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Magnetic oxide thin filmsB. Warot, V. Serin, L. Calmels, E. Snoeck, J.F. Bobo PhD : R. Arras Programme : GDR “Multiferroïque” (2007-2010) |
The magnetic oxides are studied to analyze the effects of strain resulting from the epitaxial growth, composition and morphology surface on the magnetic properties of the layers. The thin films are considered from an experimental point of view but also from a theoretical aspect with the calculation of electronic structures by first-principles methods. |
The sputtering chamber available at the laboratory can deposit thin epitaxial oxide layers (Fe3O4, CoFe2O4 ..) associated with different non-metallic layers according to the applications. The magnetic measurements are made by VSM and / or Kerr rotation. |
| The main work on thin oxide layers concern: -Thin layers of magnetite (Fe3O4) and cobalt ferrite (CoFe2O4) |
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Calculation of electronic structures of magnetic oxides The spinel ferrimagnetic oxides form a class of materials exhibiting original performances that make it possible to envisage their use in components for spintronic. These oxides have a wide variety of electronic structures and magnetic behaviour: magnetite is half-metallic and has a high Curie temperature, property that can be used in magnetoresistive devices. The cobalt ferrite CoFe2O4 is a ferrimagnetic insulator which can be used a spin filter in spintronic devices. The properties of these oxides strongly depend on their structural defects. |
For example, a slight modification of the Fe/Co ratio or a variation of the cationic sites occupation (octahedral or tetrahedral) in CoFe2O4 destroys the magnetic anisotropy of this oxide. Magnetoresistive effects in junctions containing magnetite layers are also disturbed by the presence of antiphase boundaries (APB). The electronic structure changes induced by changes in stoechiometry, cationic sites occupancy and by the APB should be analysed in detail to understand how these defects alter the ferrite physical behaviour. The electronic structure calculations are performed using the DFT code Wien2k in LSDA and LSDA + U approximations. |
Layer magnetite: Fe3O4
We are interested in the changes in antiphase boundary density
(APBs) in Fe3O4 thin films correlated with changes in magnetic
properties when the layers are deposited on two types of buffer
layers (Fe and Cr) themselves grown on MgO (001) Our structural analysis shows much lower APB density in Fe/Fe3O4
and Cr/Fe3O4 bilayers compared with that observed in single layers
(Figure 3). These differences in density are probably due the
difference between the Fe/Fe3O4 and Cr/Fe3O4 misfit and the MgO/Fe3O4
misfit. This low APB density reduces the antiferromagnetic interactions
between antiphase domains explaining the best magnetic properties
of magnetite layers deposited on Fe and Cr.
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Figure 1. Hysteresis cycles of Fe3O4 layers deposited on (a) MgO, (b) MgO / Fe and (c) MgO / Cr |
Figure 3. Antiphase boundaries observed in dark field mode on the Fe3O4 (20 nm) layers deposited on (a) MgO, (b) MgO / Fe (8 nm) and (c) MgO / Cr (4 nm) |
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 BiFeO3 layer deposited on LaSrMnO3/SrTiO3 |
Matériaux multiferroïques : BiFe3O4 Chemical and structural studies on these materials can validate growth conditions and explain magnetic behaviour. The structural analysis of a BiFeO3 layer deposited on LaSrMnO3 (Figure 4) by HREM gives information on the crystal symmetry and the state of strain in the layer, which is essential for understanding the origin of multiferroicity, property linked to cristalline distortion. Quantitative measurements show that the layer of BiFeO3 is strained in the growth plane and its pseudo- cubic cell undergoes a quadratic deformation. |
The chemical analysis on a FeTiO3 layer deposited on Al2O3 gives information on the quality of the interface between the substrate and eliminates any diffusion of titanium or iron in alumina and diffusion of aluminium in the magnetic layer. An analysis of the L23 iron edge provides information on the degree of oxidation of iron (2 + or 3 +) to control the composition of the deposited layer. |
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Spectres EELS à la traversée de l’interface Al2O3/FeTiO3 montrant l’absence de diffusion du titane et du fer |
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Publications : Effect of metallic buffer layers on the antiphase boundary
density of epitaxial Fe3O4 Tunnel magnetoresistance and robust room temperature exchange
bias with multiferroic BiFeO3 epitaxial thin films Growth of the magnetic semiconductor Fe2- xTixO3± d
thin films by pulsed laser deposition
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