Long range antiferromagnetic order and lattice dynamics in a rocksalt high entropy oxide

High entropy oxides are of intense interest due to their importance in fundamental research including phase formation and modeling, and potential multifunctional physical properties. Here we report the magnetic, thermal and mechanical properties of the rocksalt high entropy oxide (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O as well as their lattice dynamics for the first time. Long range antiferromagnetism with q=(½, ½, ½) was identified using neutron powder diffraction. The shear modulus obtained from resonant ultrasound spectroscopy is about 1/5 of MgO, 1/3 of CoO and 1/4 of NiO. The thermal conductivity is an order of magnitude smaller than that of MgO. Inelastic neutron scattering results indicate strong magnetic excitations at 100 K that survive up to room temperature. In addition, the phonon density of states (PDOS) is considerably softer (vibrational frequencies) than in MgO.