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 Mn3O4 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. DOI: 10.1103/PhysRevB.86.224420