Giant magnetic anisotropy in Mn₃O₄ investigated by 55Mn2⁺ and 55Mn3⁺ NMR

In Mn₃O₄, the magnetization along the c axis is different from that along the ab plane even in the strong field of 30 T. To investigate the origin of the huge magnetic anisotropy, Mn2⁺ and Mn3⁺ nuclear magnetic resonance spectra were measured in the 7-T magnetic field. The canting angle of the magnetic moments was estimated for various directions of field by rotating a single-crystalline Mn ₃O₄ sample. One of the main results is that Mn3 ⁺ moments lie nearly in the ab plane in the external field perpendicular to the plane, meaning that the macroscopic magnetic anisotropy of Mn₃O₄ originates from the magnetic anisotropy of Mn3 ⁺ in the ab plane. The anisotropy field is estimated to be about 65 T. It is obvious that the Yafet-Kittel structure made of Mn2⁺ and Mn3⁺ spins lies in the ab plane due to this huge magnetic anisotropy, contrary to the previous reports. By the least-squares fit of the canting angle data for various field directions to a simple model, we obtained that J _BB=1.88J_AB-0.09 meV and K_A=-14.7J _AB+2.0 meV, where J_AB, J_BB, and K_A are the exchange interaction constants between Mn2⁺ moments, Mn2 ⁺ and Mn3⁺ moments, and an anisotropy constant of Mn2 ⁺, respectively.