Search Results - (Author, Cooperation:M. Aeschlimann)

2014-08-26

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-11-27

American Physical Society (APS)

1098-0121

1095-3795

Physics

Surface physics, nanoscale physics, low-dimensional systems

2018-11-15

American Physical Society (APS)

1098-0121

1095-3795

Physics

Magnetism

2011-08-13

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7550

AIP Digital Archive

Physics

We use time-resolved spin-polarized photoemission to investigate thermomagnetic writing of domains in magneto-optic media, focusing on the relaxation time of the magnetization and the dynamic behavior of the nucleation process. In our initial studies, we examine a 90-nm-thick GdTbFe film using a pulsed excimer laser (pulse duration: 16 ns) as the light source for the photoemission process. We find that the thermomagnetic switching behavior is different above and below the compensation temperature Tcomp. When the sample temperature is held above Tcomp, the spin polarization of the electrons emitted during the writing pulse has the sign of the initial state even though subsequent examination shows that a reversed magnetization domain has been formed. Therefore, the domain is thermomagnetically nucleated during the trailing edge of the 16 ns writing pulse or even later when the irradiated domain cools down. On the other hand, if the initial temperature is slightly below Tcomp, the electrons emitted during the writing pulse have reversed polarization showing that the reversal of the magnetization takes place quickly compared to the pulse duration. This difference shows that compensation-point writing is much faster than Curie-point writing. Based on these measurements we propose a model to interpret the different thermomagnetic switching processes which take place above and below Tcomp. The results can be explained by different thermal relaxation times between the excited electrons and the lattice and between the electrons and the spin system.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

With pulsed lasers as light sources, a new and important element is added to photoemission: time resolution. In order to make use of the high light intensities available, the experiment should measure the spin polarization of the total photoyield since this quantity is insensitive to space-charge effects. The following examples are discussed: (1) For amorphous GdTbFe, a magneto-optical storage material, it is shown that the speed of thermomagnetic writing critically depends on whether the initial temperature of the sample is above or below the compensation temperature. (2) The response of the magnetic order to pico- and nanosecond heating pulses has been measured in iron and other materials. It is found that the spin polarization of the electrons emitted from Fe by 30-ps pulses does not depend on the pulse energy even if melting occurs. Accordingly, ferromagnetic relaxation in Fe takes place on a time scale which is considerably longer than 30 ps. (3) Using optical spin orientation, the dynamics of the band bending induced by high-intensity laser pulses has been studied in a p-GaAs/Cs Schottky barrier. The buildup time for the photovoltage created by the electron-hole pairs excited in the band-bending region is of the order of several hundred picoseconds.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Domains written thermomagnetically in TbFeCo thin films are studied with Lorentz transmission electron microscopy (TEM) and scanning electron microscopy with polarization analysis (SEMPA). Four different rare-earth/transition-metal compositions TbxFeyCo1−x−y are examined. The domain structures observed with both techniques are similar even though TEM Lorentz only detects the transverse component of the net magnetic field along the electron's trajectory through the sample, while SEMPA detects the surface electron-spin polarization (magnetization). We find that the magnetic contrast in the SEMPA measurements is proportional to the magnetization of the transition-metal (TM) subnetwork which is antiferromagnetically coupled to the rare-earth (RE) subnetwork. This allows high-contrast SEMPA images to be acquired even when the system is magnetically compensated (Ms=||MRE−MTM||=0). The surface magnetization can be explained by assuming that the surface of the TbFeCo alloy consists of an outermost thin oxide layer followed by an Fe-rich subsurface layer. The importance of the demagnetizing field on the switching and domain nucleation process for thermomagnetically written bits is examined.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

Theoretical and experimental investigations for a new design of an ultrashort pulsed laser activated electron gun for time resolved surface analysis are described. In addition, a novel electron detection and image analysis system, as it applies specifically to time resolved reflection high-energy electron diffraction in the multiple-shot operation, are reviewed. Special attention is directed to minimize the photoelectron transit-time spread from the electron gun, in spite of an unusually long focal length and a small convergence angle of the pulsed electron beam. Both requirements are necessary to use the electron gun for diffraction techniques. The design value for the temporal resolution in the synchroscan operation is 1.3 ps. Based on a thorough theoretical investigation, a new electron gun has been designed, constructed, and tested. © 1995 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Thin epitaxial iron overlayers and sandwiches on Cu(001) have been investigated by spin-polarized photoemission. The magnetization was measured as a function of perpendicularly applied field and temperature. Up to 14 monolayers (ML) of iron grow epitaxially on Cu(001) in the fcc phase and are ferromagnetic. At 30 K a remanence magnetization perpendicular to the plane of the film is observed for both the Fe/Cu(001) and the Cu/Fe/Cu(001) systems with Fe layers thicker than 2 ML. The coercive field, measured as function of temperature for a 10-ML Fe film, decreases rapidly from 2.7 kOe at 30 K to about 50 Oe at 200 K. The Curie temperature is 300 K for films thicker than 8 ML and increases for thinner films up to 500 K. The transition from fcc to bcc iron occurs at 15 ML. It is recognized by a jump of the Curie temperature to 1000 K and by the disappearance of the remanence magnetization at 30 K. Simultaneous evaporation of Fe and Cu at various rates also results in epitaxial overlayers with characteristic magnetic features. The fcc Fe films are suitable for thermomagnetic recording. Writing and reading has been carried out by a UV excimer laser. Reading was performed by analyzing the sign of the spin polarization of the photoelectrons.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Epitaxial bcc Fe films, grown on Ag(001), ranging from 0.8 to 10 monolayers (ML), are ferromagnetic. At 30 K a remanent magnetization along the surface normal is observed for the 3–4-ML films. No perpendicular remanence is observed for the same films above 100 K and for films thicker than 5 ML or thinner than 2 ML. Above 5 ML the Curie temperature is around 1000 K as for bulk bcc Fe; for thinner films it is reduced and amounts to 400 K only for the 1 ML film.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Using spin-polarized photoemission with a pulsed laser as light source, it is shown that the time for a thermally induced magnetization reversal depends critically on the temperature of the sample. For amorphous GdTbFe the time is shorter (longer) than the duration of the 16 ns laser pulses if the initial temperature is below (above) the compensation temperature.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Photoelectrons with excess kinetic energy corresponding to several absorbed photons above the work function have been measured from atomically clean Cu(110) and Cu(100) surfaces under ultrahigh vacuum conditions. The power dependence of the photoemission yield does not follow a simple power law dependence corresponding to the number of photons absorbed. This behavior is reminiscent of other above threshold ionization (ATI) or tunnel ionization (TI) processes observed for atoms in the gas phase. The photoelectrons are generated with laser pulsewidths less than 100 fs in duration and peak powers as low as 100 MW/cm2. These intensities are on the order of 105 times lower than that required to observe similar phenomena in the gas phase. The relatively low intensities and correlation with surface roughness suggests a contribution from a surface enhancement mechanism. Thermal heating and space charge effects have been ruled out, and the possibility of electric field enhancement at the surface due to the coupling of photons into surface plasmons is discussed. The nonlinear yield and enhancement of the photoemission produced by short pulse excitation needs to be considered when discussing photoinduced hot electron reaction channels at metal surfaces. © 1995 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The spin polarization of the photoelectrons emitted from Sn and Fe during picosecond (ps) and nanosecond (ns) laser pulses is measured as function of the laser intensity. For Sn the optically induced spin polarization is defined through the lattice symmetry. No difference is found between ps and ns heating. From this it is concluded that the melting of a metal like tin occurs on a time scale which is short compared to the duration of a 70 ps laser pulse. In Fe the spin polarization probes the magnetic order. It is found that Fe cannot be demagnetized within the duration of a 30 ps laser pulse, even if the melting point is reached in the laser focus. During a ns laser pulse the spin system and the lattice are in thermal equilibrium.

Electronic Resource

0304-8853

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Physics

Electronic Resource

0304-8853

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Physics

Electronic Resource

0304-8853

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Physics

Electronic Resource

1572-879X

nanolithography ; model catalyst ; palladium ; copper ; stability ; spin-coating ; SEM ; AFM ; XPS

Springer Online Journal Archives 1860-2000

Chemistry and Pharmacology

Abstract Metal clusters arranged on nanostructured oxidized silicon wafers are presented as new model catalyst systems. A photoresist layer spun on top of a wafer was patterned by laser interference exposure. The grid obtained after removing the exposed parts of the resist is used as an etching mask. Hollows with diameters of 300 nm and depths between 50 and 60 nm were etched into the oxide layer using wet chemical methods. Two methods were applied to deposit metal clusters (Pd or Cu) in a defined way within the hollows. The particles ranged from 10 to 50 nm in height and from 80 to 200 nm in diameter. The model catalyst systems were characterized by atomic force microscopy and X-ray photoelectron spectroscopy. The method presented here allows us to produce 4 inch wafers that are covered completely by nanometer-sized structures in a reasonable period of time.

Electronic Resource

1432-0630

PACS: 72.15.Lh; 78.47.+p; 73.50.Bk

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Abstract. Time-resolved two-photon photoemission, based on the equal-pulse correlation technique, is used to measure the energy relaxation and the transport of the photoexcited carriers in thin Ag and Au films. The energy-dependent relaxation time shows a significant thickness dependence in the Ag film, whereas for Au a much smaller effect is observed. These experimental observations are compared with a theoretical model based on the Boltzmann equation, which includes secondary (Auger) electrons and transport. A good agreement between experimental and theoretical results is found for Au. However, in our calculations, we did not find any significant change in the thickness dependence in the case of Ag. In order to explain the strong effect in Ag, we discuss the possibility of surface excitations.

Electronic Resource

1432-0649

PACS: 78.47.+p; 71.24.+q; 72.15.Lh

Springer Online Journal Archives 1860-2000

Physics

Electronic Resource