Interfacial Control of Ferromagnetism in Ultrathin La 0.67 Ca 0.33 MnO 3 Sandwiched between CaRu 1-x Ti x O 3 (x =0-0.8) Epilayers Binbin Chen, † Pingfan Chen, † Haoran Xu, † Feng Jin, † Zhuang Guo, † Da Lan, † Siyuan Wan, † Guanyin Gao, † Feng Chen, ‡ and Wenbin Wu* ,†,‡,§ † Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China ‡ High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China § Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China * S Supporting Information ABSTRACT: Controlling functionalities in oxide heterostructures remains challeng- ing for the rather complex interfacial interactions. Here, by modifying the interface properties with chemical doping, we achieve a nontrivial control over the ferromagnetism in ultrathin La 0.67 Ca 0.33 MnO 3 (LCMO) layer sandwiched between CaRu 1-x Ti x O 3 [CRTO(x)] epilayers. The Ti doping suppresses the interfacial electron transfer from CRTO(x) to LCMO side; as a result, a steadily decreased Curie temperature with increasing x, from 262 K at x = 0 to 186 K at x = 0.8, is observed for the structures with LCMO fixed at 3.2 nm. Moreover, for more insulating CRTO(x ≥ 0.5), the electron confinement induces an interfacial Mn-e g (x 2 -y 2 ) orbital order in LCMO which further attenuates the ferromagnetism. Also, in order to characterize the heterointerfaces, for the first time the doping- and thickness-dependent metal- insulator transitions in CRTO(x) films are examined. Our results demonstrate that the LCMO/CRTO(x) heterostructure could be a model system for investigating the interfacial multiple interactions in correlated oxides. KEYWORDS: oxide interface, ferromagnetism, chemical doping, charge transfer, orbital order ■ INTRODUCTION The interface is an inevitable topic for the implantation of functional oxides into electronic devices, and manipulating the interface states at will becomes a main goal of current research. 1 The La 0.67 Sr 0.33 MnO 3 /SrTiO 3 (LSMO/STO) interface has received considerable interest for its relevance to magnetic tunnel junctions (MTJs), and a long-standing issue concerning its performance in MTJs is the deteriorated ferromagnetism (FM) of LSMO at the interfacial region, known as the “dead layer”. 2-8 Aside from those extrinsic defects, such as the oxygen deficiency and cation disorder, 2 interfacial reconstructions are thought to play a critical role on the dead-layer formation. 3-8 For instance, the crystal symmetry mismatch between LSMO and STO leads to modulations of the interfacial octahedral rotations yielding an unusual elongation of the c-axis parameter within the first two unit cells of LSMO, and the resultant Mn- e g (3z 2 -r 2 ) orbital order at the interface deteriorates the FM by forming the C-type antiferromagnetism. 3,4 Moreover, the polarity mismatch can also trigger an interfacial electronic reconstruction to alleviate the divergent electric potential in the LSMO layer. 5,6 This is similar to the case of LaAlO 3 /STO interface, and the transferred electrons are confined at the interfacial Mn sublattice because of the higher energy level of Ti t 2g relative to Mn e g states. 6 In this regard, the Mn valence change due to the electron transfer can also be compensated or prevented by inserting different buffer layers, 6,7 and beneficial effects have indeed been obtained when the manganites are sandwiched between metallic ruthenates. 9-14 First, the polar discontinuity can be screened by the mobile charge carriers because both components are nominally metallic. 12-14 Second, the metallic ruthenates can serve as electron donors to enhance the double exchange (DE) interaction for the interfacial manganite and improve the FM. Consistently, weak FM with spin canting has been induced at the interface between the antiferromagnetic insulator CaMnO 3 (CMO) and the para- magnetic metal CaRuO 3 (CRO). 9-11 Room-temperature FM is stabilized in LSMO/SrRuO 3 superlattices with the LSMO layer down to at least two unit cells, 12 which is lower than the limit of five unit cells in optimized LSMO/STO superlattices despite the same strain states. 2,4 Clearly, the distinct behaviors for FM observed in manganite/titanate and manganite/ruthenate heterostructures critically depend on the interface properties. Therefore, the “hybrid” interfaces between the manganite and Ti-doped ruthenate (or Ru-doped titanate) deserve to be investigated so as to understand and achieve a full control over the interfacial reconstruction and dead-layer formation at the manganite interfaces. Received: October 16, 2016 Accepted: November 28, 2016 Published: November 28, 2016 Research Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/acsami.6b13158 ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX