H. Fujimori

Tunnel-type GMR in metal-nonmetal granular alloy thin films

Abstract Previously, a tunnel type of giant magnetoresistance (GMR) was discovered in a sputtered Co-base CoAlO granular alloy thin film, which was characteristic in exhibiting GMR of about 8% accompanying a large specific electrical resistivity of the order of 10 4 μΩ cm, due to a weak tunnel conductance in the metal-nonmetal granular structure of this alloy. In this paper, it has been found that GMR changes depending on O content and appears with a maximum near the percolation threshold point from metallic conductance to tunnel conductance in CoAlO granular alloy thin films. In addition, superparamagnetic behavior in the magnetic field and temperature dependencies of magnetization have been found for the GMR alloy films. The observations support the spin-dependent tunneling effect for this new GMR.

High‐frequency magnetic properties in metal–nonmetal granular films (invited)

The structure and properties of Co–N and Co–O based films, prepared by rf magnetron reactive sputtering using nitrogen or oxygen and argon gases, have been studied. Transmission electron microscopy (TEM) observation reveals that each Co–(Al or Si)–(N or O) film is a typical film with granular structure, with grain size less than 5 nm. It is found by micro‐focused energy‐dispersive x‐ray and electron energy loss spectroscopy analysis that the grains are mainly composed of Co and the intergranular regions are ceramics of N or O. In Co–N based films, soft magnetic properties are found in both Si and Al containing films over a wide range of film preparation conditions and compositions. Only the films with Al show soft magnetic properties in Co–O based films, which have ρ of 500–1000 μΩ cm, Hk of about 80 Oe and Bs of about 10 kG. By adding about 10 at. % Pd, the soft magnetic properties and Hk of Co–O based films are significantly improved, with Hk more than 180 Oe. These films exhibit a remarkable constant f...

High resistive nanocrystalline Fe-M-O (M=Hf, Zr, rare-earth metals) soft magnetic films for high-frequency applications (invited)

Microstructure, soft magnetic properties, and applications of high resistive Fe-M-O (M=Hf, Zr, rare-earth metals) were studied. The Fe-M-O films are composed of bcc nanograins and amorphous phases with larger amounts of M and O elements which chemically combine each other. Consequently, the amorphous phases have high electrical resistivity. The compositional dependence of magnetic properties, electrical resistivity, and structure have been almost clarified. For example, the high magnetization of 1.3 T, high permeability of 1400 at 100 MHz and the high electrical resistivity of 4.1 μΩ m are simultaneously obtained for as-deposited Fe62Hf11O27 nanostructured film fabricated by rf reactive sputtering in a static magnetic field. Furthermore, Co addition to Fe-M-O films improves the frequency characteristics mainly by the increase in the crystalline anisotropy of the nanograins. The Co44.3Fe19.1Hf14.5O22.1 film exhibits the quality factor (Q=μ′/μ′′) of 61 and the μ′ of 170 at 100 MHz as well as the high Is of ...

Spin-dependent tunneling phenomena in insulating granular systems

Abstract We studied giant magnetoresistance (GMR) and the related properties in Co-Al-O insulating granular films prepared by reactive sputtering. Microstructural observation revealed that the films were composed of Co base metallic granules and Al 2 O 3 base insulating network-like boundaries. The GMR varied as a function of oxygen content (or Co/Al 2 O 3 ratio) and it took a maximum of 8% at room temperature and 17% at 4.2 K for a Co 52 Al 20 O 28 film, accompanying huge specific electrical resistivities of the order of 10 5 –10 6 μΩ cm. It has been considered that the observed GMRs are attributed to a spin-dependent tunneling and that the spin diffusion of electrons during tunneling plays an important role of the composition dependence of GMR. In addition, it has been noted that the temperature dependence of GMR is smaller than the T −1 dependence predicted theoretically by Helman et al. but rather close to the theory by Inoue et al.

Artificial fabrication of an L10‐type ordered FeAu alloy by alternate monatomic deposition

An FeAu ordered alloy has been fabricated artificially by depositing alternately monatomic layers of Fe and Au. Neither intermediate phase nor intermetallic compound is known for the Fe–Au system in the equilibrium state. Nevertheless, the x‐ray diffraction for prepared FeAu films shows definite superlattice lines corresponding to the L10 ordered structure, and it is metastable at room temperature. The obtained FeAu ordered alloy is ferromagnetic, and it has the Fe moment of 2.5±0.3 μB, a considerably high Curie temperature over room temperature, and a large uniaxial magnetic anisotropy perpendicular to Fe and Au atomic planes.

Microstructure of Co–Al–O granular thin films

We have investigated the microstructures of Co–Al–O granular thin films, which were prepared by the sputter-deposition technique with various oxygen partial pressures. The constituent phases, grain sizes of granular particles, and width of insulating channels have been evaluated quantitatively. The specimen with the optimum giant magnetoresistance (GMR) is composed of nanoscale Co particles, and these are completely isolated by amorphous aluminum oxide channels. The GMR behavior observed in the Co–Al–O films has a close correlation with the width of the insulating channel and the grain size of the magnetic particles, which is consistent with the spin-dependent tunneling conduction mechanism of GMR

Crystal structure and magnetic properties of the compound CoN

Abstract The compound CoN was prepared as a single phase by the d.c. reactive sputtering. Its crystal structure determined by X-ray diffraction measurements is the zinc blende type f.c.c. structure. The compound CoN is stable up to 504 K and decomposes into CoN+δ-Co 2 N above 509 K. This compound exhibits a Pauli paramagnetic character.

Co-Zr-O nano-granular thin films with improved high frequency soft magnetic properties

A study was made of the properties and structure of Co-Zr-O nano-granular films prepared by rf reactive sputtering in an atmosphere of argon and oxygen gases. We found soft magnetic Co-Zr-O films in a wide compositional range near Co-ZrO/sub 2/ and investigated systematically the compositional dependence of their magnetic properties and electrical resistivity. The films near Co/sub 60/Zr/sub 10/O/sub 30/ simultaneously have anisotropy fields more than 150 Oe, coercivities less than 3 Oe, saturation magnetizations more than 9 kG, and electrical resistivities more than 1000 /spl mu//spl Omega/cm. The frequency response of permeability of these films is excellent. And they show a high resonance frequency exceeding 3 GHz.

Crystal structure and magnetic properties of the compound FeN

Abstract The compound FeN was prepared as a single phase by d.c. reactive sputtering. Its crystal structure was determined by X-ray diffraction measurements to be the zinc blende type f.c.c. structure. This compound FeN is stable up to 593 K and decomposes into FeN+Fe2N above 593 K. The present sample exhibits a micromagnetic character at low temperatures, which is considered to be related to the antiferromagnetism of the FeN compound.

Crystal structure and magnetic properties of the compound MnN

Abstract The compound MnN was prepared as a single phase by DC reactive sputtering. Its crystal structure is determined to be face-centered tetragonal one with the NaCl type by X-ray diffraction measurements. The compound MnN is stable up to 753 K and decomposes to tetragonal Mn3N2 at 758 K. This compound exhibits a antiferromagnetism.