Partial Up-Up-Down Order with the Continuously Distributed Order Parameter in the Triangular Antiferromagnet TmMgGaO_{4}

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 TmMgGaO_{4}, which entails non-Kramers Tm^{3+} ions arranged on a geometrically perfect triangular lattice. The crystal electric field randomness caused by the site-mixing disorder of the nonmagnetic Mg^{2+} and Ga^{3+} ions merges two lowest-lying crystal electric field singlets of Tm^{3+} into a ground-state quasidoublet. Well below T_{c}∼0.7 K, a small fraction of the antiferromagnetically coupled Tm^{3+} 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 μ_{0}H_{c}∼2.6 T, with highly anisotropic correlation lengths, ξ_{ab}≥250a in the triangular plane and ξ_{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 ground-state quasidoublets.