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

摘要: We examine the effect of small amounts magnetic substituents in $A$ sites frustrated spinels MgCr$_2$O$_4$ and ZnCr$_2$O$_4$. Specifically we look for effects spin lattice disorder on structural changes accompanying ordering these compounds. Substitution Co$^{2+}$ non-magnetic Zn$^{2+}$ site Zn$_{1-x}$Co$_{x}$Cr$_2$O$_4$ where 0\,$<$\,$x$\,$\leq$\,0.2 completely suppresses spin-Jahn-Teller distortion ZnCr$_2$O$_4$ although systems remain frustrated, occurs at very low temperatures $T$\,$<$\,20\,K. On other hand, substitution Jahn-Teller active Cu$^{2+}$ Mg$^{2+}$ Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ induce well above N\'eel solid solutions, yet interactions with long-range occurring below 20\,K without any further distortion. The distorted solutions adopt orthorhombic $Fddd$ structure ferrimagnetic CuCr$_2$O$_4$. Total neutron scattering studies suggest that there are local $A$O$_4$ distortions Cu$^{2+}$-containing room temperature become cooperative when average occur. Magnetism evolves from compensated antiferromagnetism to uncompensated cations cation