Kwaku Eason

Channel Models and Detectors for Two-Dimensional Magnetic Recording

Two-dimensional magnetic recording (TDMR) is a novel recording architecture intended to support densities beyond those of conventional recording systems. The gains from TDMR come primarily from more powerful coding and signal processing algorithms that allow the bits to be packed more tightly on the disk, and yet be retrieved with acceptable error rates. In this paper, we present some preliminary results for an advanced channel model based on micromagnetic simulations, coined the Grain Flipping Probability model. This model requires a one-time computationally complex characterization phase, but subsequently provides fast and accurate two-dimensional (2-D) readback waveforms that include effects captured from micromagnetic simulations and the statistical effects derived from the granularity of the recording medium. We also show the performance of several detectors over a pre-existing TDMR channel model directly as a function of channel density rather than the signal-to-noise ratio (SNR).

Modeling of 2-D Magnetic Recording and a Comparison of Data Detection Schemes

Two-dimensional magnetic recording (TDMR) together with shingled magnetic recording (SMR) are technologies proposed to extend the life of conventional granular magnetic recording. The grain flipping probability (GFP) model has been proposed to mimic the performance of micromagnetic ( μ-mag) simulations for the purpose of signal reproduction. Other work in TDMR includes the proposal of a Gaussian mixture model (GMM) that produces improved likelihood information at the output of the detector, combined with low density parity check (LDPC) codes. The contribution of this paper is threefold. First, we aim to simulate a TDMR/SMR recording system with the GFP model, both with and without the GMM detector, and with various random and structured LDPC codes, of both 4 k and 16 k block lengths, to determine areal densities that might be achieved. Second, we perform a comparison of the various model order reduced (MOR) GFP implementations to compare the effect of writing with various factors taken out of the picture. Third, we perform a validation of the GFP model and the setup as a whole, by running the system with a parameter set close to that of conventional recording. The results of these experiments give an assurance of the validity of our model, give an indication of the expected density that might be achieved in a TDMR/SMR system, and give a direction for which parameter(s) in magnetic recording systems might be optimized to yield the most gain.

Seed influence on the ferromagnetic resonance response of Co/Ni multilayers

The effect of Pd and Ru seed layers on the magnetic properties of [Co/Ni]N multilayers with varying number of bilayer repeats N is investigated using vector network analyzer ferromagnetic resonance. The effective anisotropy field HKeff is found to increase with N for Ru seed, but decreases for Pd until N = 15. As N is increased beyond 15, HKeff decreases for both seeds. In contrast, the damping parameter α decreases with N regardless of the seed, showing a 1/N dependence. Taking spin pumping into account, the intrinsic damping α0 for both Pd and Ru seeds reduce to α0 ≈ 0.01. These results demonstrate that there can be a strong influence of the seed/Co interface on anisotropy, especially for sufficiently low N, but not necessarily on α0.

Topological Hall conductivity of vortex and skyrmion spin textures

We analyze the topological Hall conductivity experienced by conduction electrons whose spins are strongly coupled to axially symmetric spin textures, such as magnetic vortex and skyrmion of types I and II, theoretically by gauge theory, and numerically via micromagnetic simulations. The numerical results are in agreement with the theoretical predictions. Divergence between the two is seen when the vortex/skyrmion core radius is comparable or larger than the element size, and when the skyrmion configuration breaks down at high Dzyaloshinskii-Moriya interaction strength.

Optimal Design of MAMR and HAMR by Applying Response Surface Methodology

In this paper, we systematically perform optimal design of microwave-assisted magnetic recording (MAMR) and heat-assisted magnetic recording (HAMR) by applying response surface methodology (RSM) to optimize the signal-to-noise ratio (SNR) of the recording process. We evaluate the performance of design of MAMR and HAMR through micromagnetic modeling. Design of experiments (DoE) is conducted by using composite central design (CCD), and the influence of the design parameters on the SNR is investigated. The optimal designs of MAMR and HAMR systems are obtained by solving for the best-fit quadratic response-surface to the sampled points and solving for the parameters that maximize this cost function.

Control of offset field and pinning stability in perpendicular magnetic tunnelling junctions with synthetic antiferromagnetic coupling multilayer

In a magnetic tunnelling junction (MTJ) with perpendicular magnetic anisotropy (PMA), offset field (Ho) of the free layer is usually controlled by using a synthetic antiferromagnetic (SAF) coupling structure, which is composed of an antiferromagnetic coupling (AFC) layer sandwiched by two ferromagnetic (FM) layers. However, Ho increases significantly as the size of MTJ devices shrinks to accommodate high density. In addition, magnetostatic field in PMA SAF structure tends to destabilize the antiferromagnetic (AFM) alignment of the SAF layers, in contrast to the in-plane anisotropy SAF, where the closed flux forms stable AFM magnetic configuration. Here, we present a double SAF structure to control Ho, while maintaining high magnetic stability of the reference layer (RL). The double SAF consists of FM1/AFC/FM2/AFC/FM3 multilayer. An AFM layer like PtMn is added to further stabilize the magnetic configuration of the double SAF. As the magnetization of other FM layers (FM1 and FM2) is aligned oppositely, the...

Channel Characterization and Performance Evaluation of Bit-Patterned Media

Bit-patterned media (BPM) is a promising approach to push back the onset of the superparamagnetic limit faced by conventional continuous granular media. Today, BPM islands can be fabricated at densities higher than can be characterized by existing methods and full bit-patterned media recording (BPMR) is still a long way off. In this work, we rely on simulations to predict how such islands would perform in a real recording scenario. The grain flipping probability (GFP) model is trained via micromagnetic simulations and reproduces the magnetic profiles used to generate readback signals for channel simulations. The geometrical parameters to the micromagnetic simulations, such as the island size variations and island position jitter are characterized from measurements of islands fabricated via e-beam at various channel densities.

MRAM Device Incorporating Single-Layer Switching via Rashba-Induced Spin Torque

We designed and modeled a nonvolatile memory device that utilizes the Rashba spin-orbit coupling (SOC) to write data onto a free ferromagnetic (FM) layer and uses the tunneling magnetoresistive (TMR) effect for data read-back. The magnetic RAM (MRAM) device consists of a free (switchable) FM multilayer stack, in which a large internal electric field is induced at the interfaces between the oxide and the FM layer. In the FM layer, data writing by magnetization switching occurs via the Rashba-induced spin torque, while the data reading process in the system could be fulfilled via the current-perpendicular-to-plane TMR response. A general equation of motion for the local moments has been obtained by formally deriving the SU(2) spin-orbit gauge field arising due to SOC and the critical current density is estimated to be 1.2 ×108 A/cm2. Micromagnetic simulations were performed to demonstrate the Rashba-induced switching mechanism. By choosing or fabricating alloys with a lower magnetocrystalline anisotropy and enhancing the Rashba coupling strength via surface or interfacial engineering, the critical current may be further reduced to well below 107 A/cm2, a level that may enable the practical realization of a single-layer Rashba-induced magnetization switching memory.

Recording Performance and Comparison of Graded-

Heat-assisted magnetic recording (HAMR) signal-to-noise ratio results are presented based on Landau-Lifshitz-Bloch recording simulations using a double-layer composite Tc media. An exchanged-coupled-composite Ku media is also considered in a HAMR system. Mean Tc and σTc are varied in conditions of high and low interlayer exchange coupling, using material parameters consistent with FePt. Results suggest that using a composite Tc media may potentially regain ~1-2 dB from a single layer HAMR system with dB loss of ~3 dB, varying cure temperature distributions up to 6%. However, some of the benefits are due to creating conditions of writing above Tc. A significant performance degradation is found to occur if grains are not heated strictly above the Curie temperature, in both single and double-layer media. And use of a Ku composite media beyond deploying a composite Tc media shows insignificant gain in the recording performance.

{T_{c}}

The effects of Dzyaloshinskii-Moriya interaction (DMI) on the ferromagnetic resonance response are investigated in nanometer-sized disks using 3D micromagnetics with the inclusion of DMI energy. A rich complexity is found in the effects on the spinwave eigenmodes and their behavior when varying parameters. Two distinct results are demonstrated: first, unique DMI modes are found to form, instead of the expected modes forming in the absence of DMI and they can be uniquely accessed using field rotation; and second, modal evolution with the DMI parameter involves distinct modal twisting and rotations.