Xumin Chen

Electrically controlled exchange bias for spintronic applications

Exchange coupling between a magnetoelectric (111)-oriented Cr2O3 single crystal and a CoPt multilayer with perpendicular anisotropy exhibits an exchange bias field proportional to the applied axial electric field. Extrapolation from bulk to thin film magnetoelectric pinning system suggests promising spintronic applications due to coupling between the electric field-controlled magnetization and the magnetization of a neighbor ferromagnetic layer. Pure voltage control of magnetic configurations of tunneling magnetoresistance spin valves is an attractive alternative to current-induced magnetization switching.

Surface state engineering of molecule-molecule interactions.

Engineering the electronic structure of organics through interface manipulation, particularly the interface dipole and the barriers to charge carrier injection, is of essential importance to improve organic devices. This requires the meticulous fabrication of desired organic structures by precisely controlling the interactions between molecules. The well-known principles of organic coordination chemistry cannot be applied without proper consideration of extra molecular hybridization, charge transfer and dipole formation at the interfaces. Here we identify the interplay between energy level alignment, charge transfer, surface dipole and charge pillow effect and show how these effects collectively determine the net force between adsorbed porphyrin 2H-TPP on Cu(111). We show that the forces between supported porphyrins can be altered by controlling the amount of charge transferred across the interface accurately through the relative alignment of molecular electronic levels with respect to the Shockley surface state of the metal substrate, and hence govern the self-assembly of the molecules.

Spin polarization asymmetry at the surface of chromia

We demonstrate boundary spin polarization at the surface of a Cr2O3 single crystal using spin-polarized low-energy electron microscopy (SPLEEM), complementing prior spin polarized photoemission, spin polarized inverse photoemission, and x-ray magnetic circular dichroism photoemission electron microscopy measurements. This work shows that placing a Cr2O3 single crystal into a single domain state will result in net Cr2O3 spin polarization at the boundary, even in the presence of a gold overlayer. There are indications that the SPLEEM contrast for the two polarization states may be different, consistent with scanning tunneling microscopy spectroscopy results obtained from ultrathin films of Cr2O3.

Strain induced modulation of the correlated transport in epitaxial Sm0.5Nd0.5NiO3 thin films

We report a study of the effect of epitaxial strain on the correlated transport properties of 2-40 nm Sm0.5Nd0.5NiO3 (SNNO) films grown on different substrates. The metal-insulator transition (MIT) temperature T(MI) of the SNNO films increases with increasing tensile strain. While films on (0 0 1) LaAlO3 and (1 1 0) NdGaO3 substrates exhibit a sharp MIT and thermal hysteresis in the cooling-heating cycle, signaling a first-order transition, films on (0 0 1) SrTiO3 show a broad, second-order MIT. Hall effect measurements reveal hole-type charge carriers and thermally activated temperature dependence of the carrier density below T(MI). The corresponding activation energy is ∼80 meV for films on LaAlO3 and NdGaO3, and is suppressed to 25 meV for films on SrTiO3. The carrier mobility in the metallic state and variable range hopping (VRH) transport at a low temperature point significantly enhanced electron localization in SNNO on STO, which we believe is not simply driven by extrinsic effects such as oxygen vacancies, but rather is an intrinsic characteristic for films subject to tensile strain due to the elongated Ni-O bond and hence enhanced dynamic Jahn-Teller distortion. In ultrathin films above the electrical dead layer thickness (2-3 nm), we observe a more than 100 K increase of T(MI) for films on LaAlO3, which has been correlated with a crossover from 3D to 2D transport as revealed from VRH. We attribute the distinct transport characteristics to strain induced modulation of various energy scales associated with the Ni-O-Ni bond angle and Ni-O bond length, which collectively determine the delocalization bandwidth of the system.

Temperature dependence of metal-organic heteroepitaxy.

The nucleation and growth of 2D layers of tetraphenyl porphyrin molecules on Ag(111) are studied with variable-temperature scanning tunneling microscopy. The organic/metal heteroepitaxy occurs by strict analogy to established principles for metal heteroepitaxy. A hierarchy of energy barriers for diffusion on terraces and along edges and around corners of adislands is established. The temperature is key to activating these barriers selectively, thus determining the shape of the organic aggregates, from a fractal shape at lower temperatures to a compact shape at higher temperatures. The energy barriers for the terrace diffusion of porpyrins and the molecule-molecule binding energy were determined to be 30 meV < E(terrace) < 60 and 130 meV < E(diss) < 160 meV, respectively, from measurements of island sizes as a function of temperature. This study provides an experimental verification of the validity of current models of epitaxy for the heteroepitaxy of organics and is thus expected to help establish design principles for complex metal-organic hybrid structures.

Ultrathin BaTiO3 templates for multiferroic nanostructures

The structural, electronic and dielectric properties of high-quality ultrathin BaTiO3 films were investigated. The films, which were grown by ozone-assisted molecular beam epitaxy on Nb-doped SrTiO3(001) substrates and have thicknesses as low as 8 unit cells (u.c.) (3.2nm), are unreconstructed and atomically smooth with large crystalline terraces. A strain-driven transition to three-dimensional (3D) island formation is observed forfilms of 13u.c. thickness (5.2nm). The high structural quality of the surfaces, together with dielectric properties similar to bulk BaTiO3 and dominantly TiO2 surface termination, makes these films suitable templates for the synthesis of high-quality metal- oxide multiferroic heterostructures for the fundamental study and exploitation of magneto-electric effects, such as a recently proposed interface effect in Fe/BaTiO3 heterostructures based on Fe-Ti interface bonds.

Magnetic phases of cobalt atomic clusters on tungsten

First-principle calculations are employed to show that the magnetic structure of small atomic clusters of Co, formed on a crystalline W(110) surface and containing 3-12 atoms, strongly deviates from the usual stable ferromagnetism of Co in other systems. The clusters are ferri-, ferro- or non-magnetic, depending on cluster size and geometry. We determine the atomic Co moments and their relative alignment, and show that antiferromagnetic spin alignment in the Co clusters is caused by hybridization with the tungsten substrate and band filling. This is in contrast with the typical strong ferromagnetism of bulk Co alloys, and ferromagnetic coupling in Fe/W(110) clusters.