Electron Magnetic Resonance in Magnetic Nanoparticles: Dependence on the Particle Size and Applicability of the Modified Giant Spin Model

Abstract

Electron magnetic resonance is experimentally studied in dilute solid suspensions of iron oxide nanoparticles as the function of the particle size, and discussed in the frames of the modified giant spin approach. Gradual evolution of features specific for small nanoparticles, including a narrow component at the main resonance field and a weak half-field line, is observed with the increase in the particle size, manifesting the transition from quantum to fully classical behavior. The shape, width, position of the resonance spectra, and the specific features are described quantitatively with a single set of fitting parameters for nanoparticles of 5-20 nm size. Limitations of the quantum model at high spin values are discussed. IntroductionMagnetic nanoparticles (MNPs) attract a great interest due to various applications in a vast area from spintronics [1] to medicine [2]. This requires adequate quantitative description of the MNP features, and particularly the electron magnetic resonance (EMR) spectrum, which contains an important information on spin dynamics. Systems with small number of electron spins, such as individual spins of paramagnetic impurity or spin clusters with several spins coupled by strong exchange interactions, including molecular nanomagnets (MNMs), are commonly described with quantum approach. By increasing a number of coupled spins, one can expect a transition from purely quantum to classical behavior, with strongly different thermodynamics and a different shape of the electron magnetic resonance (EMR) spectrum. Magnetic nanoparticles containing hundreds or thousands of coupled spins are on the boundary between quantum and classical regimes. Both Zeeman and anisotropy energies of a single-domain MNP are comparable with the thermal energy k B T, resulting in a significant role of thermal fluctuations in the magnetization dynamics of MNPs. Electron magnetic resonance in such superparamagnetic nanoparticles is often described using the classical approach based on Landau-Lifshits equations for ferromagnetic resonance (FMR) taking into account rapid thermal fluctuations of MNP magnetic moments in the framework of rotary diffusion [3][4][5][6][7][8][9]. However, there are features in the EMR spectrum of small MNPs [10][11][12][13][14][15][16][17], which are not readily described by the classical approach, including a narrow component at the main resonance field, B 0 , and transitions at B 0 /n, n = 2, 3, 4. Such features are typical for electron paramagnetic resonance of single spins but not for classical FMR.Recently the "giant spin" approach was employed for description of the EMR properties of small MNPs [10]. In this approach, a nanoparticle is considered as a giant spin, S, originated from exchange coupled electron spins (equivalent to the magnetic moment µ of the nanoparticle). The EMR spectrum is the sum of the contributions from

Electron Radiation-Induced Nickel Nanoparticle Formation in KCl-ZnCl2 Eutectic Molten Salt Using X-ray Absorption Spectroscopy

“…33 Melting the powder sample increases the intensity of the main resonance line and shifts the position of the resonance line (g-factor) to higher values, which is indicative of a higher abundance of Ni 0 and a shorter distance between Ni atoms (spin coupling), respectively. The features for the high-temperature spectra consist of a high field (g ≈ 2.32) main resonance line, due to the FMR of the Ni MNP, and a small resonance at a lower field (g ≈ 3.97) (Figure 8b) due to the double absorption of photons by the metallic nanoparticles (Figure 8a), which is not present in the pristine NiCl 2 (black line), 34 confirming the fact that low-field resonance originates from metallic nanoparticles. At higher temperatures, the asymmetry of the main resonance line becomes more evident (Figure 8b), signifying an increasing metallic character of Ni in the samples.…”