Background: In nano-size α-Fe2O3 particles, the Morin transition temperature was reported
to be suppressed. This suppression of the TM in nano-size α-Fe2O3 was suggested to be due to high
internal strain and to the enhanced role of surface spins because of the enhanced surface to volume
ratio. It was reported that for nanoparticles of diameters less than 20 nm, no Morin transition was
observed and the antiferromagnetic phase disappears. In addition, annealing of samples was reported
to result in both an increase of TM and a sharper transition which were attributed to the reduction in
defects, crystal growth, or both.
Objective: In this work, we investigated the role of applied magnetic field in TM, the extent of the
Morin transition, thermal hysteresis, and the spin-flop field in synthetic α-Fe2O3 nanoparticles of
diameter around 20 nm.
Methods: Hematite nanoparticles were synthesized using the sol-gel method. Morphology and structural
studies of the particles were done using TEM, and XRD, respectively. The XRD patterns confirm
that the particles are hematite with a very small maghemite phase. The average size of the nanoparticles
is estimated from both TEM images and XRD patterns to be around 20 nm. The magnetization
versus temperature measurements were conducted upon heating from 5 K to 400 K and cooling
down back to 5 K at several applied fields between 50 Oe and 500 Oe. Magnetization versus magnetic
field measurements between -5 T and +5 T were conducted at several temperatures in the temperature
range of 2-300 K.
Results: We report three significant findings in these hematite nanoparticles. Firstly, we report the
occurrence of Morin transition in hematite nanoparticles of such size. Secondly, we report the slight
field dependence of Morin transition temperature. Thirdly, we report the strong temperature dependence
of the spin-flop. Zero-field-cooled magnetization versus temperature measurements were conducted
at several applied magnetic fields.
Conclusion: From the magnetization versus temperature curves, Morin transition was observed to
occur in all applied fields at Morin transition temperature, TM which is around 250 K with slight field
dependence. From the magnetization versus magnetic field curves, spin-flop in the antiferromagnetic
state was observed and found to be strongly temperature dependent. The results are discussed in
terms of three components of the magnetic phase in our sample. These are the paramagnetic, soft
ferromagnetic, and hard ferromagnetic components.