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Register a new userAntiferromagnetism in a nanocrystalline high entropy oxide (Co,Cu,Mg,Ni,Zn)O : Magnetic constituents and surface anisotropy leading to lattice distortion
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Added by
Subramshu Bhattacharya
Publication date
2020
fields
Physics
and research's language is
English
Authors
Nandhini J. Usharani
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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.
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In this paper we analyze in details the electronic properties of (Co/Ni) multilayers, a model system for spintronics devices. We use magneto-optical Kerr (MOKE), spin-polarized photoemission spectroscopy (SRPES), x-ray magnetic circular dichroism (XMCD) and anomalous surface diffraction experiments to investigate the electronic properties and perpendicular magnetic anisotropy (PMA) in [Co(x)/Ni(y)] single-crystalline stacks grown by molecular beam epitaxy.
We demonstrate by means of fully relativistic first principles calculations that, by substitution of Fe by Cr, Mn, Co, Ni or Cu in FePt-L10 bulk alloys, with fixed Pt content, it is possible to tune the magnetocrystalline anisotropy energy by adjusting the content of the non-magnetic species in the material. The changes in the geometry due to the inclusion of each element induces different values of the tetragonality and hence changes in the magnetic anisotropy and in the net magnetic moment. The site resolved magnetic moments of Fe increase with the X content whilst those of Pt and X are simultaneously reduced. The calculations are in good quantitative agreement with experimental data and demonstrate that models with fixed band structure but varying numbers of electrons per unit cell are insufficient to describe the experimental data for doped FePt-L10 alloys.
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We examine the effect of small amounts of magnetic substituents in the $A$ sites of the frustrated spinels MgCr$_2$O$_4$ and ZnCr$_2$O$_4$. Specifically we look for the effects of spin and lattice disorder on structural changes accompanying magnetic ordering in these compounds. Substitution of Co$^{2+}$ on the non-magnetic Zn$^{2+}$ site in Zn$_{1-x}$Co$_{x}$Cr$_2$O$_4$ where 0,$<$,$x$,$leq$,0.2 completely suppresses the spin-Jahn-Teller distortion of ZnCr$_2$O$_4$ although these systems remain frustrated, and magnetic ordering occurs at very low temperatures of $T$,$<$,20,K. On the other hand, the substitution of Jahn-Teller active Cu$^{2+}$ for Mg$^{2+}$ and Zn$^{2+}$ in Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ where 0,$<$,$x$,$leq$,0.2 induce Jahn-Teller ordering at temperatures well above the Neel temperatures of these solid solutions, and yet spin interactions remain frustrated with long-range magnetic ordering occurring below 20,K without any further lattice distortion. The Jahn-Teller distorted solid solutions Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ adopt the orthorhombic $Fddd$ structure of ferrimagnetic CuCr$_2$O$_4$. Total neutron scattering studies of Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ suggest that there are local $A$O$_4$ distortions in these Cu$^{2+}$-containing solid solutions at room temperature and that these distortions become cooperative when average structure distortions occur. Magnetism evolves from compensated antiferromagnetism in MgCr$_2$O$_4$ and ZnCr$_2$O$_4$ to uncompensated antiferromagnetism with substitution of magnetic cations on the non-magnetic cation sites of these frustrated compounds.
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