Superparamagnetism and ferrimagnetism in the Sr₂FeMoO_6–δ nanoscale powder

As a result of using combined synthesis modes and optimized conditions for ultrasonic dispersion, a single-phase nanosized Sr2FeMoO6–δ powder with a high degree of superstructural ordering of Fe/Mo cations (88%) with an average grain size of 70.8 nm was obtained. Based on the results of Mössbauer spectroscopy and magnetic measurements, it was established that the obtained nanosized strontium ferromolybdate powder is in a magnetically inhomogeneous state, consisting of superparamagnetic and ferrimagnetic phases. The ferrimagnetic component of magnetization is characterized by higher values of magnetization in comparison with the superparamagnetic component, and a smooth increase is noted for this component, which reaches saturation with decreasing temperature. It is shown that there is no exchange magnetic interaction between superparamagnetic grains in the superparamagnetic phase, which made it possible, based on the Neel-Brown model, to estimate the critical sizes of nanoparticles, dSPM, in the single-domain state. The obtained dSPM values are smaller than the sizes of single-domain particles, which confirms the absence of a frozen state in some of the superparamagnetic particles. The results of this work are important for understanding the effect of nanosized grains on the magnetic properties and features of magnetization of the Sr2FeMoO6–δ polydisperse powder that is very promising for spintronic device application.