Shigehiro Ohnuma

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...

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.

MAGNETOSTRICTION AND SOFT MAGNETIC PROPERTIES OF (CO1-XFEX)-AL-O GRANULAR FILMS WITH HIGH ELECTRICAL RESISTIVITY

We have investigated the magnetic properties of (Co1−xFex)–Al–O granular thin films prepared by a reactive sputtering method in an atmosphere of O2+Ar. The coercivity and electrical resistivity of the films are almost independent of x, but with increasing x the anisotropy field decreases linearly and the saturation induction has a broad maximum of 20 kG around x=0.7. The granular films show the same general tendency as bulk Co–Fe alloys in the compositional dependence of magnetostriction (λ). The magnitude of λ varies with increasing annealing temperature. Using a film with x=0.08 as the core of a spiral-wound inductor gave a quality factor Q=25 at a frequency of 300 MHz.

Invar characteristics of amorphous (Fe,Co and Ni)-Zr alloys

A detailed investigation on the thermal expansion, magnetic properties and electrical resistance of rapidly quenched amorphous (Fe,Co,Ni)-Zr alloys has been undertaken. Magnetic properties of these alloys are reported separately in this conference. The thermal expansion curves exhibit typical invar characteristics below the Curie temperature due to positive spontaneous volume magnetostriction us similar to those for Fe-Ni crystalline invar alloys. The reduced magnetization decreases strongly with increasing temperature being analogous to the case of other amorphous invar alloys. Moreover, w s at 0 K and electrical resistance at room temperature and Liq. N 2 temperature increase in the invar region. Amorphous ternary alloys of (Fe,Co, Ni) 90 Zr 10 with about 72 at% Fe have the invar characteristics in a wide temperature range and in this composition range, the volume magnetostriction and electrical resistance show maximum values of about 1.5 × 10-2and 160 μΩcm respectively.

Microstructures and magnetic properties of Co–Al–O granular thin films

The microstructures of Co–Al–O thin films of wide varieties of compositions are studied by transmission electron microscopy and small angle x-ray scattering (SAXS). In the superparamagnetic specimens, high resolution electron microscope images reveal that isolated spherical Co particles are surrounded by an amorphous aluminum oxide matrix. However, in the soft ferromagnetic films, the shape of the Co particles is prolate ellipsoidal. SAXS intensities from the soft magnetic specimens decrease inversely with the wave vector, q, in a low wave-vector region, while an interparticle interference peak is observed for the superparamagnetic specimens. The scattering profiles of the soft magnetic films imply that the Co particles have a cylindrical shape and are randomly oriented. The correlation between the magnetic properties and the microstructures is discussed.

Tunnel-type GMR in CoAlO insulated granular system — Its oxygen-concentration dependence

Abstract CoAlO thin films, with network-like nano granular structure consisting of Co based magnetic and metallic grains and A1 2 O 3 based nonmetallic intergrains, were fabricated by Ar + O 2 reactive sputtering. Giant magnetoresistances (GMR) were observed. From the experiments on oxygen concentration dependence of GMR, electrical resistivity and microstructure, it was confirmed that spin dependent tunneling was responsible for the GMRs.

Characteristics of a thin film microtransformer with circular spiral coils

A thin-film 3.14-mm*3.88-mm microtransformer has been fabricated, using photolithography technology, to support high-frequency switching regulators. The microtransformer is composed of sputtered amorphous magnetic layers, electroplated copper windings with 20- mu m width, and insulator layers. The coupling factor between the primary and secondary windings of the microtransformer is 0.97 for f=3 MHz. An input-output voltage ratio of 0.95 at 15 MHz is obtained at no-load. Observation of the domain images under high-frequency excitation indicates that the magnetic anisotropy of the microtransformer films is induced by rotating magnetic field annealing so as to increase the output voltage. >

Magnetic properties of (Fe, Co and Ni) - Zr amorphous alloys

Magnetic properties of melt-quenched amorphous ribbons having the general composition TM 100-x Zr x (TM= Fe,Co and/or Ni) have been investigated. Composition dependence of Curie temperature, Tc and magnetic moment, n B in Co 100-x Zr x and Ni 100-x Zr x alloys is similar to Co- and Ni-metalloid amorphous alloys. However, the decrement of Zr content in the Fe 100-x Zr x alloys causes both Tc and n B to decrease, and n B extrapolates at x=0 to almost 0 μ B . Such an unusual behavior of Tc and n B is also found on the Fe-rich portion of (Fe-Co-Ni) 90 - Zr 10 ternary system and Tc and n B of the Fe 90 Zr 10 alloy are 243 K and 1.29 μ B /Fe atom, respectively. For the Co 90 Zr 10 alloy the value of longitudinal linear magnetostriction λ is +3 × 10-6, and the zero-magnetostrictive composition lies near (Co .88 Ni .12 ) 90 Zr 10 and shifts to Fe-rich side with increasing Ni content.

(Fe-Co)-(Mg-fluoride) insulating nanogranular system with enhanced tunnel-type giant magnetoresistance

Insulating nanogranular-type tunnel magnetoresistive thin films made of (Fe or Fe–Co)–(Mg-fluoride) have been investigated. The films were prepared by a tandem deposition method, using Fe, Co, or Fe+Co metal and MgF2 insulator targets. The granular structure was found to consist of Fe or Fe–Co based nanogranules surrounded by thin intergranules of Mg based fluoride with the MgF2 crystal structure. A magnetoresistance value of 13.3% at room temperature and 10 kOe, the largest values ever reported, were obtained at the compositions of 32 vol %(Fe0.51Co0.49)–(Mg–F). To increase the magnetic field sensitivity of the magnetoresistance, a granular-in-gap film consisting of an (Fe–Co)–(Mg–F) granular thin film filling a narrow gap in a soft magnetic Permalloy thin film was prepared. A remarkably high magnetoresistance of 4% or more at 1–2 Oe was obtained.