Y. Kobayashi

Giant magnetoresistance related transport properties in multilayers and bulk materials (invited)

A systematic comparison of magnetoresistance, Hall effect, thermal conductivity, and thermoelectric power has been made on systems exhibiting giant magnetoresistance (GMR), Co/Cu/Ni(Fe) multilayers, and AgCo granular alloys, for examples. Each property exhibits field dependence characteristic of the GMR and justifies its own merit in characterizing the conduction‐electron scattering responsible for the GMR. The comparison was extended to intermetallic compounds such as REGa2 and RECo2 (RE: rare earth element) which also show a large magnetoresistance.

Transport properties in granular Co-Ag alloys

We have investigated the field dependence of the magnetoresistance, Hall effect, thermal conductivity and thermoelectric power on Co-Ag granular films made by electron gun evaporation under ultrahigh-vacuum conditions. A large field dependence correlated with the giant magnetoresistance is observed in all the transport properties. The dependence is anisotropic for 42at.% Co-Ag between parallel and perpendicular fields. At zero field, a resistivity minimum is found that varies with annealing temperature. The behaviours of the magnetothermal conductivity and the magnetothermoelectric power are generally consistent with previous studies. The extraordinary Hall resistivity exhibits an anomalous field dependence and changes sign with temperature.

The extraordinary Hall effect of Pd/Co multilayers

Abstract The magnetoresistivity and Hall resistivity of Pd/Co multilayers have been investigated down to 4.2 K, focusing on the difference between samples with in-plane and perpendicular magnetization. At 4.2 K, a sign change of the extraordinary Hall coefficient as a function of Co thickness is observed at ∼ 2 A . For a Co layer thickness of 2 A, an anomaly is found in the temperature dependence of both resistivity and Hall coefficient near 250 K, suggesting a magnetic phase transition in an alloyed interface region.

Huge magnetic field-dependent thermal conductivity in magnetic multilayer films

Abstract We have studied the magnetic field dependence of thermal conductivity in Cu/Co/Cu/Ni(Fe) and Cu/Cr/Co/Cr/Cu/ Ni(Fe) multilayer films at 10, 80 and 295 K. The magnitude of magnetoresistance (MR) ratio is close to that of the magneto-thermal resistance (MTR) ratio, which suggests that the conduction electron scattering responsible for the giant MR is mostly of large angle and elastic in nature.

Giant magnetoresistance in MBE-grown Co/Cu multilayers

Abstract We have investigated the dependences of magnetoresistance (MR) in (111)-oriented Co/Cu multilayers on Cu and Co layer thicknesses. As a function of Cu layer thickness, MR ratio shows a maximum at 9 A, while the saturation field decreases monotonously. The saturation field also has a similar dependence on Co layer thickness.

Exchange coupling in Cu(Mn)Co multilayers

Abstract We have studied the effect of 4% Mn-doped in Cu layers on the interlayer exchange coupling in Cu Co multilayers. At room temperature, no essential effect of the doping on the magnetoresistance (MR) properties has been found. Below about 100 K, a drastic increase in the saturation field and the MR ratio has been found for the sample ferromagnetically coupled at room temperature.

The giant magnetoresistance and the anomalous Hall effect in molecular-beam-epitaxy grown Co/Cu superlattices

We have investigated the dependence of the magnetoresistance (MR) and the Hall effect on both the Cu layer thickness dCu and the Co layer thickness dCo in (111)-oriented epitaxial Co/Cu superlattices. The MR ratio has a single peak near 9 AA as a function of dCu while it shows a broader faint peak near 10 AA as a function of dCo. The maximum MR ratio up to 81% was observed at 4.2 K for dCo=9.0 AA and dCu=9.3 AA. For both dCu and dCo dependence, the saturation field increases monotonically with decreasing layer thickness. As a function of dCo, the anisotropy field changes its sign from in plane for large dCo to perpendicular near dCo=9 AA. The extraordinary part of Hall resistivity shows a single peak against magnetic field for samples with a large MR ratio. The field dependence is explained in terms of the field dependence of both the skew and the side-jump scattering components associated with the giant magnetoresistance. The ratio of the two components is almost unchanged between zero field and saturation field, which suggests that the main scattering mechanisms at both fields are microscopically the same.

Anomalous Hall effect in magnetic multilayers

Abstract We have studied the field dependence of the Hall resistivity in Fe/Cr multilayer films at 4.2 K. The observed anomalous Hall resistivity is composed of both negative skew-scattering and positive side-jump components. The conduction electron scattering responsible for the giant magnetoresistance is accompanied by large left-right asymmetry.

Transport properties of Co/Cu(Mn) multilayers

Abstract Measurements of the electrical resistivity, Hall effect and thermoelectric power on Co/Cu(Mn) multilayers show anomalies around a critical temperature that appears to be associated with a magnetic phase transition due to a cooperative interaction among the Co layers and the Mn spins.

Electronic specific heat correlated with giant magnetoresistance in UNiGa

To investigate the origin of giant magnetoresistance in UNiGa, we have measured the field dependence of specific heat in the field B//c-axis. The electronic specific heat coefficient γ, being 48 mJ/K2mole in high-field ferromagnetic (F) phase, decreases by 10% in low-field antiferromagnetic (AF) phase. This is the first direct evidence of a superzone-gap formation suggesting that the large resistivity in AF phase is caused at least partly by the gap. The nuclear specific heat mainly due to Ga nuclear hyperfine field is observed.