Validating Mean Field Theory in a New Complex, Disordered High-Entropy Spinel Oxide

Abstract: The advent of novel high-entropy oxides has sparked substantial research interest due to their exceptional functional properties, which often surpass the mere sum of their constituent elements' characteristics. This study introduces a complex high-entropy spinel oxide with composition (Ni${0.2}$Mg${0.2}$Co${0.2}$Cu${0.2}$Zn${0.2}$)(Mn${0.66}$Fe${0.66}$Cr${0.66}$)O$_{4}$. We performed comprehensive structural (X-ray and Neutron diffraction), microstructural, magnetic, and local electronic structure investigations on this material. Despite the material's high degree of disorder, detailed magnetization measurements and low temperature neutron powder diffraction studies reveal long-range ferrimagnetic ordering beginning at 293 K. The sample exhibits a high saturation magnetization of 766 emu-cm${^3}$ (at 50 K), a low coercivity (H$_C$) of 100 Oe (50 K), a high transition temperature (T$_C$) around room temperature, and high resistivity value of 4000 Ohm-cm at room temperature, indicating its potential for high density memory devices. The magnetic structure is determined using a collinear-type ferrimagnetic model with a propagation vector k = 0,0,0. Various analytical techniques, including modified Arrott plots, Kouvel-Fischer analysis, and critical isotherm analysis, are employed to investigate the phase transitions and magnetic properties of this complex system. Our results indicate a second-order phase transition. Remarkably, despite the complex structure and significant disorder, the critical exponents obtained are consistent with the mean field model. The high entropy leads to a remarkably homogeneous distribution of multiple cations, validating the approximation of average local magnetic environments and supporting the mean field theory.