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Ferroelectric Control of Spin Polarization - Structural investigations at the atomic scale

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Garcia et al.have recently reported on the ferroelectric control of spin polarization in an artificial multiferroic heterostructure1, i.e. Fe/BaTiO3/La2/3Sr1/3MnO3 (Fe/BTO/LSMO). This contribution is considered as a breakthrough in the field of spintronics since it opens new perspectives to create a magnetoelectric coupling which can be obtained in heterostructures combining an ultrathin ferroelectric layer between of two ferromagnetic electrodes. It has been for the first time demonstrated experimentally the opportunity to tune the spin polarization by an electric field in these artificial multiferroic tunnel junctions.

Rapidly growing developments of the magnetic recording technology have been possible thanks to the discovery of the giant magnetoresistance (GMR)2 and the following emergence of spintronics in the 90’s. Spintronics focus on physical properties for which a strong interplay exists between charge and spin of electrons yielding the development of a large number of devices for magnetic field sensing and magnetic data storage, i.e. the magnetic random access memories (MRAM). A current drawback in spintronics devices remains the large power required for magnetic writing. These limitations can be overcome applying different strategies, e.g. by varying the spin-relation rate, however these effects are usually small and volatile.

In the present work, Garcia et al.design artificial multiferroic tunnel heterostructures in which spin polarization can be electrically controlled in a low-power and a non-volatile fashion. Ferroelectric tunnel junctions with ferromagnetic electrodes were used to demonstrate local, large, and non-volatile control of carrier spin polarization by electrically-switching ferroelectric polarization, i.e. a spin-based information control

Mechanisms at the origin of the spin polarization modulation by ferroelectricity are based on interfacial effects in oxide heterostructures. Structural and electronic investigations of these interfaces are therefore required at the atomic scale. The Fe/BTO interface structural features of the Ta/Fe/BTO/LSMO heterostructure were investigated using the NION UltraSTEM 100 scanning transmission electron microscope (STEM). Atomically-resolved HAADF image of the Fe/BTO allowed the distinction of the perovskite lattice, where the A-site columns (Ba atoms) are observed as bright spots while the B-site columns (Ti atoms) appears as relatively darker spots (see Figure). Subtle changes in the structural, chemical, and electronic features of the Fe/BTO interfacial could be evidenced over less than one nanometer. These first results yield complementary investigations on these promising oxide heterostructures. Further experiments combining imaging and spectroscopy studies are in progress for a better understanding of the microstructural, structural and electronic properties of the Fe/BTO interface, which is of main importance regarding the ferroelectric control of Fe spin polarization.

This study was partially supported by France-UK PMC Alliance program, French RTRA Triangle de la Physique, EU STRP Macomufi, EU STRPCoMePhS, UK EPSRC EP/E026206/I, French C-Nano Île de France, French ANR Oxitronics, French ANR Alicante, the European ESTEEM, and the METSA networks.This work results from a strong collaboration between two French teams, i.e. UMjCNRS/Thalès and the LPS STEM groups, the English department of materials from the University of Cambridge, and the German Bessy synchrotron research center.

1 “Ferroelectric Control of Spin Polarization” V. Garcia, M. Bibes, L. Bocher, S. Valencia, F. Kronast, A. Crassous, X. Moya, S. Eouz-Vedrenne, A. Gloter, D. Imhoff, C. Deranlot, N. D. Mathur, S. Fusil, K. Bouzehouane, A. Barthélémy. Accepted in Science, 2010, DOI: 10.1126/science.1184028

2 Discovery of the giant magnetoresistance awarded in 2007 with the physic Nobel prize attributed to Albert Fert and Peter Grünberg