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 ${\mathrm{Ni}}_{0.25}{\mathrm{Mn}}_{0.75}{\mathrm{TiO}}_3$. 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 $4\times T_N$. 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 $b$ $\left(c\right)$ axis. The magnetic phase diagrams are constructed and the uniaxial pressure dependencies of the ordering phenomena are determined. For magnetic fields $B\parallel b$ 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-$\chi$ $(T_{\mathrm{SG}}\simeq 3.7 \mathrm{K})$ and quasilinear temperature dependence of $c_{\mathrm{p}}$. 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 ${\mathrm{Mn}}^{2+}$ 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 $P\parallel c$ to $P\parallel a$ [Phys. Rev. B 90, 144429 (2014)], in particular, suggesting that the magnetoelectric effect is not directly related to magnetostriction.

• Received 3 March 2023
• Accepted 1 June 2023

DOI:https://doi.org/10.1103/PhysRevB.108.014406