The formation of a spin glass usually requires both structural disorder and frustrated magnetic interactions. Consequently, the origin of spin-glass behaviour in Y2Mo2O7− in which magnetic Mo4+ 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 Mo4+ ions displace according to a local "2-in/2-out" rule on each Mo4 tetrahedron, driven by orbital dimerisation of Jahn-Teller active Mo4+ ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O2− 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.
@article{arxiv.1603.03087,
title = {Orbital Dimer Model for Spin-Glass State in Y$_2$Mo$_2$O$_7$},
author = {Peter M. M. Thygesen and Joseph A. M. Paddison and Ronghuan Zhang and Kevin A. Beyer and Karena W. Chapman and Helen Y. Playford and Matthew G. Tucker and David A. Keen and Michael A. Hayward and Andrew L. Goodwin},
journal= {arXiv preprint arXiv:1603.03087},
year = {2017}
}