Simon Bance

Recording simulations on graded media for area densities of up to 1Tbit∕in.2

We report recording simulations on graded media with area densities of 1Tbit∕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 12nm and a track width of 53nm on graded media with total thickness of 21nm and maximum anisotropy of 1MJ∕m3. The computed values for transition jitter are around 0.65nm, depending on the intergrain exchange.

Microwave-assisted three-dimensional multilayer magnetic recording

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.

Grain-size dependent demagnetizing factors in permanent magnets

The coercive field of permanent magnets decreases with increasing grain size. The grain size dependence of coercivity is explained by a size dependent demagnetizing factor. In Dy free Nd2Fe14B magnets, the size dependent demagnetizing factor ranges from 0.2 for a grain size of 55 nm to 1.22 for a grain size of 8300 nm. The comparison of experimental data with micromagnetic simulations suggests that the grain size dependence of the coercive field in hard magnets is due to the non-uniform magnetostatic field in polyhedral grains.

Micromagnetic calculation of spin wave propagation for magnetologic devices

The propagation of magnetic wave packets in magnetic nanowires was calculated as a function of wire width, field strength, field ramp time, field area size, and geometry of a magnetic nanowire. Spin waves are excited locally by applying a small perturbation in the magnetization in a 20nm wide region. A wave packet is emitted from the input region and travels along the wire with a velocity of 740m∕s. The finite element micromagnetic simulations show that wave packets can be guided along a bent nanostructure without losses due to geometry; amplitude and frequency are exactly the same as in a straight wire with equal distance between excitation point and probe. The wave amplitude was found to decrease with increasing rise time of the excitation field with an upper limit of 100ps. For a Permalloy wire with a thickness of 10nm, the frequency peak changes from 10GHz in a wire with 60nm width to 6GHz in a wire with 140nm width.

Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets

The coercive field and angular dependence of the coercive field of single-grain Nd2Fe14B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where an Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour.

Microwave-Assisted Magnetization Reversal in Exchange Spring Media

Presented here are micromagnetic simulations of the behavior of single phase media and exchange spring media for data storage devices under the influence of a microwave field. A reduction of the switching field by about a factor of two can be found in both single phase and exchange spring media when the microwave field reaches an amplitude of 12% of the remanent coercivity without microwave assist. It is shown here that the switching time for exchange spring media is less than that for single phase media due to the influence of soft upper layer which helps in reversing the magnetization along the opposite direction. The optimum microwave frequency that leads to the most effective reduction of the switching field depends on the angle of the applied field. At higher field angle the frequency band for successful microwave assist becomes smaller. In exchange spring media the frequency that leads to a maximum switching field reduction is smaller than in single phase media.

LaBonte's method revisited: An effective steepest descent method for micromagnetic energy minimization

We present a steepest descent energy minimization scheme for micromagnetics. The method searches on a curve that lies on the sphere which keeps the magnitude of the magnetization vector constant. The step size is selected according to a modified Barzilai-Borwein method. Standard linear tetrahedral finite elements are used for space discretization. For the computation of quasistatic hysteresis loops, the steepest descent minimizer is faster than a Landau-Lifshitz micromagnetic solver by more than a factor of two. The speed up on a graphic processor is 4.8 as compared to the fastest single-core central processing unit (CPU) implementation.

Thermally activated coercivity in core-shell permanent magnets

Finite element micromagnetic simulations are used to compute the temperature-dependent hysteresis properties of Nd2Fe14B permanent magnets in order to assess the influence of a hard (Dy,Nd)2Fe14B shell. The simulations show that the 4 nm thick shell cancels out the reduction in coercivity from an outer defect layer, which is known to exist at the grain boundaries in NdFeB permanent magnets. Activation volumes are computed and shown to depend on the structures configuration as well as the temperature.

Transverse and vortex domain wall structure in magnetic nanowires with uniaxial in-plane anisotropy

Micromagnetic and analytical models are used to investigate how in-plane uniaxial anisotropy affects transverse and vortex domain walls in nanowires where shape anisotropy dominates. The effect of the uniaxial anisotropy can be interpreted as a modification of the effective wire dimensions. When the anisotropy axis is aligned with the wire axis (θ(a) = 0), the wall width is narrower than when no anisotropy is present. Conversely, the wall width increases when the anisotropy axis is perpendicular to the wire axis (θ(a) = π/2). The anisotropy also affects the nanowire dimensions at which transverse walls become unstable. This phase boundary shifts to larger widths or thicknesses when θ(a) = 0, but smaller widths or thicknesses when θ(a) = π/2.

High energy product in Battenberg structured magnets

Multiphase nano-structured permanent magnets show a high thermal stability of remanence and a high energy product while the amount of rare-earth elements is reduced. Non-zero temperature micromagnetic simulations show that a temperature coefficient of remanence of −0.073%/K and that an energy product greater than 400 kJ/m3 can be achieved at a temperature of 450 K in a magnet containing around 40 volume percent Fe65Co35 embedded in a hard magnetic matrix.