K. Bussmann

SWITCHING OF VERTICAL GIANT MAGNETORESISTANCE DEVICES BY CURRENT THROUGH THE DEVICE

Experiments are reported that demonstrate current-perpendicular-to-the-plane giant magnetoresistance devices can be switched repeatably between the high- and low-resistance states by passing current vertically through the structure. The lithographically patterned devices, having diameters in the range of 0.3–0.7 μm, operate at room temperature and exhibit distinctly separate switching of the soft and hard layers. Designs for magnetoelectronic random access memory can utilize this scheme for storing and reading information.

Magnetic, structural, and transport properties of thin film and single crystal Co2MnSi

The magnetic, structural, and transport properties of the Heusler alloy Co2MnSi are reported for sputtered thin films and a single crystal. X-ray diffraction reveals a phase pure L21 structure for all films grown between 573 and 773 K. Films grown at 773 K display a four-fold decrease in the resistivity relative to those grown at lower temperatures and a corresponding 30% increase in the residual resistivity ratio (ρ300 K/ρ5 K). We show that the higher growth temperature results in lattice constants, room temperature resistivities, and magnetic properties that are comparable to that of the bulk single crystal.

Large magnetic field effects in organic light emitting diodes based on tris(8-hydroxyquinoline aluminum) (Alq3)/N,N′-Di(naphthalen-1-yl)-N,N′diphenyl-benzidine (NPB) bilayers

Magnetic field effects (MFEs) in the luminescence and photoconductivity of organic crystals such as anthracene have been known since the 1960s. While engaged in a project to use spin polarized current to manipulate electroluminescence (EL) in organic light emitting diodes based on a bilayer of tris(8-hydroxyquinoline) aluminum (Alq3) and N,N′-Di(naphthalen-1-yl)-N,N′diphenyl-benzidine (NPB), we observed MFE similar to those reported in anthracene. The MFE consist of an increase in EL with increasing magnetic field of a few percent for small magnetic fields, a decrease in EL of greater than 20% at high fields and an increase in conductivity, i.e., negative magnetoresistance (MR), for all magnetic fields. The high field effect (HFE) is enhanced at lower temperatures and higher current densities and is similar to that reported for delayed luminescence in anthracene single crystals suggesting that triplet–triplet annihilation (TTA) influences light emission in Alq3. Transient EL studies were performed in an a...

A room-temperature semiconductor spaser operating near 1.5 μm

Room temperature spasing of surface plasmon polaritons at 1.46 μm wavelength has been demonstrated by sandwiching a gold-film plasmonic waveguide between optically pumped InGaAs quantum-well gain media. The spaser exhibits gain narrowing, the expected transverse-magnetic polarization, and mirror feedback provided by cleaved facets in a 1-mm long cavity fabricated with a flip-chip approach. The 1.06-μm pump-threshold of ~60 kW/cm2 is in good agreement with calculations. The architecture is readily adaptable to all-electrical operation on an integrated microchip.

Guided resonances in asymmetrical GaN photonic crystal slabs observed in the visible spectrum

We demonstrate that guided resonant modes can be readily observed in asymmetrical photonic crystal slabs on high-index substrates. In spite of the high radiative loss associated with all optical modes in these cases, the guided resonant modes are found to give rise to strong high-Q features in the transmission spectra. Since these photonic crystal structures are far more robust and easier to fabricate than the free-standing photonic crystal membranes used in previous studies of guided resonant modes, detailed studies of relevant optical phenomena and the implementation of proposed applications are greatly simplified.

Growth and magnetic properties of single crystal Co/sub 2/MnX (X=Si,Ge) Heusler alloys

Half-metallic (HM) ferromagnetic materials have recently drawn intense interests due to their potential use in magnetoelectronic devices. Co-based Heusler alloys of the type CO/sub 2/MnX (X=Si,Ge), predicted to be HM by first principles band structure calculations, are of particular interest since they alone possess Curie temperatures in excess of 900 K. Since the spin polarization (P) is believed to be sensitive to antisite defects that are likely to occur in vapor-quenched thin film synthesis, single crystals of Co/sub 2/MnX (X=Si, Ge) were prepared using the tri-arc Czochralski method. X-ray diffraction, including Laue backscattering, was employed to determine the high crystalline quality of these crystals. SQUID magnetometry measured a magnetic moment per formular unit that is close to the calculated value indicating that these alloys may in fact be HM. However, point contact Andreev reflection, a technique that has reliably measured high P in CrO/sub 2/, measures P values for these crystals of 50-60%, well below their theoretical values. The reduced spin polarization may be due to the effects of crystal symmetry breaking at the surface or the presence of anti-site defects, or that these materials are not truly half-metallic.

Organic luminescent devices and magnetoelectronics

Developments in magnetoelectronics are advancing by combination of once disparate areas of research in magnetic materials, semiconductor electronics, and optoelectronics. We explore the integration of magnetic materials with organic semiconductors. Because small spin–orbit coupling in these materials minimizes spin relaxation, they may be useful in spintronic applications. Motivated by a theoretical investigation into spin-dependent exciton formation that predicts a magnetoluminescence valve effect, we attempt to manipulate spin-polarized holes and electrons in an effort to generate magnetic field dependent luminescence in organic light emitting diodes (OLEDs). We have fabricated various functional OLEDs consisting of ferromagnetic electrodes sandwiching a organic semiconducting bilayer, thus demonstrating that hole and electron injection from magnetic electrodes is possible. However, magnetic transition metal anodes produce higher turn-on voltages and significantly reduced lifetimes compared to indium–ti...

Exploring magnetic roughness in CoFe thin films

The behavior of chemical and magnetic interfaces is explored using diffuse x-ray resonant magnetic scattering (XRMS) for CoFe thin films with varying interfacial roughnesses. A comparison of the chemical versus magnetic interfaces shows distinct differences in the behavior of these two related interfaces as the chemical roughness is increased. Such changes appear to be correlated with the behavior of the magnetic hysteresis of the interface, measured by tracking the diffuse XRMS intensity as a function of applied magnetic field.

Improved atomic force microscope cantilever performance by ion beam modification

The performance of atomic force microscopy cantilevers, as measured by the resonant frequency and spring constant, is directly dependent on the shape of the cantilever. Here we have improved the performance of conventional silicon nitride cantilevers by using focused ion beam milling to minimize the width of the cantilever legs. The resonant frequency in solution for any given spring constant is increased by two- to threefold, and the thermal noise in a given bandwidth is correspondingly reduced.

Evidence for charge-carrier mediated magnetic-field modulation of electroluminescence in organic light-emitting diodes

Electroluminescence (EL) from organic light-emitting diodes can be surprisingly sensitive (∼20%) to modest magnetic fields B (0–2T). The origin of this magnetic-field effect has not been clearly identified, although the magnetic-field effect in some devices resembles that of delayed fluorescence in anthracene, which originates from magnetic-field-dependent singlet-exciton production via triplet-triplet annihilation (TTA). Here, we test the role of TTA at low magnetic fields (∼80mT) by measuring transient EL and by employing dc drive levels so low that the bimolecular rate of TTA is unimportant. Under these conditions, we find enhancements of EL exceeding 14% at the lowest drives, which excludes TTA-mediated magnetic-field effects and indicates a role for charge-carrier pair states.