Magnetic resonance in nanoparticles: between ferro- and paramagnetism

TL;DR

This study investigates the magnetic resonance properties of gamma-Fe2O3 nanoparticles across a temperature range, revealing insights into their anisotropy, relaxation times, and surface effects through electron magnetic resonance analysis.

Contribution

It provides a detailed analysis of EMR spectra of magnetic nanoparticles, introducing a quantization model and exploring surface anisotropy effects, which are novel in understanding nanoparticle magnetism.

Findings

Spectra become broader and shift to lower fields upon cooling.

A narrow spectral component obeys Arrhenius law with T_A ≈ 850 K.

Uniaxial magnetic anisotropy dominates, with formation of dipolar-coupled chains.

Abstract

Magnetic nanoparticles of gamma-Fe2O3 coated by organic molecules and suspended in liquid and solid matrices, as well as a non-diluted magnetic fluid have been studied by electron magnetic resonance (EMR) at 77-380 K. Slightly asymmetric spectra observed at room temperature become much broader, symmetric, and shift to lower fields upon cooling. An additional narrow spectral component (with the line-width of 30 G) is found in the diluted samples, its magnitude obeying the Arrhenius law with the activation temperature of about 850 K. The longitudinal spin-relaxation time, T1 >> 10 ns, was determined by the specially developed modulation method. Angular dependence of the EMR signal position in field-freezing samples unambiguously points to the domination of the uniaxial magnetic anisotropy. Substantial alignment is achieved in moderate freezing fields of 4-5 kG, suggesting formation of dipolar-coupled chains consisting from several particles separated by organic nanolayers. The shift and broadening of the spectrum upon cooling are ascribed to the role of the surface layer, which is considered with taking into acount the strong surface-related anisotropy. To describe the overall spectrum shape, a quantization model is used which includes summation of the resonances corresponding to varios orientations of the particle magnetic moment. This approach, supplemented with some phenomenological assumptions, provides satisfactory agreement with the experimental data.