This invention relates to media for storing information and, in particular, to high-density information storage media and methods for making the same.
Magnetic recording is an important part of modem computer technology. Conventional magnetic recording systems such as computer hard disk drives typically use a continuous magnetic thin film on a rigid substrate as the recording medium. Each bit of information is stored by magnetizing a small area on the magnetic film using a write head that provides a writing magnetic field. The magnetization strength and the location of each magnetic bit should be defined precisely to allow a flying magnetic sensor (read head) to retrieve the written information.
Each magnetic bit in the magnetic recording medium contains one magnetized region that consists of many small magnetized grains. Because of the trend toward higher recording density, the magnetic bit size is continuously being reduced. In order to reduce the size of the magnetic bits while maintaining a satisfactory signal-to-noise ratio, the size of the grains is also being reduced. Unfortunately, substantial reduction of the size of the weakly coupled magnetic grains will make their magnetization unstable due to the superparamagnetic phenomena occurring at ambient operating temperatures.
In order to overcome superparamagnetic limits, patterned magnetic media with discrete magnetic regions have been prepared. See U.S. Pat. No. 5,820,769 to Chou et al., U.S. Pat. No. 5,5587,223 to White et al., and U.S. Pat. No. 6,440,520 B1 to Baglin et al.
In patterned magnetic media, the conventional continuous magnetic film that covers the rigid disk substrate is replaced by an array of discrete magnetic regions, each of which serves as a single magnetic bit. Typical prior art approaches for preparing patterned magnetic media include photolithography, laser interference lithography and electron beam lithography. The lithographic techniques are used to form isolated regions of magnetic material surrounded by areas of non-magnetic material. See C. A. Ross et al., “Micromagnetic behavior of electrodeposited cylinder arrays”, Phy. Rev., Vol. B65, p. 1417 (2002).
Conventional photolithography and laser interference lithography are more convenient than the e-beam lithography. They produce fine, discrete magnetic structures. The bit size, however, is typically larger than ˜100 nm. Hence the magnetic recording density is unduly limited.
Electron beam lithography is capable of producing a finer structure with a bit size as small as ˜10 nm. However, current electron beam lithography with a single-beam writing process is a slow, expensive process which is not amenable to industrial mass production.
Desirable nanomagnet arrays can also be obtained using porous anodic alumina membranes containing periodically arranged vertical pores. (The term “nano” as used herein, refers to components having sub-100 nm operative dimensions). Cobalt or iron nanomagnet wire arrays so fine as ˜10-15 nm diameter have been obtained by electroplating magnetic metals into such pores. See H. Zeng et al., “Magnetic properties of self-assembled nanowires of varying length and diameter”, J. of Appl. Physics, Vol. 87, p. 4718 (2000), and Y. Peng et al., “Magnetic properties and magnetization reversal of alpha-iron nanowires deposited in alumina film”, J. of Appl. Physics, Vol. 87, p. 7405 (2000). However, the aluminum oxide membrane is a fragile, brittle structure that can easily break or distort from the flat surface required of a magnetic hard disk. The disk must be sufficiently flat that a flying read/write head can slide over it with a gap distance of less than ˜30 nm. The difficulty of filling nanopores with aqueous solution against surface tension of liquid, especially for nanopores of ˜50 nm or smaller in diameter, often causes reliability and reproducibility problems from pore to pore.
Carbon nanotubes have been used as a template to deposit nanowires of magnetic material. Various techniques were utilized—arc discharge synthesis (by M. Terrones, MRS Bulletin, Vo. 24, No. 8, page 43, August 1999), metal impregnation by electrolysis (by Ye, et al, Advanced Materials, Vol. 15, page 316, 2003), high temperature decomposition of metal-containing salt (by Govindaraj et al., Chemistry of Materials, Vol. 12, page 202, 2000), two-step deposition consisting of thermal decomposition deposition of carbon tubules and then MOCVD deposition of Ni nanowires into the vertical pores of anodic aluminum oxide membrane, followed by etching of alumina membrane, (by Pradhan et al, Chemical Communications, Issue 14, page 1317, 1999), and decomposition of (Co, C)-containing precursor (by Liu et al, Chemistry of Materials Vol. 12, page 2205, 2000). However, most of these techniques use loose, isolated nanotubes, instead of aligned and fixed nanotubes, so the magnetic metal filled nanotubes are randomly configured and the desired periodic arrangement and vertical alignment of nanomagnets suitable for magnetic recording media can not be achieved.
Accordingly there is a need to create ultrafine scale, nano-magnets in an aligned parallel array configuration on a solid substrate manner in order to fabricate industrially viable ultra-high-density magnetic recording media.
In accordance with the invention, a high density recording medium is fabricated by novel methods. The medium comprises an array of nanomagnets disposed within a matrix or on the surface of a substrate material. The nanomagnets are advantageously substantially perpendicular to a planar surface. The nanomagnets are preferably nanowires of high coercivity magnetic material inside a porous matrix or an array of vertically aligned nanotubes, or on the surface of flat substrate. Such media can provide ultra-high density recording with bit size less than 50 nm and preferably less than 20 nm. A variety of techniques are described for making such media.
The nature, advantages and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail with the accompanying drawings. In the drawings:
It is to be understood that these drawings are for the purpose of illustrating the concepts of the invention and are not to scale.
This invention describes the structure and fabrication of recording media particularly useful for high-density recording. By “high-density recording”, is meant recording at 50-nanometer information bit size or less, and preferentially 20 nanometer bit size or less.
Referring to the drawings,
In this embodiment, the conventional magnetic disk material comprising a continuous magnetic film is no longer utilized. Instead, a plurality of discrete, nanoscale magnetic elements 12 are employed to overcome the superparamagnetic limits in recording density. Each discrete magnetic element, or several elements as a block, can be magnetized along the same direction, thus constituting a magnetic bit. Each of the 12 elements are preferably separated from other elements by a nonmagnetic matrix material 14. The inter-element spacing is kept large enough to minimize exchange interaction between neighboring elements. Each magnetic element 12 preferably has the same size and shape, and is preferably made of the same magnetic material.
The elements 12 are preferably regularly arranged on the substrate, although it is not an absolute requirement where plural elements are magnetized and used as a single bit. Each magnetic element has a small size and a preferred shape anisotropy so that the magnetization of each discrete magnetic element will be automatically aligned along the long axis of the anistropic element. Instead of shape anisotropy, crystal anisotropy can also be utilized to align the magnetic moment of the discrete element preferentially along the vertical direction. This means that the magnetic moments of each nano-scale discrete magnetic element 12 is quantized and has only two states with the same magnitude but two opposite directions. Such a discrete magnetic element can be a single magnetic domain. Each direction of a quantized magnetic moment represents one value of a binary bit. A magnetic recording (or writing) operation involves flipping the magnetic moment direction of the single domain element. A reading operation involves sensing the quantized magnetic moments. The moments are preferably oriented perpendicular to the medium surface rather than longitudinally along the surface. A magnetic storage system, such as a hard disk system in a computer, consists of the magnetic storage medium, write heads, and read heads.
In the second step, Block B, the substrate with aligned nanowires is processed so as to make an array of magnetic nanowires. For example if the nanowires are hollow nanotubes, the substrate can be placed inside a supercritical CO2 deposition chamber, and the nanotubes made magnetic by filling their hollow cores with a high coercivity magnetic metal or alloy, such as Co, Co—Cr, Co—Cr—Ta, Fe—Pt, Co—Pt, rare earth cobalt, rare earth iron, or rare earth iron boron alloy.
The cores of nanotubes are desirably fully filled with magnetic material, at least to such length that the aspect ratio of the resulting nanomagnet is at least 3, and preferably at least 10. Such nanotubes filled with a magnetic material such as Ni have been reported, albeit without the desired alignment, by N. Grobert et al., Appl. Phys. Vol. A67, page 595 (1998), and by M. Terrones et al. in MRS Bulletin, August 1999 issue, page 43. The desired aligned and metal-filled carbon nanotubes can be obtained by control of nucleation and growth on a flat substrate to co-deposit metal and carbon nanotube simultaneously. The alignment can be accomplished by applying electric or magnetic field. In such a filled configuration, the diameter of nanomagnets can be even smaller, and a higher density of magnetic recording can thus be accomplished. However, too small a diameter may not be desirable because of the onset of superparamagnetic behavior. The magnetic metal filling inside a tube-shaped, non-magnetic nano material (not necessarily carbon nanotubes) desirably has a diameter of at least 0.5 nanometer, preferably at least 1 nanometer, even more preferably at least 3 nm. To fill the core of nanotubes with magnetic material, the tubes are desirably open at the top end. The nanotube tips can be opened by acid treatment or by localized burning in oxygen atmosphere. See Ajayan et al., Nature, Vol. 362, page 522, 1993, and Tsang et al., Nature, Vol 372, page 159, 1994.
While there are several ways of filling the nanotubes with a metal, the preferred way for the sake of reliability and uniform deposition is to use carbon dioxide supercritical fluid chemical deposition. The supercritical fluid behaves like a hybrid of liquid and gas. It can dissolve desired solutes like a liquid, and yet conveniently behave like a gas. It exhibits low viscosity, high diffusivity and high pressure for easy penetration into small pores. See an article by Darr, Chemical Review, Vol 99, page 495-542, 1999 and U.S. Pat. No. 6,132,491 issued to Wai et al on Oct. 17, 2000. An article by Ye, et al, Advanced Materials, Vol. 15, page 316, 2003 describes an example of supercritical fluid deposition of a metal inside carbon nanotubes. For magnetic recording media, filling of nanotubes is advantageously accomplished in a densely arrayed nanotube configuration with the nanotubes securely attached on a flat substrate, and the nanotube array is preferably immobilized and planarized in a solid form rather than in a free-moving nanotube shape. One exemplary processing of supercritical CO2 fluid deposition involves the dissolution of metal-containing precursor such as metal(hfa)22.xH2O [where (hfa)=hexafluroacetyl-acetonate] in a mixture of CO2 and H2, which is then reduced by hydrogen in supercritical CO2, followed by reaction time and then drying (removal) of CO2. The reaction can be carried out at a slightly elevated temperature of for example, as high as 200-300° C. Another alternative way is to do the gap-filling and planarization first, and then to do the nanotube filling with magnetic metal.
In the third step, Block C of
In the final step, Block D of
The resulting magnetic storage medium is schematically shown in
Once a magnetic island array is formed, the next step is to transform the nanoislands into elongated nanowire shapes 12. The ferromagnetic nature of the nanoislands is utilized to enforce the nanowire growth in the subsequent physical vapor deposition such as sputtering or evaporation. When an external vertical magnetic field H is applied, e.g., 100-10,000 Oe field as by using a permanent magnet 41 as illustrated in
The individual atoms of ferromagnetic metal are generally nonmagnetic or superparamagnetic due to thermal instability. In order to be ferromagnetic and to respond to the applied magnetic field and to the pole field from nanoislands, the atoms need to cluster to a certain critical size of at least several atoms. In order to enhance the probability of such cluster formation, a specific evaporation or sputtering process is utilized. For example, very high pressure evaporation or sputtering is utilized so that evaporated or sputtered atoms bounce off many times and hence have time to form clusters which can respond to the magnetic field from the island tips. The desired gas pressure for the vacuum evaporation or sputtering is at least 0.001 torr, preferably at least 0.01 torr, and even more preferably at least 0.1 torr. Comparing to the typical vacuum in sputtering or evaporation, this pressure is many orders of magnitude higher.
The desired material for nanoislands 40 should be relatively magnetically soft with the coercivity in the range of 0.01-500 Oe, preferably 0.01-100 Oe, and even more preferably 0.01-20 Oe. Exemplary materials include Ni, Fe, Co, and their alloys (especially Ni—Fe permalloy which is a well known soft magnetic alloy), nanocrystalline magnetic alloys with low coercivities, or amorphous alloys such as Co—Fe—B. The desired magnetic material to be selectively deposited on top of the soft magnetic island poles should be a permanent magnet material with a relatively high coercive force to serve as recording media. Desired magnet materials to be grown on the soft magnetic islands include Co, Co—Cr, Co—Cr—Ta, Fe—Pt, Co—Pt, rare earth cobalt, rare earth iron, or rare earth iron boron alloy. The desired coercivity of such a nanowire material after all the processing steps are carried out is at least 500 Oe, preferably at least 1000 Oe, and even more preferably at least 3000 Oe. However, during the synthesis of nanowire by applied magnetic field, the growing nanowire does not have to exhibit high coercive force, and in fact, low coercive force values are preferred for the ease of magnetic attraction and attachment of magnetic clusters. The desired, final high coercive force can be obtained by annealing heat treatment after the nanowire growth is completed.
The finished composite material workpiece of
The magnetic nanowire array of
Having such a soft magnet base underneath the permanent magnet nanowire is additionally beneficial since it can enhance magnetic recording performance. The soft magnetic base serves to reduce the self demagnetizing effect and also provides flux return paths.
The magnetic nanowires so fabricated are then optionally assembled into a rigid composite structure as illustrated in
In the final step,
The nanomagnets of
Instead of nanowire arrays, a flat substrate with a recessed and vertically aligned pore array can be used to prepare the ultra-high-density nanomagnet array. Silicon substrates are flat and commercially available. Vertical nanopores in the regime of 2-20 nanometer diameter can be fabricated in silicon by known techniques. It has been shown that such pores can be filled with magnetic materials such as Ni or Fe—Co alloy, but the techniques previously used may not be suitable for this invention. See articles by Gusev et al, Journal of Applied Physics, Vol. 76, page 6671, 1994, and by Hamadache et al, Journal of Material Research, Vol. 17, page 1074, 2002. It is not clear what portion of the pore diameter or depth was filled with magnetic metal. The penetration of electrolyte into such a fine nano-scale diameter pores for electrodeposition is difficult because surface tension of a liquid tends to prevent it from getting inside pores especially nanopores. The reported magnetic properties by Gusev et al were rather poor and not suitable for magnetic recording media applications, as the coercivity value measured was only ˜200 Oe or less.
In this invention, carbon dioxide supercritical fluid chemical deposition, as described above, is advantageously used for meaningful and reliable deposition of nanomagnet into the porous silicon nanopores. High coercivity magnetic materials preferably with high magnetocrystalline anisopropy are used to fill up the nanopores.
As shown in
Yet another variation of inventive method of utilizing a planar array of nanoparticles of elemental metal to form an eventual alloy nanomagnet array is illustrated in 7. In this case, an array of nanoparticles 70 is utilized as a basis to add a second metal 71,
As an example, an array of Pt nanoparticles 70 can be placed on the substrate using surfactant to separate and periodically arrange the particles (
Examples of desirable nanomagnet materials to be supercritically filled in the silicon nanopores include Fe, Co, Ni, rare earth elements and alloys such as Co—Cr, Co—Cr—Ta, Fe—Pt, Co—Pt, rare earth cobalt, rare earth iron, or rare earth iron boron. High coercivity metal and alloys are preferred for magnetic bit stability.
Instead of porous silicon, other types of membrane materials with vertically aligned nanopores can also be used for the physical vapor nanopore filling. Illustrated in
The first step is to deposit a magnetic film 111 comprising a high-coercive force permanent magnet recording material such as Co—Cr, Co—Cr—Ta, Fe—Pt, Co—Pt, rare earth cobalt, rare earth iron, or rare earth iron boron.
Referring back to
The third step is to apply the nanoparticles 110 onto the surface of the resist layer 112 as a mono-layer. The liquid containing the nanoparticles of metals or ceramics such as Pt, Co, Ni, Fe, W, Mo, Fe2O3, TiO2, and SiO2 are dispensed on the substrate surface, e.g., using spin coating technique. For the desired periodic arrangement, the particles are preferably coated with a surfactant such as a fatty acid type material, e.g., oleic acid and oleyamine. The dispersed particles 110 are allowed to dry before beam exposure. The residual fatty acid material in the vicinity of dried nanoparticles transmits electrons or optical beams much better than metal or ceramic particles, so the residual material does not hinder the nanoparticle-mask lithography.
After exposure of the resist layer with e-beam or optical beam, the resist is developed into nano island patterns 113 as illustrated in
The spaced nanomagnet array of
In order to serve as a magnetic recording medium and store information with stability, the nanomagnet array in this invention should have a high magnetic coercivity and desirably a high magnetization squareness ratio (defined here as the ratio of remanent magnetization over the saturation magnetization). The desired value of coercivity for the inventive ultra-high-density recording media is in the range 500-6000 oersteads, and preferably in the range of 1000-3000 Oe. The desired squareness is at least 0.7, and preferably at least 0.9. High magnetic saturation of at least 2000 gauss is desirable, preferably at least 8000 gauss. Materials with high magnetocrystalline anisopropy such as the Fe—Pt alloy compound with the L1o phase, Co—Pt, rare earth cobalt or rare earth iron based compounds, hexaferrites, cobalt based alloy materials are preferred.
The as-deposited nanomagnet material in the nanopores or on the flat substrate according to the invention may not have desirable crystal structure and magnetic properties due to the defective crystal formation for deposition at or near ambient temperature. Post-deposition annealing treatment of at least 200 degrees C. for at least 10 minutes, and preferably at least 500 degrees C. for at least 1 hour can restore and maximize the magnetic properties of the deposited material. A neutral or inert gas atmosphere such as argon or nitrogen, a reducing atmosphere such as a hydrogen-containing gas, or a mixed gas at various compositions can be used for the annealing treatment with minimal oxidation of the magnetic metals involved.
While high coercivity materials provide more stability of magnetically recorded information bits, writing on high coercivity recording media with a given magnetic field from the magnetic write head can be a problem. For high coercivity materials with coercivity values in excess of ˜2000 Oe, thermally assisted magnetic recording can be used. Here, a laser pulse can be applied to heat a local region so that the coercivity is momentarily lowered by the local heating and magnetic switching (writing) is done with the available write field.
Referring to
In both modes of operations (
It is understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the invention. Numerous and varied other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.
This application is a continuation-in-part of U.S. patent application Ser. No. 10/262,462 filed by Sungho Jin on Sep. 30, 2002 and entitled “Ultra-High-Density Information Storage”, which is incorporated herein by reference.
Number | Date | Country | |
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Parent | 10262462 | Sep 2002 | US |
Child | 10969273 | Oct 2004 | US |