Presaturation phase with no dipolar order in a quantum ferro-antiferromagnet

Abstract

Magnetization, magnetocaloric, calorimetric, neutron and X-ray diffraction and inelastic neutron scattering measurements are performed on single crystals of BaCdVO(PO4)2. The low-temperature crystal structure is found to be of a lower symmetry than previously assumed. The result is a more complicated model spin Hamiltonian, which we infer from measurements of the spin wave dispersion spectrum. The main finding is a novel spin state which emerges in high magnetic fields after antiferromagnetic order is terminated at Hc1 4.0 T. It is a distinct thermodynamic phase with a well-defined phase boundary at Hc2 6.5 T and is clearly separate from the fully saturated phase. Yet, it shows no conventional (dipolar) magnetic long range order. We argue that it is fully consistent with the expectations for a quantum bond-nematic state. * vvivek@phys.ethz.ch † zhelud@ethz.ch; http://www.neutron.ethz.ch/ arXiv:1908.01734v3 [cond-mat.str-el]

Quantum magnetism of ferromagnetic spin dimers in $α$-KVOPO$_4$

“…4, inset). Similar features have been reported in BaCdVO(PO 4 ) 2 [36][37][38] and assigned to a spin-nematic state expected in a strongly frustrated square-lattice antiferromagnet in the vicinity of saturation [39]. However, in the α-KVOPO 4 case a more trivial explanation, the distribution of saturation fields depending on the field direction, could be an equally plausible reason for the residual curvature above H s .…”

Phase diagram and spin waves in the frustrated ferro-antiferromagnet SrZnVO(PO4)2

“…The former compound is highly frustrated [11,12] with a much reduced ordered moment in zero field [13], and a very broad pre-saturation phase. This phase appears in all field orientations and clearly lies above the single-magnon instability field [9,12]. All the information available to date is consistent with it being a quantum spin-nematic.…”

“…To date, unconventional pre-saturation phases have been identified in two species, namely BaCdVO(PO 4 ) 2 [9] and Pb 2 VO(PO 4 ) 2 [10]. The former compound is highly frustrated [11,12] with a much reduced ordered moment in zero field [13], and a very broad pre-saturation phase.…”

Magnon binding in BaCdVO(PO$_4$)$_2$

“…There are several candidate materials for the realisation of a bond-nematic phase [16][17][18][19][20] , with one of the most promising being BaCdVO(PO 4 ) 2 [21][22][23][24] . Intriguingly, as the field applied to BaCdVO(PO 4 ) 2 is lowered through the saturation value of approximately 6.5T, there is an apparently continuous phase transition from the fullypolarised state to a partially-polarised but otherwise magnetically-silent state [22][23][24] . This phase persists down to approximately 4T, where there is a transition to a lowfield magnetically-ordered state.…”

“…This phase persists down to approximately 4T, where there is a transition to a lowfield magnetically-ordered state. At the same time the material is strongly frustrated, with competition between ferromagnetic nearest-neighbour exchange interactions and antiferromagnetic second-neighbour exchange [21][22][23][24] . Taken together, this is suggestive of the formation of a bond-nematic state in a field window of approximately 4-6.5T, but not conclusive proof.…”