Intertwined dipolar and multipolar order in the triangular-lattice magnet TmMgGaO$_4$

TL;DR

This study combines neutron scattering and thermodynamics to reveal intertwined dipolar and multipolar order in TmMgGaO$_4$, demonstrating a hidden multipolar phase with unique spin excitations on a triangular lattice.

Contribution

It provides the first experimental evidence of a hidden multipolar order in a triangular-lattice magnet and models it using a transverse field Ising framework.

Findings

Observation of magnetic Bragg peaks at K points

Detection of dispersive spin excitations not explained by dipolar order

Modeling of the system with a transverse field Ising model

Abstract

A phase transition is often accompanied by the appearance of an order parameter and symmetry breaking. Certain magnetic materials exhibit exotic hidden-order phases, in which the order parameters are not directly accessible to conventional magnetic measurements. Thus, experimental identification and theoretical understanding of a hidden order are difficult. Here we combine neutron scattering and thermodynamic probes to study the newly discovered rare-earth triangular-lattice magnet TmMgGaO$_4$. Clear magnetic Bragg peaks at K points are observed in the elastic neutron diffraction measurements. More interesting, however, is the observation of sharp and highly dispersive spin excitations that cannot be explained by a magnetic dipolar order, but instead is the direct consequence of the underlying multipolar order that is "hidden" in the neutron diffraction experiments. We demonstrate that the observed unusual spin correlations and thermodynamics can be accurately described by a transverse field Ising model on the triangular lattice with an intertwined dipolar and ferro-multipolar order.