Ferrimagnetism in the double perovskite Ca2FeOsO6: a density functional study

TL;DR

This study uses density functional calculations to reveal the ferrimagnetic ground state of Ca2FeOsO6, driven by specific electron hopping mechanisms and suppressed magnetic frustration, with potential ferromagnetic transition if exchange paths are straightened.

Contribution

It provides a detailed theoretical analysis of the magnetic interactions and electronic structure of Ca2FeOsO6, highlighting the mechanisms behind its ferrimagnetism and the conditions for ferromagnetic coupling.

Findings

Ca2FeOsO6 has a high-spin Fe3+ and Os5+ state with a stable ferrimagnetic ground state.

Strong antiferromagnetic coupling between Fe3+ and Os5+ ions suppresses magnetic frustration.

Straightening the exchange path could induce ferromagnetic coupling.

Abstract

Using density functional calculations, we find that the newly synthesized Ca$_2$FeOsO$_6$ has the high-spin Fe$^{3+}$ ($3d^5$)-Os$^{5+}$ ($5d^3$) state. The octahedral Os$^{5+}$ ion has a large intrinsic exchange splitting, and its $t_{2g\uparrow}^3$ configuration makes the spin-orbit coupling ineffective. Moreover, there is a strong antiferromagnetic (AF) coupling between the neighboring Fe$^{3+}$ ($S$ = 5/2) and Os$^{5+}$ ($S$ = -3/2), but the AF couplings within both the fcc Fe$^{3+}$ and Os$^{5+}$ sublattices are one order of magnitude weaker. Therefore, a magnetic frustration is suppressed and a stable ferrimagnetic (FiM) ground state appears. This FiM order is due to the virtual hopping of the $t_{2g}$ electrons from Os$^{5+}$ ($t_{2g\downarrow}^3$) to Fe$^{3+}$ ($t_{2g\uparrow}^3e_{g\uparrow}^2$). However, if the experimental bended Fe$^{3+}$-O$^{2-}$-Os$^{5+}$ exchange path gets straight, the $e_g$ hopping from Fe$^{3+}$ ($t_{2g\uparrow}^3e_{g\uparrow}^2$) to Os$^{5+}$ ($t_{2g\uparrow}^3$) would be facilitated and then a ferromagnetic (FM) coupling would occur.