Zeeman Split Kramers Doublets in Spin-Supersolid Candidate Na2BaCo (PO4)2

Na2BaCo(PO4)2 is a triangular antiferromagnet that displays highly efficient adiabatic demagnetization cooling [Junsen Xiang et al., Nature (London) 625, 270 (2024)NATUAS0028-083610.1038/s41586-023-06885-w] near a quantum critical point at μ0Hc∼1.6 T, separating a low-field magnetically disordered from a high-field fully polarized ferromagnetic phase. We apply high resolution backscattering neutron spectroscopy in an applied field to study the magnetic excitations near μ0Hc. At large fields we observe ferromagnetic fluctuations that gradually transition to being overdamped in energy below μ0Hc where the magnetism is spatially disordered. We parametrize the excitations in the high-field polarized phase in terms of coupled Zeeman split Kramers doublets originating from the presence of spin-orbit coupling. On reducing the field, the splitting between the Kramers doublets is reduced and if done adiabatically, provides a mechanism for reducing temperature. On lowering the applied field through the μ0Hc the excitations characterize a textured phase that we suggest is inefficient for cooling. Low temperature disordered frustrated magnets built on Kramers doublets with nearby quantum critical points provide a route for efficient magnetocalorics.