Magnetic Order in Ga-Substituted Spinel Type High Entropy Oxide (MnFeCrCoNi)_3-xGa_xO₄

High entropy oxides (HEOs) are crystalline materials with a high degree of cation disorder and novel properties. It has been proposed that the high configurational entropy in HEOs, which can be achieved by having five or more equimolar cations with random occupancy, results in an entropy term large enough to dominate the enthalpy term at high temperatures and minimizes the Gibbs free energy resulting in a single phase material. However, the degree of configurational disorder, its role in stabilizing HEOs, and its affect on the properties remain open questions. Here we attempt to shed light on these questions using  the 3d transition metal based AB₂O₄ spinel-type HEO (s-HEO) (MnFeCrCoNi)₃O₄ as our platform.  In particular, we attempt to elucidate the relationship between chemical disorder and magnetism using a number of experimental probes, which are sensitive to different length scales, including neutron diffraction, X-ray absorption spectroscopy (XAS), and soft X-ray magnetic circular dichroism (XMCD). With XAS and XMCD, we identify preferred cation occupancy of the tetrahedral A site and octahedral B site in (MnFeCrCoNi)₃O₄ and with neutron diffraction, we characterize its ferrimagnetic ordered state below 400 K. Furthermore, we study the affect of Ga substitution on the cation A and B site occupancy and the magnetism by comparing s-HEOs (MnFeCrCoNi)_3-xGa_xO₄, with x=0.0,0.1,0.2,0.4. Finally, we comment on the role of entropy in stabilizing the s-HEOs and its affect on the magnetic order.