Static and dynamic XY-like short-range order in a frustrated magnet with exchange disorder

Abstract

A single crystal of the Co2+ based pyrochlore NaCaCo2F7 was studied by inelastic neutron scattering. This frustrated magnet with quenched exchange disorder remains in a strongly correlated paramagnetic state down to one 60th of the Curie-Weiss temperature. Below T_f = 2.4 K, diffuse elastic scattering develops and comprises 30 +/- 10% of the total magnetic scattering, as expected for J_{eff} = 1/2 moments frozen on a time scale that exceeds \hbar/δE=3.8 ps. The diffuse scattering is consistent with short range XY antiferromagnetism with a correlation length of 16 Å. The momentum (Q) dependence of the inelastic intensity indicates relaxing XY-like antiferromagnetic clusters at energies below ~ 5.5 meV, and collinear antiferromagnetic fluctuations above this energy. The relevant XY configurations form a continuous manifold of symmetry-related states. Contrary to well-known models that produce this continuous manifold, order-by-disorder does not select an ordered state in NaCaCo2F7 despite evidence for weak (~12 %) exchange disorder. Instead, NaCaCo2F7 freezes into short range ordered clusters that span this manifold.

9 pages, 9 figures. This updated version features modified figures and some new discussion

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• Ground State Phase Diagram of Generic XY Pyrochlore Magnets with Quantum Fluctuations
• A single crystal high-temperature pyrochlore antiferromagnet
• Nature of finite-temperature transition in anisotropic pyrochlore Er2Ti2O7
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• A single crystal Co2+ fluoride pyrochlore antiferromagnet
• NaCaNi2F7: A frustrated high temperature pyrochlore antiferromagnet with S=1 Ni2+

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• Defect-induced spin textures in magnetic solids