Field-induced quantum first-order transition in a two-dimensional frustrated antiferromagnet

Fujihara, Masayoshi ; Hagihara, Masato   ; Ishikado, Motoyuki*; Sari, D. P.*; Watanabe, Isao*; Higemoto, Wataru  ; Tabata, Chihiro; Kaneko, Koji   

We report a field-induced quantum first-order transition in the antiferromagnet KCoPOHO. Using highly oriented microcrystals with , polarized neutron diffraction directly reveals coexistence of an incommensurate cycloid and a commensurate collinear state, together with a discontinuous jump of the ordering wave vector from incommensurate to commensurate. Magnetization and specific heat show a step and a collapse of the anomaly near ~T with no measurable hysteresis, while LF-SR detects two-component relaxation and slow fluctuations around . The - phase diagram exhibits a narrow coexistence region. Symmetry of space group constrains the DM vector to the plane and, together with XY anisotropy, nearly degenerates the incommensurate and commensurate states, enabling a first-order switch in field. KCoPOHO thus serves as a platform for quantum first-order transitions in frustrated magnets.