Heteroepitaxial spinel ferrites NiFe 2 O 4 and CoFe 2 O 4 films have been prepared by pulsed laser deposition (PLD) at various temperatures (175 -690 °C) under ozone/oxygen pressure of 10 mTorr. Due to enhanced kinetic energy of ablated species at low pressure and enhanced oxidation power of ozone, epitaxy has been achieved at significantly lower temperatures than previously reported. Films grown at temperature below 550 °C show a novel growth mode, which we term "vertical step-flow" growth mode. Epitaxial spinel ferrite films with atomically flat surface over large areas and enhanced magnetic moment can be routinely obtained.Interestingly, the growth mode is independent of the nature of substrates (spinel MgAl 2 O 4 , perovskite SrTiO 3 , and rock salt MgO) and film thicknesses. The underlying growth mechanism is discussed.
The electronic structures of (Fe,Mn)3O4
spinel oxides have been investigated by employing soft-x-ray absorption
spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD). We have
determined the valence states as well as the occupation sites of Mn and Fe ions in
Fe0.9Mn2.1O4
and MnFe2O4. Fe0.9Mn2.1O4
is found to be close to the inverse spinel (the inversion parameter
y≈0.85), while
MnFe2O4 is close to the
normal spinel (y≈0.2). In Fe0.9Mn2.1O4, Fe ions are mainly trivalent and the majority of
Fe3+
ions occupy the octahedral B sites, while Mn ions are mixed-valent with approximately 45%
MnA2+ at the tetrahedral A
sites and 55% MnB3+ ions at
the octahedral B sites. In MnFe2O4, Mn ions are mainly divalent and the majority of
Mn2+
ions occupy the tetrahedral A sites, while Fe ions are mainly trivalent and the majority of
Fe3+
ions occupy the octahedral B sites.
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