Publication: Spin-dependentant transport in oxide multiferroic tunnel junctions
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2014-11-06
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Universidad Complutense de Madrid
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Abstract
A lo largo de esta tesis se han investigado las propiedades interfaciales de óxidos de metales de transición con orden ferroico usando estructuras de unión túnel La0.7Sr0.3MnO3/BaTiO3/ La0.7Sr0.3MnO3. Al cambiar la dirección de la polarización ferroleléctrica de la barrera túnel se produce una electroresistencia túnel superior al 1000%, lo que evidencia una fuerte asimetría en las uniones túnel LSMO/BTO/LSMO. Se ha comprobado que dicho efecto es mediado por un efecto de dopado electrónico generado por vacantes de oxígeno. Asimismo, cuando la dirección de la polarización ferroeléctrica apunta hacia abajo se ha encontrado que la magneto resistencia túnel a bajos voltajes está fuertemente deprimida como resultado de un filtrado de espín en la corriente túnel a consecuencia del momento magnético inducido en el Ti acoplado antiferromagneticamente con el momento magnético del Mn. Este efecto se desvanece cuasndo la polarización ferroeléctrica se invierte debido a un vaciamiento de carga en la interfase inferior evidenciando un fuerte acoplamiento magneto-eléctrico interfacial mediado por las vacantes de oxígeno. Nuestros resultados muestran la importancia de las vacantes de oxígeno en las interfases de la heteroestructuras de óxidos complejos, y su fuerte influencia en el transporte túnel. Se ha comprobado que es posible modificar la corriente túnel dependiente de espín usando campos eléctricos gracias al carácter ferroelectrico de la barrera túnel. Con esto se demuestra que estas propiedades interfaciales en las uniones túnel multiferroicas basadas en óxidos de metales de transición no son solo interesantes desde un punto de vista fundamental, sino que también pueden ser de gran utilidad a la hora de diseñar nuevos dispositivos funcionales en el campo de la espintrónica
Throughout this thesis we have explored the interfaces properties between functional oxides using tunnel junction architecture exploring the variation of the spin dependent transport by an electric field through the reversal of the ferroelectric polarization of the barrier. We found large tunnel electro resistance of near 1000% switching the direction of the ferroelectric polarizations revealing an asymmetry in the LSMO/BTO/LSMO tunnel junction. The reversal of the ferroelectric polarization of the BTO causes accumulation or depletion of the electron density generated by the oxygen vacancies at the bottom interface to screen the polarization charges giving rise to a significant modulation of the width of the tunneling barrier. Furthermore, for the down-polarization, for which lower resistance values are found, tunnel magnetoresistance (TMR) is also strongly depressed, as a result of a depolarization of the tunneling current. When the ferroelectric polarization is pointing upward the magnetic moment disappears because the charge density at the bottom interface is depleted cancelling the negative spin filter effect and reaching larger TMR values at low bias. Due to the interfacial charge density generated by the oxygen vacancies, the ferroelectric polarization reversal produces a large modulation of the TMR at low bias revealing a strong magneto-electric coupling in our LSMO/BTO/LSMO heterostructures. Our results reveal the importance of oxygen vacancy at the interfaces in complex oxide heterostructure, and its huge effect in tunneling transport. We verified that it is possible to modify the spin-dependent transport using electric fields due to the ferroelectric character of the tunnel barrier. We demonstrated that these interfacial properties in all-oxide multiferroic tunnel junction are not only interesting from fundamental point of view but are also important to design novel functional spintronic devices
Throughout this thesis we have explored the interfaces properties between functional oxides using tunnel junction architecture exploring the variation of the spin dependent transport by an electric field through the reversal of the ferroelectric polarization of the barrier. We found large tunnel electro resistance of near 1000% switching the direction of the ferroelectric polarizations revealing an asymmetry in the LSMO/BTO/LSMO tunnel junction. The reversal of the ferroelectric polarization of the BTO causes accumulation or depletion of the electron density generated by the oxygen vacancies at the bottom interface to screen the polarization charges giving rise to a significant modulation of the width of the tunneling barrier. Furthermore, for the down-polarization, for which lower resistance values are found, tunnel magnetoresistance (TMR) is also strongly depressed, as a result of a depolarization of the tunneling current. When the ferroelectric polarization is pointing upward the magnetic moment disappears because the charge density at the bottom interface is depleted cancelling the negative spin filter effect and reaching larger TMR values at low bias. Due to the interfacial charge density generated by the oxygen vacancies, the ferroelectric polarization reversal produces a large modulation of the TMR at low bias revealing a strong magneto-electric coupling in our LSMO/BTO/LSMO heterostructures. Our results reveal the importance of oxygen vacancy at the interfaces in complex oxide heterostructure, and its huge effect in tunneling transport. We verified that it is possible to modify the spin-dependent transport using electric fields due to the ferroelectric character of the tunnel barrier. We demonstrated that these interfacial properties in all-oxide multiferroic tunnel junction are not only interesting from fundamental point of view but are also important to design novel functional spintronic devices
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Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 13-05-2014