Magnetic anisotropy, magnetoelastic coupling, and the magnetic phase diagram of Ni_0.25Mn_0.75TiO₃

Thermodynamic and magnetic studies on high-quality single crystals are used to investigate the magnetic phase diagram and magnetostructural coupling in the mixed-spin system Ni_0.25Mn_0.75TiO₃. Clear anomalies in the thermal expansion at the spin ordering and spin reorientation temperatures, T_N and T_R, evidence pronounced magnetoelastic effects. The magnetic entropy is released mainly above T_N implying considerable short range magnetic order up to about 4 × T_N . This is associated with a large regime of negative thermal expansion of the c axis. Both T_N and T_R exhibit the same sign of uniaxial pressure dependence, which is positive (negative) for pressure applied along the b (c) axis. The magnetic phase diagrams are constructed and the uniaxial pressure dependencies of the ordering phenomena are determined. For magnetic fields B II b axis, a sign change and splitting of anomalies implies further magnetic phases. In addition to short-range magnetic order well above T_N, competing anisotropies yield a glasslike behavior as evidenced by a maximum in AC-χ (T_SG ≃ 3.7 K) and quasilinear temperature dependence of c_p. High-field magnetization up to 50 T demonstrates that in addition to antiferromagnetically ordered spins there are also only weakly coupled moments at 2 K with a sizable amount of about 15% of all Mn²⁺ spins present in the material. The observed changes in the pressure dependence and the magnetostrictive effects shed light on the recently observed flop of electric polarization from P II c to P II a [Phys. Rev. B 90, 144429 (2014)], in particular, suggesting that the magnetoelectric effect is not directly related to magnetostriction.