Patent Application
20110211271
This invention relates generally to a field of high-density magnetic data storage systems and methods, and more particularly to magnetic recording storage systems and methods including a hard disk drive to write to ultra high anisotropy magnetic storage media.
In a hard disk drive (HDD) within a magnetic recording system, recording bits consist of one or many of single domain islands or particles. As recording density increases, area of each recording bit reduces proportionally together with volume of each recording particle. Magnetic energy of each magnetic particle also tends to reduce which contributes to thermal agitation at room temperature. This thermal agitation contributes to a higher probability to flip the magnetization direction of recorded particles which results in losing recorded data.
In order to prevent the thermal induced instability of recorded bits, anisotropy of magnetic particles has to be increased for keeping sufficient ratio of magnetic energy of the recording particle over the thermal energy. In this environment, a recording head of an HDD requires higher magnetic field to write to the magnetic storage media. When areal density approaches 1 Tb/in2 and beyond, conventional write heads do not produce a sufficiently strong magnetic field to write the high anisotropy magnetic storage media with sufficient thermal stability. This kind of constraint is called superparamagnetic limit of a magnetic recording system. In magnetic recording systems, to achieve improved performance and increased areal density, there is a tradeoff among signal to noise ratio, thermal stability of recording bits, and writability. The most significant challenge to overcome superparamagnetic limit among the trilemma is how to increase the writability of magnetic recording systems.
To help maximize the areal density, another type of energy is required to be injected into a write bubble produced by conventional write head and assists it to write the magnetic particles with ultra high anisotropy. Attempts of assisted energy have been proposed such as thermal energy, microwave energy, exchange coupled composite (ECC) media or graded media, etc. The principle is to use extra energy to lower anisotropy barrier of magnetic particle and help the conventional writer head to record during the writing process. All prior attempts have low energy transmission efficiency and also cause many other engineering implementation difficulties.
For example, in the heat assisted magnetic recording (HAMR), ferromagnetic material of magnetic storage media is heated up close to Curie temperature and coercivity of it is significantly reduced for the magnetic recording head to write. Use of high temperature produces many engineering challenges and makes this technology hard to be implemented. The microwave assisted magnetic recording (MAMR) applies transverse oscillating field at an order of Larmor frequency of ferromagnetic medium material to assist magnetic recording head to write. As the Larmor frequency is proportional to the anisotropy field of ferromagnetic medium material, total writeability is limited.
Therefore, there is a need for a system and a method to lower the anisotropy barrier of magnetic particles and help the conventional writer head to record during the writing process in magnetic recording.
An aspect of the invention is a magnetic recording system comprising a magnetic storage media having a recording layer comprising a material having magnetic particles having a magnetic anisotropy energy that changes in the presence of an electrical field; a write head having a write pole for applying an alternating current (AC) magnetic field for writing magnetic information to the magnetic storage media; and a power supply for generating a negative direct current (DC) electrical bias between the magnetic storage media and the write head for applying a DC electric field to the recording layer to reduce the magnetic anisotropy energy and switching field of the material of the recording layer during the writing of magnetic information to the magnetic storage media.
In an embodiment, the magnetic recording system further comprises a soft magnetic underlayer under the recording layer. The magnetic storage media may further comprise an interlayer between the recording layer and the underlayer. The material of the recording layer may be CoCrP(SiO2), CoCrPt(TiO2), FePt and FePt with TiO2, FePt and FePt with SiO2, FePt and FePt with any oxide, CoPt and CoPt with TiO2, CoPt and CoPt with SiO2, CoPt and CoPt with any oxide, or the like.
In an embodiment, the negative electrical bias may be applied to the magnetic storage media to provide a source of free electrons for the magnetic particles in the recording layer to trap the electrons and fill an electronic shell of the magnetic particles to reduce the magnetic anisotropy energy of the magnetic particles. The magnetic particles that trap electrons also reduce the switching field of the recording layer for the write head to write magnetic information in the magnetic storage media. The magnetic particles that trap electrons also increase the signal to noise ratio of the recording layer magnetic particles. The magnetic particles that trap the free electrons are located at a surface of the recording layer.
An aspect of the invention is a magnetic recording method that comprises providing a magnetic storage media having a recording layer comprising a material having magnetic particles having a magnetic anisotropy energy that changes in the presence of an electrical field; applying an AC magnetic field for writing magnetic information to the magnetic storage media with a write head having a write pole; and generating a negative DC electrical bias between the magnetic storage media and the write head for applying a DC electric field to the recording layer to reduce the magnetic anisotropy energy and switching field of the material of the recording layer during the writing of magnetic information to the magnetic storage media.
An aspect of the invention is a magnetic storage media comprising a recording layer comprising a material having magnetic particles having a magnetic anisotropy energy that changes in the presence of an electrical field; and a soft magnetic underlayer under the recording layer.
An aspect of the invention is a hard disk drive comprising a magnetic storage media having a recording layer comprising a material having magnetic particles having a magnetic anisotropy energy that changes in the presence of an electrical field; and a write head having a write pole for applying an AC magnetic field for writing magnetic information to the magnetic storage media, and arranged to receive a power supply for generating a negative DC electrical bias between the magnetic storage media and the write head for applying a DC electric field to the recording layer to reduce the magnetic anisotropy energy and switching field of the material of the recording layer during the writing of magnetic information to the magnetic storage media.
In order that embodiments of the invention may be fully and more clearly understood by way of non-limitative examples, the following description is taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions, and in which:
A hard disk drive (HDD) 10 consists of magnetic read/write heads 14 and magnetic storage media 16 as illustrated in
Due to the relatively very low flying height of magnetic recording, for example less than 10 nm, the electrical field on the magnetic storage media surface is relatively very high, for example 4-5×108 V/m with only a few volts bias applied, for example below 5V. Such high electrical field can trap enough electrons on the surface of the grains which compose of the magnetic storage media. The trapped electrons are localized and therefore can modify the magnetic properties of the magnetic storage media. Using an isolated iron atom as an example illustrated in
An embodiment of the invention uses a high electrical field to trap the free electrons into an unfilled electronic shell of magnetic particles, in particular, the 3d shell of transition elements, 4f shell of rare earths of lanthanides series, and 5f shell of actinides series. The trapped electron decreases the anisotropy of magnetic particles due to the reduced number of Bohr magnetron. As a result, the conventional head is able to write very high anisotropy magnetic storage media.
In an example, an electrically controlled magnetism in a real recording system with CoCrPt—TiO2 nanocomposite thin films are used as magnetic storage media. In a spin-stand test at 10 mA writing current, with a voltage of 3 V applied across the head-media gap during recording, the amplitude of the readback signal was almost doubled and the read back waveforms showed sharper transitions. These account for the 3 dB improvement in read back signal-noise-ratio (SNR) of the written magnetic information. The improved recording performance is mainly attributed to the reduction of anisotropy of the magnetic storage media in the presence of electrical field. Simulations were also carried out to understand the magnetization reversal process under applied electric and magnetic fields. In a spin-stand test on a recording system with a voltage of 3 V applied across the head-media gap during recording results in an appreciable reduction of media switching field. The read back waveforms showed sharper transitions and a 3 dB improvement in read back SNR was achieved as well. In order to apply electric field to a magnetic storage media, read/write heads from commercially available HDD products may be modified to allow direct electrical access to the writer or recording pole. In addition, the usually grounded slider main body may be electrically isolated also by modification. This is to prevent the presence of large electrostatic forces that can affect the flying height and stability of the slider or damaging electric discharge between slider and magnetic storage media. As the slider main body is already electrically isolated, and the alumina around the writer main pole is nonconducting, the applied electric field is strongest at the main pole region. A schematic for the experimental set-up is given in
The recording and read back measurements were performed on a commercial Guzik spinstand. During the recording process, combinations of different electric potentials applied across the head media gap, for example via the said connections to the write pole as well as media substrate, with different writer currents were used. The applied electric potential was supplied by a Keithley Sourcemeter. For each experimental run, an all “ones” pattern track was first written. Subsequently, the same track is read back and the magnitude of the read back pattern was recorded. During the read back process, no gap electric potential was applied.
Density-functional calculations show spin dependent screening of the electric fields led to spin imbalance of the excess surface charge. As the electric field does not penetrate into the bulk of metals, the excess electric charge is confined to a depth of lattice constant level. Because the excess charges remained localized near the surface atoms, the effect of local properties such as inter-atomic bonding or the atomic magnetic moments may conceivably be quite large. The electrical field distribution at the metallic grain surfaces under the bias from writer pole can be calculated from the output data of electric force over electrical field. In an embodiment, the electric conductive pole is separated by the air gap and media overcoat material to reach the metallic grains grounded through metallic underlayer. The metallic grains are separated by dielectric grain boundary materials. In an embodiment, in order to have significant magnetic switching field reduction, the charge density may reach 0.3 to 0.5 electrons per unit cell for FePt material for example. In a magnetic storage media structure, the overcoat and the metallic underlayer are connected by the metallic grain and the dielectric grain boundary material. Inside the metallic grains and underlayer, there is no electric potential difference. Due to the electrical shielding effect of metallic grains, it is very hard for the dielectric grain material to lead down the electrical field down to the underlayer. The higher permittivity grain boundary material helps to lead down more of the electrical field downwards. With this configuration, although the higher permittivity overcoat material traps more electrons in each unit cell, the charge density decreases relatively quickly and only the top few atoms of each grain traps a meaningful number of electrons for magnetic switching field reduction.
It will be appreciated that it has been shown that contribution of any heating effects of the gap current in the increase in recording capability is not significant in comparison to the electrically modified anisotropy effects. Based on experimental conditions the power from the heating effect is estimated to 9 μW for a 3 μA gap current. If this heating power is transferred to the magnetic storage media, the energy absorbed when the pole passes the magnetic bit is 6.74×10−14 J. Such heating power roughly raises the magnetic storage media temperature by less than 1.6 K. This temperature rise is not significant and is not high enough to provide significant assisted writing. Therefore the improvement of recording performance is not due to the heating effect of the gap current but is the result of a pure electric-field induced effect. Thus, the effect is in the influence of an electric field on electron filling of the magnetic storage media, which reduces the magnetic anisotropy of the magnetic storage media. Additionally, substantial electric-field induced effects may be present in nanosystems where the surface-to-volume ratio is high, as in the case of magnetic thin film media with grain size of about 8 nm or less.
Density-functional calculations were also applied to ferromagnetic Fe(001), Ni(001), and Co(0001) films in the presence of an external electric field. These showed spin-dependent screening of the electric fields led to spin imbalance of the excess surface charge. As the electric field does not penetrate into the bulk of metals, the excess electric charge is confined to a depth of lattice constant level. Because the excess charges remain localized near the surface atoms, the effect on local properties such as the inter-atomic bonding or the atomic magnetic moments may conceivably be quite large. This offers us an opportunity to modify the intrinsic magnetic properties by applying electrical field. The applied electric field modifies the magnetic properties of magnetic storage media by trapping electrons into the surface of magnetic grains. The assisted recording approach discussed herewith is named trapping electron assisted magnetic recording (TEAMR).
To understand the effect of trapped or induced electric charges at the interface on the magnetization reversal process of magnetic grains, a simple model is used illustrate and to represent individual magnetic grains whereby the topmost layer of atoms of the magnetic grain has been modified to be magnetically soft, for example with a lower anisotropy Ku1 for the surface portions, by the applied electric field. Simulated hysteresis loops of a conventional media with different Ku1 for the surface portion are given in the graph 170 shown in
In an embodiment, system implementation of TEAMR requires little modification to conventional recording configuration, where a change to conventional heads is the electrical disconnection of the writer pole from the common ground of the slider body and has a separate wire out to control the electrical bias. As the spacing between the writer pole and the grain top surface is much less than the head keeper spacing, the bubble size of the electrical field is smaller than that of the magnetic field. Because both field bubbles are mainly determined by the size of the writer pole, the bubble size difference is small. If it is necessary to enlarge the electrical field bubble for optimization of effective field, the non-magnetic metallic layer can be added on the side surfaces of the writer pole. The materials of the overcoat and the grain boundary on the magnetic storage media may be high permittivity materials to ensure the charge density at the surfaces of the grains. For TEAMR writing within an embodiment, the outmost layer of grain atoms is magnetically softer than the grain core and the magnetic switching is preferred to start near the grain boundary, to have a sharper field gradient. As such, TEAMR assists the granular media to further reduce the grain number per bit through improved quality of the writing field, which contributes to areal density: Additionally, for maximizing writing capability, the media may be bit patterned media (BPM). Filler material for the planarization of BPM grooves can be high permittivity materials as well. Due to the relatively larger island to island spacing, the electrical shielding effect from protruded islands is much smaller than the case of granular media. The electrical field can go deeper towards the bottom of BPM islands. There are more areas on the side surfaces of BPM islands trapping electrons at sufficiently high charge density. This increases the interface area of the soft to hard exchange coupling and also total volume of soft layer, which both are beneficial for switching field reduction. The media overcoat may assist in producing high charge density for metallic grains trapping free electrons. At higher areal density where there is little or no room for the media overcoat, the dielectric charge producing layer can be put under the metallic grains, with corresponding changes to media fabrication process.
In summary, the electrically controlled magnetism at spin-stand level is applied in real magnetic recording systems. An electrical bias is applied to the main pole of the write head with the media and the other part of the head slider grounded. As the main pole area is relatively small, the electrostatic force produced by electrical potential is a few orders smaller than the air bearing force at rear pad. Therefore, it will not affect the flying performance of the head slider. At the nanometer head media spacing, a relatively strong electrical field is produced in the head media interface. By using a sufficiently high electric field across the head-media gap, electrons are trapped into the surface of magnetic grains in the media. The strong electrical field traps free electrons to accumulate at the interfacial surfaces of metallic magnetic grains. The trapped electrons are localized in the surface atoms of magnetic grains and alter the valance-electron band filling of those surface atoms. The extra band-filling electrons, trapped electrons reduce the anisotropy energy of the magnetic grains and in turn reduce the switching field of the magnetic storage media which makes it easier to be magnetically switched. The softened portions in the magnetic grains narrow down the media switching field distribution and therefore reduce the transition width. The gain in SNR is due to better write-ability and narrower transitions. The demonstrated concept is easily implementable through slight modification to existing recording heads. It will be appreciated that more significant improvement can be observed for higher recording density media with smaller grain size where surface-to-volume ratio is higher. This electric field assisted approach with demonstrated 3 dB SNR gain makes it a viable alternative to other more complex assisted recording schemes such as HAMR or microwave assisted recording.
In an embodiment, a DC electric field is negatively biased to the magnetic storage media. The magnetic storage media is a material such as for example a metal material such as CoCrPt, FePt and the like. The DC field is applied to trap electrons on the surface of the grains by which the magnetic storage media is composed. The trapped electrons effectively reduce the anisotropy energy of the magnetic storage media and therefore make the media easier to write. The negative DC potential is directly applied to the magnetic storage media and at the same time, the write pole is grounded. Due to the low flying height which can be less than 8 nm for example, a low potential can generate high electrical field at the media surface, and therefore can trap electrons on the magnetic storage media surface. The media used may be CoCrPt+SiO2(TiO2), FePt, CoPt, and the like.
While embodiments of the invention have been described and illustrated, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SG09/00399 | 11/2/2009 | WO | 00 | 5/3/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61193177 | Nov 2008 | US |
