Anticollinear order and degeneracy lifting in square lattice antiferromagnet LaSrCrO4

TL;DR

This study investigates the unusual anticollinear magnetic order in LaSrCrO4, revealing how subtle interactions lift degeneracy and stabilize a non-collinear antiferromagnetic structure in a frustrated square-lattice system.

Contribution

It demonstrates the role of biquadratic interlayer exchange and dipolar interactions in degeneracy lifting, providing a detailed experimental and theoretical analysis of magnetic order in LaSrCrO4.

Findings

Discovery of anticollinear magnetic order below 170 K.

Identification of a 0.5 meV spectral gap and spin-flop transition.

Evidence that weak interactions (~10^{-4} meV) select the magnetic structure.

Abstract

We report the static and dynamic magnetic properties of LaSrCrO$_4$, a seemingly canonical spin-3/2 square-lattice antiferromagnet that exhibits frustration between magnetic layers -- owing to their AB stacking -- and offers a rare testbed to investigate accidental-degeneracy lifting in magnetism. Neutron diffraction experiments on single-crystal samples uncover a remarkable anticollinear magnetic order below $T_N$ = 170 K characterized by a N\'eel arrangement of the spins within each layer and an orthogonal arrangement between adjacent layers. To understand the origin of this unusual magnetic structure, we analyze the spin-wave excitation spectrum by means of inelastic neutron scattering and bulk measurements. A spectral gap of 0.5 meV, along with a spin-flop transition at 3.2\, T, reflect the energy scale associated with the degeneracy-lifting. A minimal model to explain these observations requires both a positive biquadratic interlayer exchange and dipolar interactions, both of which are on the order of 10$^{-4}$ meV, only a few parts per million of the dominant exchange interaction $J_1 \approx 11$ meV. These results provide direct evidence for the selection of a non-collinear magnetic structure by the combined effect of two distinct degeneracy lifting interactions.