Partial up-up-down order with the continuously distributed order parameter in the triangular antiferromagnet TmMgGaO4

TL;DR

This study investigates the unconventional magnetic order in TmMgGaO4, revealing a partial up-up-down order with continuously distributed parameters due to CEF randomness and quasidoublets, using thermodynamic and neutron diffraction methods.

Contribution

It provides the first comprehensive analysis of the magnetic structure and order parameter distribution in TmMgGaO4, highlighting the role of CEF disorder and quasidoublets in forming unconventional magnetic states.

Findings

Partial up-up-down magnetic order with continuous distribution of order parameters.

Highly anisotropic 2D magnetic correlations with long in-plane and short inter-plane correlation lengths.

Field-induced uniform polarization of nonmagnetic Tm ions at low temperatures.

Abstract

Frustrated quasidoublets without time-reversal symmetry can host highly unconventional magnetic structures with continuously distributed order parameters even in a single-phase crystal. Here, we report the comprehensive thermodynamic and neutron diffraction investigation on the single crystal of TmMgGaO$_4$, which entails non-Kramers Tm$^{3+}$ ions arranged on a geometrically perfect triangular lattice. The crystal electric field (CEF) randomness caused by the site-mixing disorder of the nonmagnetic Mg$^{2+}$ and Ga$^{3+}$ ions, merges two lowest-lying CEF singlets of Tm$^{3+}$ into a ground-state (GS) quasidoublet. Well below $T_c$ $\sim$ 0.7 K, a small fraction of the antiferromagnetically coupled Tm$^{3+}$ Ising quasidoublets with small inner gaps condense into two-dimensional (2D) up-up-down magnetic structures with continuously distributed order parameters, and give rise to the \emph{columnar} magnetic neutron reflections below $\mu_0H_c$ $\sim$ 2.6 T, with highly anisotropic correlation lengths, $\xi_{ab}$ $\geq$ 250$a$ in the triangular plane and $\xi_c$ $<$ $c$/12 between the planes. The remaining fraction of the Tm$^{3+}$ ions remain nonmagnetic at 0 T and become uniformly polarized by the applied longitudinal field at low temperatures. We argue that the similar model can be generally applied to other compounds of non-Kramers rare-earth ions with correlated GS quasidoublets.