This is a copy of the slides presented at the meeting but not formally written up for the volume.

We propose a novel approach to correlate electronic properties with superconductivity in high critical temperature cuprates. We have disassembled the superconductor in the constituent blocks and investigated separately the charge reservoir (CR) and the conduction planes or infinite layer (IL) blocks, which consist of BaCuO2+x (BCO) and CaCuO2 (CCO) respectively. Artificial BCO/CCO superconducting superlattices [1], obtained by stacking in a sequence the two blocks, have been also measured to study the effect of charge transfer (from the CR to the IL block). On such samples, in form of c-axis oriented thin films, we have performed polarization dependent X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) measurements at the Cu L3 edge, to investigate the hole doping effect and the low energy excitations. XAS results clearly show a significant number of Cu d(z2) holes, both in the pure BCO samples and the BCO/CCO superlattices linked to the presence of apical oxygen. The high energy shoulder related to the existence of itinerant Cu 3d holes hybridised with O 2p (doping holes) is evident in the XAS spectra of the pure BCO and the BCO/CCO superconducting superlattices. The relative shoulder intensity decreases by increasing the CCO block thickness, in agreement with the expected decrease of the doping level. RIXS spectra show similar dd-excitations in the ab-planes for BCO, CCO and 2X2 superlattice, as a consequence of similar in-plane crystal field effects [2]. On the contrary, the charge transfer excitations [3] in ab-plane look similar only for the BCO film and the 2X2 superlattice. Namely, fluorescence activates at the absorption edge because of the metal character of both the CR block and the superconducting superlattice. On the contrary in CCO (insulator) the onset of the fluorescence signal is moved at higher photon energies, in agreement with the presence of a charge transfer band gap. In addition, we have detected an excitation at 0.5 eV in BCO and BCO/CCO superlattices, but not in CCO. This excitation could be interpreted as the stabilization energy of the Zhang-Rice (ZR) singlet [4] in doped compounds, or as the x2-y2 to z2 dd excitation. All the results are in agreement with the scenario of charge transfer from the BCO charge reservoir block to the CCO infinite layer block. [1] G. Balestrino et al., Phys. Rev. B 62, 9835 (2000) [2] P. Kuiper et al., Phys. Rev. Lett. 80, 5204 (1998) [3] G. Ghiringhelli et al. Phys. Rev. Lett. 92, 117406 (2004) [6] N.B.Brookes et al., Phys. Rev. Lett. 87, 237003 (2001).

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Highly efficient control of magnetism in terms of genuinely-electric action in a solid has been a big challenge in condensed matter science and will also widen the bottle-neck of the state-of-art spin-electronics technology. Multiferroics, the materials in which both ferromagnetism and ferroelectricity can coexist, are the prospective candidate which can potentially host the gigantic ME effect. A useful hint to designing of the strong M-P coupling has been gained by the recent discovery of the ferroelectricity in the frustrated magnets, such as perovskite manganites, spinel chromites, etc. The direction of the polarization can be totally determined by the clockwise (CW) or counter-clockwise (CCW) rotation of the transverse-spiral (cycloidal) spin in proceeding along the spiral propagation axis, that is called spin helicity or vector chirality. In those compounds, the spontaneous P can be easily controlled by an external magnetic field of specific direction, such as the generation and/or flopping of the spontaneous polarization. Conversely, the spin helicity, CW or CCW, can be controlled with genuine electric field via the reversal of spontaneous polarization. Furthermore, the clamping between the magnetic and ferroelectric domains can show up, enabling the magnetic (electric) control of the ferroelectric (ferromagnetic) domains. Multiferroics with the strong M-P correlation may also provide a unique arena to test new optical effects, say the magnetoelectic optical effects. This includes the so-called magnetization-induced second harmonic generation (MSHG) and the nonreciprocal dichroism dependent on the light propagation direction (but not on the light polarization). The former can be applied to probe the interface magnetism as well, while the latter may have potential of producing new optical devices with nonreciprocal functions. Strategy for exploring such multiferroics as showing strong M-P coupling and novel optical functions is argued in terms of the designed spin superstructure and tailor-made materials.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Complex oxides show a broad spectrum of intrinsic functionalities, such as ferroelectricity, magnetism, superconductivity or multiferroic behavior, which can be utilized and combined in electronic devices by the growth and design of heterostructures. Physical properties may arise in such multilayers that are not found in either of their constituents. A spectacular example for such a phenomenon, a conducting and highly mobile electron gas, is formed at the interface between the two insulating, dielectric perovskites LaAlO3 and SrTiO3 [1] which can be easily tuned by transverse electric fields [2]. In our contribution we will present recent studies of the microstructure of the multilayer and we will correlate them with the transport properties of the electron gas. [1] A. Ohtomo, H. Y. Hwang, Nature 427, 423-426 (2004). [2] S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, J. Mannhart, Science 313, 1942-1945 (2006).

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Description: Epitaxial thin films of the multiferroic BiFeO3 have been grown by molecular beam epitaxy in an adsorption-controlled growth regime where substrate temperature and bismuth oxide over pressure establish phase and stoichiometry control. 30 nm thick BiFeO3 films have been deposited directly on (001) SrTiO3 and SrRuO3/(110) DyScO3, and on (0001) GaN containing a 1 nm thick TiO2 overlayer to enable the epitaxial transition between (0001) GaN and (0001) BiFeO3. Films grown on (001) SrTiO3 possess rocking curves identical to that of the underlying substrate, e.g., a full width at half maximum (FWHM) in ù of 25 arc sec in the best case. This is over 40 times narrower than the best published result for epitaxial BiFeO3 films deposited by any technique. On all of the above substrates, the BiFeO3 films exhibit rhombohedral symmetry. The in-plane epitaxial alignment observed for BiFeO3/TiO2/(0001) GaN differs by a 30° in-plane rotation from that observed in previous work for BiFeO3/SrTiO3/TiO2/(0001) GaN. Specifically, our (0001) BiFeO3/TiO2/(0001) GaN is oriented in-plane with [10-10]BiFeO3 [11-20]GaN, accompanied by a 180° in-plane rotational twin variant. The results of this work will be discussed in the context of the interfacial and crystallographic orientation dependence on the ferroelectric, antiferromagnetic, and dielectric properties of this multiferroic.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

In order to pursue device applications of magnetoelectric Cr2O3, we have fabricated epitaxial Cr2O3 thin films on (001), (110) and (101) oriented Nb doped TiO2 by pulsed laser deposition. The Cr2O3 films with different thicknesses (0.3 1 ¦Ìm) showed extremely smooth surfaces with rms roughness ¡Ö 0.3 nm (for 10 X 10 ¦Ìm2) as measured by AFM for all 3 different orientations. The films display robust insulating properties at room temperature with leakage current density of 8.9 X 10-6 A/cm2 at 10 kV/cm for 300 nm thick films. In order to investigate exchange bias, we fabricated bilayer films of Cr2O3/Co with all 3 orientations. The magnetic properties of the films were measured using SQUID and the magnetic optical Kerr effect (MOKE). From the Cr2O3/Co film grown on a (110) oriented TiO2, we clearly observe exchange bias of ¡Ö 13 Oe with a coercive field of 115 Oe upon cooling from 320 K to 30 K in a 1 T magnetic field. The microstructural properties of the bilayers and the effect of electric field on the exchange bias behavior were investigated using TEM, VSM and MOKE. Comparison of exchange bias with BiFeO3 and TbMnO3 multiferroic thin films will also be discussed. This work is supported by W. M. Keck Foundation, ONR grant No. N00014-01-1-0761, N00014-04-1-0085, and the NSF under grants DMR-00-94265 (CAREER), NSF DMR-00-0231291, NSF 0095166, NSF-MRSEC Award No. DMR-00-0520471. We acknowledge use of the Nanoscale Imaging, Spectroscopy, and Properties (NISP) Laboratory for TEM characterization.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Interactions at magnetic interfaces are central to the operation of virtually all magnetic heterostructures. When the interface is between two magnetic materials, the exchange interaction between spins at the interface is often a dominant force, and can dramatically change the magnetic response of the overall heterostructure. In ferromagnet (FM)/antiferromagnet (AFM) heterostructures, this interaction is often referred to as exchange anisotropy or bias and it has been widely used over the past decade in a wide array of applications such as magnetic recording heads, MRAMs, etc. The powerful implications of interactions between an AFM and a FM have been realized in a wide range of thin film heterostructure with both metallic and oxide constituents. There is, however, much less work on oxide-oxide FM/AFM systems. On the other hand, the development and understanding of functional oxide materials, especially multifunctional materials like BiFeO3 (BFO), have piqued the interest of researchers worldwide with the promise of coupling between order parameters such as ferroelectricity and antiferromagnetism. Recent research suggests that there is exchange coupling and anisotropy between the metallic ferromagnet Co0.9Fe0.1 (CoFe) and the multiferroic, antiferromagnet BFO, showing the possibility to create highly desirable multifunctional systems with new possibilities for device design. Such a result provides the driving force to create multifunctional oxide-oxide systems where exchange interactions could be much stronger then in metal/oxide structures due the added epitaxial nature of the interface. In this study, we use La0.7Sr0.3MnO3(LSMO)/BFO thin film heterostructures as a model system to explore the exchange interaction at an oxide interface. The heterostructures are grown on various vicinal cuts of SrTiO3 single crystal substrates using laser MBE. Structural analysis using x-ray diffraction, transmission electron microscopy and Rutherford backscattering spectrometry reveals high quality films with the pristine interfaces required for exchange coupling. First results from photoemission electron microscope (PEEM) studies reveal that the magnetic LSMO domain structure mimics underneath ferroelectric BFO domain structure, i..e, it is strongly pinned by the underlying AFM structure. The coupling behavior is being characterized by magnetic measurements (SQUID, VSM), which shows a strong enhancement in the coercivity of the LSMO layer, suggesting the existence of exchange bias coupling. We are probing the strength of this coupling using a combination of careful laser MBE growth experiments and physical property measurements. In this paper, we will report results of experiments in which the LSMO layer has been grown by laser MBE in the thickness range of 2-50nm on a [001] BFO layer.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

I will discuss theoretical and experimental results pointing to possibly new magnetic materials without resorting to the use of traditional magnetic elements. These ideas will involve the use of special surfaces and interfaces as well as defects. We show that a simple uninteresting oxide like CaO can at least theoretically be turned into a half metallic ferromagnet by introducing Ca vacancies. This mechanism opperates via a molecular analog to Hunds rule coupling and longer range exchange interactions. Polar surfaces and interfaces between polar mismatched layers can develop metallic and magnetic 2 dimensional properties. We also suggest that the use of atomic Hunds rule coupling in O and N which is as large as in the 3d transition metal compounds can be used to generate interesting ferromagnetic materials. Experimental evidence for the fasibility of such materials will also be presented.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Multiferroics belong to a class of compounds which show simultaneous electric and magnetic orders. Consideration has been focused on this family of materials due to their potential for applications in electronics resulting from both the ferroelectric and magnetic orders and additional functionalities resulting from the coupling between these two orders. We will present experiments performed on multiferroic based heterostructures in order to check the potential of such multifunctional materials in the field of spintronics. Two kind of materials have been studied. The ferromagnetic-ferroelectric LaxBi1-xMnO3 (LBMO) (x ≤ 0.1) and the antiferromagnetic-ferroelectric BiFeO3 (BFO). The ferromagnetic and ferroelectric of LBMO offers the unique opportunity to encode information independently in electric polarization and magnetization to obtain four different logic states. To achieve this objective, ferromagnetic-ferroelectric (LBMO) (x ≤0.1) thin layers have been used as tunnel barriers in magnetic tunnel junction. In such device, the ferromagnetic and insulating character allows to filter the spin by an active barrier presenting a barrier height different for spin up and spin down tunneling electrons. To demonstrate spin filtering through LBMO barriers, we have analyzed the spin polarization of the current tunnelling from a nonmagnetic electrode (Au) with a half-metallic ferromagnetic oxide counter-electrode (La2/3Sr1/3MnO3 (LSMO)) through such ultrathin LBMO barrier (1 - 3.5 nm). A 100% change of the tunnel resistances is obtained at 3K depending on whether the magnetizations of LBMO(x = 0) and LSMO are parallel or opposite [1]. In addition, the ferroelectric character of the compound is exploited to give rise to two different resistance states related to the direction of the polarization. The resulting electroresistance phenomena amount to 20% at low temperature [3]. On the other hand, we have taken advantage of the antiferromagnetic character of BFO thin films [3], as evidenced by X-ray linear magnetic dichroism experiments, to induce a sizeable (~60 Oe) exchange bias on ferromagnetic films of NiFe and CoFeB, at room temperature. In parallel we have check the possibility to obtain large tunnel magnetoresistance through such antiferromagnetic barrier in LSMO/BFO/Co heterostructures. Perspectives of these findings for multiferroics-based spintronics devices will be discussed. [1] M. Gajek, M. Bibes, A. Barthélémy, K. Bouzehouane, S. Fusil, M. Varela, J. Fontcuberta and A. Fert, Phys. Rev. B 72, 020406(R) (2005) [2] M. Gajek, M. Bibes, S. Fusil, K. Bouzehouane, J. Fontcuberta, A. Barthélémy and A. Fert, Cond.Mat. 0606444 (2006) [3] H. Béa et al.; Appl. Phys. Lett., 88, 062502 (2006) and to be published in Phys. Rev. B.