Magnetization reversal of bit patterned media: Role of the angular orientation of the magnetic anisotropy axes
TL;DR: In this paper, the influence of angular dispersion of the magnetic anisotropy axis on the reversal behavior of tilted bit patterned media with an areal density of about 1 Tbit/in.
Abstract: Micromagnetic modeling was performed to study the influence of an angular dispersion of the magnetic anisotropy axis on the reversal behavior of tilted bit patterned media with an areal density of about 1 Tbit/in.2. Thereby, the angular dispersion was realized by having the anisotropy axes of the individual bits lying on the surface of a cone with a specific opening angle. In addition, a distribution of the magnetic anisotropy value within the array of magnetic nanostructures was taken into account. The effect of the angular variation in the magnetic anisotropy orientation on the switching field distribution was investigated. Two optimized geometries were suggested in order to keep the switching field distribution as narrow as possible: (1) uniaxial perpendicular bits with the magnetic field applied under an angle of 45° and (2) tilting the anisotropy axis to about 45°–75° and applying a perpendicular magnetic field. However, mixing both situations results in a drastic increase in the switching field distribution.
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Journal Article•
TL;DR: It is shown that changing the deposition angle with respect to the particle ensemble allows tailoring of the orientation of the magnetic anisotropy, which results in tilted nanostructure material.
Abstract: Thin-film technology is widely implemented in numerous applications1. Although flat substrates are commonly used, we report on the advantages of using curved surfaces as a substrate. The curvature induces a lateral film-thickness variation that allows alteration of the properties of the deposited material2,3. Based on this concept, a variety of implementations in materials science can be expected. As an example, a topographic pattern formed of spherical nanoparticles4,5 is combined with magnetic multilayer film deposition. Here we show that this combination leads to a new class of magnetic material with a unique combination of remarkable properties: The so-formed nanostructures are monodisperse, magnetically isolated, single-domain, and reveal a uniform magnetic anisotropy with an unexpected switching behaviour induced by their spherical shape. Furthermore, changing the deposition angle with respect to the particle ensemble allows tailoring of the orientation of the magnetic anisotropy, which results in tilted nanostructure material.
272 citations
TL;DR: In this paper, a detailed analysis of the experimental data has been performed based on a phenomenological model taking into account the influence of the nanowire shape anisotropy added to the dipolar magnetostatic interactions among them.
Abstract: Highly ordered arrays of Ni nanowires have been prepared by pulsed electrochemical deposition into nanopores of anodic alumina membranes (NAAMs) used as templates. They have been experimentally characterized by magnetic torque measurements and vibrating sample magnetometer (VSM) techniques in order to determine the magnetic anisotropy of the hexagonal array of nanowires. A detailed analysis of the experimental data has been performed based on a phenomenological model taking into account the influence of the nanowire shape anisotropy added to the dipolar magnetostatic interactions among them. An overall agreement is obtained between the simulations derived from the model and the experimental magnetic torque anisotropy curves.
28 citations
TL;DR: The present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media and provides an easy cost-effective micro/nanopatterning of recording bits.
Abstract: L10 FePt is an important material for the fabrication of high density perpendicular recording media, but the ultrahigh coercivity of L10 FePt restricts its use. Tilting of the magnetic easy axis and the introduction of a soft magnetic underlayer can solve this problem. However, high temperature processing and the requirement of epitaxial growth conditions for obtaining an L10 FePt phase are the main hurdles to be overcome. Here, we introduce a bilayered magnetic structure ((111) L10 FePt/glassy Fe71Nb4Hf3Y2B20/SiO2/Si) in which the magnetic easy axis of L10 FePt is tilted by ∼36° from the film plane and epitaxial growth conditions are not required. The soft magnetic underlayer not only promotes the growth of L10 FePt with the preferred orientation but also provides an easy cost-effective micro/nanopatterning of recording bits. A detailed magnetic characterization of the bilayered structure in which the thickness of (111) L10 FePt with the soft magnetic Fe71Nb4Hf3Y2B20 glassy underlayer varied from 5 to 60 nm is carried out in an effort to understand the magnetization switching mechanism. The magnetization switching behavior is almost the same for bilayered structures in which FePt layer thickness is >10 nm (greater than the domain wall thickness of FePt). For FePt film ∼10 nm thick, magnetization reversal takes place in a very narrow field range. Magnetization reversal first takes place in the soft magnetic underlayer. On further increase in the reverse magnetic field, the domain wall in the soft magnetic layer compresses at the interface of the hard and soft layers. Once the domain wall energy becomes sufficiently large to overcome the nucleation energy of the domain wall in L10 FePt, the magnetization of the whole bilayer is reversed. This process takes place quickly because the domain walls in the hard layer do not need to move, and the formation of a narrower domain wall may not be favorable energetically. Our results showed that the present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media.
22 citations
TL;DR: In this paper, the use of CoFeTaB metallic glass thin film as a soft magnetic underlayer which promotes the growth of L10 FePt along the preferred (111) crystallographic direction is reported.
Abstract: We report on the use of CoFeTaB metallic glass thin film as a soft magnetic underlayer which promotes the growth of L10 FePt along the preferred (111) crystallographic direction. The preferred oriented phase of L10 FePt is obtained by either in situ deposition upon a heated CoFeTaB/SiO2/Si or at room temperature (FePt/CoFeTaB/SiO2/Si) followed by annealing at 400–450 °C for 15 min. The latter process is shown to be advantageous in terms of the fabrication of patterned media. Pattern sizes ranging from 25–100 nm made from L10 FePt (111)/glassy CoFeTaB were fabricated by photo/electron beam lithography. The structural and magnetic characterizations strongly suggest the applicability of the present bilayered structure in the fabrication of high density bit-patterned magnetic recording media.
8 citations
20 Nov 2013
TL;DR: In this article, an experimental application of soft x-ray Fourier-transform holography (FTH) for imaging magnetic samples with nanometer structure size was discussed. But the authors focused on the impact of the experimental apparatus on the image and the influence of the reference structure, the area detector, and the coherence of the radiation delivered by the xray source.
Abstract: This thesis deals with the application of soft x-ray Fourier-transform holography (FTH) for imaging magnetic samples with nanometer structure size. FTH can be set up as a lensless method by exploiting the coherence properties of the illumination. An image of the sample is obtained by first recording a hologram which is the interference pattern of the light scattered at the sample with light originating from a reference structure. The hologram is then digitally reconstructed by a Fourier transform. The used x-ray probe facilitates nanometer spatial resolution of the images due to the small wavelength of the radiation and delivers magnetic contrast via the x-ray magnetic circular dichroism effect. The first part of the thesis contains a theoretical investigation of the function and properties of soft x-ray FTH. The fundamental image formation process with focus on achieving magnetic contrast is deduced. Furthermore, the impact of the experimental apparatus on the image is studied in a formal way. In particular, the influence of the reference structure, the area detector, and the coherence of the radiation delivered by the x-ray source on the point spread function of the imaging system is considered. Finally, the conclusions of the findings for the actual FTH experiment are discussed. In the second part of the thesis, an experimental application of FTH in the field of magnetic research on the nanometer scale is presented. The switching behavior of magnetic islands of a bit-patterned media (BPM) sample is studied via direct imaging of the islands’ magnetic state under an applied magnetic field. The data analysis focuses on the switching field distribution of the island ensemble. The origins for the broadening of this distribution are found in an intrinsic variation, the magnetostatic interaction between the islands and thermal fluctuations. For the latter two effects, models describing the experimental findings were developed. The intrinsic variations were further investigated by structural analysis using transmission electron microscopy.
5 citations
References
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04 May 1948-Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
TL;DR: In this paper, the effect of shape anisotropy on magnetization curves was studied for the case of ellipsoidal spheroids of revolution (e.g., ellipses of revolution).
Abstract: The Becker-Kersten treatment of domain boundary movements is widely applicable in the interpretation of magnetization curves, but it does not account satisfactorily for the higher coercivities obtained, for example, in permanent magnet alloys. It is suggested that in many ferromagnetic materials there may occur ‘particles’ (this term including atomic segregates or ‘islands’ in alloys), distinct in magnetic character from the general matrix, and below the critical size, depending on shape, for which domain boundary formation is energetically possible. For such single-domain particles, change of magnetization can take place only by rotation of the magnetization vector, I O . As the field changes continuously, the resolved magnetization, I H , may change discontinuously at critical values, H O , of the field. The character of the magnetization curves depends on the degree of magnetic anisotropy of the particle, and on the orientation of ‘easy axes’ with respect to the field. The magnetic anisotropy may arise from the shape of the particle, from magneto-crystalline effects, and from strain. A detailed quantitative treatment is given of the effect of shape anisotropy when the particles have the form of ellipsoids of revolution (§§ 2, 3, 4), and a less detailed treatment for the general ellipsoidal form (§ 5). For the first it is convenient to use the non-dimensional parameter such that h = H /(| N a - N b |) I O , N a and N b being the demagnetization coefficients along the polar and equatorial axes. The results are presented in tables and diagrams giving the variation with h of I H / I O . For the special limiting form of the oblate spheroid there is no hysteresis. For the prolate spheroid, as the orientation angle, θ , varies from 0 to 90°, the cyclic magnetization curves change from a rectangular form with | h O | = 1, to a linear non-hysteretic form, with an interesting sequence of intermediate forms. Exact expressions are obtained for the dependence of h θ on θ , and curves for random distribution are computed. All the numerical results are applicable when the anisotropy is due to longitudinal stress, when h = HI 0 /3λδ, where λ is the saturation magnetostriction coefficient, and δ the stress. The results also apply to magneto-crystalline anisotropy in the important and representative case in which there is a unique axis of easy magnetization as for hexagonal cobalt. Estimates are made of the magnitude of the effect of the various types of anisotropy. For iron the maximum coercivities, for the most favourable orientation, due to the magneto-crystalline and strain effects are about 400 and 600 respectively. These values are exceeded by those due to the shape effect in prolate spheroids if the dimensional ratio, m , is greater than 1·1; for m = 10, the corresponding value would be about 10,000 (§7). A fairly precise estimate is made of the lower limit for the equatorial diameter of a particle in the form of a prolate spheroid below which boundary formation cannot occur. As m varies from 1 (the sphere) to 10, this varies from 1·5 to 6·1 x 10 -6 for iron, and from 6·2 to 25 x 10 -6 for nickel (§ 6). A discussion is given (§ 7) of the application of these results to ( a ) non-ferromagnetic metals and alloys containing ferromagnetic ‘impurities’, ( b ) powder magnets, ( e ) high coeravity alloys of the dispersion hardening type. In connexion with ( c ) the possible bearing on the effects of cooling in a magnetic field is indicated.
4,382 citations
IBM1
TL;DR: In this article, the authors present a review of the literature on high Ku alternative media, both for longitudinal and perpendicular recording, with data on sputtered and evaporated thin FePt films, with coercivities exceeding 10000 Oe.
Abstract: High K/sub u/, uniaxial magnetocrystalline anisotropy, materials are generally attractive for ultrahigh density magnetic recording applications as they allow smaller, thermally stable media grains. Prominent candidates are rare-earth transition metals (Co/sub 5/Sm,...), and tetragonal intermetallic compounds (L1/sub 0/ phases FePt, CoPtY,...), which have 20-40 times higher K/sub u/ than today's hexagonal Co-alloy based media. This allows for about 3 times smaller grain diameters, D, and a potential 10-fold areal density increase (/spl prop/1/D/sup 2/), well beyond the currently projected 40-100 Gbits/in/sup 2/ mark, Realization of such densities will depend on a large number of factors, not all related to solving media microstructure problems, In particular it is at present not known how to record into such media, which may require write fields in the order of 10-100 kOe. Despite this unsolved problem, there is considerable interest in high Ku alternative media, both for longitudinal and perpendicular recording. Activities in this area will be reviewed and data on sputtered and evaporated thin FePt films, with coercivities exceeding 10000 Oe will be presented.
1,310 citations
TL;DR: In this article, a method to pattern a substrate with dense periodic nanostructures that combine top-down lithographic tools and self-assembling block copolymer materials is described.
Abstract: Methods to pattern substrates with dense periodic nanostructures that combine top-down lithographic tools and self-assembling block copolymer materials are provided. According to various embodiments, the methods involve chemically patterning a substrate, depositing a block copolymer film on the chemically patterned imaging layer, and allowing the block copolymer to self-assemble in the presence of the chemically patterned substrate, thereby producing a pattern in the block copolymer film that is improved over the substrate pattern in terms feature size, shape, and uniformity, as well as regular spacing between arrays of features and between the features within each array compared to the substrate pattern. In certain embodiments, the density and total number of pattern features in the block copolymer film is also increased. High density and quality nanoimprint templates and other nanopatterned structures are also provided.
1,089 citations
TL;DR: Faceted surfaces of commercially available sapphire wafers were used to guide the self-assembly of block copolymer microdomains into oriented arrays with quasi–long-range crystalline order over arbitrarily large wafer surfaces, opening a versatile route toward ultrahigh-density systems.
Abstract: Generating laterally ordered, ultradense, macroscopic arrays of nanoscopic elements will revolutionize the microelectronic and storage industries. We used faceted surfaces of commercially available sapphire wafers to guide the self-assembly of block copolymer microdomains into oriented arrays with quasi-long-range crystalline order over arbitrarily large wafer surfaces. Ordered arrays of cylindrical microdomains 3 nanometers in diameter, with areal densities in excess of 10 terabits per square inch, were produced. The sawtoothed substrate topography provides directional guidance to the self-assembly of the block copolymer, which is tolerant of surface defects, such as dislocations. The lateral ordering and lattice orientation of the single-grain arrays of microdomains are maintained over the entire surface. The approach described is parallel, applicable to different substrates and block copolymers, and opens a versatile route toward ultrahigh-density systems.
740 citations
01 Apr 1997
TL;DR: In this paper, the authors review the recent advances in patterned magnetic nanostructures, a fast-emerging field, including state-of-the-art technology for patterning of magnetic nano-structures as small as 10 nm, and a new paradigm for ultra-high-density magnetic storage based on patterned single-domain elements.
Abstract: Nanofabrication, offering unprecedented capabilities in the manipulation of material structures and properties, opens up new opportunities for engineering innovative magnetic materials and devices, developing ultra-high-density magnetic storage, and understanding micromagnetics. This paper reviews the recent advances in patterned magnetic nanostructures, a fast-emerging field, including (1) state-of-the-art technology for patterning of magnetic nanostructures as small as 10 nm; (2) engineering of unique magnetic properties (such as domain structures, domain switching, and magnetoresistance) by patterning and controlling the size, shape, spacing, orientation, and compositions of magnetic materials; (3) quantized magnetic disks-a new paradigm for ultra-high-density magnetic storage based on patterned single-domain elements that have demonstrated a storage density of 65 Gb/in/sup 2/ (nearly two orders of magnitude higher than that in current commercial magnetic disks) and a capability of 400 Gb/in/sup 2/; (4) novel magnetoresistance sensors based on unique properties of magnetic nanostructures; (5) other applications of nanoscale patterning in magnetics such as the quantification of magnetic force microscopy (MFM) and a new ultra-high-resolution MFM tip; and (6) sub-10-nm imprint lithography-a new low-cost, high-throughput technology for manufacturing magnetic nanostructures.
386 citations