Magnetism in a high-entropy oxide compound investigated via muon spin relaxation*

High entropy oxides (HEOs), which are compounds containing five or more types of metal ions, have attracted interest in recent years for their promising mechanical, electronic, and optical properties. Magnetic HEOs have also emerged as an exciting new research direction. Despite the extreme amounts of atomic-scale disorder, multiple HEOs containing magnetic metals have shown antiferromagnetic (AF) order at low temperatures. We have used muon spin relaxation (µSR) – a sensitive probe of the local magnetic structure of a sample – to study (Mg, Mn, Fe, Co, Ni)O, which we call Fe-HEO for its inclusion of iron (relatively rare in HEOs). From the µSR data, we have identified the AF transition region of Fe-HEO to extend from 207 K and 240 K, with different regions within the sample volume transitioning at different temperatures within this range. We ascertained that the full sample volume becomes magnetically ordered below 207 K, but we found evidence for inhomogeneous spin fluctuations, which we attribute to the variation of the individual magnetic moment for each type of metal ion. These results provide a detailed, microscopic view of the magnetism in this fascinating class of emerging materials.