Orbital Dimer Model for Spin-Glass State in Y$_2$Mo$_2$O$_7$

TL;DR

This study reveals that orbital dimerization and atomic displacement disorder, rather than compositional disorder, can induce spin-glass behavior in Y$_2$Mo$_2$O$_7$, through neutron and X-ray PDF analysis.

Contribution

It introduces a disorder model based on orbital dimerization and atomic displacements, explaining spin-glass formation without compositional disorder in Y$_2$Mo$_2$O$_7$.

Findings

Mo$^{4+}$ ions displace following a local '2-in/2-out' rule.

Orbital dimerization driven by Jahn-Teller effects causes local displacements.

Disorder in magnetic interactions arises from atomic displacement disorder.

Abstract

The formation of a spin glass usually requires both structural disorder and frustrated magnetic interactions. Consequently, the origin of spin-glass behaviour in Y$_2$Mo$_2$O$_7$ $-$ in which magnetic Mo$^{4+}$ ions occupy a frustrated pyrochlore lattice with minimal compositional disorder $-$ has been a longstanding question. Here, we use neutron and X-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously-reported PDF, EXAFS and NMR studies and provides a new and physical mechanism for spin-glass formation. We show that Mo$^{4+}$ ions displace according to a local "2-in/2-out" rule on each Mo$_4$ tetrahedron, driven by orbital dimerisation of Jahn-Teller active Mo$^{4+}$ ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O$^{2-}$ displacements yield a distribution of Mo$-$O$-$Mo angles, which in turn introduces disorder into magnetic interactions. Our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition.