Yoshihisa Obi

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.

Magnetic Domain Structure of an Amorphous Fe-P-C Alloy

The domain structure of an amorphous Fe80P13C7 alloy ribbon produced by the centrifugal solidification technique was investigated using the magnetic powder pattern technique. Two different types of domains (a maze domain and a 180°-domain) were observed on the specimen surface. The relationship between the domain structure and the magnetization process was also investigated. The results showed that some of the 180°-walls, which ran nearly parallel to the long axis of the ribbon, caused the hysteresis in the magnetization curve, while the maze domain was responsible for the difficulty in obtaining the saturation in magnetization. The maze domain arises probably from the uniaxial magnetic anisotropy having the direction of easy magnetization perpendicular to the surface. This anisotropy seems to be caused by the magnetoelastic coupling between positive magnetostriction and internal stress in the specimen.

Soft Ferromagnetic properties of some amorphous alloys

Abstract In rapidly quenched amorphous alloys (Fe 1 − x Co x ) 80 P 13 C 7 and (Fe 1 − x Co x 75 Si 15 B 10 , the zero magnetostrictive alloys are remarkably soft magnetic materials, while the magnetostrictive alloys have rather large coercive forces [1]. To investigate the effect of magnetostriction on the properties of the BH loop, some fundamental magnetic properties of these alloys were studied. The magnetic domain pattern observed by Bitters method shows a periodic domain structure consisting of 180° and maze domain walls in all the magnetostrictive alloys, suggesting the existence of a magnetic anisotropy with an easy axis that varies locally. The estimated value of the anisotropy energy varies with x similarly to the concentration dependence of the magnetostriction. The coercive force also varies with x and is nearly proportional to the ratio of the magnetostriction to magnetization. It is, therefore, considered that the anisotropy is caused by the anisotropic internal stress field, and that the stress fluctuation which inevitably exists is responsible for the large coercive force.

Relationship between Magnetoresistance and Lattice Uncertainty at the Interface in Sputtered Fe/Cr Multilayer Films

We have investigated the relationship between the magnetoresistance and the paracrystalline disorder of lattice plane spacings at the interface in Fe/Cr multilayer films prepared by rf sputtering. The X-ray linewidths for the (110) and (220) peaks were analysed using a modified paracrystalline theory to obtain the grain size and the lattice uncertainty, which represents the degree of paracrystalline disorder. We have found that the magnetoresistance increases with decreasing lattice uncertainty.

On the Magnetically Induced Anisotropy in Amorphous Ferromagnetic Alloys

Compositional dependence of the magnetically induced magnetic anisotropy in the Fe and Co based amorphous alloys has been studied experimentally. The induced anisotropy constant is 30–60 erg/g for Fe78Si10B12 and for Co78Si10B12 but is 400–500 erg/g for (Fe1−xCox)78Si10B12 in 0.25<x<0.8. The addition of elements such as Ni, Pd and Cr into Co78Si10B12 results in the increase in induced anisotropy constant. The directional order of metal-metal pairs is most likely to be responsible for the induced anisotropy.

Advanced experimental study on giant magnetoresistance of Fe/Cr superlattices by rf-sputtering

Abstract The study on MagnetoResistance (MR) has been performed for the Fe/Cr SuperLattice (SL) produced by the rf-sputtering method. Especially the effect of the preparation condition on MR has been investigated in detail. The MR oscillates with respect to the Cr layer thickness (tCr) as was reported by Parkin et al. [1]. The characteristic experimental results is that the MR depends strongly on the Ar pressure. This indicates that the size of the MR is greatly affected by the interface roughness of the SL induced by the different Ar pressure during sputtering.

Oscillation phenomenon of transition temperatures in Nb/Co and V/Co superconductor/ferromagnet multilayers

Abstract Tc oscillation phenomenon of Nb/Co and V/Co superconductor/ferromagnet multilayers has been observed as a function of Co layer thickness dCo. Spontaneous magnetization appeared for dCo≳7 A in both Nb/Co and V/Co systems. Tc exhibited double minimum oscillations with the first local minimum at ∼12 A for Nb/Co and at ∼18 A for V/Co and with the second local minimum at ∼32 A for both systems. The first minimum may be caused by a combined effect of magnetic spin-flip scattering, resonance scattering by Co virtual levels in the interfacial region and thickness dependent exchange field. The second minimum is found to be consistent with the appearance of so-called π-phase.

Magnetic properties of amorphous Co-Mn-B alloys

The effects of B and Mn contents on the magnetic moment and the Curie temperature of Co-Mn-B amorphous alloys have been studied in a wide range of B and Mn concentrations. The magnetic moment at 4.2K decreases monotonically with increasing Mn content for the alloys with low B content, while it increases initially and then decreases similarly to the low B alloys for the alloys with high B content. Ferromagnetic moments vanish at about x=0.4 irrespective to B content. The Curie points also reach zero absolute temperature near the same Mn content. These Mn and B composition dependences of magnetic moment are analysed on the basis of the local environment model.

Effect of Pressure on the Giant Magnetoresistance of Fe/Cr Magnetic Superlattices

The effect of pressure on the electrical resistance and the magnetoresistance of (Fe 30 A/Cr 11 A) 22 superlattices has been investigated at high pressures up to 16 kbar and high magnetic fields up to 50 kOe. It is found that the electrical resistivity ρ decreases with increasing pressure, having a rate of (1/ρ)(∂ρ/∂ P )=-8.7×10 -3 kbar -1 at room temperature, and that the magnetoresistance (MR) ratio Δρ/ρ s at 4.2 K is approximately constant below 8 kbar but decreases rapidly with increasing pressure above 8 kbar. The saturation field H s increases with pressure.

Anomalous thermal expansion of MnN

Abstract The MnN compound is prepared as a single phase by d.c. reactive sputtering. The crystal structure of MnN is tetragonally distorted NaCl type (fct) one. The temperature variation of lattice constants for MnN is measured by X-ray diffraction experiments in the temperature range from 289 to 803 K. It is found that the MnN compound shows anomalous thermal expansion and the crystal structure of MnN has changed from fct structure to fcc one at about 650 K. On the other hand, we found formerly that MnN was an antiferromagnetic compound with a Neel temperature of 650 K. The tetragonal distortion below about 650 K is well explained by the strain dependence of the exchange interaction on the basis of molecular field theory.