Yukio Nozaki

Microwave-assisted magnetization reversal in a Co/Pd multilayer with perpendicular magnetic anisotropy

Microwave-assisted magnetization reversal in a rectangle of a Co/Pd multilayer with a perpendicular magnetic anisotropy is examined using vector network analyzer ferromagnetic resonance (FMR) spectroscopy. A microwave field is applied along the in-plane direction of the rectangle together with a negative dc easy-axis field smaller than the coercive field. Broadening or splitting of the peak profile in the FMR spectrum suggesting the formation of multidomain structure appears after the microwave field is applied. The dominance of microwave-assisted nucleation of magnetization is supported by the frequency dependence of the probability with which the multidomain structure appears.

Magnetic force microscopy study of microwave-assisted magnetization reversal in submicron-scale ferromagnetic particles

Changes in magnetic-domain configurations caused by the application of radio-frequency (rf) fields ranging from 3to18GHz were observed using magnetic force microscope. rf fields barely altered the magnetization of Co particles with scattered large crystalline anisotropy; however, for NiFe particles, magnetization switching occurred when the rf approached the natural ferromagnetic-resonance frequency of 5.92GHz. However, the reduction of the switching field fluctuated widely among the particles. Scattering of the effective energy barrier to thermal fluctuations caused by an applied rf field can explain the stochastic switching properties.

Microwave-assisted magnetization reversal in 0.36-μm-wide Permalloy wires

Microwave-assisted magnetization reversal in a 0.36μm wide Ni80Fe20 wire with a natural ferromagnetic resonance frequency of 8.42GHz was investigated experimentally. Magnetization switching could be detected from a discontinuous change in the ferromagnetic resonance frequency of the wire. The application of microwaves with a frequency of 7.5GHz and a power of 19.2dBm reduced the switching field from 217to142Oe. While high-power microwave-assisted switching reduced the dc switching field, it substantially broadened the switching field distribution. This is mainly attributed to inhomogeneous resonance properties of the nucleation sites in the wire.

Room temperature microwave-assisted recording on 500-Gbpsi-class perpendicular medium

Microwave-assisted recording on a 500-Gbpsi-class perpendicular medium was experimentally demonstrated at room temperature. Magnetization reversal under a radio-frequency magnetic field was measured by an electrically shorted coplanar waveguide, which enabled us to evaluate the change in the mediums ferromagnetic resonance spectrum. A frequency-dependent reduction in the switching field was clearly observed in response to a microwave impulse 50 ns in duration. A significant reduction of up to 30% in the coercive field was achieved by applying a microwave impulse with an amplitude of 25 dBm and a frequency of 15 GHz.

Electrical demonstration of spin-wave logic operation

Spin-wave logic operation in a ferromagnetic thin film was investigated by using a three-antenna device. Binary output 1/0 signals were realized with the constructive and destructive spin-wave interference signals. It was found that the configuration of the RF antenna determines the phase of the input spin wave, which provides a control method of logic operation in spin-wave devices.

High‐resolution observation of magnetization processes in 2 μm × 2 μm × 0.04 μm permalloy particles

The magnetization process in an array of 2 μm × 2 μm × 0.04 μm permalloy particles fabricated on a 100 nm carbon membrane was investigated by means of high‐resolution Lorentz microscopy and electron holography in magnetic fields parallel and perpendicular to the film surface. For perpendicular fields, the magnetization reversal was found to begin with the nucleation of a vortex which gradually transformed into a closure domain configuration. At the remanent state, the flux closure was achieved with the generation of a new type of doubled domain wall.

Spin wave-assisted reduction in switching field of highly coercive iron-platinum magnets

Recent rapid progress in spintronic and magnetic storage nanodevices has required nanomagnets to balance competing goals for high coercive field and low switching field. However, a decisive route for highly efficient magnetization switching has not been established yet. Here we propose a novel switching method using a spin wave of magnetic structures twisted in a nanometre scale. We have experimentally demonstrated extremely low field-magnetization switching in a highly coercive FePt by using a spin wave excited in a soft magnetic permalloy (Ni81Fe19), where permalloy is exchange-coupled to FePt through the interface. We can tune the switching field by varying the magnitude and frequency of the radio frequency magnetic field, and a significant decrease in switching field by one order of magnitude is achieved under the optimum conditions. The spin wave-assisted magnetization switching is a promising technique for ultralow-energy magnetization manipulation.

Magnetic properties of epitaxially grown Fe3Si/Ge(111) layers with atomically flat heterointerfaces

We study magnetic properties of epitaxial Fe3Si layers grown on Ge(111) with atomically flat interfaces. An unexpected uniaxial magnetic anisotropy is observed in the film plane for all as-grown samples, and the direction of the uniaxial easy axis is different for each of these samples. By postgrowth annealing, surprisingly, the random orientation of the uniaxial easy axis is aligned to a direction along about [01¯1], together with a reduction in the saturation magnetization. We discuss a possible mechanism of the variation in the magnetic properties after the annealing.

Magnetic flux penetration process in two‐dimensional superconductor covered with ferromagnetic particle array

The magnetostatic effect of a ferromagnetic particle array on an underlying two‐dimensional superconducting Nb thin film was investigated in terms of flux penetration processes by means of high sensitive magnetometry. For bare Nb film, a large drop in magnetization due to a flux rearrangement appeared in the low field region below 2.5 mT. However, once magnetic particles were fabricated on the surface of the Nb film, the drop was not observed. In this configuration, the ferromagnetic particles seem to behave as flux guides for the external magnetic field, whereby the flux distribution inside the superconductor is well regulated. Furthermore, a periodical oscillation was observed in the hysteresis loops. This behavior is associated with a flux quantization in the regions where the superconductivity is suppressed by the stray field of the ferromagnetic particles. The local flux density distribution around the ferromagnetic particles was evaluated by using the London equation, and the magnetization curve of ...

Frequency dependence of critical switching asteroid of CoCrPt–SiO2 granular film under 50-ns microwave impulse

Understanding the dynamic process of microwave-assisted magnetization reversal (MAMR) in a dipole–dipole coupled granular perpendicular medium is essential for its application to future magnetic storage devices. We experimentally investigated the magnetization reversal process in CoCrPt–SiO2 granular films in terms of the dependence of the switching field on the strength of an applied in-plane radio-frequency magnetic field. The reversal process changed from non-uniform rotation to uniform rotation when the frequency increased toward the ferromagnetic resonance frequency. In the resonant condition, the switching field agreed well with a model assuming uniform rotation of the magnetization in each grain. The agreement suggests that the decoherence of ferromagnetic resonance due to a dynamic dipolar field is weak in adequately exchange-coupled granular films with a saturation magnetization of 415 emu/cm3. This finding is significant for the development of the practical medium necessary for MAMR writing.