Shiho Okuno

Preferred crystal orientation of cobalt ferrite thin films induced by ion bombardment during deposition

Thin films of CoxFe3−xO4−y (x=0.8, 1.0, 1.15, 1.25, 1.5, 1.8) were prepared under 100–500 eV Ar+‐ion bombardment during film deposition. Strong crystallization enhancement and preferred crystallographic orientation were observed as the effects of ion bombardment. Crystal orientation normal to the substrate depended on the composition x; 〈110〉‐axis orientation was obtained for x=0.8 and 1.0 and 〈111〉 orientation for x=1.15, 1.25, 1.5, and 1.8. The orientation was not affected significantly by the incident angle of the ions and did not vary with ion energy in the range 100–500 eV. The mechanism of crystal orientation is thought to have a close relationship with the preferential sputtering of oxygen by Ar+‐ion impact.

Oscillatory Behavior of the Interlayer Exchange Coupling as a Function of Au Cap Layer Thickness in Au/Fe/Au/Fe/Au(001)

We have found that the interlayer exchange coupling between two Fe layers separated by a Au spacer oscillates as a function of the Au cap layer thickness in a MBE-grown Au/Fe/Au/Fe/Au(001) sandwich structure. Two oscillations with periods of 2.6 ML and 8.0 ML were observed. The characteristics of the Au cap layer thickness dependence, namely the appearance of two oscillations and the values of their oscillation periods are the same as those for the Au interlayer. The behavior can be explained as arising from the quantum interference effect in the Au cap layer.

Two spin‐freezing behavior in concentrated amorphous ferrite oxides

Amorphous CoFe2O4 with metal ions coordinated in a manner similar to the spinal crystal was prepared by the ion‐beam sputter deposition method. The study of magnetic properties was carried out by dc‐ and ac‐susceptibility measurements and Mossbauer spectroscopy. The strong spin‐freezing behavior was observed in 80‐Hz, 1‐Oe ac susceptibility at two spin‐freezing temperatures: 284 K (Tf1), which is considered to be caused by a random anisotropy of Co ions coordinated in distorted octahedral sites, and 86 K (Tf2), which might be caused by the frustration of antiferromagnetic interactions due to the amorphous structure. The dc susceptibility showed irreversibility below Tf1 which decreased with increasing applied field. However, a distinct change was not observed at Tf2. The Mossbauer spectrum revealed that some of the paramagnetic Fe ions were frozen below Tf1, with relaxation phenomena, and that the residual paramagnetic ions were frozen at Tf2. The time decay of the thermoremanent magnetization observed be...

Spin-Dependent Electron Tunneling in Ferromagnetic Metal/Insulator/Semiconductor Junctions Using Optical Spin Orientation

We examined the spin-dependent tunneling of photoexcited electrons in FM/Al2O3/p-GaAs (FM=Ni and Co) junctions by using the optical spin orientation of p-GaAs. We have measured the phase-sensitive current modulation which corresponds to the polarization modulation of the excitation light. The relative changes of conductance ΔG/G, with respect to the magnetization reversal of the ferromagnet, were found to be 0.3 and 0.8% for Ni and Co, respectively. From bias-dependence of ΔG/G, we assigned that the observed changes were responsible for the spin-dependent tunneling where the photoexcited electrons in the conduction band of the GaAs enter into the unoccupied s,p-states of the ferromagnet.

Magnetoresistance in FeSi films grown by molecular beam epitaxy

We have used molecular beam epitaxy apparatus to grow Fe-Si films. The films were grown by first depositing a nominal 40 A of Fe, then a nominal thickness of Si, then a further nominal 40 A of Fe. A plot of the ratio (remanent magnetisation/saturation magnetization) versus nominal Si layer thickness shows a minimum for a nominal thickness of 25 A, and a plot of saturation field versus nominal Si thickness has a maximum for a nominal thickness of 20 A. We find a room temperature magnetoresistance of more than 2% in films with nominal Si layer thicknesses of 20 A. Raising the temperature causes a decrease in (remanent magnetization/saturation magnetisation) for films with nominal Si thicknesses of 20 A and 25 A.

Oscillatory interlayer couplings as functions of a ferromagnetic metal layer and a semiconducting spacer in magnetic superlattices

Abstract Fe layer thickness dependence of the interlayer exchange coupling across a Au spacer is investigated for MBE-growth Au/Fe/Au/Fe/Au(001) films involving two step-shaped Fe layers with each step 1 ML high. The coupling strength oscillates as a function of each Fe layer thickness with a 2 ML Fe period, which is attributed to the resonance state involving the Fe majority spin band. The interlayer coupling is also studied for Fe/Si and Fe/FeSi multilayers as functions of semiconducting spacer thickness and temperature.

Spin-Glass Behavior in Co-Ferrite Amorphous Films

Amorphous Co-ferrite thin films not containing vitrifying elements were prepared by ion beam sputter deposition. Their magnetic properties were investigated by means of low-field dc magnetization and ac susceptibility measurements, which gave evidence of spin glass behavior. The temperature dependence of magnetization in a 50 Oe external field exhibited an irreversibility just below room temperature, which decreased with increasing field strength. A cusp in the ac susceptibility (80 Hz, 1 Oe) corresponding to the spin freezing temperature was observed at 284 K. Below this temperature, relaxation of the thermoremanent magnetization was observed. Besides this transition at 284 K, a sharp peak in the ac susceptibility at 86 K, suggestive of another transition, was observed. It is thought that spin glass ordering with a high freezing temperature is caused mainly by transition to an amorphous structure for strong superexchange interactions.

Domain processes in antiferromagnetically-coupled Co9Fe/Cu/Co9Fe thin-film sandwiches

We discuss ways in which domain processes are important in determining the properties of “giant magnetoresistive” thin-film structures. Be describe some Kerr observations of domain processes in Co9Pe/Cu/CO2,Fe sandwiches and apply simple nucleation theory to the problem of switching between ferromagnetically- and antiferromagnetically- aligned states.

Amorphous oxides with ferromagnetic exchange coupling and optical transparency

0.5La1−xSrxMnO3⋅0.5B2O3 (0.2≤x≤1.0) oxides were melt quenched using a twin‐roll technique. The resultant ribbons were amorphous for thicknesses below 30 μm, which was confirmed by x‐ray diffraction, anomalous x‐ray scattering, and transmission electron microscopy. They were characterized by high crystallization temperatures above 920 K and optical transparency. Their Curie temperatures are higher than 380 K over a wide range of Sr concentrations (0.3≤x≤1.0), thus differing from crystalline La1−xSrxMnO3 perovskites. Magnetic susceptibility above the Curie temperature follows the Curie–Weiss law with a positive paramagnetic Curie temperature, suggesting ferromagnetic spin coupling in the present amorphous oxides. The ferromagnetic behavior of the amorphous state may be explained by positive double‐exchange interactions of Mn3+ and Mn4+ ions.