Papers by Author: H. Nishihara

Abstract: X-ray powder diffraction, permeability, magnetization and differential scanning calorimetry measurements were carried out on the magnetic shape memory alloys Ni2MnGa1−xCux (0 ≤ x ≤ 0.25). On the basis of the experimental results, the phase diagram in the temperature– concentration plane was determined for this alloy system. The determined phase diagram is spanned by the paramagnetic austenite phase (Para-A), paramagnetic martensite phase (Para-M), ferromagnetic austenite phase (Ferro-A), ferromagnetic martensite phase (Ferro-M) and the premartensite phase. It was found that the magnetostructural transition between the phases Para-A and Ferro-M can occur in the concentration region 0.12 < x ≤ 0.14 and that Ni2MnGa1−xCux has the characteristics of the phase diagram similar to those of the phase diagrams of Ni2+xMn1−xGa and Ni2Mn1−xCuxGa. In order to understand the phase diagram, the phenomenological free energy as a function of the martensitic distortion and magnetization was constructed and analyzed.

Abstract: The effect of mechanical stress on Barkhausen noise from 45-permalloy plate is reported for as-rolled plate and plates which are annealed at various temperatures up to 1000 K. The data are compared with data of magnetic permeability, coercive force and Vickers hardness. The Barkhausen noise energy detected has been found to increase linearly with nominal stress. The larger the annealing temperature, the less the rate of increase was observed. On the other hand, the magnetic permeability has been found to decrease linearly with nominal stress in a small stress region. Overall features are explained qualitatively by a model in which tensile stress makes the number of effective pinning sites which have appropriate activation energies for discontinuous motion of domain wall more, while annealing process makes just the number of pinning sites such as defects and grain boundaries less.

Abstract: X-ray powder diffraction and magnetization measurements were done on the magnetic shape memory alloys Ni2Mn1+xSn1-x. The alloys with 0≤x≾0.4 crystallize in the cubic L21 structure and exhibit the ferromagnetic behavior. X-ray diffraction patterns indicate that the excess Mn atoms occupy the Sn sites. Furthermore, magnetization measurements make clear that the Mn atoms, which substitute for Sn sites, are coupled antiferromagnetically to the ferromagnetic manganese sublattices. The alloys with 0.4≾x≤0.6 undergo a martensitic transition from the high temperature L21 structure to the orthorhombic 4O one. These alloys show a variety of magnetic transitions. A magnetic phase diagram of Ni2Mn1+xSn1-x system is discussed qualitatively on the basis of the interatomic dependence of the exchange interactions.