Signs of superparamagnetic cluster formation in LuFe1−xCrxO3 perovskites evidenced by magnetization reversal and Monte Carlo simulations

In this paper, we study the magnetic properties of orthorhombic (Pbnm) perovskites $\mathrm{Lu}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ with $x=0.25$, 0.45, 0.55, and 0.75 by magnetization vs temperature and neutron powder diffraction measurements at room temperature. The magnetic moments are oriented along the $x$ direction with a G-type antiferromagnetic (AFM) arrangement that corresponds to irreducible representation ${\mathrm{\ensuremath{\Gamma}}}_{4}$ (${G}_{x}{A}_{y}{F}_{z}$). Magnetization reversal (MR) is observed for $0.45\ensuremath{\le}x\ensuremath{\le}0.75$. The MR phenomenon was modeled using Monte Carlo simulations, modeling both homogeneous and inhomogeneous cation distributions. For $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_{3}$, the presence of MR could only be explained using an inhomogeneous distribution with well-defined magnetic clusters. Looking at the staggered magnetization of each cluster, we propose a superparamagnetic regime of ${\mathrm{Fe}}^{3+}$ clusters in a ${\mathrm{Cr}}^{3+}$-rich matrix for $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_{3}$. By calculating the specific heat, we found that, in $\mathrm{Lu}{\mathrm{Fe}}_{0.45}{\mathrm{Cr}}_{0.55}{\mathrm{O}}_{3}$ and $\mathrm{Lu}{\mathrm{Fe}}_{0.25}{\mathrm{Cr}}_{0.75}{\mathrm{O}}_{3}$ the clusters order at nearly the same temperature ${T}_{N}^{\mathrm{Clus}}\ensuremath{\sim}535$ and 520 K, respectively, while the matrix orders at ${T}_{N}^{\mathrm{Mat}}\ensuremath{\sim}115\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. There is considerable reduction in the cluster ordering temperature as compared with the bulk, as ${T}_{N}\ensuremath{\sim}628\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{LuFe}{\mathrm{O}}_{3}$, while the matrix orders at the expected temperature as ${T}_{N}\ensuremath{\sim}115\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\mathrm{LuCr}{\mathrm{O}}_{3}$.