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In this work we study the magnetic properties of 4-nm-thick permalloy films deposited into flat elliptical holes with an aspect ratio of 2:1 and short axis ranging from 2000 down to 70 nm. The holes are patterned into 50- and 200-nm-thick SiO 2 dielectric by e-beam lithography. The magnetization switching characteristics of the embedded magnetic elements were probed by localized magneto-optical Kerr effect and magnetic force microscopy measurements. It is shown that the switching mode, field and distribution depend on the hole depth and very strongly on the element's size, especially when the short axis shrinks below 100 nm. The results are compared with micromagnetic simulations.

Ballistic electron emission microscopy (BEEM) is a three terminal extension of scanning tunneling microscopy and yields topographic and spectroscopic information on high-energy electron transport in semiconductors at nm-resolution. In BEEM on GaAs–AlGaAs double barrier resonant tunneling diodes (DBRTDs) ballistic electrons which tunnel through a resonant state inside the DBRTD result in a characteristic linear behavior in the BEEM spectrum. On DBRTDs nanostructured into narrow quantum wires, however, this tunneling is quenched for electron energies below the AlGaAs barrier heights. This quenching of the ballistic current can be explained in terms of a transfer Hamiltonian formalism applied to tunneling processes between electron systems of different dimensionality. We measured BEEM spectra on InAs self-assembled quantum dots (SAQDs) for positions on the dots and for “off-dot” regions on the so-called InAs wetting layer. From these data, we determined the local InAs–GaAs band offsets on the dots and on the wetting layer and investigated the temperature dependence of the InAs–GaAs barrier height.

In this Letter we report on the spin-torque-driven magnetic switching and dynamic excitations in spin-valve, circular nanocontacts, incorporating an unpinned CoFe/Ru/CoFe artificial antiferromagnet as polarizer. Our investigation concentrates on the influence of the magnetic free layer moment and of the nanocontact size on these spin-torque related phenomena. Two multilayers are therefore investigated, one with an amorphous CoFeB and the other with a CoFe/CoFeB free layer. The nanocontact radii range from 40–130 nm. Both multilayers show clear current-induced magnetic switching for small in-plane magnetic fields. On the other hand, sub-gigahertz dynamic spectra, related to magnetic vortex precession, are only observed for multilayers employing the CoFeB free layer and only for contact radii below 100 nm. The current and frequency dependence of the oscillation on the point contact radius and magnetic field is discussed. Multiple excitation modes are shown to exist at zero or small magnetic fields as a result of a highly inhomogeneous magnetic state below the nanocontact. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, ballistic electron transport through the lowest miniband of a biased GaAs–AlGaAs superlattice is investigated in transverse magnetic fields. As method we employ a solid-state version of ballistic electron emission microscopy/spectroscopy using a metal-insulator-metal injector structure that replaces the tip of the scanning tunneling microscope (STM). The ballistic electron current measured as a function of the collector bias shows a peak at flatband conditions indicating coherent transport through the superlattice miniband. With increasing transverse magnetic fields, this peak is quenched and evidence of sequential LO-phonon scattering inside the superlattice is found. Using an extended transfer matrix method, the observed effects are quantitatively explained; differences to previous STM based measurements are discussed.

In this work, buried Al 0.4 Ga 0.6 As/GaAs superlattices are employed as an energy filter in order to study the energy distribution of the ballistic electron current in ballistic electron emission microscopy (BEEM). As the measured total transmission of the superlattice is in excellent agreement with the calculated transmission, the superlattice is a promising tunable energy filter for studying the energy distribution of ballistic electrons. We further show that due to the large difference in electron masses between the Au base layer and the GaAs collector, parallel momentum conservation leads to considerable electron refraction at the Au/GaAs interface. As a consequence, the energy distribution of ballistic electrons is inverted beyond the Au/GaAs interface.

Ballistic electron emission microscopy (BEEM) has been used to study MBE grown Al 0.4 Ga 0.6 As/GaAs superlattice structures at variable temperature. The influence of the superlattice miniband manifests itself even at 300 K by a considerable lowering of the collector current threshold voltage compared to the threshold measured on samples with a thick Al 0.4 Ga 0.6 As barrier. On the other hand, reference samples with neither a superlattice nor a barrier give low thresholds, as expected for a Au/GaAs Schottky barrier. The temperature dependence of the threshold for the superlattice sample agrees excellently with self-consistent Schrodinger–Poisson calculations of the structure.

We introduce a solid-state version of ballistic electron emission microscopy/spectroscopy (BEEM/BEES) on GaAs–AlGaAs heterostructures using a metal–insulator–metal (MIM) injector structure that replaces the tip of the scanning tunneling microscope (STM). In the present work, the MIM injector is realized by an Al–Al2O3–Al tunnel junction yielding an easy-to-fabricate three-terminal device for ballistic electron spectroscopy. The device principle is applied to several GaAs–AlGaAs structures. The barrier heights obtained from the onsets of the ballistic current spectra are in good agreement with self-consistent calculations as well as earlier experimental results achieved with STM-based BEES.

In Ballistic Electron Emission Microscopy, ballistic electrons are injected from a metallic base contact into a semiconductor. In the this experiment, the miniband of a GaAsAlGaAs superlattice is employed as energy filter in order to study the energetic distribution of the ballistic electrons injected into the semiconductor. It is found, that due to the large difference in electron mass between Au and GaAs, parallel momentum conservation leads to considerable electron refraction at the AuGaAs interface. Moreover, a resonant tunneling structure directly at the sample surface can act as momentum filter for electrons injected at k // =0.

In this work, ballistic electron transport through the lowest miniband of a biased GaAs–AlGaAs superlattice is investigated by ballistic electron emission microscopy (BEEM). In the BEEM spectra the miniband manifests itself as clear peak in the second derivative of the ballistic electron current. Biasing the superlattice results in a shift of the miniband position and the corresponding peak position. It is shown that the measured total transmission of the superlattice is in excellent agreement with the calculated transmission, which makes the superlattice a promising tunable energy filter for studying the energetic distribution of ballistic electrons.

Ballistic electron emission spectroscopy and ballistic electron emission microscopy offer the unique possibility of probing subsurface quantum states. To improve the spectroscopic sensitivity, it is important to increase the amount of electrons, which are able to penetrate into the sample. In this work, we show that the transmission coefficient and the attenuation length of the base layer can be enhanced by more than one order of magnitude, if the commonly used thin metal film is replaced by a molecular beam epitaxy grown InAs layer. At low temperatures (T=100 K), a passivated InAs layer yields an attenuation length in the order of 70–90 nm instead of 5 nm obtained on Au films.

Spin valve structures, as employed in base layers of spin valve transistor devices, are characterized by ballistic electron emission microscopy (BEEM). In detail, Co–Cu–Permalloy–Au layers sputtered onto n-type GaAs bulk substrates were studied. BEEM spectra taken on these multilayers show that magnetocurrents on the order of 600% can be achieved even at room temperature. Small area images (400nm×400nm) show that the spin filtering effect of the spin valves is quite homogeneous on the submicron scale. On larger scales, magnetic domains were imaged close to the switching field of the spin valve structure.

Buried ${\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ superlattices on Au-GaAs Schottky diodes have been used as an energy filter to study the energetic current distribution in ballistic electron-emission microscopy (BEEM) at room temperature and $T=100\mathrm{K}.$ Due to the large difference in electron masses in Au and GaAs we find that parallel momentum conservation leads to considerable electron refraction at the Au-GaAs interface. As a consequence, the energetic distribution of the ballistic electron current is inverted beyond the Au-GaAs interface and an almost linear behavior of the BEEM spectrum is observed in the energetic regime of the superlattice miniband.