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 TmMgGaO4, 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 Tc ∼ 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 columnar magnetic neutron reflections below µ0Hc ∼ 2.6 T, with highly anisotropic correlation lengths, ξ ab ≥ 250a in the triangular plane and ξc < 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.

Detailed structural investigation of Ba 2 CoGe 2 O 7 was performed in its low-temperature multiferroic state combining neutron diffraction with magnetization measurements and the optical study of lattice vibrations on single crystals. The crystal structure above (10.4 K) and the crystal and magnetic structures below (2.2 K) the antiferromagnetic transition temperature of T N = 6.7 K were determined using neutron diffraction. The tetragonal space group (SG) P -42 1 m, corresponding to the average structure at room temperature, was found to also describe the structure at low temperatures well. Neutron diffraction data and infrared phonon mode analysis imply no structural phase transition down to 2.2 K. Orthorhombic polar SG Cmm2 is proposed as a true crystal structure. Below T N , the spins of the Co 2+ ions form a square-lattice Néel order within the (a,b) plane, while their alignment is ferromagnetic along the c axis. The magnitude of the ordered moment, fully lying within the (a,b) plane, is found to be 2.9(1) μ B /Co 2+ and the easy axis of the sublattice magnetizations corresponds to the [110] direction. A noncollinear spin structure due to small canting is allowed.

The crystal structure of a new superconductor UTe 2 has been investigated using single-crystal neutron diffraction for the first time at the low temperature (LT) of 2.7 K, just above the superconducting transition temperature of $1.6 K, in order to clarify whether the orthorhombic structure of type Immm (No. 71), reported for the room-temperature (RT) structure persists down to the superconducting phase and can be considered as a parent symmetry for the development of spin-triplet superconductivity. In contrast to the previously reported phase transition at about 100 K [Stö we (1996). J. Solid State Chem. 127, 202-210], our high-precision LT neutron diffraction data show that the bodycentred RT symmetry is indeed maintained down to 2.7 K. No sign of a structural change from RT down to 2.7 K was observed. The most significant change depending on temperature was observed for the U ion position and the U-U distance along the c direction, implying its potential importance as a magnetic interaction path. No magnetic order could be deduced from the neutron diffraction data refinement at 2.7 K, consistent with bulk magnetometry. Assuming normal thermal evolution of the lattice parameters, moderately large linear thermal expansion coefficients of about = 2.8 (7) Â 10 À5 K À1 are estimated.