Half Semimetallic Antiferromagnetism in the Sr$_2$CrTO$_6$ System, T=Os, Ru

TL;DR

This study investigates the electronic and magnetic properties of double perovskites Sr$_2$CrOsO$_6$ and Sr$_2$CrRuO$_6$, revealing the significant role of spin-orbit coupling and the existence of near-half metallic antiferromagnetism with small net moments.

Contribution

The paper demonstrates how spin-orbit coupling influences magnetic moments and reveals near-half metallic antiferromagnetism in Sr$_2$CrRuO$_6$, a novel finding in these compounds.

Findings

Sr$_2$CrOsO$_6$ exhibits a net moment of 0.54 μ_B influenced by SOC.

Sr$_2$CrRuO$_6$ is nearly a perfect half (semi)metallic antiferromagnet with negligible net moment.

Cr moments are significantly more resistant to fluctuations than Os moments.

Abstract

Double perovskite Sr$_2$CrOsO$_6$ is (or is very close to) a realization of a spin-asymmetric semimetallic compensated ferrimagnet, according to first principles calculations. This type of near-half metallic antiferromagnet is an unusual occurrence, and more so in this compound because the zero gap is accidental rather than being symmetry determined. The large spin-orbit coupling (SOC) of osmium upsets the spin balance (no net spin moment without SOC): it reduces the Os spin moment by 0.27 $\mu_B$ and induces an Os orbital moment of 0.17 $\mu_B$ in the opposite direction. The effects combine (with small oxygen contributions) to give a net total moment of 0.54 $\mu_B$ per cell in \scoo, reflecting a large impact of SOC in this compound. This value is in moderately good agreement with the measured saturation moment of 0.75 $\mu_B$. The value of the net moment on the Os ion obtained from neutron diffraction (0.73 $\mu_B$ at low temperature) differs from the calculated value (1.14 $\mu_B$). Rather surprisingly, in isovalent Sr$_2$CrRuO$_6$ the smaller SOC-induced spin changes and orbital moments (mostly on Ru) almost exactly cancel. This makes Sr$_2$CrRuO$_6$ a "half (semi)metallic antiferromagnet" (practically vanishing net total moment) even when SOC is included, with the metallic channel being a small-band-overlap semimetal. Fixed spin moment (FSM) calculations are presented for each compound, illustrating how they provide different information than in the case of a nonmagnetic material. These FSM results indicate that the Cr moment is an order of magnitude stiffer against longitudinal fluctuations than is the Os moment.