Magnetic properties of hematite nanoparticles

The magnetic properties of hematite (alpha-Fe2O3) particles with sizes of a bout 16 nm have been studied by use of Mossbauer spectroscopy, magnetizatio n measurements, and neutron diffraction. The nanoparticles are weakly ferro magnetic at temperatures at least down to 5 K with a spontaneous magnetizat ion that is only slightly higher than that of weakly ferromagnetic bulk hem atite. At T greater than or similar to 100 K the Mossbauer spectra contain a doublet, which is asymmetric due to magnetic relaxation in the presence o f an electric field gradient in accordance with the Blume-Tjon model, Simul taneous fitting of series of Mossbauer spectra obtained at temperatures fro m 5 K to well above the superparamagnetic blocking temperature allowed the estimation of the pre-exponential factor in Neel's expression for the super paramagnetic relaxation time, tau(0) = (6 +/- 4) X 10(-11) s and the magnet ic anisotropy energy barrier, E-bm/k = 590+/-(150)(120) K. A lower value of the pre-exponential factor, tau(0) = 1.8+/-X-3.2(1.3) 10(-11) s, and a sig nificantly lower anisotropy energy barrier E-bm(magn)/k = 305 +/- 20 K was derived from simultaneous fitting to ac and de magnetization curves. The di fference in the observed energy barriers can be explained by the presence o f two different modes of superparamagnetic relaxation which are characteris tic of the weakly ferromagnetic phase. One mode involves a rotation of the sublattice magnetization directions in the basal (111) plane, which gives r ise to superparamagnetic behavior in both Mossbauer spectroscopy and magnet ization measurements. The other mode involves a fluctuation of the net magn etization direction out of the basal plane, which mainly affects the magnet ization measurements.