Eiji Kita

Ni and NiFe nanocrystalline films prepared with gas-deposition method

The magnetic properties and structures of Ni and NiFe nanocrystalline films prepared with the gas-deposition method (GDM) are investigated. From the results of STM and SEM, fine particles, approximately 10 nm in diameter, were seen in the films. Furthermore, aggregates several tens of nm in diameter, composed of the small particles, are also observed. The magnetization of the Ni nanocrystalline films is about 50 emu/g, or 90% of that of bulk Ni. Coercivity of the Ni nanocrystals ranged from 4 to 20 Oe. In the case of NiFe, the concentration of Fe is slightly increased as the evaporation temperature is increased.

Coexistence of the uniaxial anisotropy and the antiferromagnetic coupling in Co/Ir(111) superlattices

Abstract We have investigated the structural and the magnetic properties of carefully grown Co/Ir(1xa01xa01) superlattices composed of a few atomic mono-layers (MLs) of each element. We found that the superlattice of [Co(3xa0ML)/Ir(3xa0ML)] has its magnetic easy-axis perpendicular to the surface and has quite large antiferromagnetic (AF) coupling between adjacent Co layers. As a result, the magnetic hysteresis curve shows a spin-flip at H∼53 kOe . Adopting a simple two sublattice coherent rotation model, we analyzed the MH curves and obtained the AF coupling strength J AF and the perpendicular anisotropy K as well. The estimated K of [Co(3xa0ML)/Ir(3xa0ML)](1xa01xa01) is even larger than that of Co/Pt multilayer which is known as a typical strong perpendicular anisotropy system.

3D transition ferromagnetic metal nano-crystals prepared with gas deposition method (GDM)

The application of gas deposition method (GDM) to the formation of 3d transition metal nano-crystals and their structure is reported. A He circulation system was installed in order to purify the inert gas used during the preparation. The metals of Fe, Co, and Ni were evaporated by RF heating using a ZrO2 linear set in a tungsten crucible and a carbon crucible. X-ray diffraction study reveals the same crystal structure as those of bulk states for Fe and Ni (bcc and fcc), while the Co nanocrystal has modified structure from the fcc structure. The averaged grain sizes are ranged about 5 nm. The magnetization are more than 95 % of bulk except for samples with loose packing, which were prepared with lower pressure difference between the evaporation and deposition chambers. It is found that oxidization is remarkably suppressed during the fabrication process.

Magnetoresistance of zigzag-shaped cobalt wires

A new approach to detect the domain wall resistivity in a ferromagnetic zigzag bent wire is demonstrated. Two different domain configurations are artificially created in the zigzag wire by changing the orientation of magnetic fields. A comparison between the resistivities in the two configurations shows an additional 0.05% negative magnetoresistance related to the domain walls.

Quadrupolar nutation NMR on a compound semiconductor gallium-arsenide

Lattice defects in a compound semiconductor, gallium-arsenide are evaluated by two-dimensional nutation nuclear magnetic resonance. Especially in the case of indium doped gallium-arsenide, analysis of the nutation patterns indicates that the electric field gradient exists in the whole crystal. Asymmetry parameters and quadrupolar coupling constants are determined as approximately 1.0 and 93 kHz, respectively. These results suggest that the whole crystal is under slight strain. Through this work, it is demonstrated that a two-dimensional nutation nuclear magnetic resonance is the useful method to investigate the lattice defects in gallium-arsenide.

Magnetic properties and structures of Fe/MgF2 multilayered films

Abstract Multilayers composed of Fe and MgF 2 with layer thicknesses between 9xa0A and 100xa0A and of 30xa0A, respectively, were prepared with an ultrahigh-vacuum deposition technique. Medium-angle X-ray data show that the Fe layers in the BCC phase have considerable (1xa01xa00) texture. Periodicity due to multilayered structures was confirmed by a small-angle X-ray diffraction study and cross-section transmission electron microscope for films with Fe layer thicknesses >45xa0A. In an Fe/MgF 2 (9xa0A/30xa0A) sample, an island structure for the Fe layers was suggested by the existence of superparamagnetism in a film. At 4.2xa0K, enhancements of both magnetization and hyperfine field were observed in films having Fe layers thinner than 40xa0A. The maxima in the magnetization (233xa0emu/g of Fe) and in the average hyperfine field (390xa0kOe) at 4.2xa0K were found in an Fe/MgF 2 (9xa0A/30xa0A) film and were approximately 105% and 115% that of the bulk α-Fe, respectively. The thickness dependence suggests a 12% enhancement in the magnetic moment of interface Fe atoms. No exchange bias was found in the films, implying that antiferromagnetic fluorides are not formed at the interface, which is different from the case of Fe/LiF and Fe/CaF 2 multilayers.

Perpendicular magnetic anisotropy of Co∕Rh(111) distorted superlattices

We have investigated the relationship between perpendicular magnetic anisotropy (PMA) and the magnetoelastic effect of Co∕Rh(111) epitaxial superlattices as a large mismatch system and found the PMA for the thin Co layer to be less than 12A. The magnitude of the PMA is comparable to those of typical PMA systems such as Co∕Pd and Co∕Pt. By taking account of the Co-thickness-dependent magnetoelastic anisotropy KME, arising from the lattice strain due to the 7% mismatch between Co and Rh, we found that the thickness-dependent PMA can be reproduced with no adjustable parameter.

Experimental determination of inter-sublattice exchange constants in amorphous FeGd alloys

High-field-magnetization measurements were assessed for melt-spun amorphous Fe100−xGdx alloys. Samples with X=22 and 24 clearly revealed ferrimagnetic spin-flip transition around 100–150 kOe at 4.2 K and their magnetizations linearly increased with increasing magnetic field above the transition. This magnetization process with spin flipping is well understood with molecular field approximation assuming a simple two-sublattice ferrimagnet. Inter-sublattice exchange constants JRT were obtained directly and amplitudes were −16.5×10−16u2009erg for X=22 and −18.1×10−16u2009erg for X=24 samples. These agree with previously reported values estimated by molecular field analysis.

Field Dependence of Magnetoresistance in Granular Ag–Co Thin Films

The proportionality of the giant magnetoresistance (MR) to 2, where θ is the angle between the local magnetization and the applied field, was tested in granular Ag–Co (20 at.%) thin films. The assumption that 2=(M/Ms)2 does not give a linear relation. To deduce the granule size distribution, magnetization data for the superparamagnetic state were fitted using a set of Langevin functions on a granule size histogram. Assuming that the scattering rate is proportional to the surface area of each granule in such a system and that the granules are spherical, we weight the average of cos θ in the size distribution by the surface area. If granules containing more than 900 Co atoms are ignored when determining the average value of cos θ, the MR varies linearly with this quantity, which is denoted as sur.av.2. We suggest that the surface roughness enhances the contribution of smaller granules to the MR.

Appearance of Resistance Minimum in Nanocrystalline Films Prepared by Gas Deposition Method

Iron nanocrystalline films were prepared by a gas deposition method. The nanocrystal has a diameter of about 300 A and its surface was covered with a 40-A-thick oxide layer. The electric resistance versus temperature curve showed a minimum in the low-temperature region. The resistance minimum was analyzed from the viewpoint of electron hopping through the oxide layer on the nanocrystal surface.