Direct observation of interfacial coupling of oxygen octahedra and its impact on ferromagnetic order in La0.7Sr0.3MnO3/SrTiO3 epitaxial heterostructures

Abstract

La 0.7 Sr 0.3 MnO 3 /SrTiO 3 (LSMO/STO) heterostructures have been intensively studied because of large values of tunnel magnetoresistance (TMR), which make it a promising material for spintronic devices based on magnetic tunneling junctions. However, the “dead‐layer” behavior of the manganite layers, a quick decay of the magnetoresistance and metallicity with the decreasing thickness and eventually insulating behavior below certain critical thickness (4‐7 u.c.), hinder the applications. The origins of this behavior were reported to be strongly related to the interfacial reconstruction, such as magnetic and orbital coupling, epitaxial strain, and carrier doping charge‐effect.[1‐2] Recently it has been also recognized that the metal‐oxygen bonding modification due to oxygen octahedral tilt can also play an important role in the magnetism double‐exchange interaction.[3‐4] However, an atomistic understanding of how oxygen octahedral distortions are introduced to accommodate octahedral mismatch at a heterointerface is still a challenge because it requires a precise simultaneous determination of the positions of metal cations and oxygen. We used the annular bright‐field (ABF) imaging in a Cs‐corrected scanning transmission electron microscope (STEM, JEOL ARM200F) to image directly both heavy cations and oxygen atoms in the real space. Two samples grown on cubic STO(100) substrate were studied: (a) a sandwich‐like sample consisting of two LSMO layers of 10 and 20 u.c thick, separated by a 3 u.c thick STO middle layer; (b) a multilayer sample with LSMO‐STO consisting of 15 bilayers of 3‐4 u.c LSMO and 6 u.c. STO. Magnetometry measurements showed that the LSMO layers of the trilayer sample had large Curie temperature for the onset of ferromagnetic order (larger than 250 K) while the LSMO‐STO multilayer had a very low Curie temperature of less than 50 K and a very low magnetization, indicating very poor magnetic order. In Fig.1(a), a high magnification STEM‐ABF image of LSMO at the 2 nd layer (20 u.c.) is shown which is far from the strained interface and well present a bulk‐like LSMO structure. The corresponding atomic model of LSMO with MnO 6 tilt a ‐ a ‐ c ‐ is also shown under the ABF image, where we can see clearly the oxygen octahedra with the anti‐phase tilting of ~10° along [110] axis. The quantitative analysis of oxygen octahedral tilt angle is done for both samples, as shown in Fig.1 (b) and (c). At the interface between STO substrate and LSMO in both samples, a clear suppression of oxygen octahedral tilt of LSMO in the first 2‐3 u.c. is found, and then the tilt gradually recovers when it is away from the interface, until reaching the bulk value at around 9 th u.c. (as seen in the Fig.1 (b) 2 nd LSMO layer). Moreover, the 3 u.c. thick LSMO layers in the multilayer are highly distorted and have a suppressed octahedral tilt of lower than 5°, which can well explain the degradation of their ferromagnetic order and transport properties. Interestingly, we also found that the top 2‐3 u.c. of the STO have a non‐zero apparent oxygen octahedra tilt, especially in the middle layer, of up to ~3°, and this finding correlates with the previous reported magnetic moment found at the interfacial Ti cations. [1] Here, we directly evidenced the strong relation between the oxygen octahedral tilt and the magnetism. We found a greatly suppressed tilting in LSMO due to the interfacial coupling of adjacent oxygen octahedral of cubic STO, which gives rise to dramatic deterioration of the magnetic and transport properties when the LSMO layer is thinner than 3 u.c.. Therefore, for good TMR properties of LSMO‐STO heterostructures, a critical thickness of LSMO of at least 7 u.c. (3 u.c. away from both interfaces) is required for preserving ferromagnetism, and a critical thickness of STO of at least 5 u.c. (2 u.c. away from both interfaces) for a good insulating barrier.