Antiferromagnetism in a nanocrystalline high entropy oxide (Co,Cu,Mg,Ni,Zn)O: Magnetic constituents and surface anisotropy leading to lattice distortion

For the first time, this study shows that distortion in a crystal structure due to magnetic effects is possible in a lattice with extreme chemical disorder. The transition metal-high entropy oxide (TM-HEO), (Co,Cu,Mg,Ni,Zn)O, has been attracting a lot of attention due to its unique application potential in many fields including electrochemical energy storage. In the present investigation, nanocrystalline TM-HEO was synthesised by three bottom-up methods. The presence of distortion in the rocksalt crystal structure, revealed by X-ray diffraction and Raman spectroscopy, and correlated with magnetic measurements from Superconducting quantum interference device (SQUID) and Electron paramagnetic resonance (EPR) studies could be attributed to the additive effects of exchange striction (from the magnetic constituents) and magnetic anisotropy (from the decreased crystallite size). Iron has been added to the TM-HEO to show that a higher amount of magnetic constituent increases the distortion in the lattice. Nanocrystalline TM-HEO also showed a “core-shell” magnetic behaviour below the bifurcation temperature arising from the uncompensated or canted spin at the surface. Néel temperature of the nanocrystalline TM-HEO is reported for first time to be as high as 700 K. This study helps unravel the structure and magnetic properties of such high entropy materials, and augurs a definite scope for better understanding of the factors influencing the crystal structure in high entropy oxides.