Half semimetallic antiferromagnetism in the Sr₂CrTO₆ system (T=Os,Ru)

Double perovskite Sr₂CrOsO₆ 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 0.27μ_B and induces an Os orbital moment of 0.17μ_B in the opposite directions. The effects combine (with small oxygen contributions) to give a net total moment of 0.54μ_B per cell in Sr₂CrOsO₆, reflecting a large impact of SOC in this compound. This value is in moderately good agreement with the measured saturation moment of 0.75μ_B. The value of the net moment on the Os ion obtained from neutron diffraction (0.73μ_B at low temperature) differs from the calculated value (1.14μ_B). Rather surprisingly, in isovalent Sr₂CrRuO₆, the smaller SOC-induced spin changes and orbital moments (mostly on Ru) almost exactly cancel. This makes Sr₂CrRuO₆ 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.