Spin-Peierls instabilities of antiferromagnetic rings in a magnetic field

TL;DR

This paper investigates the spin-Peierls instability in antiferromagnetic rings under magnetic fields, revealing strong distortions at higher level crossings and angular-dependent effects that align well with experimental data.

Contribution

It introduces a microscopic spin model coupled to lattice distortions via Dzyaloshinskii-Moriya interaction, highlighting new instability behaviors and angular effects in magnetic rings.

Findings

Stronger tendency to distort at higher level crossings

Sharp torque anomalies near in-plane magnetic fields

Good agreement with experimental torque and NMR data

Abstract

Motivated by the intriguing properties of magnetic molecular wheels at field induced level crossings, we investigate the spin-Peierls instability of antiferromagnetic rings in a field by exact diagonalizations of a microscopic spin model coupled to the lattice via a distortion dependent Dzyaloshinskii-Moriya interaction. We show that, beyond the unconditional instability at level crossings for infinitesimal magnetoelastic coupling, the model is characterized by a stronger tendency to distort at higher level crossings, and by a dramatic angular dependence with very sharp torque anomalies when the field is almost in the plane of the ring. These predictions are shown to compare remarkably well with available torque and Nuclear Magnetic Resonance data on CsFe8.