Control of magnetic properties of NiMn₂O₄ by a microwave magnetic field under air

NiMn₂O₄ prepared by conventional heating was irradiated with a microwave H-field using a single-mode cavity under air and magnetic properties of the microwave-irradiated material were investigated. X-ray diffraction and transmission electron microscopy demonstrated that the phase and microstructure are not affected by H-field irradiation. Measurements of the magnetization as a function of temperature revealed that the antiferromagnetic sublattice disappeared and electron spin resonance showed the existence of Mn²⁺, suggesting that Mn³⁺ is partially reduced. Moreover, the magnetization of NiMn₂O₄ was controlled from 35.3 to 18.2 emu/g and the coercivity from 140 to 750 Oe by changing the sample temperature during microwave irradiation. The reduction reaction of NiMn₂O₄ is controlled by microwave H-field irradiation, resulting in control over the magnetic properties.