Tuning the dead-layer behavior of La_0.67Sr_0.33MnO₃/SrTiO₃ via interfacial engineering

Abstract

Transition metal oxide hetero-structure has great potential for multifunctional devices. However, the degraded physical properties at interface, known as dead-layer behavior, present a main obstacle for device applications. Here we present the systematic study of the dead-layer behavior in La 0.67 Sr 0.33 MnO 3 thin film grown on SrTiO 3 substrate with ozone assisted molecular beam epitaxy. We found that the evolution of electric and magnetic properties as a function of thickness shows a remarkable resemblance to the phase diagram as a function of doping for bulk materials, providing compelling evidences of the hole depletion in near interface layers that causes dead-layer. Detailed electronic and surface structure studies indicate that the hole depletion is due to the intrinsic oxygen vacancy formation. Furthermore, we show that oxygen vacancies are partly caused by interfacial electric dipolar field, and thus by dopingengineering at the single-atomic-layer level, we demonstrate the dead-layer reduction in films with higher interfacial hole concentration. PACS numbers: 75.47.Lx, 75.30.Kz, 79.60.Dp The complexity in transition metal oxides and their heterostructures, due to the entangled correlation of charge, spin, orbital, and lattice degrees of freedom, is a double-edged sword. On one hand, it brings out emergent phenomena in condensed matter physics 1-4 and various possibilities for multifunctional device applications 5,6 . On the other hand, it often conceals the physical mechanism of new phenomena due to theoretical difficulties and hinders the solutions to problems in device applications. One typical example is La 0.67 Sr 0.33 MnO 3 /SrTiO 3 (LSMO/STO) hetero-structure. LSMO is well-known for the colossal magnetoresistance, half metallic behavior, room temperature ferromagnetism, and high conductivity. These exotic properties make LSMO the most promising material for metal-based spintronic device, magneto-tunneling junctions, magnetic memory, etc. Success has been achieved in developing LSMO-based field effect transistors 7 and metal-base transistors 8 . However, in spite of a few reported working devices, a large variety of applications are restricted due to the dead-layer behavior, that is, the degraded ferromagnetism and metallicity with decreasing thickness and eventually insulating below certain critical thickness 9,10 . Therefore, it is crucial to investigate the mechanism of the dead-layer behavior in LSMO/STO, for both device application and fundamental understanding of oxide interface.Intensive studies have been conducted, and several scenarios have been proposed to explain the dead-layer behavior, such as magnetic 11,12 and orbital reconstruction at the interface [13][14][15] , and substrate-induced strain 10,16 . However, the situation is still very perplexing, and many issues remain to be understood. For example, it is not clear why the complicated electric and magnetic properties in ultra-thin films are extremely sensitive to film thickness. Extrinsic imperfections induced during fabricati...

Correlating interfacial octahedral rotations with magnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices

“…In our case, the increase of the out-of-plane Mn-O-Mn bond angle with decreasing N is apparent in ( The current finding may provide a new insight to the 'dead' layer issue that are commonly found at the interface between manganite oxides and many perovskite substrates 36 , and has attracted renewed interests. In recent studies, orbital ordering, 37 polar discontinuity 38,39 , magnetic coupling to the titanate layer, 40 stoichiometry 41,42 and so on, have all been evidenced to couple to the interface magnetic property of the manganite layer, while a unified scenario is still lacking. Some recent studies on La 0.67 Sr 0.33 MnO 3 /SrTiO 3 interfaces have merged on the finding of increased amount of Mn 3 þ near the interface [43][44][45] and 3z 2 À r 2 orbital ordering irrespective of the strained state 37,45,46 .…”

Increased Curie Temperature Induced by Orbital Ordering in La0.67Sr0.33MnO3/BaTiO3 Superlattices

“…This is the so-called “dead layer,” defined as the thinnest layer for which ferromagnetic behavior is observed [6–8]. This dead layer phenomenon may be related to electronic and/or chemical phase separation [9, 10], to growth characteristics and microstructure [11, 12], or to manganese e g orbital reconstruction [13, 14].…”

Impact of interfacial coupling of oxygen octahedra on ferromagnetic order in La0.7Sr0.3MnO3/SrTiO3 heterostructures

“…thick film adjacent to STO19; as an alternative, the breakdown of ferromagnetism and metallic conductivity was attributed to the effect of strain and tetragonal distortions leading to spin canting and ultimately antiferromagnetic order132021; as a further alternative, phase separation into coexisting antiferromagnetic insulating and ferromagnetic metallic mesoscopic regions was proposed2223; moreover, interfacial orbital reconstruction might occur destroying the double-exchange interaction close to the interface242526; additionally, there might be interfacial charge transfer18272829. The intimate coupling between metallic conductivity and ferromagnetism that is implied by the double exchange mechanism30 could even be absent in very thin layers, since intrinsic defects such as cation disorder lead to the localization of charge carriers117.…”