The Low-Temperature Magnetic State and Magnetic Ordering Temperature of -Fe 2 O 3 Iron Oxide Nanoparticles

The -Fe2O3 iron oxide polymorph is a well-known magnetic material with a complex magnetic structure, which undergoes a series of magnetic transitions in different temperature ranges. However, the -Fe2O3 phase diagram is still unclear. We report on the magnetic properties of a sample consisting of -Fe2O3 nanoparticles with an average size of 8 nm embedded in a SiO2 xerogel matrix without an admixture of foreign phases. Along with the features typical of the well-known -Fe2O3 magnetic transition in the temperature range 80150 K, the temperature dependence of magnetization M(T) of -Fe2O3 includes other low-temperature anomalies. In an external field of H 70kOe, there is a noticeable temperature hysteresis of magnetization at 5090 K, and near T & approx; 50 K, the M(T) curves have a characteristic bending, which may be indicative of an additional magnetic transition. The ferromagnetic resonance spectra shows that, near 500 K, a magnetic phase transition occurs, which was previously thought to be a transition to the paramagnetic state. An analysis of the temperature dependence of the ferromagnetic resonance spectra shows that the magnetically ordered phase in -Fe2O3 exists up to about 800 K.