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 \(\epsilon \)-Fe₂O₃ nanoparticles have been studied by the nuclear forward scattering technique using synchrotron radiation. The sample consists of isolated \(\epsilon \)-Fe₂O₃ nanoparticles with an average size of \(\langle d\rangle = 3.8\) nm immobilized in a SiO₂ 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 \(\epsilon \)-Fe₂O₃ 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) \(\epsilon \)-Fe₂O₃ particles, and the magnetic structure is noncollinear in the range of 4–300 K.