H. Miyajima

Conversion of spin current into charge current at room temperature: Inverse spin-Hall effect

The inverse process of the spin-Hall effect (ISHE), conversion of a spin current into an electric current, was observed at room temperature. A pure spin current was injected into a Pt thin film using spin pumping, and it was observed to generate electromotive force transverse to the spin current. By changing the spin-current polarization direction, the magnitude of this electromotive force varies critically, consistent with the prediction of ISHE.

Current-induced resonance and mass determination of a single magnetic domain wall.

A magnetic domain wall (DW) is a spatially localized change of magnetization configuration in a magnet. This topological object has been predicted to behave at low energy as a composite particle with finite mass. This particle will couple directly with electric currents as well as magnetic fields, and its manipulation using electric currents is of particular interest with regard to the development of high-density magnetic memories. The DW mass sets the ultimate operation speed of these devices, but has yet to be determined experimentally. Here we report the direct observation of the dynamics of a single DW in a ferromagnetic nanowire, which demonstrates that such a topological particle has a very small but finite mass of 6.6 × 10-23 kg. This measurement was realized by preparing a tunable DW potential in the nanowire, and detecting the resonance motion of the DW induced by an oscillating current. The resonance also allows low-current operation, which is crucial in device applications; a DW displacement of 10 µm was induced by a current density of 1010 A m-2.

Physics of magnetic fluids

Abstract This paper reviews various fundamental physical properties of magnetic fluids, such as superparamagnetic behavior, neutron-depolarization, magneto-viscosity, anisotropic attenuation of sound waves and magneto-optical properties and discusses how these properties are explained in terms of the nature of colloidal particles, particularly in relation to their cluster formation. Emphasis is placed on the explanation of recent experiments on the magneto-optical dichroism and birefrengence in terms of Rayleigh scattering of the ray from the elongated clusteres of the magnetic particles.

2e2/h to e2/h switching of quantum conductance associated with a change in nanoscale ferromagnetic domain structure

We demonstrate the electrical conductance quantization in a Ni nanowire formed in a break junction between a ferromagnetic Ni wire and a Ni plate in applied magnetic fields. The conductance of the nanowire is clearly quantized in units of 2e2/h in a zero magnetic field, but it is switched to e2/h by applying magnetic fields above 60 Oe. This switching behavior seems closely related to a ferromagnetic domain formation in the vicinity of a nanowire, suggesting that nanoscale magnetic domain walls play an important role in determining nanoscale spin-dependent transport. The effect offers the possibility of a new device, a nanoscale colossal magnetoresistive sensor.

Magnetization reversal in submicron magnetic wire studied by using giant magnetoresistance effect

The magnetization reversal phenomenon in a submicron magnetic wire with a trilayer structure consisting of NiFe(200 A)/Cu(100 A)/NiFe(50 A) was investigated by measuring the electric resistance in an external magnetic field. A giant magnetoresistance (GMR) effect of about 0.8% was observed when the magnetizations in two NiFe layers are oriented antiparallel. It is demonstrated that magnetization reversal phenomena can be very sensitively investigated by utilizing the GMR effect.

Rectification of radio frequency current in ferromagnetic nanowire

The authors report the rectification of a constant wave radio frequency (rf) current by using a single-layer magnetic nanowire; a direct-current voltage is resonantly generated when the rf current flows through the nanowire. The mechanism of the rectification is discussed in terms of the spin-torque diode effect reported for magnetic tunnel junction devices, and the rectification is shown to be directly attributable to resonant spin wave excitation by the rf current.

Manipulation of vortex circulation in decentered ferromagnetic nanorings

A method to control the direction of the magnetic-moment circulation in ferromagnetic nanoscale rings is presented. This method involves controlling the domain-wall (DW) motion using the gradient of the DW energy along the circumference in cooperation with an external magnetic field. Since it is free from the DW pinning and depinning process, the method is applicable even under strong thermal disturbance. Magnetic-force-microscopic observation was performed for Ni81Fe19 rings 500 nm in diameter in which the gradient of the DW energy was introduced by decentering the rings. The result indicates perfect controllability of the magnetic-moment circulation at room temperature, demonstrating its potential for a wide range of applications in magnetic memory devices.

Induced Magnetic Moment in Ferromagnetic Fe Alloys by Tetragonally Elongated Lattice Expansion

Magnetization and Mossbauer spectra were measured for Fe-C, Fe-N and Fe-Ni-C systems. The mass ratio of the tetragonal martensite in the mixed phases was determined by means of Mossbauer spectroscopy, leading to the average magnetic moment of Fe atoms in the body centered tetragonal (bct) structure. The magnetic moment of Fe atoms increases from 2.2 µ B to 2.6 µ B as the axial ratio and the volume of unit cell increase. In other words, the volume expansion and/or the tetragonal elongation seem to cause an increase in the magnetic moment of Fe in bct alloys. It is concluded that the variation of the magnetic moment of Fe in the bct structure can be explained as a superposing effect of both volume expansion and tetragonal expansion. The mechanism of these effects is discussed in terms of energy band splitting of d electrons and magnetovolume effects.

Magneto-Volume and Tetragonal Elongation Effects on Magnetic Phase Transitions of Body-Centered Tetragonal FeRh1-xPtx

Magnetic properties of FeRh 1- x Pt x system with body-centered cubic and body-centered tetragonal (bct) structures were studied by means of X-ray diffractometry, magnetometry and Mossbauer spectroscopy, and the phase diagram in a temperature-composition plane was determined. With substituting Pt for Rh, the axial ratio c / a was increased and the bct phase was stabilized. With increasing temperature, the bct alloys with 0.72< x <0.81 underwent a first-order phase transition from the antiferromagnetic state to the ferromagnetic state, while the alloys with 0.3< x <0.72 exhibited a first-order phase transition from the antiferromagnetic state to the paramagnetic state with a volume expansion and a tetragonal lattice distortion. The mechanism of those phase transitions was discussed in terms of the magneto-volume and the tetragonal elongation effect.

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).