Ko Mibu

Effect of Joule heating in current-driven domain wall motion

It was found that high current density needed for the current-driven domain wall motion results in the Joule heating of the sample. The sample temperature, when the current-driven domain wall motion occurred, was estimated by measuring the sample resistance during the application of a pulsed current. The sample temperature was 750 K for the threshold current density of 6.7×1011A∕m2 in a 10-nm-thick Ni81Fe19 wire with a width of 240 nm on thermally oxidized silicon substrate. The temperature was raised to 830 K for the current density of 7.5×1011A∕m2, which is very close to the Curie temperature of bulk Ni81Fe19. When the current density exceeded 7.5×1011A∕m2, an appearance of a multidomain structure in the wire was observed by magnetic force microscopy, suggesting that the sample temperature exceeded the Curie temperature.

Magnetic force microscopy observation of antivortex core with perpendicular magnetization in patterned thin film of permalloy

The cross-tie wall is a kind of magnetic domain wall composed of a main straight wall and crossing subwalls and observed in magnetic thin films. This wall contains two kinds of magnetic vortex structures: “circular vortex” and “antivortex.” At the cores of both vortices, the existence of a spot with perpendicular magnetization has been theoretically predicted. We have detected the perpendicular magnetization spots at each vortex core and identified the direction of it by applying magnetic force microscopy imaging to cross-tie walls in patterned rectangular thin permalloy (Ni80Fe20) films. We also fabricated magnetic structures that contain only antivortex by engineering the shape of thin films.

Epitaxial growth of ferromagnetic La2NiMnO6 with ordered double-perovskite structure

Epitaxial thin films of ordered double-perovskite La2NiMnO6 were deposited on SrTiO3, (LaAlO3)0.3–(Sr2AlTaO6)0.7, and LaAlO3 substrates by a pulsed-laser deposition method. A rock-salt-type ordering for Ni2+ and Mn4+ ions was confirmed through structural and magnetic measurements. Despite the difference in heteroepitaxial constraints on the crystal structure, the magnetic properties of the films were quite similar to each other and also to those of bulk La2NiMnO6.

Magnetic Properties of Fe/Dy Artificial Superstructured Films

Artificial superstructured films composed of two magnetic constituents, Fe and Dy, were prepared by means of alternate deposition in ultrahigh vacuum, and magnetic properties have been studied from Mossbauer spectroscopy and SQUID magnetization measurements. When Fe layers are thinner than 20 A, the structure is amorphous and the magnetic transition temperature is about 270 K, which agrees with that of amorphous pure Fe. When Fe layers are thicker (e.g. 30 A) and Dy layers are thinner than 20 A, a perpendicular magnetic anisotropy becomes significant and the spin direction of Fe changes with temperature.

Dynamics of a magnetic domain wall in magnetic wires with an artificial neck

Magnetization reversal in submicron magnetic wires consisting of a NiFe/Cu/NiFe trilayer with an artificial neck was investigated by utilizing the giant magnetoresistance effect. A magnetic domain wall was injected into the wire by a local magnetic field applied at the end of the wire. Pinning and depinning of the magnetic domain wall were detected as sharp changes in resistance. It was found that the neck works as a pinning site of a domain wall and that the depinning field increases with a decrease of the neck width.

Significant growth-temperature dependence of ferromagnetic properties for Co2FeSi/Si"111… prepared by low-temperature molecular beam epitaxy

We study ferromagnetic properties of Heusler-alloy Co2FeSi epilayers grown on silicon (Si). The magnetic moment and in-plane magnetic anisotropy of the Co2FeSi/Si(111) epilayers vary significantly with the growth temperature (TG) even in the low-temperature region (TG≤200 °C). These features are induced by reaction phases formed at the interface between Co2FeSi and Si. At TG=100 °C, however, we can obtain both highly ordered L21 structures on Si and high-quality Co2FeSi/Si heterointerfaces at the same time. This fact will open a road to realize a Co-based half-metallic spin injector and detector for Si-based spintronic devices.

Propagation of a magnetic domain wall in magnetic wires with asymmetric notches

The propagation of a magnetic domain wall (DW) in a submicron magnetic wire consisting of a magnetic/nonmagnetic/magnetic trilayered structure with asymmetric notches was investigated by utilizing the giant magnetoresistance effect. The propagation direction of a DW was controlled by a pulsed local magnetic field, which nucleates the DW at one of the two ends of the wire. It was found that the depinning field of the DW from the notch depends on the propagation direction of the DW.

Magnetic and transport studies on RPdSn (R= rare earth)

Abstract Magnetic and electrical properties of a series of ternary compounds, RPdSn (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy or Ho) were studied from electrical resistivity, thermopower, magnetic susceptibility and 119 Sn Mossbauer absorption. They were found to be antiferromagnetic. However, the values of T N are not in accordance with the De Gennes factor of rare earth elements. CePdSn was found to be a Kondo compound, SmPdSn and DyPdSn to have a secondary phase transition at a temperature below T N .

Geometrical confinement of a domain wall in a nanocontact between two NiFe wires

A nanocontact structure (typically 22×34 nm2) between two NiFe wires was fabricated by an electron-beam lithography and a lift-off method, and the magnetoresistance was measured. The magnetization switching process was artificially controlled by engineering the sample geometry to realize a magnetic structure with a single domain wall (DW) trapped in the nanocontact area. This domain structure was confirmed by magnetic force microscopy observations. The magnetization rotation of 180° was realized within the nanocontact area. The contribution of the DW to the resistance was negative, which can be understood on the basis of anisotropic magnetoresistance.

Evidence for antiferromagnetic coupling between Fe layers through Cr from neutron diffraction

Small-angle neutron diffraction measurements have been made for a multilayer, [Fe(27 A)/Cr(12 A)]×30, at room temperature. It was found that the magnetizations of adjacent Fe layers are coupled in antiparallel. The decrease of antiferromagnetic peak intensity as a function of external field was observed. It is confirmed that the giant magnetoresistance in Fe/Cr multilayers is due to the antiferromagnetic ordering of Fe layers caused by the interlayer coupling across an intervening Cr layer.