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The reduction of the transition curvature of written bits in heat-assisted magnetic recording is expected to play an important role for the future areal density increase of hard disk drives. Recently, a write head design with flipped write and return poles was proposed. In this design, a large spatial field gradient of the write head was the key to significantly reduce the transition curvature. In this work, we optimized the write pole of a heat-assisted magnetic recording head in order to produce large field gradients as well as large fields in the region of the heat pulse. This is done by topology optimization. The simulations are performed with dolfin-adjoint. For the maximum field gradients of 8.1 mT/nm, 8.6 mT/nm, and 11.8 mT/nm, locally resolved footprints of an FePt-like hard magnetic recording medium are computed with a coarse-grained Landau-Lifshitz-Bloch model, and the resulting transition curvature is analyzed. Additional simulations with a bilayer structure with a 50 % hard and 50 % soft magnetic material are computed. The results show, that for both recording media, the optimized head design does not lead to any significant improvements in the written track. Thus, we analyze the transition curvature for the optimized write heads theoretically with an effective recording time window model. Moreover, we check how higher field gradients influence the curvature reduction. The results show that a simple optimization of the conventional head design is not sufficient for effective curvature reduction. Instead, new head concepts will be needed to reduce the transition curvature. [ABSTRACT FROM AUTHOR]

We optimize the recording medium for heat-assisted magnetic recording by using a high/low T c bilayer structure to reduce AC and DC noise. Compared to a former work, small Gilbert damping α = 0.02 is considered for the FePt-like hard magnetic material. Atomistic simulations are performed for a cylindrical recording grain with diameter d = 5 nm and height h = 8 nm. Different soft magnetic material compositions are tested, and the amount of hard and soft magnetic material is optimized. The results show that for a soft magnetic material with α SM = 0.1 and J i j , SM = 7.72 × 10 − 21 J / link , a composition with 50 % hard and 50 % soft magnetic material leads to the best results. Additionally, we analyze how much areal density can be improved by using the optimized bilayer structure compared to the pure hard magnetic recording material. It turns out that the optimized bilayer design allows an areal density that is 1 Tb / in. 2 higher than that of the pure hard magnetic material while obtaining the same signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

It is assumed that heat-assisted magnetic recording is the recording technique of the future. For pure hard magnetic grains in high density media with an average diameter of 5 nm and a height of 10 nm, the switching probability is not sufficiently high for the use in bit-patterned media. Using a bilayer structure with 50% hard magnetic material with low Curie temperature and 50% soft magnetic material with high Curie temperature to obtain more than 99.2% switching probability leads to very large jitter. We propose an optimized material composition to reach a switching probability of Pswitch >99.2% and simultaneously achieve the narrow transition jitter of pure hard magnetic material. Simulations with a continuous laser spot were performed with the atomistic simulation program VAMPIRE for a single cylindrical recording grain with a diameter of 5 nm and a height of 10 nm. Different configurations of soft magnetic material and different amounts of hard and soft magnetic material were tested and discussed. Within our analysis, a composition with 20% soft magnetic and 80% hard magnetic material reaches the best results with a switching probability Pswitch >99.2%, an off-track jitter parameter σoff,80/20=0.46 nm and a down-track jitter parameter &#963down,80/20=0.49 nm. [ABSTRACT FROM AUTHOR]

Topology optimization offers great opportunities to design permanent magnetic systems that have specific external field characteristics. Additive manufacturing of polymer-bonded magnets with an end-user 3D printer can be used to manufacture permanent magnets with structures that had been difficult or impossible to manufacture previously. This work combines these two powerful methods to design and manufacture permanent magnetic systems with specific properties. The topology optimization framework is simple, fast, and accurate. It can also be used for the reverse engineering of permanent magnets in order to find the topology from field measurements. Furthermore, a magnetic system that generates a linear external field above the magnet is presented. With a volume constraint, the amount of magnetic material can be minimized without losing performance. Simulations and measurements of the printed systems show very good agreement. [ABSTRACT FROM AUTHOR]

The spatially-inhomogeneous magnetization dynamics in a cylindrical magnetic nanodot driven by ac spin-torque is analyzed. To this end, the Landau-Lifshitz-Gilbert-Slonczewski equation is reformulated as a system of coupled nonlinear ordinary differential equations which describe the time-evolution of normal modes amplitudes. This approach provides a class of models with reduced number of degrees of freedom and incremental accuracy between macrospin and full micromagnetics. By using this approach, the onset of foldover effect for fundamental and higher-order modes is demonstrated. The results are confirmed by full micromagnetic simulations. [ABSTRACT FROM AUTHOR]

The switching probability of magnetic elements for heat-assisted recording with pulsed laser heating was investigated. It was found that FePt elements with a diameter of 5 nm and a height of 10nm show, at a field of 0.5 T, thermally written-in errors of 12%, which is significantly too large for bitpatterned magnetic recording. Thermally written-in errors can be decreased if larger-head fields are applied. However, larger fields lead to an increase in the fundamental thermal jitter. This leads to a dilemma between thermally written-in errors and fundamental thermal jitter. This dilemma can be partly relaxed by increasing the thickness of the FePt film up to 30 nm. For realistic head fields, it is found that the fundamental thermal jitter is in the same order of magnitude of the fundamental thermal jitter in conventional recording, which is about 0.5-0.8nm. Composite structures consisting of high Curie top layer and FePt as a hard magnetic storage layer can reduce the thermally written-in errors to be smaller than 10-4 if the damping constant is increased in the soft layer. Large damping may be realized by doping with rare earth elements. Similar to single FePt grains in composite structure, an increase of switching probability is sacrificed by an increase of thermal jitter. Structures utilizing first-order phase transitions breaking the thermal jitter and writability dilemma are discussed. [ABSTRACT FROM AUTHOR]

We investigate the effect of curvature on the energy and stability of domain wall configurations in curved cylindrical nanotubes and nanowires. We use micromagnetic simulations to calculate the phase diagram for the transverse wall (TW) and vortex wall (VW) states in tubes, finding the lower energy configuration and the metastability region where both types of walls can exist. The introduction of curvature shifts the range for which the TW is the ground state domain wall to higher diameters and increases the range of metastability. We interpret this behavior to be primarily due to the curvature-induced effective Dzyaloshinskii–Moriya term in the exchange energy. Furthermore, we demonstrate qualitatively the same behavior in solid cylindrical nanowires. Comparing both tubes and wires, we observe how while in tubes curvature tends to suppress the transformation from the TW to VW, in wires it promotes the transformation of the VW containing the Bloch point into the TW. These findings have important implications in the fundamental understanding of domain walls in 3D geometries and the design of future domain wall devices. [ABSTRACT FROM AUTHOR]

Demagnetization curves of sub-micron Nd2Fe14B particles with rectangular prism shapes were simulated by micromagnetic simulations. Coercivity decreases with increasing particle thickness for same particle volumes. This unexpected behavior can be explained by different resulting angles of the total effective field when superimposing the applied field vector with the stray field vector at the point of the first reversal event. The angular dependence of the nucleation field follows a Stoner-Wohlfarth like behavior. [ABSTRACT FROM AUTHOR]

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]

In micromagnetic simulations of spin waves in thin film geometries special boundary conditions have to be applied at the boundary of the computational domain in order to avoid the reflection of spin waves. In this paper we proposed to use an artificially rough surface that will suppress spin wave reflections in order to treat spin torque oscillators with point contact geometry numerically. It is shown that the surface roughness model disperses the reflected spin waves and improves the signal to background noise ratio. The model is tested in comparison to conventional approaches such as extended layer systems, variable damping constant, and focal point methods for double point contacts. The surface roughness model gives solutions that are stable in time, in qualitative agreement with experiments and capable to reproduce phenomena such as phase locking in double point contacts. [ABSTRACT FROM AUTHOR]

The magnetization reversal process of Co nanowires was investigated using a moving mesh technique. The nucleation and expansion of reversed domains is calculated by solving the Gilbert equation of motion for different damping constants. The adaptive finite element method reduces the total CPU time by more than a factor of 4 as compared to a uniform mesh. Two different domain wall types are observed. For a wire diameter of d = 10 nm transverse walls occur and gyromagnetic precession limits the domain wall velocity. The domain wall velocity increases from 50 to 520 m/s as the Gilbert damping constant increases from α=0.05 to α= 1 at an applied field of 500 kA/m. For a diameter greater than 20 nm vortex walls are formed. The vortex mobility increases with decreasing damping constant. Thus velocities up to 2000 m/s are reached for a wire diameter of 40 nm, α=0.05, and an applied field of 250 kA/m. [ABSTRACT FROM AUTHOR]

A skyrmion racetrack design is proposed that allows for thermally stable skyrmions to code information and dynamical pinning sites that move with the applied current. This concept solves the problem of intrinsic distributions of pinning times and pinning currents of skyrmions at static pinning sites due to geometrical constrictions or disordered film. The dynamical pinning sites are realized by a skyrmion carrying wire, where the skyrmion repulsion is used in order to keep the skyrmions at equal distances. The information is coded by an additional layer where the presence and absence of a skyrmion is used to code the information. The lowest energy barrier for a data loss is calculated to be ΔE = 55 kBT300 which is sufficient for a long time thermal stability. [ABSTRACT FROM AUTHOR]

Adding Pt into CoCr-oxide alloy for controlling magnetic softness in a soft layer on hard/soft (H/S) stacked composite media significantly affects magnetic properties and microstructure. Hc values for capping layer (CL) media with a relatively high-Hk soft layer initially increase with increasing tcap. However, Hc values are relatively constant for H/S media with Pt 3% and 10% soft layers and reduced for the media with a CoTaZr–SiO2 soft layer having in-plane Hk. The simulation results explain these experimental Hc trends by the compensation effect between the decrease in average Ku due to the use of less-Ku soft layer and the increases in volume with increasing tsoft. Reduction in Hc with increasing tsoft is anticipated when Ku in a soft layer reduces by >27%. As the Pt content in a soft layer increases from 3% to 18%, higher Hc and Hn but lower Hs are observed. Plan-view transmission electron microscopy images for the medium with Pt 18% soft layer show no subgrains and thinner grain boundary width and explain the significant increase in lateral Aex. Thus, the Pt content in a soft layer needs to be optimized. Compared to CL media, H/S composite media with the Pt 5% and 8% soft layers improve both SNRme and media writability. [ABSTRACT FROM AUTHOR]

The advancement of 3D-printing opens up a new way of constructing affordable custom terahertz (THz) components due to suitable printing resolution and THz transparency of polymer materials. We present a way of calculating, designing, and fabricating a THz waveplate that phase-modulates an incident THz beam (λ0 = 2.14 mm) in order to create a predefined intensity profile of the optical wavefront on a distant image plane. Our calculations were performed for two distinct target intensities with the use of a modified Gerchberg-Saxton algorithm. The resulting phase-modulating profiles were used to model the polylactide elements, which were printed out with a commercially available 3D-printer. The results were tested in a THz experimental setup equipped with a scanning option, and they showed good agreement with theoretical predictions. [ABSTRACT FROM AUTHOR]

Fast stray field calculation is commonly considered of great importance for micromagnetic simulations, since it is the most time consuming part of the simulation. The Fast Multipole Method (FMM) has displayed linear O(N) parallelization behavior on many cores. This article investigates the error of a recent FMM approach approximating sources using linear—instead of constant—finite elements in the singular integral for calculating the stray field and the corresponding potential. After measuring performance in an earlier manuscript, this manuscript investigates the convergence of the relative L2 error for several FMM simulation parameters. Various scenarios either calculating the stray field directly or via potential are discussed. [ABSTRACT FROM AUTHOR]

The Landau-Lifshitz-Gilbert equation and quasistatic Maxwell equations were solved simultaneously to calculate the read back signal of giant magnetoresistance read heads with a hybrid finite-element/boundary element method. The finite-element simulations show the influence of the sense current on the linearity of the reader, the effect of the exchange bias field on the sensor performance, and the influence of the Gilbert damping constant on the decay time of the read back voltage. All parts of the system, the layers of the giant magnetoresistance sensor, the hard bias magnets, the shields, and the recording layer are treated micromagnetically. In addition, the influence of the sense current onto the magnetization is taken into account self-consistently. The current distribution in the giant magnetoresistance stack is calculated from local resistivity which depends on the magnetization of the free and of the pinned layer. [ABSTRACT FROM AUTHOR]

A method for the calculation of the effective damping parameter as a function of particle size and electric conductivity is presented. An extended three-dimensional finite element/boundary element micromagnetic model is used to solve the Landau-Lifshitz-Gilbert equation together with the eddy current diffusion equation. From the change of energy with time an effective damping parameter is derived from which the net eddy current contribution is calculated. It is shown that the eddy current effects are reduced with an increasing applied field and are increased for materials with a low resistivity. Furthermore it is shown that the size of the particles, which designates the coherence of the spin structure, is essential for the eddy current net contribution. [ABSTRACT FROM AUTHOR]

Detailed studies of the structure of magnetic nanoclusters are crucial for understanding their magnetic properties. We have investigated the structure of CoxAg1-x nanoparticles by means of molecular dynamics simulations utilizing the embedded atom method. Starting from a completely random distribution of Co and Ag atoms, the clusters were heated up to 1300 K and subsequently cooled down. The size of the resulting particles was 2.8 nm (864 atoms). A clear segregation of the Ag atoms on the surface of the Co core was obtained. [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]

We developed a micromagnetic eddy current method that allows arbitrary geometries, requires no mesh outside the ferromagnet, and uses a stable integration scheme. We simultaneously solve the Landau-Lifshitz-Gilbert equation and the quasistatic Maxwell equations using a hybrid finite element/boundary element method sFEM/BEMd. The eddy current field is directly calculated from the space time behavior of the magnetization rate of change. The boundary conditions of the eddy current field at infinity are taken into account using a FEM/BEM scheme. The resulting system of differential algebraic equations is solved using a backward differentiation method. [ABSTRACT FROM AUTHOR]

Micromagnetic calculations at nonzero temperatures depend on the computational cell size. This paper shows that the spontaneous magnetization MS of exchange-coupled moments has to be scaled by a Bloch-like law, which is similar to the well-known temperature dependence of MS. Using this scaling law, nonatomistic Metropolis Monte Carlo and stochastic Landau-Lifshitz-Gilbert simulations are performed in an external field of 0.1 T. The error of the equilibrium magnetization at a temperature of T/TC=0.38 and a cell size of 1.5 nm is then 0.9% as compared with atomistic calculations. In contrast, a cell size-independent MS leads to an overestimation of the temperature of 3.2%. [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]

FePt-based nanostructured materials are excellent candidates for high density recording beyond 1 Tbit/in[SUP2]. We calculate remanence, coercivity, and loop shape of annealed monodisperse FePt nanocrystals, using a modified Stoner--Wohlfarth model. To justify the simplifications of a Stoner-Wohlfarth model detailed finite element micromagnetic simulations were performed. Magnetic measurements on arrays of chemically synthesized FePt nanoparticles show remanence ratios of about 0.6 which is greater than that predicted for a series of noninteracting Stoner--Wohlfarth particles. A small fraction of the particles (5%) is assumed to remain in the disordered fcc phase with low magnetocrystalline anisotropy. Both remanence and coercivity are highly sensitive to the strength of the exchange interactions within a multiple twined nanocrystal. The calculated values are in the range from J[SUBr]/J[SUBs] = 0.52, H[SUBc] = 0.77 MA/m to J[SUBr]/J[SUBs] = 0.61, H[SUBc] = 1.2 MA/m. The results of the modified Stoner--Wohlfarth model are confirmed by finite element micromagnetic simulations taking into account magnetostatic interactions and allowing nonuniform magnetic structures within a particle. [ABSTRACT FROM AUTHOR]

The hysteresis properties of epitaxial SmCo/Co and SmCo/Fe bilayers are calculated by the solution of the Landau -- Lifshitz Gilbert equation. The thin film grain structure is taken into account using appropriate finite element techniques. The J(H) curve shows the typical exchange spring behavior for the bilayer if the soft magnetic layer thickness exceeds 10 nm. However, the reversible rotations of the magnetization for low external field deteriorate the maximum energy density product. Straight B(H) curves are obtained only for a Fe layer thickness of 5 nm. Magnetization reversal starts with the reversible rotation of the soft layer magnetization. Initially, the magnetization rotates in opposite directions in different regions of the film. The reversible rotations penetrate substantially into the hard layer. [ABSTRACT FROM AUTHOR]

The dynamic response of a 80 nm×400 nm×25 nm Co film with and without polycrystalline grains is calculated. The numerical method combines a finite element scheme for space discretization with a preconditioned backward differentiation method for the time integration of the Landau-LifshitzGilbert equation. The use of proper preconditioning techniques for the backward differentiation formulas increases its efficiency by a factor of 40. The speed up factor compared to the Adams method is more than three orders of magnitude. In the polycrystalline thin film the nucleation of reversed domains occurs throughout the sample at grain boundaries and sharp edges. For the single crystalline film reversal starts at particle ends. Surface roughness does not significantly change the reversal mode but reduces the switching time by 40%. [ABSTRACT FROM AUTHOR]

Nanostructured granular Ni80Fe[sub 20] and Co films are studied using a 3D hybrid finite element/ boundary element model. Switching dynamics are calculated for external fields applied unidirectional after a rise time of 0.1 ns and for a 10 GHz rotational field with a field strength of H[sub ext] = 0.2 J[sub s]/µ[sub 0] (160 kA/m for NiFe and 280 kA/m for Co). The transient magnetization patterns show that reversal in the unidirectional field proceeds by the nucleation and propagation of end domains towards the center of the element. The switching time strongly depends on the Gilbert damping parameter α. Materials with uniaxial anisotropy (Co), require larger field, but exhibit shorter switching times. Reversal in rotational fields involves inhomogeneous rotation of the end domains towards the rotational field direction leading to partial flux-closure structures. Shorter switching times are obtained by the application of the 10 GHz rotational field (t[sub sw] = 0.05 ns). Precessional oscillation effects after abruptly switching off the external field which occurred in the NiFe square element, were suppressed in the granular Co film. Reducing the field to zero slowly inhibits the high frequency excitations in NiFe. [ABSTRACT FROM AUTHOR]

We have investigated domain wall pinning in Sm(Co,Fe,Cu,Zr)[sub z] based magnets with 3D micromagnetic simulations using a hybrid finite element/boundary element method. Transmission electron micrographs of magnets with Sm[sub 2] (Co,Fe) [sub 17] matrix and Sm(Co,Cu)[sub 5-7] precipitation phases were used to model the cellular microstructure. The pinning field was investigated for varying thickness of the intercellular phase. [ABSTRACT FROM AUTHOR]

Nanocrystalline exchange coupled Pr[sub 11.76]Fe[sub 82.36]B[sub 5.88] single-phase and Pr[sub 9]Fe[sub 85]B[sub 6] two-phase magnets containing 20 vol % α-Fe were produced using mechanical alloying. Micromagnetic finite element simulations were used to analyze the temperature dependence of the magnetic properties. In the single-phase exchange-coupled magnet a significant enhancement of the remanence J[sub r]=1.1 T was achieved at room temperature. A further enhancement of the remanence was observed in two-phase α-Fe containing magnets reaching a remanence of J[sub r]=1.23 T. The corresponding values of the coercive field are μ[sub 0]H[sub c]=0.81 T and μ[sub 0]H[sub c]=0.57 T for the single-phase magnet and the two-phase magnet, respectively. Remanence enhancement becomes more effective with increasing temperature, compensating the decrease of the saturation polarization. In the two-phase magnet the remanence increases from J[sub r]=1.12 to J[sub r]=1.23 T as the temperature is increased from 200 to 300 K. Micromagnetic calculations clearly show that the increase of the exchange length with increasing temperature improves the effective coupling between the Pr[sub 2]Fe[sub 14]B and the α-Fe phase. The decrease of the coercive field with increasing temperature has to be attributed to the temperature dependence of the anisotropy field. © 2000 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]

Conjugate gradient methods for energy minimization in micromagnetics are compared. The comparison of analytic results with numerical simulation shows that standard conjugate gradient method may fail to produce correct results. A method that restricts the step length in the line search is introduced, in order to avoid this problem. When the step length in the line search is controlled, conjugate gradient techniques are a fast and reliable way to compute the hysteresis properties of permanent magnets. The method is applied to investigate demagnetizing effects in NdFe12 based permanent magnets. The reduction of the coercive field by demagnetizing effects is μ0ΔH = 1.4 T at 450 K. [ABSTRACT FROM AUTHOR]

3D print is a recently developed technique, for single-unit production, and for structures that have been impossible to build previously. The current work presents a method to 3D print polymer bonded isotropic hard magnets with a low-cost, end-user 3D printer. Commercially available isotropic NdFeB powder inside a PA11 matrix is characterized, and prepared for the printing process. An example of a printed magnet with a complex shape that was designed to generate a specific stray field is presented, and compared with finite element simulation solving the macroscopic Maxwell equations. For magnetic characterization, and comparing 3D printed structures with injection molded parts, hysteresis measurements are performed. To measure the stray field outside the magnet, the printer is upgraded to a 3D magnetic flux density measurement system. To skip an elaborate adjusting of the sensor, a simulation is used to calibrate the angles, sensitivity, and the offset of the sensor. With this setup, a measurement resolution of 0.05 mm along the z-axes is achievable. The effectiveness of our calibration method is shown. With our setup, we are able to print polymer bonded magnetic systems with the freedom of having a specific complex shape with locally tailored magnetic properties. The 3D scanning setup is easy to mount, and with our calibration method we are able to get accurate measuring results of the stray field. [ABSTRACT FROM AUTHOR]

We study spin torque oscillators comprised of a perpendicular spin injection layer (SIL) and a planar field generating layer to reveal the influence of the spin polarization of SIL material on the critical current density, JC, to induce microwave oscillation. Two systems with different SIL are compared: one with a highly spin-polarized Heusler alloy, Co2Fe(Ga0.5Ge0.5) (CFGG), and the other a prototypical Fe2Co alloy. Cross sectional scanning transmission electron microscopy observations show the B2-ordered structure in a 3-nm-thick CFGG SIL, a prerequisite for obtaining half-metallic transport properties. Current induced microwave oscillations are found at frequencies of ~15 GHz for both systems. However, the current needed to cause the oscillations is ~50% smaller for films with the CFGG SIL compared to those of the Fe2Co SIL. These results are in accordance with micromagnetic simulations that include spin accumulation at the SIL. [ABSTRACT FROM AUTHOR]

Exchange-coupled composite media were realized by combining perpendicular hard magnetic FePtCu alloy films with perpendicular Co/Pt multilayers which are magnetically softer. We demonstrate that the switching field of the hard layer can be efficiently altered by modifying the material properties of the soft layer by varying the number of Co/Pt bilayers. Moreover, the possibility of effectively tuning the interlayer exchange coupling using rapid thermal annealing was shown. These studies were supported by theoretical modeling revealing the relevant factors to reduce the switching field of the hard layer which are important for future media design. [ABSTRACT FROM AUTHOR]

Layer-selective writing of two layer bit patterned media is demonstrated by performing micromagnetic finite element simulations. Selectivity is achieved by controlling the frequency of an oscillating magnetic field in the gigahertz range, applied in addition to the head field. Generation of the microwave field by means of a wire next to the tip of a single pole head is proposed. The Oersted field from the alternating current induces magnetic oscillations in the pole tip which create a high frequency field that is superimposed to the perpendicular write field. The amplitude of the ac field component is in the order of 0.1 T. [ABSTRACT FROM AUTHOR]

The thermal stability of graded exchange spring media for perpendicular recording is investigated. It is found that the energy barrier linearly decreases with the external field strength. This leads to a high thermal stability in the low frequency recording limit, where large demagnetizing fields act on the media. The attempt frequency of graded media is about a factor of 5 larger than the attempt frequency of comparable single phase media. [ABSTRACT FROM AUTHOR]

Repeated writing causes adjacent track erasure. During multiple write cycles, the grains of neighboring tracks are exposed to the write field, which will lower the energy barrier. Grains in an adjacent track are likely to reverse their magnetization when the cumulated field expose time exceeds the transition time. The energy barrier of a grain subject to the write field is computed as a function of distance from the track center. For the same perpendicular writer and the same intergranular exchange field, the number of passes before erasure is orders of magnitudes higher in exchange spring media than in single phase media. [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]

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]

A novel type of exchange spring media is proposed for magnetic recording systems consisting of a hard/soft bilayer. Finite element micromagnetic simulations show that the reversal modes induced by the external write field are significantly different from the thermally activated switching processes. Thus, the bilayers can be optimized in order to achieve a high thermal stability without increase of coercive field. In grains with identical size and coercivity an optimized bilayer reaches an energy barrier exceeding those of optimized single phase media by more than a factor of two. Additionally the lower angular dependence of coercivity of exchange spring media will improve the signal to noise ratio. [ABSTRACT FROM AUTHOR]

We present an analytical derivation of the switching field distribution (SFD) at finite temperature for a single domain particle from the Néel-Brown model in the presence of a linearly swept magnetic field. By considering the field dependence of the attempt frequency f0 in the rate equation, we find enhancement of coercivity compared to models using constant f0. The contribution of thermal fluctuations to the standard deviation of the switching field HC derived here reaches values of 10% HC. Considering this contribution, which has been neglected in previous work, is important for the correct interpretation of measurements of switching field distributions. [ABSTRACT FROM AUTHOR]

A numerical implementation of the transition state theory is presented which can be used to calculate the attempt frequency f0 of arbitrary shaped magnetic nanostructures. The micromagnetic equations are discretized using the finite element method. The climbing image nudged elastic band method is used to calculate the saddle point configuration, which is required for the calculation of f0. Excellent agreement of the implemented numerical model and analytical solutions is obtained for single domain particles. The developed method is applied to compare f0 for single phase and graded media grains of advanced recording media. f0 is predicted to be comparable if the maximum anisotropy is the same in these two media types. [ABSTRACT FROM AUTHOR]

In this article we describe the thermal relaxation in anti-ferromagnetic/ferromagnetic bilayers using a hybrid method that combines a kinetic Monte Carlo technique with magnetization dynamics following the Landau Lifshitz Gilbert equation. A granular anti-ferromagnetic layer is exchange coupled to an amorphous ferromagnetic layer and discretized using a finite element method. Calculations are made to help clarify how the underlying magnetic structure is related to the measured exchange bias fields as a function of temperature for the case of amorphous Co65.5Fe14.5B20/granular Ir22Mn78 bilayers. Our calculations are in excellent agreement with experimentally measured macro-magnetic properties of these bilayers. [ABSTRACT FROM AUTHOR]

Heat assisted recording is believed as a key future recording technology. In the work of H. J. Richter et al. [J. Appl. Phys. 111, 033909 (2012)] it is stated that storage densities will be limited to 15-20 Tbit/in2 due to thermally induced write errors. In this letter we propose a composite structure consisting of two materials with different Curie temperatures. In this composite material the thermal write error is negligible up to areal densities of about 50-100 Tbit/in2. [ABSTRACT FROM AUTHOR]

A correction to the article "Multilayer exchange spring media for magnetic recording" published in the Number 89, 2006 issue is presented.

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]

Non-Markovian Quantum State Diffusion (NMQSD) has turned out to be an efficient method to calculate excitonic properties of aggregates composed of organic chromophores, taking into account the coupling of electronic transitions to vibrational modes of the chromophores. NMQSD is an open quantum system approach that incorporates environmental degrees of freedom (the vibrations in our case) in a stochastic way. We show in this paper that for linear optical spectra (absorption, circular dichroism), no stochastics is needed, even for finite temperatures. Thus, the spectra can be obtained by propagating a single trajectory. To this end, we map a finite temperature environment to the zero temperature case using the so-called thermofield method. The resulting equations can then be solved efficiently by standard integrators.