Anisotropy in the magnetic interaction and lattice-orbital coupling of single crystal Ni₃TeO₆

This investigation reports on anisotropy in the magnetic interaction, lattice-orbital coupling and degree of phonon softening in single crystal Ni₃TeO₆ (NTO) using temperature- and polarization-dependent X-ray absorption spectroscopic techniques. The magnetic field-cooled and zero-field-cooled measurements and temperature-dependent Ni L_3,2-edge X-ray magnetic circular dichroism spectra of NTO reveal a weak Ni-Ni ferromagnetic interaction close to ~60 K (T_SO: temperature of the onset of spin ordering) with a net alignment of Ni spins (the uncompensated components of the Ni moments) along the crystallographic c-axis, which is absent from the ab-plane. Below the Néel temperature, T_N~ 52 K, NTO is stable in the antiferromagnetic state with its spin axis parallel to the c-axis. The Ni L_3,2-edge X-ray linear dichroism results indicate that above T_SO, the Ni 3d e_g electrons preferentially occupy the out-of-plane 3d_3z²_-r² orbitals and switch to the in-plane 3d_x²_-y² orbitals below T_SO. The inherent distortion of the NiO₆ octahedra and anisotropic nearest-neighbor Ni-O bond lengths between the c-axis and the ab-plane of NTO, followed by anomalous Debye-Waller factors and orbital-lattice in conjunction with spin-phonon couplings, stabilize the occupied out-of-plane (3d_3z²_-r²) and in-plane (3d_x²_-y²) Ni e_g orbitals above and below T_SO, respectively.