THz-Driven Ultrafast Spin-Lattice Scattering in Amorphous Metallic Ferromagnets

We use single-cycle THz fields and the femtosecond magneto-optical Kerr effect to, respectively, excite and probe the magnetization dynamics in two thin-film ferromagnets with different lattice structures: crystalline Fe and amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of comparable magnitude in both systems, but only the amorphous sample shows ultrafast demagnetization caused by the spin-lattice depolarization of the THz-induced ultrafast spin current. Quantitative modeling shows that such spin-lattice scattering events occur on similar time scales than the conventional spin conserving electronic scattering (∼30  fs). This is significantly faster than optical laser-induced demagnetization. THz conductivity measurements point towards the influence of lattice disorder in amorphous CoFeB as the driving force for enhanced spin-lattice scattering.

Correlation-Driven Insulator-Metal Transition in Near-Ideal Vanadium Dioxide Films

We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction, and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO_{2}. We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO_{2} films are preceded by the purely electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K (±0.3  K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically promising material.

Field Effect and Strongly Localized Carriers in the Metal-Insulator Transition Material VO(2)

The intrinsic field effect, the change in surface conductance with an applied transverse electric field, of prototypal strongly correlated VO(2) has remained elusive. Here we report its measurement enabled by epitaxial VO(2) and atomic layer deposited high-κ dielectrics. Oxygen migration, joule heating, and the linked field-induced phase transition are precluded. The field effect can be understood in terms of field-induced carriers with densities up to ∼5×10(13)  cm(-2) which are trongly localized, as shown by their low, thermally activated mobility (∼1×10(-3)  cm(2)/V s at 300 K). These carriers show behavior consistent with that of Holstein polarons and strongly impact the (opto)electronics of VO(2).

Liquid crystal alignment on carbonaceous surfaces with orientational order

We used near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to link the orientational bond order at three carbonaceous surfaces-rubbed polyimide, ion beam-irradiated polyimide, and ion beam-irradiated diamondlike carbon films-with the direction of liquid crystal (LC) alignment on these surfaces. We show that, in general, LC alignment can be created on any carbonaceous substrate by inducing orientational order at its surface. Our results form the scientific basis for LC alignment layers consisting of amorphous carbon films in which orientational order near the surface is induced by a directional low-energy ion beam.

Domain-wall induced coupling between ferromagnetic layers

The remanent magnetization of a hard ferromagnetic CoPtCr layer is progressively decreased by repeated switching of a neighboring soft magnetic layer. We show that this effect depends strongly on the thickness of the CoPtCr layer and the spacing between the hard and soft layers. We propose a model that accounts for these results: An interlayer magnetostatic coupling is induced by large stray fields from domain walls that form within the soft layer during its magnetization reversal.

Element-Specific Magnetic Microscopy with Circularly Polarized X-rays

Circularly polarized soft x-rays have been used with an imaging photoelectron microscope to record images of magnetic domains at a spatial resolution of 1 micrometer. The magnetic contrast, which can be remarkably large, arises from the fact that the x-ray absorption cross section at inner-shell absorption edges of aligned magnetic atoms depends on the relative orientation of the photon spin and the local magnetization direction. The technique is element-specific, and, because of the long mean free paths of the x-rays and secondary electrons, it can record images of buried magnetic layers.

Study by Synchrotron Radiation of the Structure of a Working Catalyst at High Temperatures and Pressures

A relation among activity, composition, and structure was determined for a working catalyst by means of a stainless-steel reactor cell of novel design that permitted operation at temperatures and pressures similar to those in industrial reactors. Molybdenum K-edge x-ray absorption spectra were used to probe the structural environment of molybdenum in CoMoS/[unknown]-alumina catalysts while hydro-desulfurization of benzothiophene was proceeding at high temperature and pressure. For catalyst samples with different contents of cobalt, radial structure functions obtained from extended x-ray absorption fine structure data presented the same features as those obtained from the spectra of MoS(2)/[unknown]-alumina reference samples. Moreover, Mo-S and Mo-Mo coordination numbers were maximum for the sample with an atomic ratio of Co to (Co + Mo) of 0.33; this sample was also the most active catalyst tested.