Papers by Author: Tomoyuki Terai

Abstract: Magnetic structure in a layered perovskite manganite, La_2-2xSr_1+2xMn₂O₇ (x = 0.307) has been investigated by neutron diffraction measurements. We found that the ground state (at 4 K) has a ferromagnetic structure with magnetic moment of Mn ions being aligned in a direction inclined by 10 degree from the c-axis. The magnetic structure changes to a canted antiferromagnetic one (CAFM-I) at about 20 K and then to another canted antiferromagnetic one (CAFM-II) at about 80 K. Here the magnetic moment lies in the ab-plane in CAFM-II but not in CAFM-I. The magnetic structure then changes to an antiferromagnetic one with magnetic moment lies in the ab-plane at about 90 K, and then to a paramagnetic one at about 100 K.

Abstract: The ferromagnetic shape memory alloy Ni2MnGa exhibits a successive martensitic transformation from the L21-type structure to the so-called intermediate phase and then to the martensite phase with an incommensurate structure during cooling under zero stress. In addition to these phases, a new phase, which we call the X-phase, appears when Ni2MnGa is cooled under compressive stress applied in the [001] direction. In this paper, we discuss the structural relations between the X-phase and the other phases on the basis of experimental results of compressive tests, transmission electron microscope observations and neutron diffraction patterns. It is likely that a multicritical point exists in the stress-temperature phase diagram of Ni2MnGa.

Abstract: We have investigated the effect of particle size on martensitic transformation by using single crystalline specimens of an Fe-30at%Ni alloy by scanning electron microscope (SEM) observation, X-ray diffraction(XRD), magnetic susceptibility and magnetization measurements. As a result, we have found that an athermal martensitic transformation changes to an isothermal martensitic transformation with decreasing particle size. Considering the result of the TTT diagram, we have estimated the driving force for the martensitic transformation and the cluster size for nucleation of the isothermal martensitic transformation based on the model previously proposed.

Abstract: We have investigated stress and temperature dependences of the structure of the X-phase in Ni2MnGa to understand structural relation between the X-phase and other phases. Position and intensity of satellites of the X-phase are different from those of the intermediate (I-) phase under compressive stress, but they approach those of the I-phase with decreasing stress. That is, the structure change associated with the I → X transformation is discontinuous under a compressive stress, while it is continuous under zero stress. In addition, the transformation from the X-phase to the L21-type parent phase is continuous regardless of applied stress. These results strongly suggest the existence of multi-critical point in Ni2MnGa. On the other hand, the transformation from the X-phase to the martensite phase is discontinuous regardless of applied stress.

Abstract: We have made crystallographic analysis for microstructures formed by charge ordering transitions in Pr0.55Ca0.45MnO3 and Nd0.5Sr0.5MnO3. From the results, it was found that all interfaces between crystallographic domains of Pr0.55Ca0.45MnO3 are {101}CO ( CO represents a charge ordered phase) twinning planes and that most of interfaces between crystallographic domains of Nd0.5Sr0.5MnO3 are {112}CO twinning planes. These twinning planes correspond to {110} planes in simple cubic perovskite structure.

Abstract: We have measured the magnetization as a function of temperature and magnetic field in layered perovskite manganites of La2-2xSr1+2xMn2O7 single crystals (x=0.313, 0.315, 0.320, 0.350) in order to know their magnetic structures. All the present manganites exhibit magnetic transitions from ferromagnetic to paramagnetic at 76K, 107K, 120K and 125K for x=0.313, 0.315, 0.320 and 0.350, respectively. For x=0.350 and 0.320, the magnetic structure is a planar ferromagnetism whose easy axis is in the ab-plane at all temperatures below the Curie temperature. On the other hand, for x=0.315 and 0.313, the magnetic structure is an uniaxial ferromagnetism whose easy axis is along the c-axis below 85K and 66K, respectively, and a planar ferromagnetism above the temperature. From the results described above, we made the detailed magnetic phase diagram of layered perovskite manganite La2-2xSr1+2xMn2O7 (0.313≤x≤0.350).