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

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.

6 pages, 3 figures

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• Spin Glass Order Induced by Dynamic Frustration
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• Frustrated Ferroelectricity in Niobate Pyrochlores
• Origin of magnetic freezing in the pyrochlore Y2Mo2O7
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