Evolution of magnetic and electrical behavior in Mg-doped MnCr2O4: A combined experimental and theoretical study

Abstract

In this study, Mg-substituted MnCr2O4 was synthesized using the solid-state reaction method, and the effects of Mg substitution on its optical, magnetic, and electrical properties were investigated. Rietveld refinement analysis confirmed the formation of a single-phase cubic spinel structure with the space group Fd3¯m. The lattice parameter systematically decreases with increasing Mg concentrations. XPS spectra reveal the presence of multiple oxidation states of Mn and Cr, along with an increase in oxygen vacancies upon Mg substitution. Raman spectroscopy studies provided further insights into the structural details of the samples. The optical study in the UV–visible region showed a bandgap variation from 3.00 to 3.26 eV, and these indicate wide bandgap semiconducting behavior. The substitution of Mg significantly influences the exchange interactions between magnetic ions. All exchange interactions calculated using the DFT + U method show antiferromagnetic (AFM) behavior. In the MnCr2O4 compound, the computed exchange constants demonstrate AFM characteristics, with values of JMn–Mn = −20.2 K, JMn–Cr = −10.76 K, and JCr–Cr = −52.4 K. At x = 0.75, the exchange constants are JMn–Cr = −5.49 K and JCr–Cr = −28.00 K. Notably, JCr–Cr dominates over other exchange interactions in all samples. This suggests that the Mg-substituted MnCr2O4 samples exhibit magnetic frustration. The frequency and temperature-dependent ac and dc conductivity measurements provided insights into the conduction mechanism, revealing that charge transport occurs through a hopping process. Additionally, the activation energy (Wn) and hopping distance (R) calculations supported the applicability of the Mott variable range hopping model for describing the dc conductivity behavior of these samples.