Session A31: Focus Session: Oxide Interfaces I

Electronic and magnetic effects at complex oxide interfaces

Remarkable electronic and magnetic behavior results from electronic reconstruction at interfaces in complex oxides. In recent years this has been strikingly shown especially for interfaces to SrTiO$_{3}$, for example with LaTiO$_{3}$ and LaAlO$_{3}$. Depending on the atomic arrangement of these interfaces, they can become conducting, remain insulating, show magnetic activity and even become superconducting. An important aspect concerns the role of oxygen vacancies, it is not completely clear yet to what extent they are decisive for some of these properties. In this talk I will discuss recent experimental and theoretical studies on such interfaces.   

Magnetic anisotropy modulation in Fe$_3$O$_4$/BaTiO$_3$(100) epitaxial structures

Renewed interest in `classical' ferroic materials has been accompanied by the study of a new class of multiferroic composite materials, based on magnetic and dielectric multilayer structures. One motivation is the search for materials that allow independent control of both the magnetic and electric properties. In this context, we investigate the modulation of the magnetic anisotropy of a 10 nm magnetite (Fe$_3$O$_4$) film grown epitaxially by off-axis magnetron sputtering on (001)BaTiO$_3$ (BTO). SQUID and magnetoresistance measurements as a function of temperature show a series of discontinuities that are attributed to changes in the strain of the magnetite film via elastic coupling with the substrate, as the latter undergoes a series of structural phase transitions. Magnetic hysteresis loops carried out at temperatures above and below each transformation in the BTO elucidate the variation of the effective anisotropy of the Fe$_3$O$_4$ film. The possibility of using the piezoelectric response of BTO to modulate the magnetic anisotropy of magnetite films is discussed.   

Epitaxial strain-mediated spin-state transitions: can we switch off magnetism?

We use first-principles density functional theory calculations to explore spin-state transitions in epitaxially strained LaCoO$_3$. While high-spin to low-spin state transitions in minerals are common in geophysics, where pressures can reach over 200~GPa, we explore whether heteroepitaxial strain can achieve similar transitions with moderate strain in thin films. LaCoO$_3$ is known to undergo a low-spin ($S$=0, t$_{2g}^6e_g^0$) to intermediate-spin ($S$=1, t$_{2g}^5e_g^1$) or high-spin ($S$=2, t$_{2g}^4e_g^2$) state transition with increasing temperature, and thus makes it a promising candidate material for strain-mediated spin transitions. Here we discuss the physics of the low-spin transition and changes in the electronic structure of LaCoO$_3$, most notably, the metal-insulator transition that accompanies the spin-state transitions with epitaxial strain. As thin film growth techniques continue to reach atomic-level precision, we suggest this is another approach for controlling magnetism in complex oxide heterostructures.   

Density functional study of ferromagnetic ferroelectric LaMnO$_{3}$/BaTiO$_{3}$ superlattice

Using the GGA + $U$ density functional, we have calculated the lattice constants, atomic positions, magnetization, and ferroelectric polarization of a (LaMnO$_{3})_{4.5}$(BaTiO$_{3})_{4.5}$ superlattice containing five LaO and TiO$_{2}$ planes and four MnO$_{2}$ and BaO planes. Although LaMnO$_{3}$ is antiferromagnetic, it is ferromagnetic in the superlattice. An approximation to the ferroelectric polarization, obtained from a comparison of superlattice and bulk crystal atomic displacements and unit cell volumes, is found to agree reasonably well with the polarization obtained from a Berry phase calculation. The electric polarization points along the longer in-plane lattice vector, while the atomic spins, after the spin-orbit interaction and spin noncollinearity are included, all point in directions close to that lattice vector.   

Electron Localization and Interface Magnetism in CaRuO$_3$/CaMnO$_3$ Superlattices

We present a study of interface physics in superlattices composed of a G type anti-ferromagnetic insulator (CaMnO$_3$) and a paramagnetic metal (CaRuO$_3$). Using laser MBE, ultra-thin superlattices were grown with the structure [CaRuO$_3$(N u.c)/CaMnO$_3$(10 u.c.)]x6 on LaAlO$_3$(100) substrates. Polarized x-ray probes clearly show a large Mn magnetic moment, which is attributed to a canted anti-ferromagnetic state at the interface. The electronic state of the system was probed by current-in-plane transport measurements. Due to the large resistivity of CMO, magneto-transport measurements probe primarily the behavior of the carriers in the CRO layer, which show a non-bulk like insulating behavior that is attributed to disorder induced localization from interface scattering. Field dependence shows a large negative magneto-resistance (MR), which results from strong scattering of the carriers by Mn spins at the interface since the MR drops with increasing CRO layer thickness. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. JC is funded by U.S. DOD-ARO under Contract No. 0402-17291.   

Exploring artificial layered heterostructures of LaM'O3/LaM''O3 (M'M''= NiCr, FeCr and NiV).

Digital synthesis of atomically sharp interfaces between strongly correlated electron systems can provide a template to build completely new materials. Here we present our results on magnetism and electronic structure in LaM'O3/LaM''O3 (M'M''= NiCr, FeCr and NiV) superlattices by using polarized X-ray spectroscopies. Using laser MBE, the (111) and (100) oriented ultra-thin superlattices were grown with alternating layer thicknesses of 1 unit cell. In the bulk, LaMO3 (M=Cr,Fe,V) are antiferromagnetic insulators while LaNiO3 is a paramagnetic metal. The evolution of element specific magnetism and charge at the interface of LFO/LCO, LNO/LVO and LNO/LCO superlattices with temperature and an applied magnetic field will be discussed in detail. The superlattice results will be contrasted to the bulk magnetic properties of the constituent layers. The work has been supported by U.S. DOD-ARO under Contract No. 0402-17291.   

Strain effect on Magnetism at the Manganite Interfaces: SrMnO$_3$/LaMnO$_3$

Recently it has been shown that new magnetic and electronic phases can be produced by varying the strain condition at the manganite interfaces[1]. From the density-functional studies of the electronic structure at the interface of SrMnO$_3$ and LaMnO$_3$ we show that the epitaxial strain, which enforces a tetragonal distortion, splits the itinerant interface Mn-e$_g$ states to x$^2$-y$^2$ and 3z$^2$-1 states. If the strain is tensile in the plane the x$^2$-y$^2$ orbital becomes more occupied, enhancing thereby the ferromagnetic double exchange which overcomes the antiferromagnetic super exchange between the core t$_{2g}$ states to produce a net in-plane ferromagnetic interaction. Due to the lower occupancy of the 3z$^2$-1 orbitals, the super exchange supercedes the double exchange to produce out-of-plane antiferromagnetic ordering. For in-plane compressive strain higher occupancy of 3z$^2$-1 orbital results in the out-of-plane ferromagnetic ordering while in-plane ordering remains antiferromagnetic. Without any epitaxial strain, the itinerant x$^2$-y$^2$ and 3z$^2$-1 states are more or less equally occupied and ferromagnetic ordering prevails both in-plane and out-of-plane. While for the tensile strain we find the heterostrutcure to be metallic, for the compressive strain an insulating phase is obtained if the strain is sufficiently large. This work was supported by DOE-DE-FG02-00ER45818. \\ 1. H. Yamada et al. Appl. Phys. Lett. {\bf 89}, 052506 (2006)   

Asymmetric and modulated magnetic profiles in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices

We have determined the magnetic depth profile of MBE-grown ferromagnetic (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices, where $n$ is nominally equal to 3 and 5. Polarized neutron reflectivity measurements reveal the existence of a modulated magnetic structure that repeats with the superlattice period in both samples. For $n$=5, a moment of $\sim$2.6 $\mu_B$/Mn is measured in the LaMnO$_3$ (LMO) layer, while the moment in the middle of the SrMnO$_3$ (SMO) layer is negligible. The magnetization at the interfaces is found to be asymmetric with an enhanced moment residing at the LMO/SMO interfaces but not at the SMO/LMO interfaces. The origin of the magnetic asymmetry at the interfaces is elucidated from comparison with the structural properties determined by x-ray reflectivity and transmission electron microscopy.   

Magnetic depth profiles of complex oxide F/S/F trilayers.

The origin of the large magnetoresitance in epitaxial F/S/F trilayers composed of highly spin polarized ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ and high-T$_{c}$ superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (YBCO) is investigated by characterizing the magnetic structure. Polarized neutron reflectometry experiments have determined the detailed magnetization depth profiles in trilayers with varying YBCO layer thicknesses. In addition to inhomogeneous magnetization profiles, rotation of the magnetization during the magnetization reversal for the films with thick ($\ge $ 17.7 nm) YBCO layers has been observed. The results are consistent with the presence of an (in plane) easy-axis tilted away from the (100) direction.   

Origins of Anomalous Ferromagnetism in F/AF LCMO Multilayers

Unexpected behavior can emerge from magneto-electronic interactions at the interface between two different strongly correlated electron systems. Exchange bias - giving a ferromagnet (F) a preferred direction via coupling with an antiferromagnet (AF) - is a phenomenon of great fundamental and applied research interest. Both topics are pertinent in the case of the interface between F and AF La[1-x]Ca[x]MnO3 (LCMO) layers. Depending on x, LCMO can be F (x = 1/3 Ca) or AF (x = 2/3 Ca), and exchange bias has been reported in superlattices consisting of such layers. Surprisingly SQUID magnetometry has shown that the saturation moment of such a structure increases as the nominally AF layer thickness is increased [1]. This has been attributed to electronic effects that cause F order to extend into the nominally AF layer. However, the location of the extra moment cannot be determined with bulk magnetometry techniques. Thus, we have used polarized neutron and x-ray reflectometry to measure the magnetic and structural depth profiles in an exchange biased x=1/3 LCMO / x=2/3 LCMO bilayer. Our results suggest that the magnetic profile extends beyond the x = 1/3 layer, implying that some F order indeed exists in the nominally AF x = 2/3 layer. [1] G. Campillo, et al., J. Appl. Phys. 97, 10K104 (2005).   

Ferroelectric field effect modulation of the magnetic properties of colossal magnetoresistive La$_{1-x}$Sr$_x$MnO$_3$

In this work, we have tuned the magnetic properties of the colossal magnetoresistive oxide La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) using a ferroelectric field effect approach. Epitaxial LSMO thin films and ferroelectric Pb(Zr,Ti)O$_3$ (PZT) / LSMO heterostructures were fabricated using off-axis RF magnetron sputtering. LSMO films with high crystalline quality and atomically smooth surfaces have been achieved. Using the magneto-optical Kerr effect (MOKE), we measured a shift in the Curie temperature (T$_{\mathrm{c}}$) of LSMO upon switching the polarization direction of PZT, in agreement with the modulation of T$_{\mathrm{c}}$ found from magnetotransport measurements. Unlike the traditional chemical doping approach, the ferroelectric field effect approach allows one to control the magnetism in the system reversibly, without introducing additional lattice disorder/distortion.   

Structure and properties of CaMnO$_3$/SrMnO$_3$/BaMnO$_3$ superlattices from first principles

Previous theoretical and experimental studies have shown that three-component, or ``tri-color'' superlattices can exhibit intrinsic electric polarization due to inversion-symmetry breaking in the layer sequence. In ferromagnetic inversion-symmetry-breaking superlattices, controlled symmetry lowering is similarly expected to lead to interesting new and tunable properties. Here, we present results of first-principles density-functional-theory calculations for short-period CaMnO$_3$/SrMnO$_3$/BaMnO$_3$ superlattices, using VASP. The ground state structure, magnetic ordering, polarization and dielectric response will be presented. The role of epitaxial strain in the individual layers and the role of layer sequence will be explored. Connections to experimental studies and prospects for future work will be discussed.   

Phase separation at the La$_{1-x}$Sr$_{x}$CoO$_{3}$ / SrTiO$_{3}$ (001) interface from thickness and doping dependent magnetotransport

Bulk La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO) has received considerable attention with regard to nanoscale magnetic phase separation. Fabrication of epitaxial films provides a means to study this phase separation under dimensional confinement and at interfaces. Moreover, the characteristic intercluster ``GMR'' effect observed in the phase-separated state of this material provides a simple means to probe phase separation even in very thin films. We have found that even at x = 0.5 (a homogeneous ferromagnetic metal in bulk), sufficiently thin films ($<$ 60 {\AA}) grown on SrTiO$_{3}$ (001) show a crossover to a reduced moment insulating phase, with the characteristic intercluster MR effect, i.e. phase separation occurs. By measuring the thickness dependence of the magnetotransport as a function of doping we have assembled a three-dimensional phase diagram in temperature-doping-thickness space. The interfacial phase-separated region increases dramatically as x is decreased (to over 250 {\AA} at x = 0.2) and the non-ferromagnetic phase boundary is approached. The increased prominence of phase separation as proximity to the non-ferromagnetic phase is increased provides a strong hint to the origin of this interfacial phase separation.