Pressure-Tuned Metamagnetism and Emergent Three-Body Interactions in CsFeCl$_3$

TL;DR

This study investigates how high magnetic fields and pressure influence the magnetic phases of CsFeCl3, revealing emergent three-body interactions and complex magnetization behavior in a quantum antiferromagnet.

Contribution

It introduces a novel high-field spin model incorporating three-body interactions, explaining asymmetric fractional steps in magnetization not captured by traditional models.

Findings

Observation of a magnetization plateau and sharp metamagnetic transition.

Emergence of fractional magnetization steps under high pressure.

Identification of three-body interactions in the effective spin model.

Abstract

We present a combined experimental and theoretical study of the triangular-lattice quantum antiferromagnet CsFeCl$_3$ under high magnetic fields and high pressure. Pulsed-field magnetization for the magnetic field along the symmetric $c$ direction at ambient pressure reveals a magnetization process from a nonmagnetic singlet ground state with a nearly linear increase between 3.7 and 10.7 T, a plateau-like region, and then a sharp stepwise metamagnetic transition near 32 T. Wide frequency--field range electron spin resonance indicates that the low-field regime originates from the $J = 1$ manifold, while the high-field metamagnetic transition suggests a level crossing between the $J = 1$ and $J = 2$ lowest states. Pulsed-field magnetic susceptibilities measured with a proximity detector oscillator under high pressure show that the low-field nonmagnetic singlet phase is gradually suppressed, while the high-field metamagnetic transition evolves into an increasingly rich pattern of fractional steps. While the observations at low to intermediate fields can be understood within the established spin-1 description, the high-field regime requires a new perspective, which we provide through a projected spin-1/2 framework built from Zeeman-selected crystal-field states not related by time reversal. This construction naturally allows emergent three-body interactions on triangular plaquettes and explains the asymmetric evolution of the fractional steps in the magnetization. Our findings reveal that high-field effective spin models in quantum magnets with separated yet accessible crystal-field multiplets are not constrained to even-body couplings, but can naturally host odd-body terms, opening a broader avenue for realizing field-asymmetric magnetization processes and exotic phases beyond conventional even-body physics.