T. Kakeshita

The influence of grain boundaries on the magnetoresistance in La0.7A0.3MnO3 (A = Ca, Sr) and La1.36Sr1.64Mn2O7

The influence of grain boundaries on the magnetoresistance in the perovskite manganites La 0.7 A 0.3 MnO 3 (A = Ca, Sr) and the layered perovskite manganites La 1.36 Sr 1.64 Mn 2 O 7 has been investigated by means of ac and dc magnetic susceptibility and magnetoresistance measurements. The following characteristics were found. (i) The magnetoresistance (MR) of single-crystal La 0.7 Ca 0.3 MnO 3 is almost independent of the magnetic field at low temperatures, and the MR of polycrystalline La 0.7 A 0.3 MnO 3 (A = Ca, Sr) and La 1.36 Sr 1.64 Mn 2 O 7 decreases drastically with the initial increment of the magnetic field up to 2.5 x 10 -1 T. (ii) The resistivity and the absolute value of the MR of polycrystalline La 0.7 Sr 0.3 MnO 3 increase with decreasing grain size at 4.2 K. (iii) The resistivity of these polycrystalline manganites shows a time-dependent nature, but the single-crystal form does not.

Magnetic field dependence of γ-α equilibrium temperature in Fe-Co alloys

We have investigated effect of magnetic field on γ(austenite)α(ferrite) equilibrium temperature T0 in Fe-xCo alloys with x = 0, 10, 20, 30 mol%. The α-phase at T0 is paramagnetism for x = 0 and 10, while it is ferromagnetism for x = 20 and 30. We have found that T0 increases almost proportional to magnetic field for the Fe-20Co and Fe-30Co alloys, while it does almost proportional to square of magnetic field for pure Fe and the Fe-10Co alloy. The increase in T0by applying magnetic field of 10 T is about 7 K for pure Fe, 18 K for the Fe- 10Co alloy, 24 K for the Fe-20Co alloy and 20 K for the Fe-30Co alloy. The results are discussed on the basis of the Clausius-Clapeyron equation.

Incommensurate structures of intermediate phase and martensite phase in Ni2MnGa

Neutron diffraction measurements using single crystal and powder synchrotron X-ray diffraction measurements revealed that an intermediate (I-) phase and a martensite (M-) phase in Ni2MnGa have incommensurate modulated structures. The modulation vectors of the I- and M-phases are nearly equal to q = 0.341 at 210 K and q = 0.427 at 100 K, respectively. Moreover, displacements of Ni, Mn and Ga atoms in both the I- and M-phase are expressed by sinusoidal waves with the same phase and almost the same amplitudes.