S. van Dijken

d0 Ferromagnetic Interface between Nonmagnetic Perovskites

We use computational and experimental methods to study d(0) ferromagnetism at a charge-imbalanced interface between two perovskites. In SrTiO(3)/KTaO(3) superlattice calculations, the charge imbalance introduces holes in the SrTiO(3) layer, inducing a d(0) ferromagnetic half-metallic 2D hole gas at the interface oxygen 2p orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a general property of hole-type d(0) perovskite interfaces. Atomically sharp epitaxial d(0) SrTiO(3)/KTaO(3), SrTiO(3)/KNbO(3), and SrTiO(3)/NaNbO(3) interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest that the behavior is due to the high density of states and exchange coupling at the oxygen t(1g) band in comparison with the more studied d band t(2g) symmetry electron gas.

Electrical Writing of Magnetic Domain Patterns in Ferromagnetic/Ferroelectric Heterostructures

Coupling between ferromagnetic and ferroelectric domains and electrical writing of magnetic domain patterns are investigated in artificial heterostructures consisting of a ferroelectric BaTiO3 substrate with a well-defined ferroelastic domain structure and a thin CoFe film. Polarization microscopy analysis and micromagnetic simulations show that ferroelastic stripe patterns are fully transferrable to neighboring magnetic films. The one-to-one domain correlations are the result of a strain-mediated coupling mechanism between the in-plane ferroelectric polarization of the BaTiO3 substrate and the magnetic anisotropy of the CoFe film. The ferromagnetic and ferroelectric domains remain coupled in an applied electric field and this enables the writing of new magnetic domain patterns by electrical means.

Energetics and structure of the stable and unstable biatomic step edges of Si(001)

The atomic geometry of Si(100) with a misorientation of 4.5° towards [110] has been studied with scanning tunneling microscopy. Special attention has been paid to the buckling registry of the dimers and atoms near the biatomic step edge. For the arrangement of the atoms near the steps and at kink positions some simple rules are given. the biatomic step edge formation energies have been determined by analyzing the step edge roughness. The ordering of the various types of step edges is in perfect agreement with total energy calculations performed by Chadi [Phys. Rev. Lett.58, 1691 (1987)]. Finally, the transition from a surface with single steps to a surface with double steps has been calculated to occur at a miscut angle of 1.5°.

Effect of epitaxy on interband transitions in ferroelectric KNbO3

Very large lattice strain and strain-induced polarization are achieved in KNbO3 using epitaxial growth of a thin KNbO3 film onto a (001)-oriented SrTiO3 single-crystal substrate. We demonstrate experimentally that epitaxy produces dramatic changes of interband transitions in the film compared to those of a reference KNbO3 crystal: the energies of transitions change, some transitions are substantially suppressed and new ones appear in the film. A comparison of the experimental observations with theoretical calculations points to yet unexplored phenomena. Our results indicate that optical refraction and electro-optical coefficients of ferroelectric films can be controlled by epitaxial growth, which is of importance for emerging photonic and optoelectronic applications.

Effects of doping and epitaxy on optical behavior of NaNbO3 films

Cube-on-cube epitaxy of perovskite sub-cell of Pr-doped and undoped NaNbO3 is obtained in 130-nm-thick films on top of (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001) substrates. Experimental studies show that the edge of optical absorption red-shifts and some interband transitions change in the films compared to crystals. Bright red luminescence is achieved at room-temperature under ultraviolet excitation in the Pr-doped film. An interband mechanism of luminescence excitation is detected in the film, which is in contrast to the intervalence charge transfer mechanism in the crystal. The results are discussed in terms of epitaxially induced changes of crystal symmetry and ferroelectric polarization in the films. It is suggested that the band structure and interband transitions in NaNbO3 and the transition probabilities in the Pr ions can be significantly modified by these changes.

Dirty limit scattering behind the decreased anisotropy of doped YBa2Cu3O7-δ thin films

We measured the resistivity of pulsed-laser-deposited BaCeO3 (BCO)-doped YBCO thin films containing spherical BCO particles in fields up to 30 T. The average diameter of the particles depends on the dopant concentration being below 4 nm in all the samples. Raised values of the upper critical field, Bc2, were observed in all the samples. Additionally, the parameter γ, describing the electron mass anisotropy, decreased from 6.2 in the undoped sample to 3.1 in the 8 wt.% BCO-doped sample. These results can be explained by the increased number of defects decreasing the mean free path of electrons and thus lowering the coherence length, which in turn increases Bc2.

Tunnelling anisotropic magnetoresistance at La0.67Sr0.33MnO3-graphene interfaces

Using ferromagnetic La0.67Sr0.33MnO3 electrodes bridged by single-layer graphene, we observe magnetoresistive changes of ∼32–35 MΩ at 5 K. Magneto-optical Kerr effect microscopy at the same temperature reveals that the magnetoresistance arises from in-plane reorientations of electrode magnetization, evidencing tunnelling anisotropic magnetoresistance at the La0.67Sr0.33MnO3-graphene interfaces. Large resistance switching without spin transport through the non-magnetic channel could be attractive for graphene-based magnetic-sensing applications.

Coupling effects in plasmonic nanoparticle arrays: The weak and the strong coupling regime and the effects of spin-orbit coupling

Summary form only given. We study the spatial coherence properties of a system composed of periodic silver nanoparticle arrays covered with fluorescent organic molecule film [1]. The evolution of spatial coherence of the structure is investigated both in weak and strong coupling regimes by systematically varying the coupling strength between the localized molecular excitons and the collective, delocalized modes of the nanoparticle array known as surface lattice resonances (SLRs). In stark contrast to pure localized excitons, the high degree of spatial coherence is maintained in the strong coupling regime, even when the mode is very exciton-like (80%). The results of our recent luminescence experiments will be presented. The effects of spin-orbit coupling are studied in periodic rectangular arrays of magnetic Ni nanoparticles [2]. We observe SLR modes in which the two directions of the lattice are coupled by the magnetic-field-controllable spin-orbit coupling in the nanoparticles. When breaking the symmetry of the lattice, we find that the optical response shows Fano-type surface lattice resonances whose frequency is determined by the periodicity orthogonal to the polarization of the incident field. In striking contrast, the magneto-optical Kerr response is controlled by the period in the parallel direction. The spectral separation of the response for longitudinal and orthogonal excitations provides versatile tuning of narrow and intense magneto-optical resonances.

Giant resistive switching effects in symmetric all-oxide tunnel junctions with La 2/3 Sr 1/3 MnO 3 electrodes

This study demonstrates the switching behavior of nominally symmetric tunnel junctions that are comprised of two La_2/3Sr_1/3MnO₃ electrodes separated by a ferroelectric PbZr_0.2Ti_0.8O₃ or BaTiO₃ tunnel barrier, or a paraelectric SrTiO₃ tunnel barrier. Transmission electron microscopy measurements indicate that the structural and chemical symmetry of the junctions is broken at the tunnel barrier interfaces. Electrical transport measurements are preformed on Hall bar structures by scanning probe microscopy.