High temperature dielectric relaxation behaviors of single phase Mn₃O₄ polycrystalline ceramics prepared by spark plasma sintering technology have been studied. Two dielectric relaxations were observed in the temperature range of 200 K–330 K and in the frequency range of 20 Hz–10 MHz. The lower temperature relaxation is a type of thermally activated relaxation process, which mainly results from the hopping of oxygen vacancies based on the activation energy analysis. There is another abnormal dielectric phenomenon that is different from the conventional thermally activated behavior and is related to a positive temperature coefficient of resistance (PTCR) effect in the temperature region. In line with the impedance analyses, we distinguished the contributions of grains and grain boundaries. A comparison of the frequency-dependent spectra of the imaginary impedance with imaginary electric modulus suggests that both the long range conduction and the localized conduction are responsible for the dielectric relaxations in the Mn₃O₄ polycrystalline samples.

Negative magnetization, dielectric and magnetodielectric properties of single phase orthorhombic perovskite EuCrO3 were investigated. The magnetization-temperature curve after zero-field-cooling process displays that an antiferromagnetic transition occurs at TN ~180 K and there exists negative magnetization below TN. Detailed experiments reveal that the negative magnetization is caused by the grain interaction and the negative trapped field. The dielectric curve exhibits two dielectric loss peaks. One is related to oxygen vacancies, and the other is associated with a thermal activation relaxation caused by space charge with an activation energy Ea ~0.322 eV. And an obvious magnetodielectric effect was observed in the temperature range from 250 to 320 K, which is attributed to the change of magnetoresistance taking place at the core by double-exchange mechanism and can be described by the Maxwell-Wagner capacitor model.

Glassy magnetic behavior and exchange bias phenomena are observed in single phase Mn₃O₄ nanoparticles. Dynamics scaling analysis of the ac susceptibility and the Henkel plot indicate that the observed glassy behavior at low temperature can be understood by taking into account the intrinsic behavior of the individual particles consisting of a ferrimagnetic (FIM) core and a spin-glass surface layer. Field-cooled magnetization hysteresis loops display both horizontal and vertical shifts. Dependence of the exchange bias field (H _E) on the cooling field shows an almost undamped feature up to 70 kOe, indicating the stable exchange bias state in Mn₃O₄. H _E increases as the particle size decreases due to the higher surface/volume ratio. The occurrence of the exchange bias can be attributed to the pinning effect of the frozen spin-glass surface layer upon the FIM core.