Werner Scholz

Vortex-state oscillations in soft magnetic cylindrical dots

We have studied magnetic vortex oscillations in soft submicron cylindrical dots with variable thickness and diameter by an analytical approach and micromagnetic simulations. We have considered two kinds of modes of the vortex magnetization oscillations: (1) low-frequency translation mode, corresponding to the movement of the vortex as a whole near its equilibrium position and (2) high-frequency vortex modes, which correspond to radially symmetric oscillations of the vortex magnetization, mainly outside the vortex core. The vortex translational eigenmode was calculated numerically in frequency and time domains for different dot aspect ratios. To describe the discrete set of vortex high-frequency modes we applied the linearized equation of motion of dynamic magnetization over the vortex ground state. We considered only radially symmetric magnetization oscillations modes. The eigenfrequencies of both kinds of the excitation modes are determined by magnetostatic interactions. They are proportional to the thickness/diameter ratio and lie in the GHz range for typical dot sizes.

Plasmonic near-field transducer for heat-assisted magnetic recording

Abstract Plasmonic devices, made of apertures or antennas, have played significant roles in advancing the fields of optics and opto-electronics by offering subwavelength manipulation of light in the visible and near infrared frequencies. The development of heat-assisted magnetic recording (HAMR) opens up a new application of plasmonic nanostructures, where they act as near field transducers (NFTs) to locally and temporally heat a sub-diffraction-limited region in the recording medium above its Curie temperature to reduce the magnetic coercivity. This allows use of very small grain volume in the medium while still maintaining data thermal stability, and increasing storage density in the next generation hard disk drives (HDDs). In this paper, we review different plasmonic NFT designs that are promising to be applied in HAMR. We focus on the mechanisms contributing to the coupling and confinement of optical energy. We also illustrate the self-heating issue in NFT materials associated with the generation of a confined optical spot, which could result in degradation of performance and failure of components. The possibility of using alternative plasmonic materials will be discussed.

Micromagnetic modeling of ferromagnetic resonance assisted switching

We studied the steady state behavior and magnetization switching process of single domain particles subject to ac and dc magnetic fields using analytical and numerical models based on the Landau–Lifshitz–Gilbert equation. We compared the analytical solutions for circularly polarized fields with a numerical single spin model and circularly and linearly polarized ac magnetic fields. It has been found, that the initial conditions and the dynamics of the external fields (field ramps and amplitude changes) strongly determine which precession orbit the magnetization converges to, if the magnetization precession is stable, and if the magnetization switches. We also studied the effects of field amplitudes, field angles, and damping on the switching behavior. The presented results can be applied to high power ferromagnetic resonance experiments and ferromagnetic resonance assisted magnetic recording schemes.We studied the steady state behavior and magnetization switching process of single domain particles subject to ac and dc magnetic fields using analytical and numerical models based on the Landau–Lifshitz–Gilbert equation. We compared the analytical solutions for circularly polarized fields with a numerical single spin model and circularly and linearly polarized ac magnetic fields. It has been found, that the initial conditions and the dynamics of the external fields (field ramps and amplitude changes) strongly determine which precession orbit the magnetization converges to, if the magnetization precession is stable, and if the magnetization switches. We also studied the effects of field amplitudes, field angles, and damping on the switching behavior. The presented results can be applied to high power ferromagnetic resonance experiments and ferromagnetic resonance assisted magnetic recording schemes.

Cluster size and exchange dispersion in perpendicular magnetic media

A computational study of the effects of the intergranular exchange field and its dispersion on the cluster size in perpendicular recording media is presented. The dispersion arises from the grain size dispersion and the dispersion of intrinsic exchange coupling between individual grains. It is found that increasing the degree of dispersion reduces the cluster size due to weakly coupled grains acting as pinning sites against the expansion of the clusters. A simple semi-analytical model is proposed which gives good agreement with the numerical calculations in the limit of the large anisotropy constant.

Role of Media Parameters in Switching Granular Perpendicular Media Using Microwave Assisted Magnetic Recording

This paper describes the role of media parameters in magnetization switching in the presence of a DC bias and an in-plane RF field using a micromagnetic model. In this study, two sets of media parameters are compared for reduction in switching field based on the Larmor precessional frequency of the medium. It is found that a significant reduction in equivalent Stoner Wohlfarth (SW) field (HSW E) is observed only when a DC field is applied at a finite angle relative to the medium anisotropy direction (thetasA) and we observe a shallow minimum between 20deg and 30deg. A small field magnitude HSW E ( = 0.50 HK) for thetasA of 20deg for medium 1 using an RF field magnitude of 0.068 HK and an even smaller magnitude HSW E (=0.22 HK) for thetasA of 25degfor medium 2 using an RF field magnitude of 0.035 HK is required to switch the media for the case of a uniform granular medium without any distributions at T = 0 K. With the inclusion of magnetic materials distributions and/or introducing finite temperature, the coherent precession is lost due to the demagnetization field of neighboring grains. We compare HSW E required to switch the medium in the presence of RF assist to the SW field (HSW) required to switch the medium when there is no assist field. It is found that medium 1 shows a reduction of 28% in SW field while medium 2 shows no reduction in presence of in-plane RF field compared to the case when no RF field is applied. This highlights the importance of medium properties in microwave-assisted magnetic recording. Selection of media parameters will depend on the potential application of microwave-assisted recording in a continuous granular or the bit patterned media.

Media Roughness and Head-Media Spacing in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording involves the transfer of energy to the recording medium via optical means. To enable high areal density, the recorded track must be smaller than the diffraction limit of focused light, which is accomplished by using a near-field transducer (NFT) with a corner or peg with small dimension. Energy transfer using such a transducer is a near-field effect, and therefore is highly sensitive to the spacing between the NFT and the medium. Since the recording medium has some surface roughness, there will be a variation in the NFT-to-medium spacing and this will impact the amount of energy transferred from the NFT. We model the effect of Gaussian surface roughness on NFT energy transfer and predict surface temperature variations for a rough surface. In addition, we illustrate how changing the head-medium spacing changes the impact that roughness has on surface temperature variation. We combine these modeled predictions with spinstand measurements of recorded data and conclude that the effect of media roughness results in only limited temperature excursions above the nominal recording medium temperature.

Fast Magnetization Switching With Circularly Polarized Fields and Short Pulses

In this paper, we study the magnetization reversal process of single-domain Stoner-Wohlfarth particles subject to circularly polarized fields and short pulses using analytical and numerical models. We investigate the effect of short unipolar, bipolar field, and circularly polarized field pulses, which are applied perpendicular to the uniaxial magnetocrystalline anisotropy axis, on the magnetization switching dynamics.

Far-field head–media optical interaction in heat-assisted magnetic recording

We have used a plane wave expansion method to theoretically study the far-field head-media optical interaction in heat-assisted magnetic recording. For the Advanced Storage Technology Consortium media stack specifically, we notice the outstanding sensitivity related to the interlayers optical thickness for media reflection and the magnetic layers light absorption. With 10 nm interlayer thickness change, the recording layer absorption can be changed by more than 25%. The 2D results are found to correlate well with the full 3D model and magnetic recording tests on a flyable disc with different interlayer thickness.

Heat assisted magnetic recording performance and integration challenges

Recent recording areal density and integrated drive performance demonstrations using Heat Assisted Magnetic Recording (HAMR) suggest that it is a viable technology to succeed conventional magnetic recording. However challenges still remain for the near field transducer, in particular reliability and sufficient thermal confinement. We explore a new NFT design, Near field Transducer Gap (NTG), which offers the potential to mitigate some of the issues in track confinement and thermal profile compared to earlier published studies [4]. The design offers efficiency improvements, and the potential to reduce unwanted background light and heating that can lead to erasure in the writing track, and neighbors.