Magnetostriction and magnetostructural domains in CoTiO$_3$

We report the magnetostrictive length changes of CoTiO$_3$ studied by means of high-resolution dilatometry in magnetic fields ($B$) up to 15~T. In the long-range antiferromagnetically ordered phase below $T_N$ = 38~K, the easy-plane type spin structure undergoes a spin-reorientation transition in the $ab$ plane in magnetic fields $B || ab \approx 2$~T. We observe pronounced length changes driven by external magnetic field in this field region indicating significant magnetoelastic coupling in CoTiO$_3$. Specifically, we observe anisotropic deformation of the lattice for fields applied in the $ab$ plane. While, for $B \lesssim 2$~T, in-plane magnetostriction shows that the lattice expands (contracts) parallel (perpendicular) to the field direction, the opposite behaviour appear at higher fields. Furthermore, there are remarkable effects of slight changes in the applied uniaxial pressure on the magnetostrictive response of CoTiO$_3$ persisting to temperatures well above $T_N$. The data evidence the presence of magnetic domains below $T_N$ as well as of structural ones in CoTiO$_3$. The presence of magnetic domains in the spin ordered phase is further evidenced by an additional 3-fold magnetic anisotropy appearing below $T_N$. We discuss the effects of rotational magnetic domains on isothermal magnetization and magnetostriction and interpret our results on the basis of a multi-domain phenomenological model.