We examine the effect of small amounts of magnetic substituents in the A sites of the frustrated spinels MgCr2O4 and ZnCr2O4. Specifically, we look for the effects of spin and lattice disorder on structural changes accompanying magnetic ordering in these compounds. Substitution of Co2+ on the nonmagnetic Zn2+ site in Zn1-xCoxCr2O4 where 0 < x ≤ 0.2 completely suppresses the spin-Jahn-Teller distortion of ZnCr2O4 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 Cu2+ for Mg2+ and Zn2+ in Mg1-xCuxCr2O4 and Zn1-xCuxCr2O4 where 0 < x ≤ 0.2 induce Jahn-Teller ordering at temperatures well above the Néel 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 Mg1-xCuxCr2O4 and Zn1-xCuxCr2O4 adopt the orthorhombic Fddd structure of ferrimagnetic CuCr2O4. Total neutron scattering studies of Zn1-xCuxCr2O4 suggest that there are local AO4 distortions in these Cu2+-containing solid solutions at room temperature and that these distortions become cooperative when average structure distortions occur. Magnetism evolves from compensated antiferromagnetism in MgCr2O4 and ZnCr2O4 to uncompensated antiferromagnetism with substitution of magnetic cations on the nonmagnetic cation sites of these frustrated compounds. The sharp heat capacity anomalies associated with the first-order spin-Jahn-Teller transitions of MgCr2O4 and ZnCr2O4 become broad in Mg1-xCuxCr2O4, Zn1-xCoxCr2O4, and Zn1-xCuxCr2O4 when x > 0. We present a temperature-composition phase diagram summarizing the structural ground states and magnetic properties of the studied spinel solid solutions.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - May 9 2014|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics