Microwave-assisted magnetic recording simulation on exchange-coupled composite medium

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.

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.

Exchange-coupled Composite (ECC) media have attracted a great deal of attention as a potential solution to the tri-lemma between the signal-to-noise (SNR), thermal fluctuations, and the writability of recording media. Several methods have been proposed to systematically design ECC media, taking into account the above-mentioned three issues. A method of evaluating the thermal stability of thin-film media, however, has not yet been established. Monte Carlo methods have been widely used to evaluate to long-term thermal decay, but their application is doubtful to strong exchange-coupled systems such as ECC media. The Langevin method is also limited to very short-term thermal decay. In this paper, we propose a simple method of evaluating thermal decay that combines an analytical method with Langevin calculations.

Hard disk drives (HDDs) have been investigated to determine how to increase their areal recording density. Recently, however, HDDs have been having problems with noise, thermal fluctuations, and writability simultaneously, commonly known as a trilemma. Microwave-assisted magnetic recording (MAMR) is a promising recording method to solve this trilemma [1-4]. In MAMR, a spin torque oscillator (STO) is placed between a conventional write head and a trailing shield. Because the AC field from the STO decreases the switching magnetic fields of the medium, the writability can be improved. As a result, the trilemma can be solved. Therefore, a high frequency AC field with strong amplitude is needed. In this study, we examined methods to increase the AC-field frequency by using a micro-magnetic simulator that takes into account magnetic interactions between the write head, the medium, and the STO.

We studied exchange coupled composite (ECC) media with an out-of-plane easy axis consisting of hard magnetic L10 chemically ordered FePtCu alloy films and magnetically softer [Co/Pt]N multilayer stacks. By tailoring the structural properties of the ternary FePtCu alloy and [Co/Pt]N multilayers, we can tune the magnetic parameters of the composite in a wide range. This allowed us to address experimentally one of the most crucial properties determining the performance of ECC media, namely the pinning field of the magnetic domain wall present at the interface between the hard and soft layers. We demonstrate that the pinning field is proportional to the difference of the magnetic anisotropy constants of the hard and soft layers, which confirms the theoretical predictions. Furthermore, we show that the pinning field can be efficiently decreased after an additional annealing step. Transmission electron microscopy investigations indicated that the origin of the observed effect is due to a heat-induced phase transformation of iron oxide present at the interface between the hard and soft layers. This study reveals that tailoring the properties of the hard/soft interface is another efficient tuning knob for optimization of the performance of ECC media.

Nanocomposite magnetic films for high-density perpendicular magnetic recording media

A potential technology for use in future hard disk drives is microwave-assisted magnetic recording (MAMR). In a MAMR drive a spin-torque oscillator (STO) is integrated into the write head and applies a high frequency (HF) magnetic field to the recording medium along with the field from the write head. The HF field reduces the switching field of the medium, allowing media with higher uniaxial anisotropy to be used. As the write head moves over the recording medium the HF field seen by the medium grains changes from elliptical to circular to linear and back again. In this work the influence of circular, elliptical and linear HF fields on recording media was examined by calculating hysteresis loops for single and multiple grains.

Ultrafine magnetically hard particles are needed for the bottom­up fabrication of anisotropic exchanged­coupled permanent magnets. In this study we have chosen Sm2 Fe17 Nx because of its high anisotropy field and large saturation magnetization. A multi­stage planetary ball milling process was used. The key is to find the right balance of energy used to mill the precursor particles; it must be high enough to break the particles and reduce their size but, not so high as to destroy the crystal structure of the nanoparticles that would lead to deterioration of the magnetic properties. First the coarse powders were subjected to a wet milling with 2.0 mm diameter media. In each subsequent stage the media size was reduced to maintain the milling balance. Using such a process produced particles in a size range from 100 - 800 nm with a coercivity as high as 10 kOe at room temperature.

The Effect of saturation magnetization and interlayer exchange coupling on the time-evolutional magnetization reversal process in exchange-coupled composite (ECC) media with high coercivity was investigated by utilizing micromagnetic simulation. In order to apply magnetic field, we used magnetic printing. As a result, the magnetization reversal time of the hard layer became shorter as the saturation magnetization of soft layer increased because the magnetostatic field generated from the soft layer increased. Thus, as the saturation magnetization of both layers increased, magnetization reversal occured for the weaker interlayer exchange field.

Microstructure and magnetic properties of grain boundary diffused Nd-Fe-B magnets with Tb-Al-Ga alloy

We investigated how a Ru underlayer contributed to the in-plane magnetic anisotropy of an FeCoB upper layer. The Ru underlayer with a higher degree of Ru (001) orientation induced higher in-plane magnetic anisotropy in the FeCoB upper layer. A Si/NiFe seed layer, which was located beneath the Ru underlayer, changed the Ru crystalline orientation from random to (001) preferential orientation and induced a remarkably high in-plane anisotropy field of 540 Oe. The in-plane anisotropy of Ru crystallites in the Ru underlayer is discussed in the paper. The Ru (002) pole_figure indicated isotropic in-plane alignment of Ru crystallites. Si/NiFe/Ru/FeCoB film was prepared using two methods, i.e., with unturned and turned underlayer deposition. The direction of anisotropic alignment of FeCo crystallites and in-plane magnetic anisotropy of the FeCoB layer was not influenced by the direction of the Ru underlayer; therefore, the Ru underlayer was not a dominant factor, and the oblique incidence effect caused by the configuration of Facing Targets Sputtering system (FTS) was a dominant factor that induced in-plane magnetic anisotropy in the FeCoB layer. The Ru underlayer may have contributed to an increment of the oblique incidence effect.

The high upper critical field and low anisotropy of the 122-type iron-based superconductor BaFe2As2 makes it promising for use in superconducting high field magnets. However, its critical current density (Jc) in high magnetic fields needs to be further improved. Here we show that for the film prepared by pulsed laser deposition method by controlling the deposition parameters (higher substrate temperature and lower growth rate), the crystallinity of BaFe2(As0.66P0.33)2 (Ba122:P) matrix is improved while maintaining a high density of incoherent BaZrO3 (BZO) nanoparticles (NPs) which together lead to significantly increased self field Jc. Our Ba122:P nanocomposite films also exhibit increased in-field Jc, reduced angular anisotropy of Jc and reduced detrimental effects of thermal fluctuations (creep rate) over a wide range of temperatures and magnetic field strength. The BZO NP doped Ba122:P films show high in-field Jc over 2.1 MA cm−2 even at 4 K and 9 T (μ0H∣∣c), which is significantly higher than that of standard Ba122:P films and conventional alloy superconducting wires. To understand the contribution of the various pinning centers, we applied a simple model, which was developed for cuprates, to Ba122:P film with all the parameters used derived by fitting to a limited set of experimental data (no free parameters) such that temperature, angle and field properties at other experimental conditions are then calculated. This simple model fits very well to the experimental results in these two very different material systems. We discuss the effectiveness of natural defect and BZO NPs on the ratio of Jc to the depairing current density. The superconducting properties for 122-type iron-based superconductors obtained through this work are considered promising for high-field applications.

Anisotropy in the magnetic properties of single-crystal Y1Ba2Cu3O7−x

Barium ferrite magnetic recording media have demonstrated the capability of achieving much higher recording densities than other particulate media. Even though the best performance is obtained with perpendicularly oriented media, longitudinally oriented and non-oriented Ba-ferrite media also exhibit exceptional recording characteristics. These media have very distinctive magnetic and switching field (SFD) characteristics, exhibiting very small SFD values, almost rectangular remanence loops (indicating no irreversible losses over a wide range of reverse fields), and minor loops very close to the major loop until the driving field becomes almost equal to the coercivity. These unique magnetic and switching field characteristics can explain the very low or negligible demagnetization losses (including recording demagnetization) which we observe in Ba-ferrite media and which result in their outstanding recording performance. The fundamental reasons for these unique magnetic and switching field properties are the predominant high uniaxial crystalline anisotropy of the platelets and their single domain character. On the negative side, the high anisotropy field is directly implicated as the main reason behind the marginal overwrite modulation which characterizes these media.

Among various ThMn12-type rare-earth iron compounds, NdFe11TiN has been known for its large magnetization and high anisotropy field, although their values are smaller than those of Nd2Fe14B and Sm2Fe17N3. Recent first-principles calculations predicted that NdFe12N has a substantially larger magnetization than NdFe11TiN with a comparable anisotropy field. To validate this prediction, the NdFe12Nx phase was successfully synthesized by nitriding the NdFe12 film that was grown on a W underlayer on a MgO(001) substrate. The NdFe12Nx phase was found to have a saturation magnetization, anisotropy field, and Curie temperature superior to those of Nd2Fe14B.