Surface and interfaces of transition metal oxides

S. CARREIRA ; A. CARRERO LOBO; A. LO GIUDICE; A. LOPEZ PEDROSO; A. ROMAN; J BRIATICO; M. AGUIRRE; L.B. STEREN

Conferencia; International Workshop Spintronics 2019- SpinPeru; 2019

 The richvariety of physical properties of transition metal oxides (TMO) made thesematerials attractive not only for fundamental science but also for the designof multifunctional  devices [1]. Leftaside in  the first studies, therelevance  of surface and interfaces onthe properties of complex oxides thin films became evident after the discoveryof unexpected phenomena like exchange bias in ferromagnetic/paramagneticstructures [2] and 2D electron gases at insulator/insulator bilayers interfaces[3]. Complex oxides  are considered amodel of strongly-correlated electron materials, in which the charge, orbitaland spin degrees of freedom are closely related and extremely sensitive tostrain and surface symmetry breaking effects.[4]   During the last five years our work has been focused on  the investigation of magnetic andmultiferroic heterostructures, in particular to their interfacial and surfaceproperties.  We have particularlyinvestigated the influence of different type of coupling through interfaces,i.e. magnetic, magneto-elastic and magneto-electric, on the properties of thewhole structure. In my talk I will  firstintroduce the  general characteristics ofcomplex oxide thin films outlining the strong influence of strains onto their physicalproperties. Then I will present in detail our last results regarding thecharacterization and study of  magneticand multiferroic interfaces. In the first case,   La0.66Sr0.33MnO3/La1-xSrxMnO3(LSMO/LSxMO with X<0.1),  we haveanalysed  the strains,   charge distribution and spin polarizationthrough these interfaces using surface sensitive techniques combined with high-resolutiontransmission electron microscopy. We have examined the surface spin-polarization (Figure 1) and latticestrains (Figure 2) at LSMO/LSxMO interfaces changing the thickness of the low-doped LSxMO cappinglayer.  The spin-polarization was found tobe enhanced ata critical capping thickness that depends on the Sr doping, owing to the combined effects of chargedoping and structural strain. More recently, we revisited the complex interfacial magnetic effectsarisen at  La0.66Sr0.33MnO3/SrTiO3interfaces using Polarized Neutron Reflectometry in epitaxialheterostructures  in order to quantifythe magnetic moment atthe LSMO interfaces with a high spatial resolution. We have found, in theseexperiments new elements that provide us a deeper insigth in the LSMOinterfacial magnetism and its correlation between local structural strains.Thesecond case I will discuss is the artificial hybrid multiferroic interface, BaTiO3/FePt(BTO/FEPT). Depending on strains, the FePt alloy can be ordered or disordered ontwo different crystalline structures. In spite of the fact that our study hasbeen performed on polycrystalline samples, our results reveal  an important magneto-elastic coupling through these interfaces.  Insummary, I will show along my presentation that surfaces and interfaces oftransition metal oxides are a source of very interesting physics to learn andtake advantage of.