Surface effects in α-Fe2O3 nanoparticles studied by ILEEMS and TMS

Abstract

To understand the particular properties of thin α-Fe 2 O 3 films, hematite nanoparticles with sizes between 20 and 140 nm have been examined by integral lowenergy electron Mössbauer spectroscopy (ILEEMS) and transmission Mössbauer spectroscopy (TMS) at room temperature and 80 K. By comparing the results of both Mössbauer variants, surface effects of the Morin transition are studied. The results clearly indicate that the Morin transition takes place at higher temperature in the interior of the particles as compared to the outer layers. It is found that the lowering of the Morin transition is much stronger at the surface of smaller particles.

“In rust we trust”. Hematite – the prospective inorganic backbone for artificial photosynthesis

“…Bulk hematite is weakly ferromagnetic at room temperature, but below the Morin transition temperature at 260 K, a rst order spin reorientation becomes important, [65][66][67][68] and there is a loss in net magnetic moment and an antiferromagnetic transformation. On the other hand, in nanocrystalline hematite (size $200 nm) no spin reorientation is observed upon cooling, and ferromagnetic behavior persists to T ¼ 2 K. [69][70][71][72][73][74] The use of nanoscale hematite for solar energy applications is largely determined by its electronic properties, particularly by the widening of the band gap energy and to a lesser extent by quantum connement effects. 75 Due to strongly localized Fe 3d states, it is very unlikely that quantum connement effects will be observed in nanoscale hematite.…”

Size-dependence of the heat capacity and thermodynamic properties of hematite (α-Fe2O3)

“…The net magnetic moment is lost in this process, and bulk hematite transforms into an antiferromagnet. In contrast, magnetization studies on nanocrystalline and mesoporous samples of hematite show that no spin reorientation occurs upon cooling [14][15][16][17][18][19][20][21], and ferromagnetic behavior persists as low as T = 2 K. Many studies also show that hematite nanoparticles display superparamagnetic properties [22][23][24].…”