Antiferromagnetism in a rocksalt high entropy oxide

High entropy oxides have attracted much attention because they not only significantly broaden the phases beyond the conventional phase diagram of doping, but also have the potential to exhibit multifunctional physical properties and emergent phenomena including superconductivity and quantum criticality [1-3]. The existence of extreme chemical disorder in these materials is expected to suppress the long range magnetic order. Here, by combining magnetometry, synchrotron x-ray and neutron powder diffraction, we report for the first time that a rocksalt high entroy oxide exhibits long range antiferromagnetic order at ~120 K with q=(½, ½, ½). Inelastic neutron scattering reveals strong magnetic excitations at 100 K that survive up to room temperature. The shear modulus obtained from resonant ultrasound spectroscopy shows an anomaly around the magnetic transition, and surprisingly it hardens with decreasing of temperature without saturation down to 3 K. References: [1] Rost, C. M. et al. Nat. Commun. 6, 8485 (2015). [2] Gao, M. C. et al. J. Mater. Res., 1-18 (2018). [3] Sales, B. C. et al. npj Quantum Mater. 2, 33 (2017).