High-resolution magnetic measurements at surfaces with spin polarized electrons

Observing the spin polarization of emitted electrons reveals surface magnetic information. In particular, high resolving power is achieved in different respects: 1) Magnetic micrography with a lateral resolution of 50 nm in a scanning electron microscope; 2) Non-destructive magnetic depth profiling in the 5–50 Å range with secondary electron emission; 3) Element specific chemical resolution using Auger electron emission; 4) Time-resolved magnetization measurements with pulsed-laser photoemission in less than 10 ns. The state-of-the-art of these techniques is illustrated with specific examples of surface magnetism.     

Visualization of localized holes in manganite thin films with atomic resolution

The magnetic and transport behaviors of manganites are critically related to the spatial distribution and correlation of doped holes. Using in situ scanning tunneling microscopy, we have imaged both occupied and unoccupied states simultaneously in a hole-doped (La(5/8-0.3)Pr0.3)Ca(3/8)MnO3 epitaxial thin film grown by laser molecular beam epitaxy. Doped holes localized on Mn4+ ion sites were directly observed with atomic resolution in the paramagnetic state at room temperature. In contrast to a random distribution, these doped holes show strong short-range correlation and clear preference of forming nanoscale CE-type charge-order-like clusters. The results provide direct visualization of the nature of intriguing electronic inhomogeneity in transition metal oxides.     

Kondo temperature of magnetic impurities at surfaces

Based on the experimental observation that only the close vicinity of a magnetic impurity at metal surfaces determines its Kondo behavior, we introduce a simple model which explains the Kondo temperatures observed for cobalt adatoms at the (111) and (100) surfaces of Cu, Ag, and Au. Excellent agreement between the model and scanning tunneling spectroscopy experiments is demonstrated. The Kondo temperature is shown to depend on the occupation of the d level determined by the hybridization between the adatom and the substrate with a minimum around single occupancy.     

Raman excitation profiles of adsorbates at roughened silver surfaces

The excitation profiles of Raman bands of pyridine (1008 cm^?1), triphenylphosphine (997 cm^?1), [CN]^? (2114 cm^?1), and [Ru₃O₂(NH₃)₁₄]⁶⁺ (ruthenium red, 275 cm^?1) adsorbed at roughened silver-aqueous interfaces have been measured over the range 458–799 nm. Apart from [CN]^? for which experimental limitations prevented measurements with excitation in the far red, each of the adsorbates shows a pronounced excitation profile maximum for excitation in the range 590–750 nm. Measurements on triphenylphosphine at a silver-aqueous interface and on the same silver surface in gaseous nitrogen detected some difference in the excitation profiles due to change of medium. These results are discussed in relation to various theories of enhanced Raman scattering by adsorbates at metal surfaces.     

Electronic structure of spiral magnetic manganite phases

An approach has been developed using the tight-binding and double-exchange Hamiltonian methods for calculating the E(k) spectrum of e _g electrons in noncollinear (spiral) magnetic structures of R _{1 − x} A _x MnO₃ (R = La, Pr, Nd, Sm; A = Ca, Sr, Ba) manganites. A magnetic structure in the form of a flat cycloid observed in TbMnO₃ below 28 K has been considered for undoped manganites (x = 0).     

Measuring concentration depth profiles at liquid surfaces: Comparing angle resolved X-ray photoelectron spectroscopy and neutral impact collision scattering spectroscopy

Measuring concentration depth profiles is important for analyzing surfaces. The surface excess, the change in concentration from the surface to the bulk and separation of the constituents are some of the features that can be derived from concentration depth profiles and are most important in particular for analyzing liquid surfaces. Angle resolved X-ray photoelectron spectroscopy (ARXPS) and neutral impact collision ion scattering spectroscopy (NICISS) are methods used for determining concentration depth profiles at liquid surfaces. Here we compare concentration depth profiles determined with both methods from the constituents of a solution of the ionic surfactant tetrabutylphosphonium bromide in the polar solvent formamide. ARXPS is performed with a laboratory X-ray source and NICISS with 4.5 keV helium ions. Agreement is found in the surface excess and in the shape of the cation concentration depth profiles. Disagreement was found in the shape of the anion concentration depth profiles. The separation of charges as found with NICISS for projectiles with low kinetic energies could not be reproduced. The advantages of each of the methods and the criteria for selecting the projectile energy in NICISS are discussed.     

Chemical resolution at ionic crystal surfaces using dynamic atomic force microscopy with metallic tips

We demonstrate that well prepared and characterized Cr tips can provide atomic resolution on the bulk NaCl(001) surface with dynamic atomic force microscopy in the noncontact regime at relatively large tip-sample separations. At these conditions, the surface chemical structure can be resolved yet tip-surface instabilities are absent. Our calculations demonstrate that chemical identification is unambiguous, because the interaction is always largest above the anions. This conclusion is generally valid for other polar surfaces, and can thus provide a new practical route for straightforward interpretation of atomically resolved images.     

High resolution study of magnetic ordering at absolute zero

High resolution pressure measurements in the zero-temperature limit provide a unique opportunity to study the behavior of strongly interacting, itinerant electrons with coupled spin and charge degrees of freedom. Approaching the precision that has become the hallmark of experiments on classical critical phenomena, we characterize the quantum critical behavior of the model, elemental antiferromagnet chromium, lightly doped with vanadium. We resolve the sharp doubling of the Hall coefficient at the quantum critical point and trace the dominating effects of quantum fluctuations up to surprisingly high temperatures.     

Elementary excitations at magnetic surfaces and their spin dependence

The elementary surface excitations are studied by spin-polarized electron energy loss spectroscopy on a prototype oxide surface [an oxygen passivated Fe(001)-p(1×1) surface], where the various excitations coexist. For the first time, the surface phonons and magnons are measured simultaneously and are distinguished based on their different spin nature. The dispersion relation of all excitations is probed over the entire Brillouin zone. The different phonon modes observed in our experiment are described by means of ab initio calculations.     

Nanoscale magnetic structure of ferromagnet/antiferromagnet manganite multilayers

We use polarized neutron reflectometry and dc magnetometry to obtain a comprehensive picture of the magnetic structure of a series of La(2/3)Sr(1/3)MnO3/Pr(2/3)Ca(1/3)MnO3 (LSMO/PCMO) superlattices, with varying thickness of the antiferromagnetic (AFM) PCMO layers (0相似文献   

Electric and magnetic properties of PMMA/manganite composites

We present the synthesis and characterization of the La_2/3Sr_1/3MnO₃ manganite in the form of tapes using polymethyl methacrylate (PMMA) as binder. We have studied their electric and magnetic properties as a function of temperature and magnetic field. The magnetization results have been shown to be dominated by the intrinsic magnetic properties of the manganite. Resistivity measurements showed an insulating behavior in the whole range of temperatures measured, indicating that the percolation threshold of manganite grains has not been reached even for the sample with 35% of PMMA relative content. The obtained magnetoresistance is largest in the sample with 35% of PMMA relative content.     

Magnetotransport in a graphene monolayer with two tunable magnetic barriers

The magnetotransport property for a monolayer graphene with two turnable magnetic barriers has been investigated by the transfer-matrix method. We show that the parameters of barrier height, width, and interval between two barriers affect the electron wave decaying length, which determine the conductance with parallel or antiparallel magnetization configuration, and consequently the tunneling magnetoresistance (TMR) for the system. Interestingly, a graphene attached by two barriers with different heights can produce a resonant TMR peak at low energy region one order of magnitude larger than that for the system with two same height barriers because that the asymmetry of magnetic barriers block the electron transmission in the case of antiparallel magnetization configuration. The results obtained here may be useful in understanding of electron tunneling in graphene and in designing of graphene-based nanodevices.     

Self-assembled monolayer growth on chemically modified polymer surfaces

We report a study of the self-assembled monolayer (SAM) growth of bis[3(triethoxysilane)propyl]tetrasulfide (Tetrasulfide) on low dielectric constant (low-k) aromatic hydrocarbon SiLK whose surface chemistry was modified using sulfuric acid, He plasma treatment, and N₂ plasma treatment. X-ray photoelectron spectroscopy (XPS) spectra show that there is no detectable growth of Tetrasulfide SAM on untreated SiLK surfaces. After the SiLK surfaces have been treated with sulfuric acid, He plasma, or N₂ plasma, the original chemically inert polymer surfaces are functionalized with polar groups resulting in a significant improvement of their wettability, which is confirmed by their reduction of water droplet contact angles. The introduction of polar functional groups thus facilitates the formation of Tetrasulfide SAM on the polymer surfaces. Atomic force microscopy (AFM) analysis shows an insignificant change in the surface morphology after the growth of Tetrasulfide SAM on the chemically modified SiLK surfaces. Quantitative XPS analysis also showed that Tetrasulfide SAM growth is more prominent on He and N₂ plasma treated surfaces than those treated by sulfuric acid.     

Study of the polymer surfaces with improved resolution FRES

We have developed a time-of-flight detector system which improves the resolution of standard He⁺ forward recoil spectrometry (FRES) to better than 300 Å. The technique was used to determine the shape of the concentration profile at the surface of polymer blends of deuterated polystyrene (d-PS) and protonated polystyrene (PS). The results are discussed in terms of the predictions of mean field theories.     

Radioactive ions for solid-state investigations at magnetic surfaces and interfaces

Hyperfine interactions observed at isomeric states of radioactive probe nuclei are used as a tool for solid-state investigations. This method is sensitive to atomic-scale properties. In recent years surface and interface investigations using radioactive probes delivered many results which can hardly be achieved by any other method. Several groups, e.g., from Konstanz, Leuven, Groningen, Aarhus, Uppsala, Tel Aviv, Pennsylvania, contributed to this field. Our group studies magnetic properties at surfaces and interfaces performing perturbed angular correlation (PAC) measurements in the UHV chamber ASPIC (Apparatus for Surface Physics and Interfaces at CERN). We take advantage of the enhanced variety of PAC probes delivered by the on-line mass separator ISOLDE. First, we report on measurements of magnetic hyperfine fields ( B _hf) at Se adatoms on a ferromagnetic substrate using ⁷⁷Se as a PAC probe. The investigation of induced magnetic interactions in nonmagnetic materials is a further subject of our studies. Here the nonmagnetic 4d element Pd is investigated, when it is in contact with ferromagnetic nickel. An outlook will be given on studies to be done in the future. The experiments were performed at the HMI, Berlin, and at CERN, Geneva. Received: 1 May 2001 / Accepted: 4 December 2001     

Effect of pressure on the magnetic properties of lanthanum manganite

The crystalline structure of pure lanthanum manganite under external hydrostatic pressure has been studied. The behavior of magnetic properties and nuclear magnetic resonance (NMR) spectra under these conditions is theoretically predicted. It is shown that an increase in the Néel temperature with pressure is not only caused by the general contraction of the crystal, but is also related to certain peculiarities in the baric behavior of the orbital structure.     

Structural and magnetic phase transitions occurring in Pr0.7Sr0.3MnO3 manganite at high pressures

The crystal and magnetic structures and the vibrational spectra of Pr_0.7Sr_0.3MnO₃ manganite are studied within the pressure range up to 25 GPa by methods of X-ray diffraction and Raman spectroscopy. Neutron diffraction studies have been performed at pressures up to 4.5 GPa. The magnetic phase transition from the ferromagnetic phase (T _C = 273 K) to the A-type antiferromagnetic phase (T _N = 153 K) is found at P ≈ 2 GPa. This transition is characterized by a broad pressure range corresponding to the phase separation. The Raman spectra of Pr_0.7Sr_0.3MnO₃ measured under high pressures significantly differ from the corresponding spectra of the isostructural doped A_{1 ? x} A′ _x MnO₃ manganites, (where A is a rare-earth ion and A′ is an alkaline-earth ion) with the smaller average ionic radius 〈r _A〉 of A and A′ cations. Namely, the former spectra do not include clearly pronounced stretching phonon modes. At P ～ 7 GPa, there appears the structural phase transition from the orthorhombic phase with the Pnma space group to the orthorhombic high-pressure phase with the Imma symmetry. In the vicinity of the phase transition, anomalies in the pressure dependences of the lattice parameters, unit cell volume, and phonon frequencies corresponding to the characteristic lattice vibration modes are observed.     

Measuring system with high spectral resolution for investigating flames

Translated from Zhurnal Prikladnoi Spektro-skopii, Vol. 56, No. 1, pp. 122–127, January, 1992.     

Tuning the metal-insulator transition in manganite films through surface exchange coupling with magnetic nanodots

In strongly correlated electronic systems, the global transport behavior depends sensitively on spin ordering. We show that spin ordering in manganites can be controlled by depositing isolated ferromagnetic nanodots at the surface. The exchange field at the interface is tunable with nanodot density and makes it possible to overcome dimensionality and strain effects in frustrated systems to greatly increasing the metal-insulator transition and magnetoresistance. These findings indicate that electronic phase separation can be controlled by the presence of magnetic nanodots.