Quantum phase diagram of theS=12triangular-lattice antiferromagnetBa3CoSb2O9

Abstract

The magnetic phases of the ideal spin-1/2 triangular-lattice antiferromagnet Ba3CoSb2O9 are identified and studied using 135,137 Ba nuclear magnetic resonance (NMR) spectroscopy in magnetic fields ranging to 30T, oriented parallel and near perpendicular to the crystallographic ab-plane. For both directions, the saturation field is approximately 33T. Notably, the NMR spectra provide microscopic evidence for the stabilization of an up-up-down spin configuration for in-plane fields, giving rise to an one-third magnetization plateau (Msat/3), as well as for a higher field phase transition near to ∼ (3/5)Msat for both field orientations. Phase transitions are signaled by the evolution of the NMR spectra, and in some cases through spin-lattice relaxation measurements. The results are compared with expectations obtained from a semi-classical energy density modeling, in which quantum effects are incorporated by effective interactions extracted from the spin-wave analysis of the two-dimensional model. The interlayer coupling also plays a significant role in the outcome. Good agreement between the model and the experimental results is achieved, except for the case of fields approaching the saturation value applied along the c-axis.

Magnetic phase diagram ofBa3CoSb2O9as determined by ultrasound velocity measurements

“…Previous work showed the existence of several phases for this field direction but only obtained a rough schematic phase diagram based on several data points. 27 As shown in Fig. 5, with the field applied normal to the basal plane (H c ) only one phase transition is observed (in our range of magnetic field) at H c1 = 12.0 T .…”

“…Based on recent ESR and NMR measurements, 10,27 there is now a clear consensus that Ba 3 CoSb 2 O 9 should rather be described as a quasi-2D triangular-lattice antiferromagnet with a weak easy-plane anisotropy.…”

“…27 Thus, a detailed study of the field-induced magnetic phase diagram down to very low temperatures can potentially shed light on the effect of quantum fluctuations on the properties of frustrated triangular antiferromagnets. To that end, we have used highly sensitive sound velocity measurements down to 50 mK with a magnetic field applied parallel and perpendicular to the basal plane.…”

“…Recent NMR relaxation 27 and magnetization 10 measurements also find clear evidence of an additional phase transition around H a3 = 22 T , which is not accounted for in isotropic triangular lattice models. Koutroulakis et al 27 have proposed that this is a transition between two different kinds of V-like orders, the primary difference being the relationship between spins in adjacent ab-planes, although the NMR spectra cannot unambiguously differentiate these two states.…”

“…Koutroulakis et al 27 have proposed that this is a transition between two different kinds of V-like orders, the primary difference being the relationship between spins in adjacent ab-planes, although the NMR spectra cannot unambiguously differentiate these two states. 27 It is possible that this additional phase could be the result of a distortion associated with magnetostriction effects which break the hexagonal symmetry whenever the field is applied in the basalplane. 25 However, for the time being there is no experimental evidence that these field induced deformations are sufficiently large to play a significant role in the magnetic properties of Ba 3 CoSb 2 O 9 .…”