Critical Neutron Scattering Study of the Compositional Phase
Transition in Mg-Doped CuGeO₃

Abstract

Cu_1-xMg_xGeO₃ undergoes a first-order phase transition at a critical concentration x_c between an antiferromagnetic (AF) state on dimerized lattice (D-AF) and an AF Néel state on undistorted uniform lattice (U-AF). Previous magnetic susceptibility measurements showed x_c = 0.023 while a recent neutron scattering study reported x_c = 0.027 ± 0.001. The present critical scattering due to antiferromagnetic fluctuations near the superlattice reflection (0, 1, 1/2) unambiguously determines x_c = 0.028 ± 0.001 at T_N = 3.4 ∼ 4K. Also, at T = 1.3K, the phase boundary was determined to be x_c = 0.028 ± 0.001 by observation of a jump of an effective magnetic moment across x_c.