K. Matsuyama

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.

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.

Crystalline structure and magnetic properties of Fe2CrSi Heusler alloy films: New ferromagnetic material for high-performance magnetic random access memory

A new Heusler alloy, Fe2CrSi, which has high spin polarization (P), low saturation magnetization (Ms), and a low Curie temperature (TC), was investigated in order to fabricate high-performance magnetic tunnel junctions (MTJs) with a high tunnel magnetoresistance ratio and with low critical current for the spin-transfer switching method, or a low switching field for the thermally assisted magnetization reversal technique. The main results are as follows: (1) P and the magnetic moment of Fe2CrSi with an L21 structure were 0.98 and 1.98μB∕f.u., respectively, according to density of states calculations. (2) Fe2CrSi films show the (100) orientation with a B2 structure on a MgO substrate upon a thermal treatment with optimum temperature and duration. (3) Fe2CrSi films have Ms and TC values of 385emu∕cm3 and 630K, respectively. (4) The (100) oriented epitaxial MTJs are produced with Fe2CrSi films fabricated with the optimized thermal treatment condition. It is found that the Fe2CrSi Heusler alloy films are a sui...

Numerical study for ballistic switching of magnetization in single domain particle triggered by a ferromagnetic resonance within a relaxation time limit

Magnetization reversal via successive ballistic rotation caused by an application of alternative field temporally varying within a relaxation time limit has been numerically investigated. As the Gilbert damping constant is smaller than 0.03, a pronounced beat appears in the temporal evolution of rotating angle of magnetization. The amplitude of the rotating angle at the first peak of the beat is larger than that achieved in the stable ferromagnetic resonance, so that the magnetization reversal occurred at the first peak of the beat. Consequently, subnanosecond switching of magnetization is realized even though a ferromagnetic resonant phenomenon is utilized to enhance the rotating angle of magnetization. In such a switching scheme, it is most important to optimize the frequency of the alternating field to ensure a practical margin of the operation current creating the switching field.

Micromagnetic Study of Microwave-Assisted Magnetization Reversals of Exchange-Coupled Composite Nanopillars

Microwave-assisted magnetization reversals of exchange-coupled composite nanopillars were studied by micromagnetic simulation. Magnetization reversal occurred in one or two frequency regions associated with magnetic resonance arising from the soft magnetic section. Significant reductions in ac field strength and frequency, which are required for magnetization switching, were obtained in cases where there was a large interface exchange constant between the soft and hard sections. It was also found that the large saturation magnetization of the soft magnetic section tends to enhance magnetization switching.

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

The potential of microwave-assisted magnetic recording on exchange-coupled composite (ECC) media is investigated by numerically simulating read/write processes with respect to the physical and magnetic properties of the soft section of ECC media. To obtain desirable recording performance, the thickness of the soft section must be comparable to the exchange length. Under such condition, the optimal microwave frequency decreases owing to the non-uniformity of the reversal mode. A relatively high anisotropy field in the soft section produces high signal-to-noise ratios. Moreover, large saturation magnetization of the soft section enhances the magnetization reversal of ECC grains but excessive enhancement is likely to induce wide writing, which decreases the high track density.

Micromagnetic study on microwave-assisted magnetic recording in perpendicular medium with intergrain exchange coupling

The influence of intergrain exchange coupling on the properties of microwave-assisted magnetization reversal has been investigated. The results of micromagnetic simulation suggest that the microwave frequency realizing the magnetization reversal is increased as the exchange stiffness constant among the magnetic grains becomes larger than 1 × 10−7 erg/cm if an inhomogeneous magnetization reversal occurred. The numerically expected increase in the microwave frequency was observed experimentally in perpendicularly magnetized Co/Pd multilayer with an exchange stiffness constant of ∼1 × 10−6 erg/cm. The simulations of signal recording and reproducing processes in a granular medium with assistance of a microwave field also suggest that an adequate intergrain exchange coupling is required to inhibit the nucleation of small island domains that will suppress the amplitude of the signal-noise ratio of the readback signal.

Computer simulation of domain wall and vertical Bloch line motion in a bubble garnet film

Vertical Bloch lines(VBLs) and wall motion are studied by using computer simulation. The effect of potential wells, necessary for stable VBLs propagation, on the hard wall dynamics is investigated by the simulation. It is proved that the wall velocity decreases or the wall motion becomes oscillatory due to the potential wells. An isolated VBL propagation under a pulsed in-plane field is investigated.