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Finite-element Langevin-dynamics simulations are performed in order to extract the attempt frequency of small magnetic particles as a function of an applied perpendicular field. The obtained values of the attempt frequency are in excellent agreement with the analytical results of [Kalmykov, J. Appl. Phys. 96, 1138 (2004)]. It is shown that an external field that is applied perpendicularly to the easy axis with a strength of just about 1% of the anisotropy field is strong enough that the framework of the transition state theory (TST) for broken symmetries can be applied. It is concluded that for most realistic structures, the attempt frequency can be numerically calculated by broken symmetry—TST formulism. [ABSTRACT FROM AUTHOR]

A theoretical model is developed to describe the pinning energy and the pinning field as a function of pore density, pore diameter and material parameters of a magnetic film. It is shown that the pinning energy and pinning field increases monotonically with increasing pore diameter. A magnetic storage concept is introduced which combines percolated media and graded media which allows to write on magnetic films with ultra high anisotropies. [ABSTRACT FROM AUTHOR]

Permanent magnet materials require a high uniaxial magneto-crystalline anisotropy. Exchange coupling between small crystallites with easy-plane anisotropy induces an effective uniaxial anisotropy if arranged accordingly. Nanostructuring of materials with easy-plane anisotropy is an alternative way to create hard-magnetic materials. The coercivity increases with decreasing feature size. The resulting coercive field is about 12 percent of the anisotropy field for a crystal size of 3.4 times the Bloch parameter. [ABSTRACT FROM AUTHOR]

In order to study the effect of surface anisotropy, we have developed a three-dimensional finite element micromagnetic model. The finite element mesh of the particles is split into a core region with bulk properties and a surface region (shell) of variable thickness with modified material parameters. We have studied the magnetization reversal in FePt nanoparticles. The effect of surface anisotropy strongly reduces their coercivity. An Fe–oxide shell with low anisotropy in the plane of the surface has been considered as well as surface anisotropy with high anisotropy constants and axis orientation perpendicular to the surface. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

One essential feature in magnetic random access memory cells is the spin torque efficiency, which describes the ratio of the critical switching current to the energy barrier. In this paper, it is reported that the spin torque efficiency can be improved by a factor of 3.2 by the use of a dual free layer device, which consists of one layer with perpendicular crystalline anisotropy and another layer with in-plane crystalline anisotropy. Detailed simulations solving the spin transport equations simultaneously with the micromagnetics equation were performed in order to understand the origin of the switching current reduction by a factor of 4 for the dual layer structure compared to a single layer structure. The main reason could be attributed to an increased spin accumulation within the free layer due to the dynamical tilting of the magnetization within the in-plane region of the dual free layer. [ABSTRACT FROM AUTHOR]

Abstract: The potential of exchange spring bilayers and graded media is reviewed. An analytical model for the optimization of graded media gives an optimal value of the magnetic polarization of J s=0.8T. The optimum design allows for thermally stable grains with grain diameters in the order of 3.3nm, which supports ultra-high density up to 5–10Tbit/in2. The switching field distribution is significantly reduced in bilayer media and graded media compared to single-phase media. For the graded media the switching field distribution is reduced by about a factor of two. For bilayer media the minimum switching field distribution is obtained for soft-layer anisotropies that are about one fifth of the hard-layer anisotropy. The influence of precessional switching on the reversal time and the reversal field is investigated in detail for magnetic bilayers. Exchange spring bilayers can be reversed with field pulses of 20ps. [Copyright &y& Elsevier]

Abstract: In order to achieve an areal density of 10Tbit/in2 in perpendicular recording, grains with diameters of about 3nm are required. However, for such grain sizes, common hard disk materials are magnetically too soft to be thermally stable. Extremely hard magnetic materials such as FePt have a too large coercive field and cannot be reversed with conventional head fields. The proposed FePt (2nm)/Fe3Pt (14nm)/FePt (2nm) trilayer reduces the coercive field by a factor of 8 compared to a single-phase FePt layer of the same thickness. At the same time the energy barrier is only decreased by 40%. A high signal to noise ratio can be expected for the multilayer structure since the switching field distribution is insensitive on the angle between the easy axis of the hard layers and the external field. A change from to 1° changes the coercive field by less than 2.5% (10% for conventional perpendicular media). Thus, small distribution of the anisotropy axes angles have less effect than in conventional perpendicular media. [Copyright &y& Elsevier]

We report recording simulations on graded media with area densities of 1 Tbit/in.2. The media are composed of a nucleation layer exchange coupled to a hard magnetic storage layer. The nucleation layer has an anisotropy K(z) that gradually varies in order to adjust the domain wall propagation field to the write field. Bits were written with a bit length of 12 nm and a track width of 53 nm on graded media with total thickness of 21 nm and maximum anisotropy of 1 MJ/m3. The computed values for transition jitter are around 0.65 nm, depending on the intergrain exchange. [ABSTRACT FROM AUTHOR]

The concept of exchange spring media is extended from two layers to N layers. The coercive field of the multilayer structure decreases with 1/N while the energy barrier is only determined by the magnetic properties of the hardest layer. A 25 nm thick trilayer has a 7.5 times smaller coercive field (1.4 T) as a single layer at the same thermal stability and for the same value of the magnetization. A continuous variation of the anisotropy in the recording media reduces the coercive field by a factor of 10 for a layer thickness of 25 nm. [ABSTRACT FROM AUTHOR]

We present density functional theory (DFT) calculations of the magnetic anisotropy energy (MAE) of FePt, which is of great interest for magnetic recording applications. Our data, and the majority of previously calculated results for perfectly ordered crystals, predict a MAE of ~3.0meV per formula unit, which is significantly larger than experimentally measured values. Analyzing the effects of disorder by introducing stacking faults (SFs) and antisite defects (ASDs) in varying concentrations we are able to reconcile calculations with experimental data and show that even a low concentration of ASDs are able to reduce the MAE of FePt considerably. Investigating the effect of exact exchange and electron correlation within the adiabatic-connection dissipation fluctuation theorem in the random phase approximation (ACDFT-RPA) reveals a significantly smaller influence on the MAE. Thus the effect of disorder, and more specifically ASDs, is the crucial factor in explaining the deviation of common DFT calculations of FePt to experimental measurements. [ABSTRACT FROM AUTHOR]

We studied the variation in perpendicular magnetic anisotropy of (111) textured Au/N × [ Co/Ni ]/Au films as a function of the number of bilayer repeats N. The ferromagnetic resonance and superconducting quantum interference device magnetometer measurements show that the perpendicular magnetic anisotropy of Co/Ni multilayers first increases with N for N = 10 and then moderately decreases for N > 10. The model we propose reveals that the decrease of the anisotropy for N < 10 is predominantly due to the reduction in the magnetoelastic and magnetocrystalline anisotropies. A moderate decrease in the perpendicular magnetic anisotropy for N > 10 is due to the reduction in the magnetocrystalline and the surface anisotropies. To calculate the contribution of magnetoelastic anisotropy in the Co/Ni multilayers, in-plane and out-of-plane x-ray diffraction measurements are performed to determine the spacing between Co/Ni (111) and (220) planes. The magnetocrystalline bulk anisotropy is estimated from the difference in the perpendicular and parallel g factors of Co/Ni multilayers that are measured using the in-plane and out-of-plane ferromagnetic resonance measurements. Transmission electron microscopy has been used to estimate the multilayer film roughness. These values are used to calculate the roughness-induced surface and magnetocrystalline anisotropy coefficients as a function of N. [ABSTRACT FROM AUTHOR]

Abstract: Global optimization of writing head is performed using micromagnetics and surrogate optimization. The shape of the pole tip is optimized for bit patterned, exchange spring recording media. The media characteristics define the effective write field and the threshold values for the head field that acts at islands in the adjacent track. Once the required head field characteristics are defined, the pole tip geometry is optimized in order to achieve a high gradient of the effective write field while keeping the write field at the adjacent track below a given value. We computed the write error rate and the adjacent track erasure for different maximum anisotropy in the multilayer, graded media. The results show a linear trade off between the error rate and the number of passes before erasure. For optimal head media combinations we found a bit error rate of 10−6 with 108 pass lines before erasure at 2.5Tbit/in2. [Copyright &y& Elsevier]

Abstract: Micromagnetic simulations were performed to investigate the influence of geometry and magnetic anisotropy constant on energy barrier and magnetization reversal mechanism of individual bits important for the bit patterned media concept in magnetic data storage. It is shown that dependency of the energy barrier on magnetic and geometric properties of bits can be described by an analytical approach in the case of quasi-coherent magnetization rotation process. However, when the bit size exceeds a critical size, for which an incoherent magnetization reversal is preferred, the analytical approach becomes invalid and no self-consistent theory is available. By systematically investigating the influence of bit size on the magnetization reversal mode, it was found that the transition from quasi-coherent to incoherent magnetization reversal mode can still be described analytically if an activation volume is considered instead of the bit volume. In this case, the nucleation volume is an important parameter determining thermal stability of the bit. If the volume of the bit is larger than twice the activation volume, the energy barrier stays nearly constant; with further increase in bit size, no gain in thermal stability can be achieved. [ABSTRACT FROM AUTHOR]

We report recording simulations on graded media with area densities of 1 Tbit/in.2. The media are composed of a nucleation layer exchange coupled to a hard magnetic storage layer. The nucleation layer has an anisotropy K(z) that gradually varies in order to adjust the domain wall propagation field to the write field. Bits were written with a bit length of 12 nm and a track width of 53 nm on graded media with total thickness of 21 nm and maximum anisotropy of 1 MJ/m3. The computed values for transition jitter are around 0.65 nm, depending on the intergrain exchange. [ABSTRACT FROM AUTHOR]