Universal Dynamic Magnetism in Yb-Pyrochlores with Disparate Ground States

TL;DR

This study reveals that Yb-based pyrochlore magnets exhibit a universal, gapless spin excitation continuum despite differing static magnetic orders, indicating a common underlying spin dynamic behavior and hidden order parameter.

Contribution

It demonstrates a universal dynamic spin behavior across different Yb pyrochlores with distinct static magnetic states, highlighting hidden magnetic order.

Findings

All three Yb pyrochlores show a gapless continuum of spin excitations.

The spin dynamics are consistent despite different static magnetic orders.

The specific heat anomalies are correlated with the spin dynamics, suggesting hidden order.

Abstract

The ytterbium pyrochlore magnets, Yb2B2O7 (B = Sn, Ti, Ge) are well described by S_eff = 1/2 quantum spins decorating a network of corner-sharing tetrahedra and interacting via anisotropic exchange. Structurally, only the non-magnetic B-site cation, and hence, primarily the lattice parameter, is changing across the series. Nonetheless, a range of magnetic behaviors are observed: the low temperature magnetism in Yb2Ti2O7 and Yb2Sn2O7 has ferromagnetic character, while Yb2Ge2O7 displays an antiferromagnetically ordered Neel state at low temperatures. While the static properties of the ytterbium pyrochlores are distinct, inelastic neutron scattering measurements reveal a common character to their exotic spin dynamics. All three ytterbium pyrochlores show a gapless continuum of spin excitations, resembling over-damped ferromagnetic spin waves at low Q. Furthermore, the specific heat of the series also follows a common form, with a broad, high-temperature anomaly followed by a sharp low-temperature anomaly at T_C or T_N. The novel spin dynamics we report correlate strongly with the broad specific heat anomaly only, remaining unchanged across the sharp anomaly. This result suggests that the primary order parameter in the ytterbium pyrochlores associated with the sharp anomaly is "hidden" and not simple magnetic dipole order.