Suppression of the Magnetic Transition in Ultrasmall ϵ-Fe2O3 Nanoparticles: the Size Effect from Nuclear Forward Scattering Data

The features of the magnetic structure of ultrasmall $$\epsilon $$ -Fe 2 O 3 nanoparticles have been studied by the nuclear forward scattering technique using synchrotron radiation. The sample consists of isolated $$\epsilon $$ -Fe 2 O 3 nanoparticles with an average size of $$\langle d\rangle = 3.8$$ nm immobilized in a SiO 2 xerogel matrix. The time-domain spectra have been measured in the temperature range of 4–300 K in zero external magnetic field and field $$H = 4$$ T applied in the longitudinal direction. The character of the change in the hyperfine field H hf as a function of the external magnetic field is the same in the entire temperature range: unlike large $$\epsilon $$ -Fe 2 O 3 particles, a monotonic increase in H hf is observed in the external field. These results indicate that there is no magnetic transition in the temperature range of 80–150 K for ultrasmall (smaller than $$ \approx $$ 9 nm) $$\epsilon $$ -Fe 2 O 3 particles, and the magnetic structure is noncollinear in the range of 4–300 K.