Electron spin resonance studies on quantum tunneling in spinel ferrite nanoparticles
Department of Physics, National Tsing Hua University, Hsin Chu 300,
Corresponding author: a email@example.com
Revised: 29 August 2003
Published online: 15 October 2003
The electron spin resonance (ESR) spectrometer, a very sensitive instrument with fast detecting window to explore quantum phase transitions for magnetic nanoparticles, was exploited to study the fascinating interplay between thermal and quantum fluctuations in the vicinity of a quantum critical point. We have measured ESR in ferrofluid samples containing nanosize particles of Fe2O3. The evolution of the ESR spectrum with temperature suggests that quantum tunneling of spins occurs in single domain magnetic particles in the low temperature regime. The effects of various microwave fields, particle sizes, and temperatures on the magnetic states of single domain spinel ferrite nanoparticles are investigated. We can consistently explain experimental data assuming that, as the temperature decreases, the spectrum changes from superparamagnetic (SPR) to blocked SPR and finally evolves quantum superparamagnetic behaviour as the temperature lowers down further. A nanoparticle system of a highly anisotropic magnetic material can be qualitatively specified by a simple quantum spin model, or by the Heisenberg model with strong easy-plane anisotropy.
PACS: 76.30.-v – Electron paramagnetic resonance and relaxation / 75.40.Cx – Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.) / 05.30.-d – Quantum statistical mechanics / 75.50.Dd – Nonmetallic ferromagnetic materials
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2003