Eur. Phys. J. B 35, 357-364 (2003)
https://doi.org/10.1140/epjb/e2003-00288-x

Electron spin resonance studies on quantum tunneling in spinel ferrite nanoparticles

Department of Physics, National Tsing Hua University, Hsin Chu 300, Taiwan

Corresponding author: ^a jtlue@phys.nthu.edu.tw

Received: 12 May 2003
Revised: 29 August 2003
Published online: 15 October 2003

Abstract

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.