Session D2: Invited Session: Harnessing Local Atomic Structure to Control Magnetic Interactions in Complex Oxides

Interface engineering in manganites: from diodes to transistors

Perovskite manganites show strong coupling between charge, spin, and lattice degrees of freedom as exemplified by `colossal magnetoresistance'. The recent advances in thin film growth techniques have enabled the generation of novel phases at oxide heterointerfaces, the atomic control of their interface electronic structure, and their incorporation in novel device platforms. We apply these techniques to manganite thin films, first emphasizing the subtleties in optimizing the growth kinetics and stoichiometry [1,2], which has enabled us to create atomically precise heterostructures exhibiting room temperature metallic ferromagnetism in superlattices composed of just 5 unit cell layers [3]. The interface electronic structure was examined using Schottky junctions formed between La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and Nb-doped SrTiO$_{3}$, where the band offset (Schottky barrier height) can be controlled by the termination layer at the interface [4]. This band engineering technique was applied in making a metal-base transistor [5], which takes advantage of the strong internal electric field at interfaces. An analysis of many devices enables the quantitative understanding of the evolution from a hot-electron transistor to a permeable base transistor. This structure provides a platform for developing devices incorporating the exotic ground states of perovskite oxides and their interfaces. \\[4pt] [1] J. H. Song, T. Susaki, and H. Y. Hwang, \textit{Adv. Mater.} \textbf{20}, 2528 (2008). \\[0pt] [2] D. A. Muller, L. Fitting Kourkoutis, M. Murfitt, J. H. Song, H. Y. Hwang, J. Silcox, N. Delby, and O. L. Krivanek, \textit{Science} \textbf{319}, 1073 (2008). \\[0pt] [3] L. Fitting Kourkoutis, J. H. Song, H. Y. Hwang, and D. A. Muller, \textit{PNAS} \textbf{107}, 11682 (2010). \\[0pt] [4] Y. Hikita, M. Nishikawa, T. Yajima, and H. Y. Hwang, \textit{Phys. Rev. B} \textbf{79}, 073101 (2009). \\[0pt] [5] T. Yajima, Y. Hikita, and H. Y. Hwang, \textit{Nature Mater.} \textbf{10}, 198 (2011).   

New material design strategies to realize strong coupling in multiferroics and beyond

Perovskite ABO$_3$ oxides display an amazing variety of phenomena that can be altered by subtle changes in the chemistry and internal structure, making them a favorite class of materials to explore the rational design of novel properties. In this talk I will review a recent advance in which rotations and tilts of the BO$_6$ octahedra give rise to a novel form of ferroelectricity. Octahedral rotations strongly influence other structural, magnetic, orbital, and electronic degrees of freedom in perovskites and related materials. Thus, I want to discuss the idea that octahedral rotation-driven ferroelectricity has the potential to robustly control emergent phenomena with an applied electric field. As one example, I will show from first principles how these ``functional'' octahedral rotations simultaneously induce ferroelectricity, magnetoelectricity, and weak ferromagnetism in a class of naturally occurring Ruddlesden-Popper (RP) (ABO$_3$)$_2$(AO) layered perovskites and discuss the challenges to realize electric field switching of magnetism in these RP and in (ABO$_3$)/(A'BO$_3$) perovskite superlattice novel multiferroics. \\[4pt] N. A. Benedek and C. J. Fennie, {\it Phys. Rev. Lett} {\bf 106}, 107204, 2011;\\[0pt] J. M. Rondinelli and C. J. Fennie, arXiv 2011; N. A. Benedek and C. J. Fennie, arXiv 2011.   

Reversal of Magnetic Interactions by Electric field

Direct magneto-electric coupling describes the interaction between magnetic and electric polarization through an intrinsic microscopic phenomenon in a single phase material. Systems which exhibit such coupling are potential candidates for use in a multistate logic memory storage device whereupon magnetic control with electric fields or ferroelectric control with magnetic fields could be used to alter memory bits. I will present x-ray resonant magnetic scattering results providing direct evidence of a magneto-electric cross field effect mediated through strong spin-lattice coupling in a single phase rare earth titinate film. Compressively strained EuTiO$_{3}$ is, as in bulk, an antiferromagnetic-paraelectric material, however through strain the balance of the magnetic interactions, both antiferromagnetic and ferromagnetic, shifts whereby the two approach energetic degeneracy. By applying an electric field \textit{in-situ} one can tip the delicate equilibrium and suppress the long range antiferromagnetic order. This is accompanied by the emergence of short range ferromagnetic order. In addition we have qualitatively replicated the microscopic preferential shift from antiferromagnetic to ferromagnetic order with electric field using first principles density functional calculations.   

Strain-induced oxygen defect formation and interfacial magnetic phase separation in SrTiO$_{3}$(001)/La$_{1-x}$Sr$_{x}$CoO$_{3}$

The remarkable functionality and epitaxial compatibility of complex oxides provides many opportunities for new physics and applications in oxide heterostructures. Perovskite manganites and cobaltites provide excellent examples, being of interest for solid oxide fuel cells, catalysis, ferroelectric RAM, gas sensing, resistive switching memory, and oxide spintronics. However, the same delicate balance between phases that provides this diverse functionality also leads to a serious problem - the difficulty of maintaining desired properties close to the interface with other oxides. Although this problem is widespread, manifests itself in several ways, and could present a significant roadblock to the development of heterostructured devices for oxide electronics, there is no consensus as to its origin, or even whether it is driven by electronic or chemical effects. In this work, using SrTiO$_{3}$(001)/La$_{1-x}$Sr$_{x}$CoO$_{3}$ as a model system, we have combined epitaxial growth via high pressure oxygen sputtering with high resolution x-ray diffraction, atomic resolution electron microscopy and spectroscopy, and detailed magnetic, transport, and neutron scattering measurements to determine the fundamental origin of the deterioration in interfacial transport and magnetism. The effect is found to be due to nanoscopic magnetic phase separation in the near-interface region driven by a significant depletion in interfacial hole doping due to accumulation of O vacancies. This occurs due to a novel mechanism for accommodation of lattice mismatch with the substrate based on formation and long-range ordering of O vacancies, thus providing a fundamental link between strain state and O vacancy density. Further impacts of the O vacancy ordering and interfacial magnetic phase separation, such as formation of a spin-state superlattice and an extraordinary coercivity enhancement, will also be discussed. Work in collaboration with M. Sharma, M. Torija, J. Schmitt, C. He, S. El-Khatib, J. Gazquez, M. Varela, M. Laver and J. Borchers.   

Directly probing the effect of strain on magnetic exchange interactions

Thin films of transition metal oxides of the perovskite type ABO$_{3}$ (B = 3d or 4d metal) have revealed abundant examples for strain-driven changes of magnetic ordering. One most popular is the strain-induced ferromagnetic ferroelectric state of otherwise antiferromagnetic paraelectric EuTiO$_{3}$. Another promising example is the strain control of orbital occupation and magnetic coupling at oxide interfaces of SrRuO$_{3}$ with manganites. In spite of strong efforts, the theoretical treatment of magnetic exchange in complex oxides has remained a challenge, and experiments continue to show unpredicted / unexplained large effects of the epitaxial strains in films. In order to provide meaningful experimental data on strain dependences, epitaxial thin films should be grown in various coherent strain states on different substrates without changing anything but the strain. This is inherently difficult: possible problems may arise from a strain-dependent oxidation level or microstructure. As a complementary approach, the in-plane strain of epitaxial oxide films can be controlled reversibly using a piezoelectric substrate, even though the accessible reversible strain of 0.1 -- 0.2{\%} is an order of magnitude smaller. In my talk, I will address reversible-strain studies on La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, La$_{1-x}$Sr$_{x}$CoO$_{3}$ (x = 0, 0.2, 0.3) und SrRuO$_{3}$ films, showing the strain response of the magnetic Curie temperature, the magnetization and the electrical resistance and discussing the current understanding of the strain effects on magnetic ordering. In La$_{0.8}$Sr$_{0.2}$CoO$_{3}$, a strain-driven phase transition between ferromagnetic and spin-glass-like could be established by combining the piezoelectric substrate with a tuned buffer system providing varied as-grown strain states. In SrRuO$_{3}$, a tetragonal tensile strain state shows a suppression of the ordered magnetic moment. Lattice parameters and symmetries of the films were determined by x-ray diffraction. It is noted that the atomic displacements (bond lengths and angles) under strain in these compounds are yet essentially unknown and subject to present research.