Weak ferromagnetism and domain effects in multiferroic LiNbO3-type MnTiO3-II

Abstract

Magnetic order consistent with multiferroism has been observed in the acentric LiNbO${}_{3}$-type, high pressure form II of MnTiO${}_{3}$ using neutron diffraction and magnetization measurements. Spin order below the 28 K magnetic transition has propagation vector (0 0 0), and spins lie in the $ab$ plane. Representation symmetry analysis shows that the antiferromagnetic Mn${}^{2+}$ spin component observed by neutron scattering, of magnitude 3.9(1) \ensuremath{\mu}${}_{B}$ at 2 K, coexists with a weak ferromagnetic component of magnitude 0.0014 \ensuremath{\mu}${}_{B}$. This magnetization is perpendicular to the electrical polarization resulting from cation displacements in this acentric structure, permitting coupled switches of the two ferroic orders. The spin order is stable to fields of at least 5 T; however, facile domain reorientation within the $ab$ plane enhances the antiferromagnetic susceptibility and a constant magnetization/field ($M$/$H$) is observed in field strengths greater than \ensuremath{\sim}1.5 T. Suppression of the {101} neutron intensity with field follows the same Brillouin-dependence as $M$/$H$ and enables the antiferromagnetic easy axes to be identified as parallel to the $ab$ plane hexagonal axes.