Inconsistency in the critical behavior and magnetic phase transition of La3/4Ca1/4Mn1/2Cr1/2O3

Abstract

In this study, we examine the magnetic properties and critical behavior of $L{a}_{3/4}C{a}_{1/4}M{n}_{1/2}C{r}_{1/2}{O}_3$(LCMCO). The magnetic analysis indicates strong ferromagnetic ordering at low temperatures, transitioning to a paramagnetic state around 280 K. Three prominent transition peaks are observed: the first at T₁ = 40.2 K, corresponding to spin reorientation between Mn${}^{3+}$/Mn${}^{4+}$ and Mn${}^{3+}$/Mn${}^{2+}$ via double and super exchange interactions; the second at T₂ = 208.3 K, leads to the competition between antiferromagnetic and ferromagnetic correlations caused by spin clusters; and the third at T₃ = 279.6 K, marking the transition to a paramagnetic state, as revealed by $\frac{d\chi \left(T\right)}{dT}$. The critical behavior analysis determined the exponents $\beta$, $\gamma$, and $\delta$ to be 0.17, 0.76, and 1.32, respectively. These values deviate from standard critical exponents model, indicating the absence of a conventional second-order phase transition. This behavior arises from the competing magnetic spin interactions between the distinct chemical states of $M{n}^{2+/3+/4+}$ and $C{r}^{3+/6+}$ due to the formation of oxygen vacancies resulting from hole doping. This leads to the magnetic spin clusters that prevent the establishment of a fully ferromagnetic interaction. The oxygen defects ($\mu$) is approximately 1.7%, influencing the average valency of O, Mn, and Cr, which play a significant role in modifying the magnetic interactions. The impact of oxygen vacancies on the magnetic properties are investigated computationally by constructing a $2\times 2\times 2$ supercell of LCMCO and removing an oxygen atom from its optimal site. DFT studies are in strong agreement with experimental observations, revealing the presence of multiple chemical states of constituent ions. These varying oxidation states are likely to result in complex magnetic interactions, which could give rise to the formation of spin clusters.