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Register a new userMagnetoresistance in nanostructures: the role of nonuniform current
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We developed a method to calculate the magnetoresistance of magnetic nanostructures. We discretize a magnetic disk in small cells and numerically solve the Landau-Lifshitz-Gilbert (LLG) equation in order to obtain its magnetization profile. We consider a anisotropic magnetoresistance (AMR) that depends on the local magnetization as the main source of the magnetoresistance. We then use it as an input to calculate the resistance and current distribution numerically, using a relaxation method. We show how magnetoresistance measurements can be useful to obtain information on the magnetic structure. Additionally, we obtain non-homogeneous current distributions for different magnetic configurations in static and dynamical regimes.
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We have observed an unusual dc current spontaneously generated in the conducting channel of a short-gated GaAs transistor. The magnitude and direction of this current critically depend upon the voltage applied to the gate. We propose that it is initiated by the injection of hot electrons from the gate that relax via phonon emission. The phonons then excite secondary electrons from asymmetrically distributed impurities in the channel, which leads to the observed current.
We investigate the electrical and magneto-transport properties of Pt-C granular metals prepared by focused-electron-beam induced deposition. In particular, we consider samples close to the metal-insulator-transition obtained from as-grown deposits by means of a low- energy electron irradiation treatment. The temperature dependence of the conductivity shows a lnT behavior with a transition to square root of T at low temperature, as expected for systems in the strong-coupling tunneling regime. The magnetoresistance is positive and is described within the wave-function shrinkage model, normally used for disordered system in the weak-coupling regime. In order to fit the experimental data spin-dependent tunneling has to be taken into account. In the discussion we attribute the origin of the spin-dependency to confinement effects of Pt nano-grains embedded in the carbon matrix.
Experimental results obtained previously for the photoluminescence efficiency (PL$_{eff}$) of Ge quantum dots (QDs) are theoretically studied. A $log$-$log$ plot of PL$_{eff}$ versus QD diameter ($D$) resulted in an identical slope for each Ge QD sample only when $E_{G}sim (D^2+D)^{-1}$. We identified that above $Dapprox$ 6.2 nm: $E_{G}sim D^{-1}$ due to a changing effective mass (EM), while below $Dapprox$ 4.6 nm: $E_{G}sim D^{-2}$ due to electron/ hole confinement. We propose that as the QD size is initially reduced, the EM is reduced, which increases the Bohr radius and interface scattering until eventually pure quantum confinement effects dominate at small $D$.
Ferromagnetic contacts are widely used to inject spin polarized currents into non-magnetic materials such as semiconductors or 2-dimensional materials like graphene. In these systems, oxidation of the ferromagnetic materials poses an intrinsic limitation on device performance. Here we investigate the role of ex-situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as an oxidation barrier for nanostructured cobalt and permalloy electrodes. The chemical state of the ferromagnets was investigated using X-ray photoemission electron microscopy owing to its high sensitivity and lateral resolution. We have compared the oxide thickness formed on ferromagnetic nanostructures covered by hBN to uncovered reference structures. Our results show that hBN reduces the oxidation rate of ferromagnetic nanostructures suggesting that it could be used as an ultra-thin protection layer in future spintronic devices.
Transitions to immeasurably small electrical resistance in thin films of Ag/Au nanostructure-based films have generated significant interest because such transitions can occur even at ambient temperature and pressure. While the zero-bias resistance and magnetic transition in these films have been reported recently, the non-equilibrium current-voltage ($I-V$) transport characteristics at the transition remains unexplored. Here we report the $I-V$ characteristics at zero magnetic field of a prototypical Ag/Au nanocluster film close to its resistivity transition at the critical temperature $T_{C}$ of $approx160$ K. The $I-V$ characteristics become strongly hysteretic close to the transition and exhibit a temperature-dependent critical current scale beyond which the resistance increases rapidly. Intriguingly, the non-equilibrium transport regime consists of a series of nearly equispaced resistance steps when the drive current exceeds the critical current. We have discussed the similarity of these observations with resistive transitions in ultra-thin superconducting wires via phase slip centres.
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