchange coupling between the recording layers. In accordance with an exemplary embodiment, the magnetic materials should be different from the magnetic materials of which the recording layers are made. The thickness of the separation layer can vary from 0.3 to over 5.0 nm depending on the material used. For example, with Pd, the thickness of 1 nm is sufficient to substantially reduce the exchange coupling between adjacent Co/Pt (or Co/Pd) layers.
i. As a design guideline for defining the thicknesses of the magnetic and non-magnetic layers in 3D media, in accordance with an exemplary embodiment, one can follow the principle of minimizing the net thickness of the 3D media. With increasing the thickness of 3D media, the strength and the gradient of the recording field is sacrificed in the remote (from the transducer) layers.
j. In accordance with an exemplary embodiment, and as earlier mentioned, a single pole head is preferably used in combination with a media with a soft underlayer. The soft underlayer can be implemented in continuous as well as patterned forms.
k. The implementation with a ring head in accordance with an embodiment is similar to the implementation with a single pole head, with the exception of the direction of the change of the magnetic permeability. In the ring head case 1700, information is recorded by the trailing edge of the field emanating from the gap 1710. As a result, the layer with the highest permeability is preferably positioned closest to the gap 1710, as illustrated in
In accordance with another exemplary embodiment, the relative magnetic permeability in different layers in the multilayered pole of the recording head is preferably varied. To simplify the description, the concept is described on the example of two magnetic layers only.
It can be appreciated that in accordance with an exemplary embodiment, that with two different magnetic layers in the pole of a single pole head (with a media with a soft underlayer) is to provide an analytical model to explain the physics underlying the above described transducer design for multilevel 3D (ML3D) recording system. As described above, according to an exemplary embodiment, the trailing pole is preferably made of more than one magnetic layer, with the highest permeability magnetic layer at the trailing edge (for the single pole head and the other way around for the ring head), so that the highest flux density (field) can be concentrated near the trailing edge and thus a relatively sharp field profile could be created in the trailing edge. It can be appreciated that the field next to the trailing edge of the head is used to record information in the spinning disk. The narrow field profile is especially beneficial when a media with a soft underlayer is used and is a means of minimizing the destructive skew angle effect.
If only two layers (with two different values of permeability) are used, the high permeability layer (with permeability μ1) is used to concentrate the magnetic flux at the trailing edge, while the low permeability layer (with permeability μ2) provides a parallel path for the flux to flow. To describe this effect, the following equivalent magnetic circuit model (often called reluctance model) can be used. The two parallel paths can be represented as two reluctances, R1 and R2, connected in parallel, as shown in
In accordance with another exemplary embodiment, the above described laminated transducer design with a 3D media 1900 with a soft underlayer (SUL) 1910 and with a patterned soft underlayer 1920, is illustrated in
It can also be appreciated that the use of a patterned SUL can not only substantially increase the recording field but also provides a much sharper field profile across the entire thickness of the recording media. In fact, the localization region is defined by the length b of each island in the patterned SUL 1920, as shown in
In addition, in certain conditions (depending on the curvature of the patterned islands and others) will result in having the maximum recording field being created in the bottom layer, which can be contrary to the conventional case when the recording field has its maximum in the top layer and goes down to its minimum in the bottom layer of the recording media. It can be appreciated that such reversing of the recording field profile across the thickness of the media can be beneficial in cases when the information needs to be first recorded in the top layer.
Principles of Writing Information into 3D Media
It can be appreciated that one of the ways to record information into 3D media is to use a single-scan method because it requires only one or two scans (spins) to record information into a track. The single-scan method relies on continuously varying the drive current during one scan (spin) along a track to vary the depth of penetration in layers in the 3D media. The statement “reaching a certain layer in 3D media” implies that the recording field in the layer has exceeded the coercivity value and therefore is sufficient to reverse the magnetization as required for recording information. This mechanism was described in more detail above and is also illustrated in
In accordance with another exemplary embodiment, each track is scanned N times where N is the number of magnetic layers in the 3D recording media. To identify each layer during the writing process, a certain electric current is driven through the drive coil. The value of the current is chosen to generate a recording field higher that the coercivity field in the respective layer. As described earlier, continuous and patterned SULs under the recording media could be used to further increase the recording field and thus the number of accessible magnetic layers in 3D media. Also, it can be appreciated that most recording systems are designed so that the field drops away from the recording head. As mentioned above, the exception to this rule is the case with a certain patterned SUL where the recording field has its maximum at the bottom layer of the 3D media, i.e. the layer closest to the SUL. For simplicity, the description below is related to the conventional cases only.
In accordance with an exemplary embodiment, information in different layers is recorded sequentially, layer after layer starting with the bottom layer of the media (as “seen” by the head). As a result, to record information in each track, the disk has to spin N times under the recording head. To be exact, N+1 times because there is one initial “reset” spin to “erase” all the information in the track. “Erasing” implies magnetizing the entire track in one direction. During the first recording spin (after the initial “reset” spin), the recording field is kept strong enough to record information in the bottom layer, i.e. Layer N. During the second spin, the field is reduced to the value sufficient to overcome the coercivity in Layer N−1 however smaller than the coercivity in layer N. This assures that new information is recorded in Layer N−1 and the previously recorded information in Layer N is intact. In a similar way, information can be recorded in Layers N−2 and all the way to Layer 1, as illustrated in
Another form of recording information in the multi-scan mode is to sequentially record different layers in a certain sector (instead of a track) of the disk. Depending on specific applications, different forms of recording might be preferred. From the above description, it is clear that the number of signal levels achieved in the multi-spin recording mode will be 2N, where N is the number of magnetic layers in 3D media.
In accordance with another exemplary embodiment, the recording process relies on using a sophisticated hysteresis loop of ML3D media. Using this method ensures a predictable way to record all the possible signal levels in no more than two scans. To ensure reversibility of the recording process, every bit of the media should be magnetized in a certain order to complete the full hysteresis loop for the magnetic material of which the media is made. In other words, irreversible minor loops should be avoided. As an example, a full hysteresis loop of a three-layer ML3D measured via magneto-optical Kerr signal is shown in
In accordance with a further exemplary embodiment, this recording method relies on exploiting so called minor hysteresis loops. A schematic below illustrates the definition of minor loops. Again, for consistency and simplicity of the description, the 3D media consists of 3 layers only. However, it can be appreciated that in a preferred embodiment, the 3D media will consist of more than 3 layers.
a)-24(c) illustrate the magnetization directions in the three layers of a S-layer ML3D media during a full hysteresis loop to choose levels 1 to 6 (2410), a minor loop to choose level 7 (2420), and a minor loop to choose level 8 (2430), respectively. The minor loops are shown by the red lines. In accordance with an exemplary embodiment, the blue, red, and green colored arrows stand for the three layers, respectively. The arrows along the hysteresis curves indicate the direction of the magnetic field. It can be seen that to initiate a minor hysteresis loop, a field reversal should be started before the full loop is closed, as illustrated in
The main idea behind exploring the minor loops is to initiate the signal levels which would not have been possible in the full loop. The number of signal levels which can be initiated using the minor-loop recording technique is 2N, where N is the number of the magnetic layers in a 3D media.
Principles of Reading Information from 3D Media
The principles of reading were described above and illustrated in
In accordance with another exemplary embodiment, SNR in 3D media can be improved by at least a factor of two, and wherein reading 2500 with a differential method is especially beneficial for 3D recording systems. With the differential reading, a read element is positioned next to a magnetically soft film which creating a field shielding effect, which results in differential reading. In accordance with an embodiment, a trivial differential reader 2510 is shown in
According to the differential reading concept, the stray signal coming from the transition of interest is amplified by approximately a factor of two, while the signal coming from other transitions in the media is cancelled out by the two main regions of the differential reader 2610, as shown in
Though information in 3D media should be recorded by a magnetic field source, it can be read back optically. In accordance with an embodiment, one way to read magnetic information from a 3D disk is to use the magneto-optical Kerr effect. According to this effect, a polarized light which passes through a region in a magnetic media changes its polarization depending on the magnetization of the region. It can be appreciated that there are several options to implement such optical reading. In accordance with an embodiment, one can use near-field optical transducers (nanolasers) to control the power of highly focused light in a wide range from 0 to over 1 microwatt. For example, nanolasers can be used to focus light into a 25-nm spot. Ideally, with this technology, light can be focused into a spot size as small as 5 nm. In accordance with an embodiment, this technology provides for writing information in heat-assisted magnetic recording (HAMR) media suitable for ultra-high densities. In the current implementation, it can be appreciated that the same concept of focusing controlled amount of light power into such small regions can be used for the purpose of reading (instead of writing). The difference between the earlier proposed write scheme and the read scheme is in the power range of nanolasers. To avoid undesirable writing during reading, in accordance with an exemplary embodiment, one can use power values of less than 100 nW to focus light into a 25-nm spot. In this case, the locally heated temperature will not exceed Curie temperature for most materials used in magnetic recording and consequently any undesirable writing during the reading process would be avoided. The proposed optical reading 2700 can be implemented either in transmission 2710 or reflection mode 2720, as illustrated
With either of the above reading mechanisms, the number of signal levels that could be read back is preferably still be less than approximately 1000. To further increase the number of signal levels that could be read back with adequate SNR, in accordance with another embodiment, the softness of a SUL can be varied, for example, by biasing it by another source of magnetic field (electric current through a wire or a set of wires nearby, by physically moving a permanent magnetic or a set of magnetic nearby, etc.). By doing this, the sensitivity field that defines the spatial resolution of a reading system 2800 changes, as illustrated in
This technique can be further extended to gradually read from all the layers (by subtracting the signal at each instance from the signal in the previous instance). In this case, the time instances should correspond to the steps of reading from two adjacent layers, respectively. In another exemplary embodiment, biasing can be performed by positioning a permanent magnet close to SUL. If the magnetic is gradually moved close to the SUL (through a mechanical fixture attached to an independent voice coil motor or a piezo-activated manipulator) so that its motion is matched with the timings of reading from adjacent layers, this operation could be used to read information from more than 10 magnetic layers (effectively providing much more than 1000 signal levels).
According to another exemplary embodiment, to create a multilevel configuration during writing, the recording field was varied via the variation of the drive current. Another way to achieve the same goal is to vary the separation between the head and the media (fly height) via the variation of temperature of the local environment between the head and the media. It can be appreciated that recently, a similar technique was developed in the data storage industry to maintain the fly height with an Angstrom precision. This technique is called thermal-fly-height adjust. In this implementation, a heater element is embedded in the head. The heater is used to controllably vary the spacing during both writing and reading.
According to this particular multilevel implementation, in accordance with an exemplary embodiment, the propose of this mechanism is not to maintain the “fly height” with such a phenomenal precision, but rather to precisely control the separation and thus physically move the field profile in the vertical direction with the same unprecedented accuracy. This way, an ultra-precise control of the physical position of the recording head in a third dimension is achieved. This approach is independent of the earlier discussed approach based on the head's drive current variation. Nevertheless, it is of importance to explore the alternative approach because it has its advantages and concerns which are complementary to the first approach since the embodiment, does not require any essential system modification (adding SUL and especially a patterned SUL), and the unprecedented repeatability of less than 0.5 nm in maintaining the fly height.
In accordance with an exemplary embodiment, an acoustic emission (AE) sensor can be used to determine when the head hits the disk. As the head flies over the disk, the AE signal is monitored while the heater's power is increased. The AE signal then “tells” when the head and the disk start to touch. As long as the head/disk contacts are minimized, the repeatability of the measurement appears to be about 0.5 nm. To put this in perspective, the nominal flying height is about 11 nm. So, the error is about 5%, which is quite low for this type of measurement. The AE signal does not abruptly increase when the head and disk start to interact due to lube and other “plastic” deformations that occur at the interface. In fact, as practice shows, asperities and overcoats may be burnished off during prolonged contact. Finally, the control and repeatability of thermal flying height adjust are quite remarkable. In accordance with an embodiment, such a unique control of the fly height is used to create a multilevel signal configuration. The transducer with a micro-heater, as described above, will be used with a 3D media which was discussed above. Again, both approaches are complementarily. Finally, such fly height control can be used for both writing as well as reading multilevel information from 3D media.
For writing, by using this thermal control to physically move the recording head in a vertical direction, recording into different layers across the thickness of 3D media is achieved. Using this method, the effective separation between the head and the media could be controlled at least in the range from 0 to 200 nm. Therefore, layers in a 3D media with a net thickness ranging from a few nanometers to over 200 nm could be achieved. Considering that the thickness of each layer could be as small as 2 to 5 nm, the number of independent layers achieved could range from 100 to 40, respectively.
For reading, by using this thermal control to physically move the recording head in a vertical direction in the range from 0 to over 200 nm, the same gradient reading technique, as described above in Embodiment Number 6, could be used. In this case, instead of changing the softness of SUL, different layers across the thickness of 3D media are achieved as a result of the physical motion of the head in the vertical direction.
In accordance with another embodiment, to further increase the write data rate in ML3D system, in this embodiment to have multiple write heads (of the designs described above) to be positioned on a slider—the standard mechanical fixture used in a conventional drive to ensure a stable separation between the disk and the head. In accordance with an embodiment, the number of the recording heads positioned on the slider should be equal to the number of magnetic layers in ML3D media. In this case, each recording head 3010 is programmed to access a certain layer in the media 3000, as schematically illustrated in
In accordance with a further embodiment, a set of multiple read heads 3110 can be used to increase the data rate during reading. However, it can be appreciated that in this case, all the read heads should be positioned one after another in the direction across the track, as shown in
To demonstrate the concept, in accordance with an exemplary embodiment, a 3D media with 3 magnetic layers was used. In accordance with an embodiment, a 3-layer media having 3 layers (or sublayers) was deposited by a standard method of co-sputtering. The layers (which are also referred to as stacks) includes a first layer (or bottom layer) of Pd(5 nm)/{Co(0.26 nm)/Pd(0.56 nm)}×10 layers, a second layer (or middle layer) of Pd(5 nm)/{Co(0.26 nm)/Pd(0.56 nm)}×7 layers; and a third layer (or top layer) of Pd(5 nm)/{Co(0.46 nm)/Pd(0.56 nm)}×2/Pd(5 nm) layers. The Co and Pd in each of the three stacks or layers (of 10, 7, and 2 pairs, respectively were deposited one after another in a very fast manner as a result of relatively fast rotation of the sample with respect to the circular holder with two targets)). It can be appreciated that the further away from the recording head a layer (or sublayer), i.e., stack, the weaker and more spread out the field profile will be in the region of the stack and therefore the recorded information would be noise. Accordingly, to compensate for the weakening of the field in the bottom layer (or stack), the number of layers in the layer (i.e., first layer or bottom layer) can be increased to maintain a certain signal-to-noise ratio, i.e., the more layers in a stack, the stronger the signal coming from the stack. The deposition rates were as follows: Co: 0.39 A/s under the 7.5% DC power, Pd rate: 0.45 A/s under the 2.5% DC power. The total DC power was approximately 750 W. The deposition environment was room temperature, 100% sample rotation. The pressure in the main Chamber was about 1E-8 Ton.
In accordance with an embodiment, ideally, 8 signal levels (states) were expected from this media configuration 3210, as illustrated in
The small discrepancy (measured 7 levels instead of predicted 8 levels) between the measurements and predictions is due to the fact that two signal levels (4 and 5 according to the illustration in
It will be understood that the foregoing description is of the preferred embodiments, and is, therefore, merely representative of the article and methods of manufacturing the same. It can be appreciated that many variations and modifications of the different embodiments in light of the above teachings will be readily apparent to those skilled in the art. Accordingly, the exemplary embodiments, as well as alternative embodiments, may be made without departing from the spirit and scope of the articles and methods as set forth in the attached claims.
This application claims priority to U.S. Patent Provisional Application No. 61/082,091, filed Jul. 18, 2008, and U.S. Patent Provisional Application No. 61/115,434, filed Nov. 17, 2008, which are incorporated herein by this reference in their entirety.
This invention was made with Government support under Grant (Contract) No. H94003-07-2-0703 awarded by Defense MicroElectronics Activity (DMEA) and Office of Naval Research (ONR). The Government has certain rights to this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61115434 | Nov 2008 | US | |
| 61082091 | Jul 2008 | US |
