Effects of Cr³⁺ doping on the structure, morphology, magnetic and dielectric properties of Mn₃O₄

Mn₃O₄ and Mn_1.8Cr_1.2O₄ single-phase polycrystalline samples were successfully prepared by the conventional solid-phase method in order to investigate the effect of Cr³⁺ doping on the magnetoelectric effect of the Mn₃O₄ system. The results from Rietveld refinement of powder X-ray diffraction patterns show a change in crystal structure after Cr³⁺ doped Mn₃O₄, from a tetragonal (space group I4₁/amd \textI\text4_\text1\text/amd \textI\text4_\text1\text/amd ) to a cubic (space group \textFd\bar\text3\textm ) crystal system. Field emission scanning electron microscopy results indicate that the morphology of Mn₃O₄ is with a tight mosaic structure, while Mn_1.8Cr_1.2O₄ manifests as discrete pentahedron-like and octahedron-like. Moreover, the grain size of Mn_1.8Cr_1.2O₄ is about 1/8 times of that of the Mn₃O₄. Magnetization measurements show that the Yafet-Kittel type ferrimagnetic phase transition temperature increases from 43 K to 60 K after Cr³⁺ doping, which is due to the enhancement of the magnetic interaction from the decrease of the Mn³⁺(Cr³⁺)/Mn²⁺−O bond length and the increase of the Mn³⁺(Cr³⁺)−O−Mn³⁺(Cr³⁺) bond angle. In addition, the coercivity of Mn_1.8Cr_1.2O₄ decreases significantly, which can be understood in terms of the reduction in force between crystalline grains. The dielectric effect study shows that the dielectric anomaly peaks are observed in Mn₃O₄ and Mn_1.8Cr_1.2O₄ near the magnetic phase transition temperature, and the dielectric anomaly peak positions don´t shift with frequency, indicating that the systems exist ferroelectric polarization related to the magnetic order, which originates from the nonlinear magnetic structure of the system.