Field-induced spin level crossings within a quasi-$XY$ antiferromagnetic state in $Ba$2 $FeSi$2 $$O$$7
We present a high-field study of the strongly anisotropic easy-plane square lattice $$S$$ = 2 quantum magnet $Ba$2 $FeSi$2 $$O$$7. This compound is a rare high-spin antiferromagnetic system with very strong easy-plane anisotropy, such that the interplay between spin level crossings and antiferromagnetic order can be studied. We observe a magnetic field-induced spin level crossing occurring within an ordered state. This spin level crossing appears to preserve the magnetic symmetry while producing a nonmonotonic dependence of the order parameter magnitude. The resulting temperature–magnetic field phase diagram exhibits two dome-shaped regions of magnetic order overlapping around 30 T. The ground statemore »