Antiparallel state, compensation point, and magnetic phase diagram of Fe3O4/Mn3O4 superlattices

The magnetic response of a ferrimagnetic/ferrimagnetic superlattice, [Fe₃O₄ (20 Å)/Mn₃O₄ (80 Å)]_x20, is measured as a function of external field (-50-50 kOe) and temperature (5-300 K). A compensation point (T_cp) is identified ~33 K at which the remanence changes sign and low-field M-T curves show minima, indicating that the net moments of Fe₃O₄ and Mn₃O₄ are antiparallel. At temperatures >~50 K, the magnetic response becomes pure Fe₃O₄-like. Detailed M-H curves (at T<50 K) further exhibit magnetic phase transition at higher external field. As H is above H^*~10 kOe, the magnetization is enhanced and then saturates at H~40 kOe. These phases are similar to the twisted phases, originating from a competition between Zeeman and exchange energies, previously observed in antiparallel metallic multilayers. A H-T magnetic phase diagram of the present superlattice is presented and five phases are included: the Mn₃O₄-aligned, Fe₃O₄-aligned, twisted, ferrimagnetic saturated, and pure Fe₃O₄ phases. M-H curves also show asymmetry at temperatures below and above T_cp, which is probably related to the anisotropy effect of Mn₃O₄ (tetragonal structure). The fundamental mechanism of the antiparallel coupling between Mn₃O₄ and Fe₃O₄ is also discussed.