Electron Spin Resonance in Magnetic Nanoparticles. Effects of Temperature and Interparticle Interactions
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Electron Spin Resonance in Magnetic Nanoparticles. Effects of Temperature and Interparticle Interactions N. Noginova1, F. Chen1, A. Andreyev1, J. McClure1, E. P. Giannelis2, A. B. Bourlinos2, and V. A. Atsarkin3 1 NSU, Norfolk, VA, 23504 2 Cornell University, Ithaca, NY, 14853 3 IRE, Moscow, 103907, Russian Federation Abstract. The electron spin resonance technique has been applied to study the magnetization dynamics in broad temperature range in both concentrated magnetic fluid and diamagnetically diluted systems containing superparamagnetic maghemite nanoparticles. The ESR spectrum demonstrates an interesting double feature shape with a narrow peak near g=2, as well as “forbidden” resonances in low fields, and reveals strong dependence on temperature and nanoparticle concentration. The longitudinal spin-relaxation time was estimated. To describe the overall spectrum shape, a “quantization” model is used which involves summation of the resonance transitions corresponding to various orientations of the particle magnetic moment. Introduction. Magnetic nanoparticles attract considerable interest due to their unusual magnetic properties and many promising applications, such as in nanoscale engineering, catalysis, mineralogy, biology, and medicine. Among many publications on magnetic nanoparticles, there are a considerable number of studies performed by electron magnetic resonance (EMR) [1-17]. The theory of magnetic resonance in superparamagnetic systems has been developed in Refs. [1-3], based on the phenomenological equation of motion for a classical magnetic moment µ under conditions of ferromagnetic resonance (FMR). However, agreement between experimental data and theoretical predictions is rather poor, in particular for low temperature range, and does not provide an opportunity of accurate quantitative analysis of the experimental results. The goal of this work is the detailed experimental study of diluted and dense systems with magnetic nanoparticles, and search for a theoretical approach to describe the magnetization dynamics in such systems. Experimental. The samples were prepared using the solvent-free ferrofluid containing surface functionalized maghemite (γ-Fe2O3 ) nanoparticles of d ~ 5 nm diameter. Such ferrofluid has been produced by attaching a corona of flexible chains onto maghemite nanoparticles [18]. To prepare experimental samples with different concentrations, the nanoparticles were dispersed in liquid (toluene) and solid (polystyrene) matrices. The electron magnetic resonance studies were performed using EPR Spectrometer Bruker EMX operating at 9.8 GHz (X band) with modulation frequency of 100 kHz. The closed-circle cryostat was used for temperature studies (6K-380K). The longitudinal relaxation time (T1) was measured by modulation technique using a home-made apparatus with detection of longitudinal magnetization component oscillating at modulation frequency 1.6 MHz [19]. Both the "phase" and "amplitude" versions were employed; in the latter case, the diphenylpicrylhydrazyl (DPPH) wa
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