Dynamic magnetization of γ- nanoparticles isolated in an amorphous matrix
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We have studied the magnetization of a system of γ-Fe2O3 (0.68 vol.%) nanoparticles isolated in an SiO2 amorphous matrix placed in an alternating magnetic field with a frequency of 640 Hz and in the temperature range of (77-300) K. Compared to temperatures closer to 300 K (where the system has a superparamagnetic behaviour), at lower temperatures, the magnetization has a dynamic hysteresis loop due to the magnetization's phase shift between the field and the magnetization. The delay of the magnetization (attributed to the Néel relaxation processes) increases with the decrease of temperature. It has been shown that the relaxation time resulting from the Néel theory is determined by an effective anisotropy constant (K ) that takes into account the magnetocrystalline anisotropy, as well as the shape, surface and strain anisotropies. In the following we will show that the surface and strain anisotropy components have the most significant influence. When the temperature decreases from 300 to 77 K, the relative increase of the saturation magnetization of the nanoparticles is much higher than that of the (spontaneous) saturation magnetization of bulk γ-Fe2O3. This increase is due to the increase of the mean magnetic diameter of the particles attached to the core of aligned spins, from 10.16 nm to 11.70 nm, as a result of the modification of the superexchange interaction in the surface layer.
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