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arXiv · 1603.03087

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

Peter M. M. Thygesen·Joseph A. M. Paddison·Ronghuan Zhang·Kevin A. Beyer·Karena W. Chapman·Helen Y. Playford·Matthew G. Tucker·David A. Keen·Michael A. Hayward·Andrew L. Goodwin
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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.

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Peter M. M. Thygesen, Joseph A. M. Paddison, Ronghuan Zhang, Kevin A. Beyer, Karena W. Chapman, Helen Y. Playford, Matthew G. Tucker, David A. Keen, Michael A. Hayward, Andrew L. Goodwin. 2016-03-09. Orbital Dimer Model for Spin-Glass State in Y$_2$Mo$_2$O$_7$. https://doi.org/10.1103/physrevlett.118.067201

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