Exchange-Flips and Order-Disorder Transitions in the Two-Dimensional Random Spin System Rb₂Co_xMg_1−xF₄

Abstract

Magnetization measurements of Rb₂Co_xMg_1−xF₄ (x=0.91, 0.82, 0.75, 0.70, 0.60, 0.30), which is a two-dimensional random Ising spin system, have been performed by pulsed high magnetic field up to 500 kOe. The abrupt magnetization changes at 190 kOe and 380 kOe, which are independent of concentration, are due to the exchangeflips of spins with 1 and 2 nearest neighbor Co²⁺ spins, respectively. Another change, whose field depends on the concentration, is ascribed to the order-disorder transition similar to that observed in three-dimensional Fe_xZn_1−xF₂. The temperature dependence of this transition field is also measured for x=0.82 and 0.75 up to the Néel temperature and phase diagrams are obtained. The Néel temperatures determined from this experiment are 68 and 60 K, respectively.