Magnetic Susceptibility and Thermal Expansion of MnSe₂ with Pyrite Structure

Abstract

The magnetic susceptibility of MnSe₂ with pyrite structure was measured from 4.2 K to 473 K. The susceptibility changes discontinuously at the Néel point T_N(47.4 K) and its change δχ is −0.016 in terms of the maximum. The linear thermal expansion of the polycrystalline sample was measured from 20 K to 280 K by a strain gauge method. MnSe₂ contracts discontinuously at T_N as it is cooled, which indicates a first order phase transition. The change ΔL(T_N)⁄L₂₇₃ is −6.5×10⁻⁵. On the basis of the assumptions that exchange interactions vary as −nth power of inter-ionic distance and that the discontinuous change of the susceptibility is due only to the isotropic volume contraction, n is calculated to be 246 from δχ and ΔL(T_N)⁄L₂₇₃ by molecular field theory. From this extraordinary large value of n, it is suggested that an anisotropic lattice distortion occurs below T_N in MnSe₂.