Magnetovolume effects and spin fluctuations in Mn_3+xRh_1-x alloys

The electrical resistivity, thermal properties, pressure effect on the Néel temperature and compressibility of Mn_3+xRh_1-x (x = 0.0, 0.12, 0.20 and 0.32) alloys have been investigated in order to discuss the magnetovolume effects in both ordered and disordered states. The electronic specific heat coefficient γ^exp of the Mn₃Rh ordered alloy is slightly smaller than that of the disordered one, qualitatively consistent with the theoretical calculations. A negative magnetovolume effect is observed below the Néel temperature T_N in both the ordered and disordered alloys. Furthermore, the magnetic contribution to the thermal expansion is significant even in the paramagnetic region. That is, the thermal expansion coefficient α in the paramagnetic region shows a large value of about 30 × 10^-6 K^-1, because of a significant contribution from spin fluctuations. For the Mn₃Rh ordered alloy, the pressure shift of the Néel temperature, dT_N/dP, is estimated to be about 7.0 K GPa^-1. The temperature dependence of α exhibits a pronounced positive peak at T_N in accordance with the positive pressure dependence of T_N. A large compressibility of 1.4 × 10^-2 GPa^-1 has been obtained from the change in the lattice constant under pressure.