Low-temperature magnetic properties of MgFe1.2Ga0.8O4 spinel nanoparticles

Abstract

A detailed study has been carried out of the static and dynamic magnetic properties of ferrimagnetic spinel MgFe1.2Ga0.8O4 nanoparticles (NPs), including the measurements of low-field dc-magnetization M(T) under zero-field-cooled (ZFC) and field-cooled (FC) conditions and ac-susceptibility χac(T) at different frequencies (f) and amplitudes of the ac-field (Hac). Below, a Curie temperature (TC), a clear magnetic irreversibility (MZFC(T) ≠ MFC(T)), as well as a broad peak in both MZFC(T)–MFC(T) magnetizations have been observed. At T ≪ TC, the two distinct, frequency-dependent peaks at Tp1 ≈ 9 K and Tp2 ≈ 14 K in the imaginary part of the ac-susceptibility χac''(T) have been found. Their positions have been consistent with those of the anomalies detected on the temperature dependence of the magnetic contribution to the heat capacity Cmag(T), and a sharp decrease of the MZFC(T) magnetization near ≈ 10–20 K. It was revealed that the fitting of the frequency dependence of the peaks’ positions (Tp1(f) and Tp2(f)) in the imaginary part of ac-susceptibility χac''(T) using the Neel-Arrhenius law yields unphysical values of the relaxation time (τ0) and, therefore, these results ruled out a possibility of an existence of superparamagnetic (SPM) clusters in the system. On the other hand, the Tp1(f) and Tp2(f) dependences followed both the Vogel-Fulcher law and the power law. These observations indicate that, a complex magnetic state in MgFe1.2Ga0.8O4, realized in the low temperature region, may likely be related to a progressive freezing of magnetic clusters.