J. Caro

Nanoconstriction microscopy of the giant magnetoresistance in cobalt/copper spin valves

We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance (GMR) of only a few Co domains. The measured data show strong device-to-device differences of the GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratio decreases with increasing bias current. For one particular device, this is accompanied by the development of two distinct GMR plateaus, the plateau level depending on bias polarity and sweep direction of the magnetic field. We attribute the observed behavior to current-induced changes of the magnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.

Transmission measurement of the photonic band gap of GaN photonic crystal slabs

A high-contrast-ratio (30 dB) photonic band gap in the near-infrared transmission of hole-type GaN two-dimensional photonic crystals (PhCs) is reported. These crystals are deeply etched in a 650 nm thick GaN layer grown on sapphire. A comparison of the measured spectrum with finite difference time domain simulations gives quantitative agreement for the dielectric band and qualitative agreement for the air band. The particular behavior of the air band arises from the relatively low index contrast between the GaN layer and the sapphire substrate. Our results call for extension of the operation of GaN PhCs to the visible range.

Local probing of the giant magnetoresistance

We have contacted the tip of a scanning tunneling microscope (STM) to a Co/Cu magnetic multilayer to locally measure the giant magnetoresistance (GMR) of the multilayer. Apart from a point-contact GMR, the measured MR also reflects a magnetostriction effect in the STM. The resulting GMR ratios are typically 10%, with occasional ratios up to 60%. We attribute spot-to-spot variations of the ratio to differences in the local structure of the magnetic multilayer.

ROLE OF BOUNDARY CONDITIONS AND DIMENSIONS ON THE MICROMAGNETICS OF A COBALT POINT CONTACT

We have calculated the micromagnetic structure of a Co point contact, for applied magnetic fields parallel (H∥) and perpendicular (H⊥) to the device’s electrodes. Different boundary conditions and dimensions have been examined. In most situations a Neel wall is formed in the constriction. When the size of the device is reduced (or the exchange stiffness is increased), then the energy barrier between the state with and without the Neel wall is increased. The domain wall magnetoresistance (DWMR) and anisotropic MR (AMR) have been estimated, and they show a maximum in the DWMR for both field directions of the applied magnetic field and a maximum and minimum in the AMR for H∥ and H⊥, respectively.

Variable range hopping via a small number of localized states in nanometer scale MOSFETs

Abstract We studied experimentally the low temperature conductance of small silicon metal-oxide-semiconductor field-effect transistors operating in the localized regime. Two different device structures were used: a short and narrow point-contact and a short, wide barrier. For both structures the conductance fluctuated rapidly as a function of gate voltage. From a detailed comparison between the temperature dependence of the fluctuations and numerical simulations we found that at the lowest temperatures the fluctuations are caused by variable range hopping via paths containing only a few (≲ 3) localized states.

Giant magnetoresistance of Co/Au multilayer point contacts

Abstract We have measured the giant magnetoresistance (GMR) of granular Co/Au point contacts, fabricated by deposition of [Co(0.3xa0nm)/Au(4xa0nm)] 10 multilayers with molecular beam epitaxy. Two Co deposition rates were used to obtain two types of contacts with different Co cluster size. The measured GMR ratios are in the range 5–12%. These GMRs come from the constriction region, since the current in-plane contribution of the multilayer was shorted by a thick Au cap. GMR curves characteristic of both superparamagnetic and ferromagnetic Co clusters were observed for either contact type. Device-to-device differences in the GMR ratio and in the magnetic behaviour, and sudden jumps in the GMR indicate that only a few Co clusters in the constriction generate the GMR.

Properties of tungsten point contacts formed with chemical vapour deposition

Abstract We have used chemical vapour deposition to form tungsten point contacts. From electron microscopy we find that nanoholes in membranes are filled from the beginning of the deposition using the CVD process for α-W. Fabricated devices show good point-contact spectra (α-W) and interesting electrical characteristics (β-W and amorphous W).

High electric field transport in bismuth nanoconstrictions

Abstract We have measured I—V curves of single-crystalline lateral Bi nanoconstrictions of a size of several hundred nanometers. The I—V curves are strongly non-linear at high currents when the voltage is measured with probes at 4 μm on either side of the constriction, the effect increasing with increasing magnetic field. Surprisingly, with voltage probes at 4 and 55 μm on one side of the constriction, we still find a weak non-linearity. We explain these observations with Zener tunnelling, made possible by the strong electrical field in the constriction. We suggest that diffusion of tunnelling-induced excess carriers causes a non-local effect, which can be measured at some distance of the constriction.

Periodic conductance oscillations in a Si-MOSFET point contact

Abstract We present data on the low temperature conductance of a new dual gate silicon device, a point-contact in a metal-oxide-semiconductor field-effect transistor. Just above pinch-off, the conductance oscillates periodically as a function of gate voltage. The parameters obtained from the conductance gate voltage traces correspond well with theoretical predictions based on the Coulomb blocked of tunneling. The dependence of the oscillation period on the width of the point contact also follows the expected behaviour. A comparison between the I-V characteristics and numerical simulations shows a discrepancy, but we give two explanations for the absence of the Coulomb staircase in the experimental curves.

The effect of diffusive trajectories on conductance fluctuations in ballistic point contacts

Abstract We have measured the non-universal conductance fluctuations (CFs) in Au and Au1−xPdx point contacts for a wide range of the degree of disorder. The scaling behaviour of the amplitude and the magnetic field scale of the CFs known for highly ballistic contacts breaks down as the scaling parameter a/le is increased from 0.03 to 0.35. This arises from diffusive remote trajectories backscattered to the constriction, which modify the properties of the CFs and the correlation function. A new formula for the derivative of the correlation function is obeyed by our experimental data.