S. Honda

X-ray photoelectron spectroscopy and magnetic properties in Fe–SiO2 granular films

We have prepared single-layered Fe–SiO2 and multilayered Fe–SiO2/SiO2 granular films by rf magnetron sputtering and investigated their magnetic properties and x-ray photoelectron spectroscopy (XPS). The magnetization depends strongly on the film thickness for the single-layered films, and on SiO2 layer thickness for multilayers. In single-layered films, a lot of Fe granules near the film surface are oxidized by air that has penetrated from the surface, resulting in reduction of the net magnetization. With going to the inner layers, the oxidization is suppressed, but upon approaching the substrate considerable oxidation of Fe occurs again by consuming SiO2. In the multilayers, the energy shift of Si 2p and O 1s, seen in XPS occurs layer by layer, indicating that the Fe granules react with O and/or Si released from SiO2. The rate of oxidation of Fe becomes larger with increasing SiO2 layer thickness, especially at the layer boundaries. Thus, the change in magnetization can be explained by the oxidization of...

Formation of nanosized Fe–Co alloys in α−Al2O3 crystals by ion implantation

Nanometer-sized Fe–Co composite clusters in α−Al2O3 matrices were synthesized by sequential implantation of Fe and Co ions with the projectiles’ energy at 100 keV. The synthesized clusters were shown to have bcc structure by the measurement of glancing angle x-ray diffraction. We have observed an apparent change in the internal magnetic field of the Fe–Co composite clusters with changing Co concentration, which exhibits the maximum at a concentration around 25 at. % Co as revealed by conversion electron Mossbauer spectroscopy. The magnetoresistance ratio of the granules was observed to increase with the Co addition to the Fe nanoclusters. The results provide clear evidences on the alloy formation of Fe–Co nanoclusters in α−Al2O3 layers synthesized by ion implantation.

Tunneling magnetoresistance in ultrathin Co–SiO2 granular films

Tunneling giant magnetoresistance (MR) properties have been examined for ultrathin Co–SiO2 granular films, forming the pseudo-two-dimensional alignment of Co granules. The resistivity follows well the relation for the tunneling transport between Co granules. The values of resistivity and the activation energy for tunneling depend on both the Co composition and the film thickness, owing to the change in morphological structure. In the 35 vol % Co films of 5 and 10 nm thickness, the MR ratio increased with reducing temperature similar to thick films. On the other hand, in the 28 and 32 vol % Co films thinner than 5 nm, the MR ratio decreased drastically below 100 K with reducing temperature reflecting the two-dimensional alignment of Co granules.

In situ observation of ordering process in FePt films during annealing in a transmission electron microscope

We have investigated the process of the phase transformation from fcc to fct in FePt thin films by in situ observation during annealing in a transmission electron microscope. The coercivity for the in-plane field increases drastically up to about 6.3kOe by annealing at 300°C in the film prepared at 0.5mTorr by multipolar magnetic plasma confinement sputtering. The as-deposited film has a flat surface and very large grains of 8–16nm, which causes the easy ordering at low temperatures. The ordering progresses accompanied with the crystal growth, which occurs laterally by the coalescence with the disordered grains. On the other hand, in the film prepared at 5mTorr by the conventional sputtering, the surface is rough and grains are smaller than 6nm. This small grain in the as-deposited state suppresses the crystal growth and also the ordering even at higher annealing temperatures.

Effects of ion irradiation on structural and magnetic properties of Fe/Si multilayers prepared by helicon plasma sputtering

Abstract Helicon plasma sputtering has been used to prepare Fe/Si MLs with an Fe layer thickness around tFe=2–5 nm for a Si spacer fixed at the thickness of tSi=1 and 1.5 nm. Present study found that the Fe/Si MLs of the Si spacer thickness at tSi=1 nm exhibit antiferromagnetic nature, but the other Fe/Si MLs are ferromagnetic. The maximum value of magnetoresistance (MR) ratio in Fe/Si MLs appears at tFe=3 nm, tSi=1 nm and is about 0.22%. We performed 400 keV Ar ion irradiation to investigate the behavior of magnetic properties in Fe/Si MLs. The magnetization measurements of Fe/Si MLs after 400 keV Ar ion irradiation show the degradation of antiferromagnetic behavior and the values of MR ratio after ion irradiation decrease. X-ray diffraction (XRD) patterns indicate that the peak intensity of a satellite peak originated in superlattice structure does not change within the range of ion dose used. These results imply that the interface structures after ion irradiation become rough although the superlattice structures remain. Therefore, we consider that the change of MR properties in Fe/Si MLs by 400 keV Ar ion irradiation is due to the thickness dependence of Si layers like metallic superlattice structures.

Ab initio calculation for magnetism of Pd nanoparticles

Magnetism of Pd nanoparticles consisting of 14-85 atoms has been discussed in terms of theoretical approach based on the ab initio calculation using the linear augmented plane wave method. The magnetic moment increases with increasing atom number, indicating that the magnetism is the resulting effect of the cooperative electron interaction in the crystals. The magnitude of the moment is as small as 0.28 /spl mu//sub B/ at the surface for the 14 atoms particle and increases up to 0.8 /spl mu//sub B/ in the 85 atoms particle. In the particle including 85 Pd atoms, it is found that the magnitude of the moment at the surface and inner atoms are almost identical, implying that, there lies only three lattice planes between the surface and center atoms and this is still insufficient to discuss the magnetism of the surface and inner atoms.

Magnetoresistance and Hall effect in Co implanted GaAs hybrid magnetic semiconductors

The hybrid Co/GaAs ferromagnetic semiconductor has been prepared by the ion-implantation method, and the sample surface has been sputter-etched for examining the magnetic and transport properties as a function of the depth. Extraordinary Hall effect increases with depth until 120 nm etching, and the Hall resistance is proportional to the square of resistance, indicating the side-jump mechanism, in the depth region of 10–60 nm. The depth profiles of the saturation magnetization and the conductivity are estimated analytically, and then the Hall coefficient is evaluated as a function of etching depth.

Enhanced ordinary magnetoresistance in Co∕Si systems

The enhancement of the ordinary magnetoresistance (OMR) in Si has been attempted by constructing two type of Co∕Si systems; one is the sputter-deposited current perpendicular to plane-type Co∕Si multilayers and the other is the sputtered-Co∕anodized-porous-Si system. The Co∕Si multilayers show the sharp OMR at a low field and the negative granular-type giant magnetoresistance at higher fields. At 50K, however, only the OMR appears with the ratio of about 3.0% for an in-plane field of 10kOe. In the Co∕anodized-Si system, a very large OMR of about 60% is observed at 50K for perpendicular field of 10kOe.

Structural and Magnetic Properties of Helicon-Sputtered Fe/Si Multilayers

In Fe/Si multilayers (MLs) prepared by helicon plasma sputtering, we have investigated the structural and magnetic properties as a function of Fe and Si layer thickness systematically. In [Fe 2 nm/ Si 1 nm] 30 and [Fe 3 nm/Si 1 nm] 20 MLs, large lateral Fe grains grow suggesting small atomic intermixing of Fe and Si, and antiferromagnetic coupling between Fe layers occurs, causing a magnetoresistance of about 0.1%. With decreasing the Fe layer thickness and increasing the Si layer thickness, X-ray diffraction patterns indicate peak broadening due to atomic intermixing between Fe and Si layers, and the magnetization curves indicate the change from antiferromagnetic to ferromagnetic coupling between Fe. Furthermore, in this case, the conversion electron Mossbauer spectra show that the hyperfine field decreases due to the solution of Si atoms in the bulk Fe phase. Doublet peaks appear because of the formation of iron silicides which cause the degradation of antiferromagnetic coupling.

Structural and Magnetic Properties of Fe and Au Ion-Implanted Al2O3 Single Crystals

Au ion implantation in Fe ion-implanted Al2O3 (Fe/Al2O3) has been performed in order to tailor the structural, magnetic and optical properties of Fe granules in Al2O3 matrix. After Au ion implantation, Rutherford backscattering (RBS) measurements indicate the decrease and the redistribution of retained Fe atoms with the inclusion of Au atoms, and the patterns of X-ray diffraction (XRD) show the formation of Au granules in the Fe/Al2O3. Besides, the magnetization curves of the Fe/Al2O3 after Au ion implantation show still the superparamagnetic characteristics and the decrease of saturation magnetization, and the optical absorption measurements indicate the formation of Au granules in the Fe/Al2O3 in accordance with the XRD result. In addition, we investigated a behavior of Fe granules in Al2O3 matrix by conversion electron Mossbauer spectroscopy (CEMS), which indicates the decrease of superparamagnetic state as a function of Au ion dose. As a result, it is suggested that Au ion implantation has potentialities to tailor the physical properties of Fe granules in Al2O3 matrix.