Exchange Coupled Composite Research Articles

Microwave-Assisted Magnetic Recording on Exchange Coupled Composite Media

Microwave-assisted magnetic recording (MAMR) on single layer and exchange coupled composite (ECC) media was simulated. The high-frequency field was modeled with a macrospin model and also micromagnetically using a spin-torque oscillator integrated into the write head. ECC media with a range of hard/soft layer thicknesses were optimized for MAMR, and the gain over a non-MAMR system was established. The results show a bigger gain from using MAMR when the hard/soft layer thickness ratio was small. MAMR systems were also less susceptible to changes in magnetostatic interactions between medium grains.

Ledge-type Co/L1-FePt exchange-coupled composites

FePt-based exchange-coupled composites consisting of a magnetically hard L10-FePt phase exchange-coupled with a soft ferromagnetic material are promising candidates for future ultra-high density (>1 Tbit/in2) perpendicular magnetic recording media, also being of interest for other applications including spin torque oscillators and micro-electro-mechanical systems, among others. In this paper, the effect of the thickness of a soft Co layer (3 < thCo < 20 nm) on the magnetic behavior of ledge-type fcc(100)-Co/L10(001)-FePt composites deposited on an MgO (100) substrate is systematically studied by combining morpho-structural analyses and angular magnetization measurements. Starting from a film consisting of isolated L10(001)–FePt islands, the ledge-type structure was obtained by depositing a Co layer that either covered the FePt islands or filled-up the inter-island region, gradually forming a continuous layer with increasing Co thickness. A perpendicular anisotropy was maintained up to thCo ∼ 9.5 nm and a significant reduction in the coercivity (about 50% for thCo ∼ 3 nm) with the increase in thCo was observed, indicating that, by coupling hard FePt and soft Co phases in a ledge-type configuration, the writability can be greatly improved. Recoil loops' measurements confirmed the exchange-coupled behavior, reinforcing a potential interest in these systems for future magnetic recording media.

Superior bit error rate and jitter due to improved switching field distribution in exchange spring magnetic recording media.

We report two effects that lead to a significant reduction of the switching field distribution in exchange spring media. The first effect relies on a subtle mechanism of the interplay between exchange coupling between soft and hard layers and anisotropy that allows significant reduction of the switching field distribution in exchange spring media. This effect reduces the switching field distribution by about 30% compared to single-phase media. A second effect is that due to the improved thermal stability of exchange spring media over single-phase media, the jitter due to thermal fluctuation is significantly smaller for exchange spring media than for single-phase media. The influence of this overall improved switching field distribution on the transition jitter in granular recording and the bit error rate in bit-patterned magnetic recording is discussed. The transition jitter in granular recording for a distribution of Khard values of 3% in the hard layer, taking into account thermal fluctuations during recording, is estimated to be a = 0.78 nm, which is similar to the best reported calculated jitter in optimized heat-assisted recording media.

Microwave-assisted shingled magnetic recording simulations on an exchange-coupled composite medium

The potential of microwave-assisted magnetic recording combined with the shingled recording scheme has been studied by simulating read/write processes on exchange-coupled composite media focusing on recording characteristics in the cross-track direction. Microwave fields enhance writability, especially at the track edge, resulting in lower noise and higher signal-to-noise ratio (SNR), which enables higher track density in the shingled recording scheme. Read/write simulations of microwave-assisted shingled recording achieve 1.4Mtracks/in. while retaining high SNR. Further increases in SNR and track density will require either a narrower reader or track edge noise reduction.

Magnetization reversal of FePt based exchange coupled composite media

Microstructure and magnetic properties of L10-ordered FePt-C nanogranular films capped with A1-disordered FePt layers with various thicknesses were investigated. After the growth of 9.5 nm thick L10-FePt-C granular layer on a MgO(001) single crystalline substrate at a substrate temperature of Ts = 600 °C, FePt capping layers with thicknesses, tFePt, ranging from 2.7 to 8.6 nm were deposited at Ts = 400 °C. The coercivity of the L10-FePt-C/A1-FePt exchange coupled composite (ECC) media decreased from 4.9 to 1.4 T with increasing tFePt. Evaluation of energy barrier, Eb, using the Sharrock equation indicated no degradation of thermal barrier in spite of the large reduction in the coercivity. The switching field distribution of the ECC media is significantly reduced from 23% for the single layer FePt-C granular film to 8% for the ECC media with tFePt = 6.5 nm. Micromagnetic simulations showed the magnetization reversal of ECC media with tFePt = 2 nm capping layer occurs by the coherent rotation while domain wall pinning becomes dominant for tFePt ≥ 5 nm. The switching field distribution (SFD) of the ECC media is substantially decreased for tFePt = 2 nm. Low coercivity with high Eb and improved SFD observed from the ECC media makes them promising as high areal density recording media for both perpendicular magnetic recording (PMR) and heat assisted magnetic recording (HAMR).

Microwave-Assisted Magnetization Reversal in Exchange-Coupled Composite Media Using Linearly Polarized Microwave Fields

We have experimentally demonstrated a microwave-assisted magnetization reversal (MAMR) in exchange-coupled composite (ECC) media using the linearly polarized microwave fields generated by an electrically shorted coplanar waveguide. The MAMR frequency exhibiting a maximum reduction of coercive field can be decreased using 16 nm-thick ECC media without the suppression of the thermal stability of magnetization. The threshold amplitude of microwave field required for the successful MAMR is only 3% of the anisotropy field of the medium. A splitting of ferromagnetic resonance spectrum, which suggests an appearance of stochastic MAMR, can be observed after applying microwave fields. The stochastic MAMR gives rise to the threshold behavior in the condition of microwave field amplitude needed for MAMR.

Deterioration of the coercivity due to the diffusion induced interface layer in hard/soft multilayers.

Hard/soft permanent magnets have aroused many interests in the past two decades because of their potential in achieving giant energy products as well as their rich variety of magnetic behaviors. Nevertheless, the experimental energy products are much smaller than the theoretical ones due to the much smaller coercivity measured in the experiments. In this paper, the deterioration of the coercivity due to the interface atomic diffusion is demonstrated based on a three dimensional (3D) micromagnetic software (OOMMF) and a formula derived for the pinning field in a hard/soft multilayer, which can be applied to both permanent magnets and exchange-coupled-composite (ECC) media. It is found that the formation of the interface layer can decrease the coercivity by roughly 50%, which is responsible for the observed smaller coercivity in both composite and single-phased permanent magnets. A method to enhance the coercivity in these systems is proposed based on the discussions, consistent with recent experiments where excellent magnetic properties are achieved.

Modeling Perpendicular Magnetic Multilayered Oxide Media With Discretized Magnetic Layers

Micromagnetic models derived from the Landau-Lifshitz-Gilbert equation and its derivatives are important tools that can be used to understand error rate performance in magnetic recording systems. With increasing linear density in perpendicular magnetic recording (PMR), exchange coupled composite (ECC) media is required and optimizing the cap layer is essential to achieve good recording performance [1]. It is important to model the cap layer and thin oxide layers accurately to obtain a reasonable evaluation of recording performance, especially when the bit length is approaching the grain pitch. The often-used macro-spin model may not be sufficient to describe the semi-continuous nature of the cap layer. The macro-spin model may lead to discrepancies with the experimental data in many key areas such as resolution, non-linear effects [2], and particularly SNR at high linear densities. Here, we evaluate the performance of a fully discretized cap model for ECC media. We compare simulation results of two discretized cap models to the traditional macro-spin model as well as to experimental data.

Thermally activated switching at long time scales in exchange-coupled magnetic grains

Rate coefficients of the Arrhenius-N\'eel form are calculated for thermally activated magnetic moment reversal for dual layer exchange-coupled composite (ECC) media based on the Langer formalism and are applied to study the sweep rate dependence of $MH$ hysteresis loops as a function of the exchange coupling $I$ between the layers. The individual grains are modeled as two exchange-coupled Stoner-Wohlfarth particles from which the minimum energy paths connecting the minimum energy states are calculated using a variant of the string method and the energy barriers and attempt frequencies calculated as a function of the applied field. The resultant rate equations describing the evolution of an ensemble of noninteracting ECC grains are then integrated numerically in an applied field with constant sweep rate $R=\ensuremath{-}dH/dt$ and the magnetization calculated as a function of the applied field $H$. $MH$ hysteresis loops are presented for a range of values $I$ for sweep rates ${10}^{5}\phantom{\rule{0.28em}{0ex}}\mathrm{Oe}/\mathrm{s}\ensuremath{\le}R\ensuremath{\le}{10}^{10}\phantom{\rule{0.28em}{0ex}}\mathrm{Oe}/\mathrm{s}$ and a figure of merit that quantifies the advantages of ECC media is proposed. $MH$ hysteresis loops are also calculated based on the stochastic Landau-Lifshitz-Gilbert equations for ${10}^{8}\phantom{\rule{0.28em}{0ex}}\mathrm{Oe}/\mathrm{s}\ensuremath{\le}R\ensuremath{\le}{10}^{10}\phantom{\rule{0.28em}{0ex}}\mathrm{Oe}/\mathrm{s}$ and are shown to be in good agreement with those obtained from the direct integration of rate equations. The results are also used to examine the accuracy of certain approximate models that reduce the complexity associated with the Langer-based formalism and which provide some useful insight into the reversal process and its dependence on the coupling strength and sweep rate. Of particular interest is the clustering of minimum energy states that are separated by relatively low-energy barriers into ``metastates.'' It is shown that while approximating the reversal process in terms of ``metastates'' results in little loss of accuracy, it can reduce the run time of a kinetic Monte Carlo (KMC) simulation of the magnetic decay of an ensemble of dual layer ECC media by $2--3$ orders of magnitude. The essentially exact results presented in this work for two coupled grains are analogous to the Stoner-Wohlfarth model of a single grain and serve as an important precursor to KMC-based simulation studies on systems of interacting dual layer ECC media.

Role of Intergrain and Intragrain Coupling on Switching Field of Granular Exchange Coupled Composite Media Using Microwave Assisted Magnetic Recording

Microwave assisted magnetic recording is one interesting magnetic recording technology to improve writability for high areal density of hard disk drive. In this work, the role of intergrain and intragrain coupling and microwave field is investigated by micromagnetic simulation based on the Landau - Lifshitz - Gilbert equation. The results show that the switching field, Hsw, of media with microwave field, Hac, of 10 GHz is lower than that for media without Hac. The exchange coupling between nearest neighbor grains hardly affects the Hsw of media and slightly affects the slope of hysteresis loop. In addition, the slope of hysteresis loops and Hsw decrease with increasing exchange coupling in each grain. The results from this work can lead to obtain the improvement writability and increasing areal density of hard disk drive.

Angular Dependence of Coercivity and Bit Error Rate Estimation for Shingled Magnetic Recording With ECC Media

Shingled magnetic recording (SMR) with exchange-coupled composite media provides a feasible way by enhancing the writing capability and reducing the media switching field for achieving ultrahigh areal density of magnetic storage. However, the media switching field is dependent on the angle between the easy axis and the applied field. The adjacent track encroachment induced by this angular disturbance may happen, which could increase recording noise and lead to measurable erasure. This problem is highlighted in SMR system because of the enlarged stray field. A trapezoidal shingled writing head is modeled to explore the spatial field distribution and how the writing performance is affected by the angular dependence of media coercivity. The results show that a larger easy axis deviation and a smaller anisotropy energy density ratio of hard region will be helpful for reducing switching field to improve writing capability. The analysis also suggests that, if properly arranged, the adjacent bits can suffer minimized encroachment caused by variation of angularly induced switching field and attain a satisfactory bit error rate of as low as <;5×10 -4 .

The Dynamics of Microwave-Assisted Magnetic Recording

Models of microwave-assisted magnetic recording are used to investigate the potential of the technology. With both single- and double-layer [exchange coupled composite (ECC)] media, the higher effective head field gradient led to reduced transition jitter and higher SNR in written tracks. To reduce the high-frequency field to a realistic frequency range, the use of ECC media is required. However, the ECC medium design must consider interactions between the spin torque oscillator and the medium.

High-Areal Density Recording Simulation of Three-Layered ECC Bit-Patterned Media With a Shielded Planar Head

Magnetic parameters of the previously proposed three-layer exchange coupled composite (ECC) dot were modified to compensate the additional perpendicular anisotropy from the shape and the stacking of the layers. The modification in the magnetic parameters resulted in a minimum switching field and applied field angle dependence of the field. Recording simulation using a multistepped shielded planar head field on the bit-patterned media with modified three-layer ECC dots suggested possibility of a high-areal density recording above 4 T dot/in\(^{2}\) with no energy assist scheme. It was also concluded that magnetic parameter compensation for additional shape and stacking anisotropy is indispensable for ECC dots.

Investigation on the Effect of an In-Plane Auxiliary Field on the SNR of HAMR Recording Systems

In this paper, we consider the application of an in-plane auxiliary field into the exchange-coupled composite medium in the heat-assisted magnetic recording (HAMR) system. The dynamic micromagnetic simulations are conducted based on the Landau-Lifshitz-Bloch equations and the signal-to-noise ratio (SNR) characterizations are performed. The effect on the SNR of the HAMR system is investigated. The improvement on the SNR with the auxiliary field in negative downtrack direction is identified.

Magnetization Reversal in Exchange Spring Bilayer System Under Circularly Polarized Microwave Field

Microwave assisted magnetization reversal are studied in the bulk bilayer exchange coupled system. We investigate the nonlinear magnetization reversal dynamics in a perpendicular exchange spring media using Landau-Lifshitz equation. In the limit of the infinite thickness of the system, the propagation field leads the reversal of the system. The reduction of the switching field and the magnetization profile in the extended system are studied numerically. The possibility to study the dynamics analytically is discussed and an approximation where two P-modes are coupled by an interaction field is presented. The ansatz used for the interaction field is validated by comparison with the numerical results. This approach is shown to be equivalent to two exchange coupled macrospins.

Magnetic properties of L10-FePt/Fe exchange-coupled composite nanodots

The magnetic properties of L10-FePt/Fe exchange-coupled composite (ECC) nanodots were investigated by Hall effect measurement due to its high sensitivity. The FePt/Fe nanodots showed coercivity changing irregularly with respect to the Fe thickness, which deviated from the simulated results, where the coercivity of the ECC dots reduced with increasing Fe soft layer thickness. It was found that the edge damage induced by ion milling affected the coercivity of the nanodots significantly and the magnetization reversal mechanism. Domain wall nucleation and propagation was revealed in the FePt single domain dots. In the FePt/Fe ECC nanodots, the magnetization gradually reversed from the out-of-plane direction towards the in-plane direction before switching to the reverse out-of-plane direction. The critical size of single magnetic domains in the FePt/Fe ECC nanodots increased with increasing Fe soft layer thickness.

Microwave Assisted Magnetization Reversal on Exchange Coupled Composite Media

To overcome superparamagnetic limit, microwave assisted magnetic recording (MAMR) is one interesting magnetic recording technology. Therefore, the effect of microwave on magnetization reversal in media should be analyzed. In this work, we propose the MAMR to decrease switching field (coercivity, Hsw) in exchange coupled composite (ECC) media by using the micromagnetic simulation based on the Landau - Lifshitz - Gilbert equation. The Hsw of single layer and ECC media without microwave field is 110.90 and 7.7 kOe, respectively. When the oscillating microwave field is added, Hsw of single layer media with microwave frequency of 2.5 - 40 GHz is lower than 110.90 kOe. Likewise, Hsw of ECC media with microwave frequency of 5 - 16 GHz is lower than 7.7 kOe and has the lowest value of 4.9 kOe at frequency of 10 GHz. The results from this work lead to solve superparamagnetic limit and increase areal density in hard disk drive.

The Effects of Magnetic Properties of &lt;i&gt;L&lt;/i&gt;1&lt;sub&gt;0&lt;/sub&gt;-FePt/Fe Based Exchange Coupled Composite Media on Switching Field

The L10-FePt/Fe based exchange coupled composite (ECC) bilayer media is one candidate to extend the areal density of magnetic recording system and solve writability issue in trilemma. L10-FePt is the great high magnetic anisotropy material. Therefore, the magnetic parameters of this material such as magnetocrystalline anisotropy constant, Ku, saturation magnetization, Ms, and exchange coupling between a soft/hard interface, are important on magnetic material properties. In this work, the effects of magnetic parameters on magnetic properties of L10-FePt/Fe based ECC bilayer media are simulated by the object oriented micromagnetic framework based on Landau-Lifshitz-Gilbert equation. The ECC bilayer media can reduce switching field, Hsw, of media lower than available writing head field. Hence, writability issues of high Ku media can be achieved. Reducing Hsw of ECC bilayer media obtains from lower Ku and higher Ms values. This work can achieve writing capability of a future magnetic recording system.

Magnetic properties of perpendicular exchange coupled composite with a synthesis antiferromagnetic sandwich structure

In this study, an exchange coupled composite (ECC) film with an antiferromagnetic sandwich structure L10-FePt/[Co/Ni]5/Ru/[Co/Ni]10 (FePt-CN-Ru) is prepared via dc magnetron sputtering and measured by vibrating sample magnetometer. The results show that FePt-CN-Ru film exhibits ECC characteristics, with excellent angular tolerance, low remanance magnetization, and a narrow switching field. Its thermal stability is higher than that of conventional ECC film. Finally, magnetization reversal is demonstrated through micromagnetic simulation.

A General Energy Barrier Model for Switching of Exchange Spring Media in an External Field

In this paper, a general energy expression is developed for a cluster of multilayer exchange spring media grains. We expand the rotation of the magnetization vectors to second order in elements of an exponential unitary transformation. The first and second derivatives of the energy are determined and a damped optimization procedure is used to efficiently locate minima and transition states on the potential energy surface. In this method, the interaction of neighboring grains is taken into account and thus the effect of inter-granular exchange on the energy barrier is explicitly taken into consideration. Thus, cluster formation is seen as coherent switching of a group of grains and can be studied as a function of anisotropy and exchange distributions in the media. The lowest energy barrier for a grain or cluster is then used to construct a rate constant. We then determine rate constants for an ensemble of interacting grains from which M-H loops and magnetization decay are simulated as a function of an applied field. Both kinetic Monte Carlo and direct integration of the rate equations are used to study the kinetics of these systems. We find that thermal decay rates have better correlation with energy barriers than with nucleation fields.