Magnetic ordering and tunable structural phase transition in the chromic compound CuMoO₄

We report the first observation of long-range magnetic order in the chromic compound CuMoO₄ at 1.75 K by means of a specific heat measurement in zero magnetic field. Magnetization measurements performed up to 57 T at 4.2 K indicate a plateau at 1/3 of the saturated magnetization consistent with a simple magnetic model of two non-interacting Cu²⁺ spins and two isolated antiferromagnetic dimers (J=k_B = 26 K). A large temperature-hysteresis in the magnetic susceptibility is observed to originate from the structural phase transition and to be closely related to chromism between α-CuMoO₄ (green) and γ-CuMoO₄ (brownish-red). This discontinuous phase transition is tunable using substitutional effects in Cu_1-xZn_xMoO₄ (0 ≥ × ≤ 0.1) and CuMo_1-yW_yO₄ (0 ≤y ≤0.1) over a wide range of temperatures.