Saskia F. Fischer

Hexagonally ordered 100 nm period nickel nanowire arrays

The magnetic behavior of 100 nm period arrays of Ni nanowires embedded in a highly ordered alumina pore matrix were characterized by magnetometry and magnetic force microscopy. Reducing the diameter of the nanowires from 55 to 30 nm while keeping the interwire distance constant leads to increasing coercive fields from 600 to 1200 Oe and to increasing remanence from 30% to 100%. The domain structure of the arrays exhibits in the demagnetized state a labyrith-like pattern. These results show that stray field interactions of single domain nanowires are entirely dependent on the nanowire diameter.

High density hexagonal nickel nanowire array

Nickel nanowires were grown in highly ordered pore channels of an alumina membrane using pulsed electrodeposition. A complete metal filling of the hexagonal arranged pores with a pitch of 100 nm and a monodisperse pore diameter of iO30 nm was obtained. The bulk-magnetic behavior of the ferromagnetic nanowire arrays was characterized by SQUID-magnetometer measurements. Magnetic-force-microscopy investigation measurements with a variable external magnetic field were applied on magnetized and demagnetized samples. In addition, magnetic wires have been locally switched by a strong MFM tip and an external magnetic field. The MFM results show a good agreement with the bulk magnetic hysteresis loop.

Switching behaviour of single nanowires inside dense nickel nanowire arrays

Summary form only given. The suitability of nickel nanowire arrays for perpendicular magnetic storage media with areal densities beyond the superparamagnetic limit (>70 Gbit/in/sup 2/) is analyzed in this paper. Highly ordered alumina pore channel arrays are used as templates for the fabrication of magnetic nanowire arrays with a periodicity of 65 (180 Gbit/in/sup 2/) and 100 nm (75 Gbit/in/sup 2/) and pore diameters between 30 and 55 nm. A nearly perfect hexagonal arrangement for the magnetic columns occurs only inside very narrow process windows for periodic distances of 65, 100 and 500 nm, and - in contrast to most publications in this field - a degree of pore filling of almost 100% was achieved. Here, we will focus on nickel as a filling material due to its negligible magneto-crystalline anisotropy, so that the interactions between the anisotropy resulting from the nanowire shape and the stray field inside the magnetic arrays can be studied in detail. The small magnetic moment and the large exchange lengths (/spl lambda//sub ex/= (A/2K/sub 1/)/sup 05/ /spl ap/ 20 nm) of nickel leads to low dipole interactions between nanowires and a huge anisotropy along the column axis which enables nickel as a suitable material for pattern perpendicular magnetic media.

Suppressing a charge density wave by changing dimensionality in the ferecrystalline compounds ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, 4.

The compounds, ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4, were prepared using designed precursors in order to investigate the influence of the thickness of the VSe2 constituent on the charge density wave transition. The structure of each of the compounds was determined using X-ray diffraction and scanning transmission electron microscopy. The charge density wave transition observed in the resistivity of ([SnSe]1.15)1(VSe2)1 was confirmed. The electrical properties of the n = 2 and 3 compounds are distinctly different. The magnitude of the resistivity change at the transition temperature is dramatically lowered and the temperature of the resistivity minimum systematically increases from 118 K (n = 1) to 172 K (n = 3). For n = 1, this temperature correlates with the onset of the charge density wave transition. The Hall-coefficient changes sign when n is greater than 1, and the temperature dependence of the Hall coefficient of the n = 2 and 3 compounds is very similar to the bulk, slowly decreasing as the temperature is decreased, while for the n = 1 compound the Hall coefficient increases dramatically starting at the onset of the charge density wave. The transport properties suggest an abrupt change in electronic properties on increasing the thickness of the VSe2 layer beyond a single layer.

Temperature-Dependent Thermoelectric Properties of Individual Silver Nanowires

The thermoelectric properties of the Ag NWs are discussed in comparison to the bulk: SAg;Pt(T) was measured with respect to platinum and is in agreement with the bulk, (T) and (T) showed reduced values with respect to the bulk. The latter are both notably dominated by surface scattering caused by an increased surface-to-volume ratio. By lowering T the electron mean free path strongly exceeds the NW’s diameter of 150 nm so that the transition from diusive transport to quasi ballistic one dimensional transport is observed. An important result of this work is that the Lorenz numberL(T) turns out to be independent of surface scattering. Instead the characteristic ofL(T) is determined by the material’s purity. Moreover, (T) and L(T) can be described by the bulk Debye temperature of silver. A detailed discussion of the temperature dependence of L(T) and the scattering mechanisms is given.

Magnetization reversal in granular nanowires

The switching process of granular Co nanowires is investigated using the finite element method. The wires have a diameter of 55 nm and a length of 1000 nm. Transmission electron microscopy (TEM) investigations show two different types of hcp-structured grains. For one, the c axis is randomly oriented in a plane perpendicular to the long axis of the wire, and the other has the c axis parallel to the long axis. The numerical results show that finite element micromagnetics can explain the influence of the microstructure in magnetic nanosystems.

Influence of processing parameters on the transport properties of quantum point contacts fabricated with an atomic force microscope

We describe a reliable technique for fabricating ballistic quantum point contacts (QPCs) with large energy separation between one-dimensional subbands. The technique is based on lithography with an atomic force microscope and wet chemical etching of a GaAs/AlGaAs heterostructure. The high-mobility two-dimensional electron gas located 55 nm below the surface is laterally confined by 20 nm or 50 nm deep grooves with separations ranging between 65–105 nm or 100–185 nm, respectively. The conductance characteristics at T = 4.2 K exhibit clear quasi-plateaux at multiples of 2e2/h. Both the conductance threshold voltage and the plateau widths are directly related to the QPC geometry. The energy separation ΔE1,2 of the lowest subbands is determined from the conductance under nonzero dc drain voltage. Upon reducing the QPC width, ΔE1,2 varies from 6 meV to 15 meV.

Enhanced magneto-thermoelectric power factor of a 70 nm Ni-nanowire

Thermoelectric (TE) properties of a single nanowire (NW) are investigated in a microlab which allows the determination of the Seebeck coefficient S, the electrical conductivity σ, and a full ZT-characterization in the validity limit of the Wiedemann-Franz-law (ZT—figure of merit). A significant influence of the magnetization of a 70 nm diameter ferromagnetic Ni-NW on its power factor S2σ is observed. We detected a strong magnetothermopower effect (MTP) of about 10% and an anisotropic magnetoresistance (AMR) as a function of an external magnetic field B in the order of 1%. At T = 295 K and B = 0 T, we determined the absolute value of S = −(19 ± 2) μV/K. The thermopower S increases considerably as a function of B up to 10% at B = 0.5 T, and with a magnetothermopower of ∂S/∂B ≈ −(3.8 ± 0.5) μV/(KT). The AMR and MTP are related by ∂s/∂r ≈ −11 ± 1 (∂s = ∂S/S). Hence, the TE efficiency increases in a transversal magnetic field (B = 0.5 T) due to an enhanced power factor by nearly 20%.

Nonlocal Aharonov–Bohm conductance oscillations in an asymmetric quantum ring

We investigate ballistic transport and quantum interference in a nanoscale quantum wire loop fabricated as a GaAs/AlGaAs field-effect heterostructure. Four-terminal measurements of current and voltage characteristics as a function of top gate voltages show negative bend resistance as a clear signature of ballistic transport. In perpendicular magnetic fields, phase-coherent transport leads to Aharonov–Bohm conductance oscillations, which show equal amplitudes in the local and the nonlocal measurement at a temperature of 1.5 K and above. We attribute this observation to the symmetry of the orthogonal cross junctions connecting the four quantum wire leads with the asymmetric quantum wire ring.

Temperature-dependent thermal conductivity in Mg-doped and undoped β-Ga2O3 bulk-crystals

For - only little information exists concerning the thermal properties, especially the thermal conductivity λ. Here, the thermal conductivity is measured by applying the electrical 3ω-method on Czochralski-grown - bulk crystals, which have a thickness of and . At room temperature (RT), the thermal conductivity along the [100]-direction in Mg-doped electrical insulating and undoped semiconducting - is confirmed as for both crystals. The thermal conductivity increases for decreasing temperature down from 25 K to . The phonon contribution of λ dominates over the electron contribution below RT. The observed function is in accord with phonon–phonon–Umklapp scattering and the Debye model for the specific heat at which is about 0.1 times the Debye temperature . Here, a detailed discussion of the phonon–phonon–Umklapp scattering for is carried out. The influence of point defect scattering is considered for .