Charge Transfer Control of Emergent Magnetism at SrMnO₃/LaMnO₃ Interfaces

Abstract

cial magnetic and electric phenomena [1][2][3][4] not present in the constituting layers. As the interplay of charge, spin, and lattice degrees of freedom [5] is believed to control phase transitions in bulk correlated materials, their interfacial reconstructions, should serve as guiding mechanisms for emergent phenomena. In particular, the charge transfer (CT)/leakage at the interface [6,7] with a following electronic reconstruction of them was thought to be intimately coupled to the formation of emergent phases. The (LaMnO 3 ) m / (SrMnO 3 ) n (LMO m /SMO n ) SLs can be viewed as one of the most intensively studied oxide heterostructures. [8,9] A longterm interest to them was motivated by the observations of interfacial or emergent ferromagnetism, [10][11][12][13][14] associated with the chemically sharp LMO(top)/SMO(bottom) interfaces. [10] Very recently, we reported a high temperature emergent ferromagnetic phase with Curie temperature T C = 360 K at the SMO/ LMO interfaces, [15] originated from the interfacial CT from the electron-rich LMO to the electron-poor SMO layers.An important but rather rarely addressed question is whether CT can be additionally influenced or controlled by the growth design of SLs, e.g. by tuning the thickness ratio of constituting layers. Garcia-Barriocanal et al. [7] have demonstrated that charge leakage in (LaMnO 3 ) m /(SrTiO 3 ) n (LMO m /STO n ) SLs, indicated by the formation of Ti 3+ within the STO layers, is favored in the SLs with a large thickness ratio LMO 17 /STO 2 and is suppressed when this ratio is small LMO 17 /STO 12 . Moreover, the LMO layers in SLs with a large thickness ratio (17/2) were found to be relaxed but those in SLs with small ratio (7/5) were coherently strained. Likely, a correlation between the CT and strain state in the LMO/STO SLs could be envisioned. In addition, it is known that magnetic properties of, e.g., LMO 6 /SMO 4 SLs, [13] can be influenced by the epitaxy strain actuated by the chosen substrate, i.e., STO, LSAT, or LaAlO 3 (LAO). Namely, a small tensile strain in case of STO (a STO = 0.3905 nm) or an almost strain-free state for the lattice matched LSAT (a LSAT = 0.3868 nm) both promote ferromagnetism. In contrast, a compressive strain produced by the LAO substrate (a LAO = 0.3787 nm) suppresses ferromagnetic ground state. However, the development of emergent magnetic phenomena in a wide range of LMO m /SMO n compositions and layer thicknesses as well as the correlations between the lattice structure and strain state of SLs are far from being well understood. Moreover, a possibility to control CT by growth design Emergent phases at the interfaces in strongly correlated oxide heterostructures display novel properties not akin to those of constituting materials. The interfacial ferromagnetism in (LaMnO 3 ) m /(SrMnO 3 ) n (LMO) m /(SMO) n superlattices (SLs) with antiferromagnetic bulk LMO and SMO layers is believed to be a result of the interfacial charge transfer (CT). By using in situ optical ellipsometry, it is demonstrated directly that ...