Abstract
The epsilon Fe2O3 phase of iron oxide has been studied to understand the spin structure and the magnetocrystalline anisotropy in the bulk and in thin films of ε-Fe2O3 and Co-doped ε-Fe2O3. The preferential magnetization direction in the nanoparticles of ε-Fe2O3 is along the a-axis [M. Gich et al., Chem. Mater. 18, 3889 (2006)]. Compared to the bulk band gap of 1.9 eV, the thin-film band gap is reduced to 1.3 eV in the Co-free films and to 0.7 eV in the film with partial Co substitution. The easy magnetization direction of the bulk and Co-free ε-Fe2O3 is along the c-axis, but it switches to the a-axis on Co substitution. All three systems exhibit in-plane anisotropies associated with the orthorhombic crystal structure of the oxide.
Highlights
The metastable oxide ε-Fe2O3, a rare polymorph of Fe2O3, has recently attracted interest as a material for photoelectrochemistry and as the only magnetoelectric Fe2O3 phase.[1]
Coercivities Hc as high as 20 kOe [2 T] have been reported for ε-Fe2O3 nanoparticles,[4] but these high coercivities may reflect the small spontaneous magnetization Ms rather than a high anisotropy constant K1,6 since Hc ∼ 2K1/Ms The coercivity of 20 kOe of nanoparticles of ε-Fe2O3 is reduced to an in-plane coercivity of 8 kOe7 in epitaxially stabilized (001) thin films on SrTiO3 (111)
Compared to the bulk band gap of 1.9 eV, the thin-film band gap is reduced to 1.3 eV in the Co-free films and to 0.7 eV in the film with partial Co substitution
Summary
The metastable oxide ε-Fe2O3, a rare polymorph of Fe2O3, has recently attracted interest as a material for photoelectrochemistry and as the only magnetoelectric Fe2O3 phase.[1]. The interatomic exchange coupling is A-type antiferromagnetic, characterized by the spin directions (β, α, α, β) for the (FeA, FeB, FeC, FeD) atoms. Since the four Fe atoms are nonequivalent, the spin structure is ferrimagnetic, with a magnetization of about 0.1 T and a Curie temperature of 510 K. It is only partially clear whether the spin structure is collinear or noncollinear.[1,3,4] Recent in-field Mossbauer experiments[5] indicate collinear ferrimagnetism. To understand the coercive behavior of ε-Fe2O3, it is necessary to explain and describe the magnetocrystalline anisotropy of the oxide
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