Wolframite-type MnMoO₄: Magnetization, sintering by Cool-SPS, and magnetodielectric effect

Dense ceramic samples of wolframite-type MnMoO4 (w-MnMoO4) have been prepared by Cool-SPS (Spark Plasma Sintering) for the first time with the goal of investigating the magnetic, dielectric, and magneto-electric properties of this thermally fragile polymorph of MnMoO4. Earlier studies found that w-MnMoO4 converts to the structurally different alpha-MnMoO4 phase if heated in air at 873 K, thus hindering fabrication of w-MnMoO4 ceramics by conventional high-temperature sintering. Unsintered powder samples, which were prepared via a hydrothermal method, and dense ceramics elaborated by Cool-SPS have the same magnetic properties. Magnetic susceptibilities display Curie-Weiss behaviour, chi = C/(T-theta), with effective moments for Mn2+ ion mu(eff) approximate to 5.9 mu(B)/Mn-atom and negative Weiss temperatures theta approximate to - 96 to - 98 K. Unlike the isostructural multiferroic compounds MnW1-xMoxO4 (0 <= x < 0.3) which exhibits at least two successive magnetic transitions below 15 K, w-MnMoO4 undergoes only one paramagnetic-to-antiferromagnetic transition at T-N approximate to 32 K. The field-dependent magnetization at 2 K shows a spin-flop transition at mu H-0(SF) approximate to 2 T. Capacitance and dielectric losses obtained for a ceramic between 2 and 300 K in zero field and in a field of 9 T reveal several weak anomalies. Dielectric anomalies are observed at the magnetic phase transition, suggesting a magnetoelectric coupling. Qualitatively different dielectric and magnetodielectric behaviours occur in alpha-MnMoO4 ceramics at low temperature.