Mössbauer spectroscopy of superparamagnetic Fe3O4 nanoparticles

• Mössbauer spectra at 300 K of Fe 3 O 4 nanoparticles clearly show superparamagnetic A- and B-sites. • A magnetic field produces ferrimagnetic hyperfine splitting in the conducting cubic phase. • Low temperature spectra show the insulating monoclinic structure, hence a Verwey transition. Spherical, mono-disperse, non-interacting iron oxide (Fe 3 O 4 ) nanoparticles, synthesized by high-temperature hydrolysis of chelated iron alkoxide complexes, have been studied by Mössbauer spectroscopy. The critical diameter for room temperature superparamagnetism, an important parameter for high frequency biomedical (MRI) and IT applications, was about 11 nm. Particles of diameter 11.9 nm and greater are ferrimagetic and showed magnetic splitting. Particles of diameter 10.6 nm and smaller are superparamagnetic and gave a non-magnetic spectrum at room temperature. The lines narrow as the particle size decreases and the spin relaxation rate increases. For the smallest particles (8.6 nm or less) the room temperature spectra could be resolved into two partially overlapping lines, one from the A-sites and one from the B-sites, the latter being broadened by the nuclear quadrupole interaction. Similar spectra have been previously reported for bulk Fe 3 O 4 above the Curie point. The isomer shifts showed anomalies possibly arising from magnetostrictive atomic displacements. On applying a magnetic field of 20 kG, hyperfine splitting was observed, confirming that the particles are single-domain with large magnetic moments of the order of 10 4 μ B . The in-field spectrum was similar to that of bulk crystals above the Verwey temperature with Fe 3+ on the A-sites and Fe 2.5+ on the B-sites, characteristic of the inverse cubic spinel structure. The lines narrowed as the temperature was decreased until magnetic splitting was observed at temperatures below the blocking temperature T B, where the magnetic fluctuations are blocked. The transition to the magnetic state is smooth, confirming that the particles are mono-disperse. At the lowest temperature (6 K) the spectra resemble those of bulk Fe 3 O 4 with Fe 3+ on the A-sites and both Fe 2+ and Fe 3+ on the B-sites corresponding to the local monoclinic distortion, indicating that the nanoparticles have undergone a Verwey transition. The values of T B found are lower than those reported by most other researchers, suggesting that magnetic interactions between our particles are small. The non-stoichiometry parameter, x, defined by the formula Fe 3-x O 4 , was estimated from the relative amounts of Fe 3+ / Fe 2+ and from isomer shifts to be less than 0.1 in the different samples.