Competing Jahn-Teller and spin Jahn-Teller ordering in $A$Cr$_2$O$_4$ spinels

ORAL

Abstract

Magnetic ordering is strongly linked to structural distortions in the frustrated antiferromagnets ZnCr$_2$O$_4$ and MgCr$_2$O$_4$. These systems undergo spin Jahn-Teller distortions at the onset of magnetic order. The addition of magnetic $A$ site cations in $A$Cr$_2$O$_4$ spinels can relieve frustration. High-resolution variable-temperature synchrotron powder X-ray diffraction, detailed magnetic studies, and heat capacity measurements show that dilute amounts of Jahn-Teller active Cu$^{2+}$ or Co$^{2+}$ on the $A$ sites of these spinels have different effects on structure but similar effects on magnetism. Partial replacement of $A$ by Cu$^{2+}$ generates Jahn-Teller distortions at temperatures above the endmember Neel temperatures, yet spin interactions remain frustrated to $\sim$ 12 K. This contrasts with Co$^{2+}$ substitution which also maintains frustration, but results in a suppression of spin Jahn-Teller ordering in ZnCr$_2$O$_4$. We report decoupled Jahn-Teller and spin Jahn-Teller ordering in the canonical frustrated systems ZnCr$_2$O$_4$ and MgCr$_2$O$_4$ that is tunable by varying the identity of the magnetic $A$ site substituent.

*Schlumberger Foundation Fellowship, NSF DMR 1105301, NSF-MRSEC DMR 1121053, DOE contract no. DE-AC02-06CH11357

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Authors

  • Moureen Kemei

    • Materials Department and Materials Research Laboratory University of California, Santa Barbara, California 93106, USA

  • Stephanie Moffitt

    • Materials Science and Engineering Department Northwestern University, Evanston, Illinois 60208, USA

  • Matthew Suchomel

    • Advanced Photon Source, Argonne National Laboratory
    • X-Ray Science Division Argonne National Laboratory, Argonne, Illinois 60439, USA

  • Daniel Shoemaker

    • Argonne National Laboratory
    • Material Science Division Argonne National Laboratory, Argonne, Illinois 60439, USA

  • Ram Seshadri

    • Materials Department and Materials Research Laboratory, University of California, Santa Barbara
    • Materials Department and Materials Research Laboratory University of California, Santa Barbara, California 93106, USA