Spin-cycloid instability as the origin of weak ferromagnetism in the disordered perovskiteBi0.8La0.2Fe0.5Mn0.5O3

Abstract

Powder neutron diffraction and magnetometry studies have been conducted to investigate the crystallographic and magnetic structure of ${\mathrm{Bi}}_{0.8}{\mathrm{La}}_{0.2}{\mathrm{Fe}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_{3}$. The compound stabilizes in the $Imma$ orthorhombic crystal symmetry in the measured temperature range of 5 to 380 K, with a transition to antiferromagnetic order at ${T}_{\mathrm{N}}\ensuremath{\approx}240$ K. The spin cycloid present for BiFeO${}_{3}$ is found to be absent with 50% Mn${}^{3+}$ cation substitution, leading to $G$-type antiferromagnetic order with an enhanced out-of-plane canted ferromagnetic component, evident from measurable weak-ferromagnetic hysteresis. Structural modifications do not solely explain this behavior, indicating that modified electron exchange interactions must be taken into account. A classical spin simulation was developed to investigate the effect of random substitution in a disordered pseudocubic perovskite. The calculations took into account the nearest-neighbor, next-nearest-neighbor, and Dzyaloshinskii-Moriya interactions, along with the local spin anisotropy. Using this framework to extend the established Hamiltonian model for BiFeO${}_{3}$, we show that only certain types of perturbations at a magnetic defect and the surrounding molecular fields trigger a simultaneous collapse of cycloidal order and the emergence of the long-range weak-ferromagnetic component. By adopting values for the Mn molecular fields appropriate for $\mathit{RE}$MnO${}_{3}$ ($\mathit{RE}=$ rare earth), simulations of BiMn${}_{0.5}$Fe${}_{0.5}{\mathrm{O}}_{3}$ exhibit the key magnetic properties of our experimental observations.