Field-induced spin level crossings within a quasi-XY antiferromagnetic state in Ba$_{2}$FeSi$_{2}$O$_{7}$

TL;DR

This study investigates the complex magnetic behavior of Ba$_{2}$FeSi$_{2}$O$_{7}$ under high magnetic fields, revealing spin level crossings, phase diagram features, and multiferroic effects in a strongly anisotropic quantum magnet.

Contribution

It provides the first detailed high-field phase diagram of Ba$_{2}$FeSi$_{2}$O$_{7}$, demonstrating spin level crossings within an ordered state and their influence on multiferroic properties.

Findings

Observation of a magnetic field-induced spin level crossing within an ordered state.

Identification of two dome-shaped magnetic phases around 30 T.

Correlation of electric polarization changes with magnetic phase boundaries.

Abstract

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 non-monotonic dependence 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 state of the lower-field dome is predominantly a linear combination of $| S^{z} = 0 \rangle$ and $ |S^{z} = 1 \rangle$ states, while the ground state of the higher-field dome can be approximated by a linear combination of $| S^{z} = 1 \rangle $ and $ | S^{z} = 2\rangle$ states. At 30 T, where the spin levels cross, the magnetization exhibits a slanted plateau, {\color {black}the magnetocaloric effect shows a broad hump, and the electric polarization shows a weak slope change}. We determined the detailed magnetic phase boundaries and the spin level crossings using measurements of magnetization, electric polarization, and the magnetocaloric effect in pulsed magnetic fields to 60 T. We calculate these properties using a mean field theory based on direct products of SU(5) coherent states and find good agreement. Finally, we measure and calculate the magnetically-induced electric polarization that reflects magnetic ordering and spin level crossings. This multiferroic behavior provides another avenue for detecting phase boundaries and symmetry changes.