Magnetometric exploration of the phase diagrams of Yb-based delafossites

The Yb-based delafossites NaYbCh₂ (Ch = O, S, Se) are planar triangular-lattice spin systems with a trigonal crystal structure (space group R-3m). In these compounds, a strong spin-orbit coupling, combined with crystalline-electric-field effects, leads to a pronounced magnetic anisotropy and a pseudospin-1/2 spin-liquid ground state at low temperatures. The chalcogen series provides the possibility for tuning the interlayer distance and the associated exchange couplings by changing the chemical composition. The absence of magnetic long-range order at zero field down to lowest temperatures is strongly suggestive of a quantum spin-liquid ground state. Relaxation measurements by means of µSR and NMR have shown persistent strong fluctuations down to 100 mK at low magnetic fields. Based on specific-heat and magnetization experiments, we have observed magnetic order for out-of-plane fields exceeding 2 T for all three compounds. For in-plane fields of several tesla, a plateau-like feature in the magnetization indicates an up-up-down spin arrangement [1-3]. Furthermore, our measurements up to fields of 30 T allow to probe the saturation fields and polarized moments and, thus, the determination of the anisotropic exchange couplings [4]. Our ²³Na NMR measurements of NaYbSe₂ aim to probe the microscopic details of the field-induced magnetic structure in this compound. Measurements of the field-dependent transition temperature to long-range order via the 1/𝑇₁-relaxation rate are in agreement with the specific-heat results. The in-plane up-up-down spin arrangement is leading to an asymmetric broadening of the NMR spectra. At elevated out-of-plane fields, an umbrella-type configuration of the magnetic moments is predicted and in agreement with a symmetric broadening of the ²³Na NMR spectra. Low-field measurements reveal a monotonic low-temperature increase of the 1/𝑇₁-relaxation rate and spectral broadening, without any signature of long-range order down to 0.3 K.

[1] M. Baenitz et al., Phys. Rev. B 98, 220409(R) (2018)
[2] K. M. Ranjith et al., Phys. Rev. B 99, 180401(R) (2019)
[3] K. M. Ranjith et al., Phys. Rev. B 100, 224417 (2019)
[4] B. Schmidt et al., Phys. Rev. B 103, 214445 (2021)