Field-induced level crossings in spin clusters: Thermodynamics and magneto-elastic instability

TL;DR

This paper investigates how quantum spin clusters with antiferromagnetic interactions exhibit level crossings under magnetic fields, and how perturbations like anisotropy and spin-phonon coupling induce magneto-elastic instabilities affecting their thermodynamic properties.

Contribution

The study derives a two-level Hamiltonian for spin clusters near level crossings, incorporating perturbations and magneto-elastic effects, revealing instability conditions and magnetic behavior.

Findings

Magneto-elastic instabilities occur at level crossings due to magnetic anisotropy.

Thermodynamic functions are calculated using the effective two-level Hamiltonian.

Perturbations significantly influence the stability and magnetic response of spin clusters.

Abstract

Quantum spin clusters with dominant antiferromagnetic Heisenberg exchange interactions typically exhibit a sequence of field-induced level crossings in the ground state as function of magnetic field. For fields near a level crossing, the cluster can be approximated by a two-level Hamiltonian at low temperatures. Perturbations, such as magnetic anisotropy or spin-phonon coupling, sensitively affect the behavior at the level-crossing points. The general two-level Hamiltonian of the spin system is derived in first-order perturbation theory, and the thermodynamic functions magnetization, magnetic torque, and magnetic specific heat are calculated. Then a magneto-elastic coupling is introduced and the effective two-level Hamilitonian for the spin-lattice system derived in the adiabatic approximation of the phonons. At the level crossings the system becomes unconditionally unstable against lattice distortions due to the effects of magnetic anisotropy. The resultant magneto-elastic instabilities at the level crossings are discussed, as well as the magnetic behavior.