The frustrated FCC antiferromagnet Ba2YOsO6: structural characterization, magnetic properties and neutron scattering studies

Abstract

We report the crystal structure, magnetization and neutron scattering measurements on the double perovskite Ba${}_2$YOsO${}_6$. The $Fm\overline{3}m$ space group is found both at 290~K and 3.5~K with cell constants $a_0=8.3541(4)$~Å and $8.3435(4)$~Å, respectively. Os${}^{5+}$ ($5d^3$) ions occupy a non-distorted, geometrically frustrated face-centered-cubic (FCC) lattice. A Curie-Weiss temperature $\theta =-772$~K suggests the presence of a large antiferromagnetic interaction and a high degree of magnetic frustration. A magnetic transition to long range antiferromagnetic order, consistent with a Type I FCC state below $T_{\mathrm{N}}\sim 69$~K, is revealed by magnetization, Fisher heat capacity and elastic neutron scattering, with an ordered moment of 1.65(6)~${\mu }_B$ on Os${}^{5+}$. The ordered moment is much reduced from either the expected spin only value of $\sim 3{\mu }_B$ or the value appropriate to $4d^3$ Ru${}^{5+}$ in isostructural Ba${}_2$YRuO${}_6$ of 2.2(1)~${\mu }_B$, suggesting a role for spin orbit coupling (SOC). Triple axis neutron scattering measurements of the order parameter suggest an additional first-order transition at $T=67.45$~K, and the existence of a second ordered state. % Time-of-flight inelastic neutron results reveal a large spin gap $\Delta \sim 17$~meV, unexpected for an orbitally quenched, $d^3$ electronic configuration. We discuss this in the context of the $\sim 5$~meV spin gap observed in the related Ru${}^{5+}$, $4d^3$ cubic double perovskite Ba${}_2$YRuO${}_6$, and attribute the $\sim 3$ times larger gap to stronger SOC present in this heavier, $5d$, osmate system.

10 pages, 9 figures

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• Magnon spectrum of the Weyl semimetal half-Heusler compound GdPtBi
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