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Temperature Dependence of Electron Magnetic Resonance Spectra of Iron Oxide Nanoparticles Mineralized in Listeria Innocua Protein Cages



Robert J. Usselman


Electron magnetic resonance (EMR) spectroscopy was used to determine the magnetic properties of maghemite (gamma-Fe2O3) nanoparticles formed within size-constraining Listeria innocua Dps protein cages that have an inner diameter of 5 nm. Variable temperature X-band EMR spectra exhibited broad asymmetric resonances with a superimposed narrow peak at a gyromagnetic factor of g{approximately equal} 2. The resonance structure, which depends on both superparamagnetic fluctuations and inhomogeneous broadening, changes dramatically as a function of temperature and the overall line width becomes narrower with increasing temperature. Here, we present comparisons of two different models to simulate temperature dependent line shape trends. The temperature dependence for both models is derived from a Langevin behavior of the line width resulting from “anisotropy melting.” The first uses a truncated log-normal distribution of particle sizes and a Landau-Liftshitz line shape to describe the nanoparticle resonances. The essential feature of this model is that small particles have narrow line widths and account for the {Ig/I} {approximately equal} 2 feature with a constant resonance field, whereas larger particles have broad line widths and undergo a shift in resonance field. The second model assumes uniform particles with a diameter around 4 nm and a random distribution of uniaxial anisotropy axes. This model uses a more precise calculation of the line width due to superparamagnetic fluctuations and a random distribution of anisotropies. Sharp features in the spectrum near {Ig/I} {approximately equal} 2 are qualitatively predicted at high temperatures. Both models can account for many features of the observed spectra but each has deficiencies. The first model leads to a nonphysical increase in the magnetic moment as the temperature is increased, but a temperature dependent magnetic moment was resolved by introducing a bi-modal distribution of particle sizes.
Physical Review B


EMR, EPR, FMR, iron oxide nanoparticles, protein cages


Usselman, R. (2012), Temperature Dependence of Electron Magnetic Resonance Spectra of Iron Oxide Nanoparticles Mineralized in Listeria Innocua Protein Cages, Physical Review B, [online], (Accessed May 23, 2024)


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Created October 16, 2012, Updated November 10, 2018