Partial Up-Up-Down Order with the Continuously Distributed Order Parameter in the Triangular Antiferromagnet TmMgGaO4

Abstract

We show that frustrated quasidoublets without time-reversal symmetry can host highly unconventional magnetic structures with continuously distributed order parameters even in a single-phase crystal. Our study comprises a comprehensive thermodynamic and neutron diffraction investigation on the single crystal of TmMgGaO4, which entails non-Kramers Tm3+ ions arranged on a geometrically perfect triangular lattice. The crystal electric field randomness caused by the site-mixing disorder of the nonmagnetic Mg2+ and Ga3+ ions merges two lowest-lying crystal electric field singlets of Tm3+ into a ground-state quasidoublet. Well below Tc∼0.7 K, a small fraction of the antiferromagnetically coupled Tm3+ Ising quasidoublets with small inner gaps condense into two-dimensional up-up-down magnetic structures with continuously distributed order parameters, and give rise to the columnar magnetic neutron reflections below μ0Hc∼2.6 T, with highly anisotropic correlation lengths, ζab≥250a in the triangular plane and ζc