Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


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New functional oxides for electronic or spintronic applications

Research of new functional oxides for electronic or spintronic applications

Due to the strong correlations of their 3d electrons, transition-metal oxides possess rich diagram phases. The competition between charges, orbital, spin and lattice degrees of freedom originates in different seek properties, like giant magnetoresistance, multiferroicity or superconductivity.
The strong interplay between electronic and structural properties makes these materials easily react to an external perturbation, and thus be highly functional. One of our goals is to perform theoretical studies, using ab initio numerical methods (based on the density functional theory, DFT) to propose new compounds for specific applications, or to propose a way to enhance the performances of existing compounds.

Spinel ferrites for spintronics:
Ferrites with the spinel structure MFe2O4 can present different electronic and magnetic behaviors, depending on their chemical composition.
Magnetite, Fe3O4, has been predicted to be half-metallic at room temperature, i.e. to display a -100% spin-polarization. With its ferrimagnetic ordering and its high Curie temperature (858 K), this material is thus an ideal candidate for spintronic applications (magnetic tunnel junctions, spin injection…). We have studied during the last years how the electronic and magnetic properties of this oxide could be modified by the presence of structural defects such as oxygen or cation vacancies, or antiphase boundaries.
Other parent compounds (Mn-, Co-, Ni-ferrites) are also ferrimagnetics with high Curie temperatures, but are insulating. They have been proposed as interesting materials for spin-filtering applications or to create extrinsic multiferroics. We are studying the magnetic (manetocrystalline anisotropy) and dielectric properties variations as a function of their cation distributions, the presence of structural defects or under epitaxial strain.


Figure 1: Density of states of Fe3O4 with (blue) and without (red) an oxygen vacancy.

References:
R. Arras, L. Calmels, and B. Warot-Fonrose, Electronic structure near an antiphase boundary in magnetite, Phys Rev B 81, 104422 (2010).
R. Arras, L. Calmels, and B. Warot-Fonrose, Half-metallicity, magnetic moments, and gap states in oxygen-deficient magnetite for spintronic applications, Appl. Phys. Lett. 100, 032403 (2012).
R. Arras, B. Warot-Fonrose, and L. Calmels, Electronic structure near cationic defects in magnetite J. Phys. Condens. Matter 25, 256002 (2013).

Spinel oxides for photovoltaic applications:
Collaboration: C. Tenailleau (CIRIMAT, Toulouse)
Using oxides could be a good choice to create photovoltaic cells: Abundants on earth, environementally-friendly, chemically stable, relatively simple to synthetise, and possessing highly-tunable electronic properties, Mn- and Co-based spinel oxides have been proposed for such applications and have been studied for many years by the team of C. Tenailleau (CIRIMAT, Toulouse). One of the main goals was to determine the structures and chemical composition which could give the best properties in terms of visible-light absorbance and electric conductivity.
We have recently calculated the electronic structure of the (Mn,Co)3O4 for the whole range of Mn/Co composition. The evolution of band-gap widths was related to the chemical composition, the magnetic ordering and the structural distortions of the lattice. This theoretical study performed in the MEM group was done in collaboration with C. Tenailleau, experimentalist in the group OVM (“mixed-valence oxides”) of the CIRIMAT laboratory (Toulouse).


Figure 2: Calculated energy differences which may correspond to optical transition in MnxCo3-xO4, as a function of the composition x.

References:
R. Arras, T. L. Le, S. Guillemet-Fritsch, P. Dufour and C. Tenailleau, First-principles electronic structure calculations for the whole spinel oxide solid solution range MnxCo3-xO4 (0 ≤ x ≤ 3) and their comparison with experimental data, Phys. Chem. Chem. Phys. 18, 26166 (2016).

These projects were granted access to the HPC resources of CALMIP supercomputing center under the allocation 2012-2017 [p1313].

(Multi-)functional oxide-based interfaces

Since the early 2000’s, the growth of oxide thin films has improved sufficiently to enable a good control of the interface terminations and of the film thicknesses. This had for consequences to open new opportunities in creating surprising interfacial properties. A perfect example is the emergence of a two-dimensional electron gas (2DEG) at the interface between the two polar oxides LaAlO3 and SrTiO3(001). At this same interface, other interesting properties have been then put to light, such as giant magnetoresistance, magnetism or superconductivity. Because of the dimensionality effects, the particular chemical environment, and the symmetry breakings, interfaces can be the place of emerging new properties, absent from bulk compounds. Interfaces can also allow to couple different functionalities, through strain effects or via the interfacial chemical bonds, as it is the case in extrinsically multiferroic heterostructures.

Two-dimensional electron gas (2DEG):
Most of the proposed conductive oxide-based interfaces have concerned perovskites and only few studies have proposed conductive interfaces with other oxide structures, the appearance of a 2DEG in oxide heterostructures remaining still rare. The study of oxide interfaces which display a controllable insulator-to-metal transition (IMT) could however open the route to new technologies with high industrial impact.
In this thematic we are interested in designing new interfaces which can display a 2DEG with specific properties (higher conductivity, high spin-polarization...).
We have for example studied theoretically by the means of ab initio calculation the ability of stabilizing a spin-polarized two-dimensional electron gas SP2DEG between the two insulating spinel oxides CoFe2O4 and MgAl2O4(001). Due to the cation distributions in these two oxides and their polar character, a charge discontinuity is present at the interface, inducing the appearance of an internal electric field diverging as a function of the thickness of CoFe2O4. Similarly to the catastrophe scenario proposed for the LaAlO3/SrTiO3(001) interface, an insulating-to-metal transition occurs at a critical thickness of CoFe2O4 (calculated around 1.9 Å), with a transfer of electron toward the Fe3+ cations. This transfer of charge only involves minority spin electrons, and due to the high TC of CoFe2O4 (793 K), we can thus expect that such 2DEG can remain spin-polarized at room temperature.


Figure 3: The (001) interface between CoFe2O4 (inverse spinel) and MgAl2O4 (normal spinel) presents a +1/-1 charge discontinuity (left panel). A built-in potential is then created between the two interfaces and lead to the so-called “polar catastrophe scenario” when the two materials have their thickness increased above a critical value tcrit (bottom-right panel): An electronic reconstruction occurs (here modeled by a charge transfer from the Co-atoms of the interface p to the Fe-atoms of interface n), as it can be seen on the band structures (top-right panel). The created electronic states at the Fermi level are fully spin-polarized and low effective masses, suggesting the stabilization of a SP2DEG at the n-type interface. Figure adapted from Ref. Phys. Rev. B 90, 045411 (2014).

References:
R. Arras and L. Calmels, Fully spin-polarized two-dimensional electron gas at the CoFe2O4/MgAl2O4(001) polar interface, Phys. Rev. B 90, 045411 (2014).

Extrinsic multiferroics:
Collaboration : S. Cherifi (IPCMS, Strasbourg)
Multiferroic materials present simultaneously at least two ferroic orderings. In this material family, the compounds possessing both a (ferro)magnetic and a ferroelectric ordering are very desirable as they can display a high magnetoelectric coupling. Such property would allow switching the magnetization by applying an external electric field, which would induce a drastic reduction of the energy cost for such process and then for the functioning of some spintronic devices.
In the framework of a collaboration with S. Cherifi (IPCMS, Strasbourg), we have calculated the effects linked with the magnetoelectric coupling at the interface between the ferroelectric oxide Pb(Zr,Ti)O3 and a metallic film of Co. The variations of spin magnetic moments and in the electronic structure as a function of the polarization state (up or down) have been calculated and explained by the change in the cation-cation distances at the interface. The contribution of the chemical bonds to the magnetoelectric coupling at the interface has been analyzed: the strength of these bonds (due to the orbital overlaps) changes as a function of the electric-polarization reversal, inducing some change in the induced magnetic moments at the interface.

Figure 4: a) Rectangular magnetoelectric hysteresis loop obtained in Co/PbZrTiO3 bilayers. The figure inset corresponds to the related atomic structure of the bilayer as deduced from DFT calculations. b) Densities of states calculated for the atomic layers at the interface and for the two polarization states. Figure adapted from ACS applied materials & interfaces 8, 7553 (2016).

Reference:
O. Vlašín, R. Jarrier, R. Arras, et al., Interface magnetoelectric coupling in Co/Pb(Zr,Ti)O3, ACS Appl. Mater. Interfaces 8, 7553 (2016).

These projects were granted access to the HPC resources of CALMIP supercomputing center under the allocation 2012-2017 [p1229].