Structural ground states of ($A,A'$)Cr$_2$O$_4$($A$=Mg, Zn; $A^{\prime}$ = Co, Cu) spinel solid solutions: Spin-Jahn-Teller and Jahn-Teller effects

Moureen C. Kemei; Stephanie L. Moffitt; Lucy E. Darago; Ram Seshadri; Matthew R. Suchomel; Daniel P. Shoemaker; Katharine Page; Joan Siewenie

doi:10.1103/PhysRevB.89.174410

paper

Structural ground states of ( $A, A^{'}$ )Cr $_{2}$ O $_{4}$ ( $A$ =Mg, Zn; $A^{'}$ = Co, Cu) spinel solid solutions: Spin-Jahn-Teller and Jahn-Teller effects

arXiv:1401.3081 · doi:10.1103/PhysRevB.89.174410

Abstract

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 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 Mg $_{1 - x}$ Cu $_{x}$ Cr $_{2}$ O $_{4}$ and Zn $_{1 - x}$ Cu $_{x}$ Cr $_{2}$ O $_{4}$ adopt the orthorhombic $F ddd$ 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.

Structural ground states of (A,A′)Cr2​O4​(A=Mg, Zn; A′ = Co, Cu) spinel solid solutions: Spin-Jahn-Teller and Jahn-Teller effects

Abstract

Structural ground states of ( $A, A^{'}$ )Cr $_{2}$ O $_{4}$ ( $A$ =Mg, Zn; $A^{'}$ = Co, Cu) spinel solid solutions: Spin-Jahn-Teller and Jahn-Teller effects