Exchange anisotropy-driven noncollinear magnetism and magnetic transitions in MnTiO3 ilmenite
Evidence for multiple magnetic transitions and unconventional spin exchange interactions in the ilmenite insulator MnTiO3 is provided via neutron scattering. On cooling, while G-type antiferromagnetic (AFM) order sets in first at 63 K with a k1 = (000) characteristic wave vector, a weaker second magnetic transition with k2 = (00 3/2 ) appears near 42 K, giving rise to a noncollinear structure. Intrinsic buckling of the honeycomb lattice along c creates bond anisotropy and a distorted crystal field that can lead to exchange paths that modulate orbital overlap and spin-orbit coupling. The inelastic spectrum is best described by magnetic exchange anisotropy that breaks the local symmetry of the honeycomb, with competing AFM Heisenberg, Dzyaloshinskii-Moriya and alternate intra-planar ferromagnetic (FM) interactions, that may yield a weakly-coupled ladder system.