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

Abstract: For the first time, this study shows that distortion in a crystal structure due to magnetic effect is possible in a lattice with extreme chemical disorder. The multicomponent equimolar transition metal oxide (ME TMO), (Co,Cu,Mg,Ni,Zn)O, which is a high entropy oxide, 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 ME TMO 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 SQUID and EPR studies could be attributed to the additive effects of exchange striction (from the magnetic constituents) and magnetic anisotropy (from the decreased crystallite size). For the first time, iron has been doped into ME TMO, to show that a higher amount of magnetic constituent increases the distortion in the lattice. Nanocrystalline ME TMO also showed a core shell magnetic behavior below the bifurcation temperature arising from the uncompensated or canted spin at the surface. Neel temperature of the nanocrystalline ME TMO 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.