Masukazu Igarashi

Magnetic and electrical properties of single‐phase, single‐crystal Fe16N2 films epitaxially grown by molecular beam epitaxy (invited)

The average magnetic moment per Fe atom for a single‐phase, single‐crystal Fe16N2(001) film epitaxially grown on a GaAs(001) substrate by molecular beam epitaxy has been confirmed to be 3.5μB at room temperature by using a vibrating sample magnetometer (VSM) and Rutherford backscattering. The value was in good agreement with that obtained by using a VSM and by measuring the film thickness (3.3μB per Fe atom). The saturation magnetization 4πMs has been found to increase with decreasing temperature, obeying T3/2 law at lower temperatures. The slope was steeper than that of a pure Fe film, suggesting a lower exchange constant for Fe16N2. The g factor for Fe16N2 has been accurately measured to be 2.17 by using ferromagnetic resonance with changing frequencies of 35.5–115 GHz, which is not unusual compared with the g factor of 2.16 for pure Fe. The resistivity for Fe16N2 has been measured to be around 30 μΩ cm at room temperature compared with 10 μΩ cm for pure Fe and decreases linearly with decreasing tempera...

Microwave assisted switching mechanism and its stable switching limit

Microwave assisted switching (MAS) of magnetization has been intensively studied as an alternative technique for ultrahigh density magnetic recording. In this paper, comparison between the Landau–Lifshitz–Gilbert simulation and the analytical model in the rotating frame {Bertotti et al., [Phys. Rev. Lett. 86, 724 (2001)]} reveals that the switching behaviors of MAS can be clearly divided into two groups, that is, stable and unstable switching regions, depending on the frequency and amplitude of the ac field. The stable switching exhibits small switching field and narrow switching field distribution, and perfectly coincides with by the analytical model. Furthermore, in this region, the Sharrock type thermal fluctuation formula can be applicable to the MAS at finite temperature. On the other hand, for the unstable switching, the switching field is rather large and the SDF becomes very broad. Obviously, the former is preferable for the practical application of MAS. The critical frequency of the ac field for ...

A method of measuring anisotropy field of polycrystalline thin-film media

Ferromagnetic resonance (FMR) is used to measure the magnetic anisotropy field Hk and its dispersion of poly-crystalline recording media. This method does not use any single crystal substrate. A high-intensity field is applied vertically to a sample surface to completely saturate the magnetization and a high-frequency (over 50 GHz) magnetic field is applied in the surface. An FMR simulator based on the Landau–Lifshitz–Gilbert equation confirms that the magnetic (exchange and/or magnetostatic) interactions have very little effect on the resonant frequency in the configuration used. The linewidth of the FMR spectrum is affected by the Hk dispersion and damping constant. Methods using a torque or magnetization curve include magnetic interactions, resulting in a systematic error to the value of Hk. This FMR method is applied to CoCrPt-alloy thin-film media with a Cr-alloy underlayer grown on a glass substrate. A conventional torque method is also used for comparison. The axis of hexagonal-close-packed Co magn...

Perpendicular uniaxial magnetic anisotropy of Fe/sub 16/N/sub 2/[001] single crystal films grown by molecular beam epitaxy

The uniaxial magnetic anisotropies by torque measurement have been measured for Fe/sub 16/N/sub 2/[001] single crystal films grown by molecular beam epitaxy (MBE). It has been found that Fe/sub 16/N/sub 2/[001] films exhibits perpendicular uniaxial anisotropy which makes the easy axis along the [001] direction, and those anisotropy constants K/sub U1/ and K/sub U2/ for Fe/sub 16/N/sub 2/ were 1.6/spl times/10/sup 7/ and 0.4/spl times/10/sup 7/ erg/cm/sup 3/ respectively. Those K/sub U1/ and K/sub U2/ for Fe/sub 16/N/sub 2/ were constant in the thickness range from 34 to 83 nm. The ferromagnetic resonance of Fe/sub 16/N/sub 2/[001] single crystal films have been measured. The saturation magnetic flux densities, 4/spl pi/M/sub s/ for Fe/sub 16/N/sub 2/ measured by the magnetic torques and resonance fields agreed well with the values measured with a vibrating sample magnetometer (VSM).

Mechanism of microwave assisted magnetic switching

The characteristic of microwave assisted switching for an isolated grain was investigated using the Landau–Lifshitz–Gilbert simulation. It was found that anticlockwise and clockwise polarized fields assist magnetization to switch and to reswitch, respectively. Using larger linear polarized field, sufficient switching is not obtained. It was confirmed the magnetic resonance effect on the reduction in the switching field. It was also found that a large assist effect and a narrowing transition effect were achieved in exchange coupled subgrains.

Reduction in intermediate layer thickness of CoCrPt–SiO2 perpendicular recording media by using Ru–SiO2

A Ru–SiO2 intermediate layer was introduced between the Ru and recording layers to reduce the intermediate layer thickness of CoCrPt–SiO2 perpendicular recording media. A structural analysis revealed that the Ru–SiO2 intermediate layer enhanced the oxide segregation in the initial growth region of the recording layer. A micromagnetic simulation and magnetic property measurements indicated that the Ru–SiO2 intermediate layer reduced the intergranular exchange coupling in the initial growth region of the recording layer without magnetic anisotropy degradation. We reduced the intermediate layer thickness down to 9.1nm by using the Ru–SiO2 intermediate layer while maintaining the signal-to-noise ratio.

Bicrystal cluster structure and magnetic properties of CoCrTa/Cr thin film media

Structures of bicrystal clusters of CoCrTa thin film media are examined using high resolution transmission electron microscopy (HRTEM), and the effects of the bicrystal clusters on magnetic hysteresis were studied using micromagnetic simulation. The bicrystal cluster of CoCrTa consists of two crystallographic variants grown on a single Cr underlayer grain with their c-axes normally-aligned in the film plane. The typical number of grains in each cluster is 2 to 3. With the bicrystal structure, coercivity decreases significantly as grain size decreases when intergranular exchange coupling is strong, and high hysteresis squareness can be obtained. Coercivity in the small grain size region can be increased by reducing the exchange coupling.

Experimental feasibility of spin-torque oscillator with synthetic field generation layer for microwave assisted magnetic recording

We investigated the oscillation characteristics of a fabricated spin-torque oscillator (STO) with a synthetic field generation layer (FGL) composed of the FGL and perpendicular anisotropy magnetic layer in microwave assisted magnetic recording (MAMR). The fabricated STO was composed of a perpendicularly magnetized reference layer/interlayer/synthetic-FGL. The STO with synthetic-FGL showed very narrow peaks caused by spin-torque oscillation. We also found oscillating frequency of the fabricated STO increased as the external field increased. This implies that the FGL magnetization rotates out-of-plane. Calculations by micromagnetic simulation were qualitatively consistent with the STO measurements. The calculated magnetization configurations in the synthetic-FGL formed a single domain but the one in the single-FGL formed multiple domains. This is one reason that the synthetic-FGL is estimated to generate a larger ac-field. In sum, we experimentally confirmed the effective oscillation of fabricated STO with ...

Effect of Elliptical High-Frequency Field on Microwave-Assisted Magnetic Switching

The characteristic of microwave-assisted switching for an isolated particle was investigated using the Landau Lifshitz Gilbert simulation. 1) An effective component of high-frequency (HF) field is one perpendicular to the magnetization, which is in the direction of the effective field under a write field. 2) The anti-clockwise polarized component should be increased in a practical field generation layer (FGL) to obtain larger assist effect. 3) It is necessary to use an elliptically polarized field and to design a microwave-assisted magnetic recording head with FGL with consideration about the direction and the efficiency of the HF field.

Oscillation Feature of Planar Spin-Torque Oscillator for Microwave-Assisted Magnetic Recording

The mechanism of oscillation or magnetization rotation of a field generation layer (FGL) in a spin torque oscillator (STO) under the large gap field between main pole and trailing shield for microwave assisted magnetic recording (MAMR) was investigated using an analytical model. The following results were obtained. The oscillation frequency at the maximum magnetic field output is decided by the vertical component of the magnetic field from the main pole. When there is a strong external magnetic field, it is difficult to restrain the FGL magnetization in the plane of FGL and the oscillation becomes unstable. A FGL with zero (or positive) anisotropy will not be suitable for use in the strong external magnetic field because it will not oscillate properly. It can be expected that the spin torque oscillator with a planar FGL with negative perpendicular magnetocrystalline anisotropic material is likely to be suitable for MAMR.