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

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.

arXiv comment13 pages, 8 figures, REVTEX4

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Cited by in corpus (5)

• Spin Jahn-Teller antiferromagnetism in CoTi${}_2$O${}_5$
• Magnetocaloric and electrocaloric properties of the Hubbard pair cluster
• Spin-Peierls instabilities of antiferromagnetic rings in a magnetic field
• Huge Transverse Magnetization in the Field-Induced Phase of the Antiferromagnetic Molecular Wheel CsFe8
• Chaotic dynamics and spin correlation functions in a chain of nanomagnets