Atomic resolution studies of
L
a0.7
S
r0.3
M
n
O
3/
B
a
T
i
O
3 multiferroic tunnel junctions
Authors: Cabero Mariona , Perez‐Muñoz Ana , Hernández David , Sefrioui Zohuair , Abrudan Radu , Valencia Sergio , Pennycook Steve John , León Carlos , Varela Maria , Santamaría Jacobo 
Published: December 2016

In complex oxide heterostructures, the interplay between magnetic, electric and transport properties often results in novel functionalities [1‐4]. The introduction of controlled densities of oxygen vacancies may allow further tuning of the interface magnetic structure, providing a new path towards enhanced device functionalities. In this work, we have fabricated multiferroic tunnel junctions combining a La
0.7
Sr
0.3
MnO
3
(LSMO) ferromagnetic manganite electrodes with BaTiO
3
(BTO) ferroelectric tunnel barriers. High quality heterostructures have been produced by high oxygen pressure RF sputtering system. In particular, we have grown samples where an ultrathin layer of a La‐Sr cuprate with a nominal composition of oxygen deficient La
0.84
Sr
0.16
CuO
3‐x
(LSCO) is inserted in between the BTO and the top LSMO electrode. The transport properties of such asymmetric LSMO/BTO/LSCO/LSMO junctions should be highly tunable via the application of external fields, since the physical properties of cuprates are highly sensitive to doping.

Aberration‐corrected scanning transmission electron microscopy (STEM) combined with electron energy‐loss spectroscopy (EELS) has been used to study the structure, chemistry and electronic properties of our systems, with atomic resolution in real space. These techniques constitute powerful tools to inspect cuprate/manganite interfaces, where charge transfer may take place [5].

Low magnification images such as the one in figure 1(a) show that the resulting magnetic tunnel junctions exhibit flat layers over long lateral distances. Atomic resolution images in figure 1(b) and 1(c) exhibit a high crystal quality and coherent growth. Figure 2(a) displays atomic resolution compositional maps obtained across the stacking, for the different elements La, Ti, Mn, Ba, Cu and O. Figure 2(b) exhibits the O K edge signal across the heterostructure, normalized and integrated laterally. A clear dip is observed within the cuprate layers, suggesting that the LSCO ultrathin film stores a large density of oxygen vacancies in the structure. Such thin cuprate layers are insulating, ensuring an electronic screening asymmetry. This behavior could yield values in excess of 10
4
% upon polarization switching, since ferroelectric switching would involve the motion of those oxygen vacancies. Vacancy accumulation would also affect the electronic structure of the interfaces. In fact, values of the tunneling magnetoresistance (TMR) of 125% have been obtained along with a change in the sign of the TMR when the BTO polarization is reversed. This finding (which is not observed in symmetric LSMO/BTO/LSMO structures) suggests that polarization switching has an effect on the spin polarization of the interface, related to the presence of the LSCO layer and the O vacancies. In summary, our results suggest that tailoring the spin polarization at interfaces by growing asymmetric heterostructures with O vacancy–rich thin cuprate layers may provide new ways to design the spin filtering devices of the future.