Satoru Oikawa

Magnetoresistance characteristics of grain-type alloy thin films of various compositions

Grain-type alloy thin films consisting of fine ferromagnetic grains, which are composed of Fe, Co, Ni, or their alloys, in non-magnetic Ag, Bi, or Mg matrices were prepared by a vacuum deposition method. Structural, magnetic, and magnetoresistance characteristics were studied on such films as a function of the composition and concentration. A giant magnetoresistance GMR effect was observed over a broad range of magnetic concentrations in the Ag-based systems, except the Ni-Ag one, at room temperature. The appearance of the GMR effect is probably correlated with the Curie temperature of the magnetic component and the atomic radius ratio of the magnetic and non-magnetic components.

Origin of giant magnetoresistance effect in granular thin films

Though the demagnetization process of granular Co‐Ag films became steep by adding permalloy layer, slow saturation in the magnetoresistance (MR) curves was maintained. In addition, the MR characteristics of granular Fe‐Ag films prepared under a magnetic field, in which strong magnetic anisotropy was induced, were isotropic. The disagreement between the MR and magnetic characteristics implies that ferromagnetic granules are not responsible for the giant MR (GMR) effect. The MR ratio of the granular Fe‐Ag films considerably increased at thicknesses less than 20 nm. In such ultrathin films, the features of the MR curves corresponded well with those of the magnetization curves with slow saturation and no hysteresis. These results suggest that the GMR effect in the granular systems is attributable to superparamagnetism.

Magnetic and Magnetoresistive Modification on Granular Co-Ag Films by Soft Magnetic Permalloy Layer.

Double-layered structure which is composed of permalloy and granular Co–Ag thin layers has been fabricated by successive sputtering method. Their magnetic and magnetoresistive (MR) characteristics have been studied as a function of the Co–Ag/permalloy thickness ratio. The coercive force decreases with increasing the permalloy layer thickness, indicating the presence of an interlayer magnetic coupling. However, the magnetic saturation field associated with the MR characteristics is maintained to be extremely high. The MR ratios of some double-layered films are larger than that of the Co–Ag single layer.

Annealing effects on the magnetoresistance characteristics of grain-type alloy thin films

Structural, magnetoresistance (MR), and magnetic characteristics were studied on FeCo-Ag grain-type alloy thin films annealed under various conditions. Internal diffusion and interfacial diffusion of the Ag atoms in the ferromagnetic FeCo grains possessed negative and positive contributions to obtaining a large MR ratio, respectively. The magnetic anisotropy field (Hk) associated with the MR characteristics decreased in accordance with the decrease in resistivity as the annealing temperature and/or annealing time increased. This suggests that the Ruderman-Kittel-Kasuya-Yosida interaction between the ferromagnetic grains dominates the Hk in such systems.

Giant magnetoresistance effect in grain-type alloy thin films

An annealing study suggested that the internal diffusion and interfacial diffusion of Ag atoms in ferromagnetic FeCo grains made negative and positive contributions, respectively, to obtaining a large magnetoresistance (MR) ratio. The magnetic saturation field (Hs) associated with MR characteristics decreased in proportion to the decrease in resistivity as the annealing temperature and/or annealing time increased. The coercive force in double-layered films with constant Co-Ag thickness decreased with increasing permalloy layer thickness. This indicates that the two layers are magnetically coupled. However, the Hs was maintained to be extremely large. The MR ratio in the double-layered films was occasionally larger than that in a Co-Ag single layer.

Thickness Dependence of Giant Magnetoresistance in Granular Materials

Granular Co–Ag films show a steep low-field magnetization process when they are covered by a permalloy layer, but their magnetoresistance (MR) curves show slow saturation. In granular Fe–Ag films prepared under a magnetic field, moreover, strong magnetic anisotropy is induced in the magnetization curves, while the MR curves are isotropic. The disagreement between the MR and magnetization curves suggests that ferromagnetic granules are not responsible for the giant MR (GMR) effect. The MR ratio of the granular Fe–Ag film is much enhanced when the thickness of the film is less than 20 nm. In such ultra-thin films, the MR and magnetization curves show slow saturation and no hysteresis. These results indicate that superparamagnetism causes the GMR effect in granular systems.

Giant Magnetoresistance Effect in Grain-Type Alloy Thin Films: II -Annealing and Double-Layering-

An annealing study suggested that the internal diffusion and interfacial diffusion of Ag atoms in ferromagnetic FeCo grains made negative and positive contributions, respectively, to obtaining a large magnetoresistance (MR) ratio. The magnetic saturation field (H s ) associated with MR characteristics decreased in proportion to the decrease in resistivity as the annealing temperature and/or annealing time increased. The coercive force in double-layered films with constant Co-Ag thickness decreased with increasing permalloy layer thickness. This indicates that the two layers are magnetically coupled. However, the H s was maintained to be extremely large. The MR ratio in the double-layered films was occasionally larger than that in a Co-Ag single layer.