arXiv subjects

Farkhad G. Aliev

Publications and source records attributed to Farkhad G. Aliev.

29 records · Page 2Linked to original sources

arXiv2017

Thermally driven inhibition of superconducting vortex avalanches

Complex systems close to their critical state can exhibit abrupt transitions, avalanches, between their metastable states. It is a challenging task to understand the mechanism of the avalanches and control their behavior. Here we investigate microwave (mw) stimulation of avalanches in the so- called vortex matter of type II superconductors, a system of interacting Abrikosov vortices close to the critical (Bean) state. Our main finding is that the avalanche incubation strongly depends on the excitation frequency, a completely unexpected behavior observed close to the so-called depinning frequencies. Namely, the triggered vortex avalanches in Pb superconducting films become effectively inhibited approaching the critical temperature or critical magnetic field when the mw stimulus is close to the vortex depinning frequency. We suggest a simple model explaining the observed counter- intuitive behaviors as a manifestation of the strongly nonlinear dependence of the driven vortex core size on the mw excitation intensity. This paves the way to controlling avalanches in complex systems through their nonlinear response.

cond-mat.supr-con↗

arXiv2017

Communication and information processing in magnetic nanostructures with edge spin waves

Low dissipation data processing with spins is one of the promising directions for future information and communication technologies. Despite a signifcant progress, the available magnonic devices are not broadband yet and have restricted capabilities to redirect spin waves. Here we propose a breakthrough approach to the spin wave manipulation in patterned magnetic nanostructures with unmatched characteristics, which exploits spin waves analogous to edge waves propagating along a water-wall boundary. Using theory, micromagnetic simulations and experiment we investigate spin waves propagating along the edges in magnetic structures, under an in-plane DC magnetic field inclined with respect to the edge. The proposed edge spin waves overcome important challenges faced by previous technologies such as the manipulation of the spin wave propagation direction, and they substantially improve the capability of transmitting information at frequencies exceeding 10 GHz. The concept of the edge spin waves allows to design broad range of logic devices such as splitters, interferometers, or edge spin wave transistors with unprecedented characteristics and potentially strong impact on information technologies.

cond-mat.mes-hall↗

arXiv2017

Photodoping-driven crossover in low-frequency noise in MoS2 transistors

Transition metal dichalcogenide field-effect transistors (FETs) have been actively explored for low-power electronics, light detection, and sensing. Albeit promising, their performance is strongly limited by low-frequency noise (LFN). Here, we report on the study of LFN in MoS$_2$ FETs on SiO$_2$ substrates in ambient conditions using photodoping. Using this external excitation source allows us to access different non-equilibrium steady states and cross over different noise regimes. We observe a dependence of the noise power spectrum with the transient decay time window, approaching $1/f$-type when the system is closer to equilibrium, and identify a dependence of the LFN with channel thickness. Monolayer/bilayer devices exhibit random telegraph noise for insulating regimes and $1/f$-type Hooge mobility fluctuations (HMF) for conductive regimes. Thicker devices exhibit mainly $1/f$-type carrier number fluctuations (CNF). In the latter, we observe a photodoping-induced change from a near parabolic to a near linear dependence of the inverse $1/f$ noise amplitude above the threshold gate voltage. This change indicates a crossover in the LFN mechanism from CNF to HMF. We demonstrate that the study of conductance and noise under photodoping is an effective tool to identify dominating carrier noise mechanisms in few-atomic-layer FETs for a wide range of doping regimes.

cond-mat.mes-hall↗

arXiv2016

Band structure of topological insulators from noise measurements in tunnel junctions

The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunneling and noise spectroscopy utilizing TI/Al$_2$O$_3$/Co tunnel junctions with bottom TI electrodes of either Bi$_2$Te$_3$ or Bi$_2$Se$_3$. We demonstrate that features related to the band structure of the TI materials show up in the tunneling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems, that can further lead to the manipulation of their spin-polarized properties for technological purposes.

cond-mat.mes-hall↗

arXiv2015

Detection of spin torque magnetization dynamics through low frequency noise

We present a comparative study of high frequency dynamics and low frequency noise in elliptical magnetic tunnel junctions with lateral dimensions under 100 nm presenting current-switching phenomena. The analysis of the high frequency oscillation modes with respect to the current reveals the onset of a steady-state precession regime for negative bias currents above $J=10^7 A/cm^2$, when the magnetic field is applied along the easy axis of magnetization. By the study of low frequency noise for the same samples, we demonstrate the direct link between changes in the oscillation modes with the applied current and the normalised low frequency (1/f) noise as a function of the bias current. These findings prove that low frequency noise studies could be a simple and powerful technique to investigate spin-torque based magnetization dynamics.

cond-mat.mes-hall↗

arXiv2015

Magnetic state dependent transient lateral photovoltaic effect in patterned ferromagnetic metal-oxide-semiconductor films

We investigate the influence of an external magnetic field on the magnitude and dephasing of the transient lateral photovoltaic effect (T-LPE) in lithographically patterned Co lines of widths of a few microns grown over naturally passivated p-type Si(100). The T-LPE peak-to-peak magnitude and dephasing, measured by lock-in or through the characteristic time of laser OFF exponential relaxation, exhibit a notable influence of the magnetization direction of the ferromagnetic overlayer. We show experimentally and by numerical simulations that the T-LPE magnitude is determined by the Co anisotropic magnetoresistance. On the other hand, the magnetic field dependence of the dephasing could be described by the influence of the Lorentz force acting perpendiculary to both the Co magnetization and the photocarrier drift directions. Our findings could stimulate the development of fast position sensitive detectors with magnetically tuned magnitude and phase responses.

cond-mat.mes-hall↗

arXiv2015

Microwave-stimulated superconductivity due to presence of vortices

The response of superconducting devices to electromagnetic radiation is a core concept implemented in diverse applications, ranging from the currently used voltage standard to single photon detectors in astronomy. Suprisingly, a sufficiently high power subgap radiation may stimulate superconductivity itself. The possibility of stimulating type II superconductors, in which the radiation may interact also with vortex cores, remains however unclear. Here we report on superconductivity enhanced by GHz radiation in type II superconducting Pb films in the presence of vortices. The stimulation effect is more clearly observed in the upper critical field and less pronounced in the critical temperature. The magnetic field dependence of the vortex related microwave losses in a film with periodic pinning reveals a reduced dissipation of mobile vortices in the stimulated regime due to a reduction of the core size. Results of numerical simulations support the validy of this conclusion. Our findings may have intriguing connections with holographic superconductors in which the possibility of stimulation is under current debate.

cond-mat.supr-con↗

arXiv2015

Magnetization reversal assisted by half antivortex states in nanostructured circular cobalt disks

The half antivortex, a fundamental topological structure which determines magnetization reversal of submicron magnetic devices with domain walls, has been suggested also to play a crucial role in spin torque induced vortex core reversal in circular disks. Here we report on magnetization reversal in circular disks with nanoholes through consecutive metastable states with half antivortices. In-plane anisotropic magnetoresistance and broadband susceptibility measurements accompanied by micromagnetic simulations reveal that cobalt disks with two and three linearly arranged nanoholes directed at 45 and 135 degrees with respect to the external magnetic field show reproducible step-like changes in the anisotropic magnetoresistance and magnetic permeability due to transitions between different intermediate states mediated by vortices and half antivortices confined to the dot nanoholes and edges, respectively. Our findings are relevant for the development of multi-hole based spintronic and magnetic memory devices.

cond-mat.mes-hall↗

arXiv2015

Band edge noise spectroscopy of a magnetic tunnel junction

We propose a conceptually new way to gather information on the electron bands of buried metal(semiconductor)/insulator interfaces. The bias dependence of low frequency noise in Fe$_{1-x}$V$_{x}$/MgO/Fe (0 $<$ x $<$ 0.25) tunnel junctions show clear anomalies at specific applied voltages, reflecting electron tunneling to the band edges of the magnetic electrodes. The change in magnitude of these noise anomalies with the magnetic state allows evaluating the degree of spin mixing between the spin polarized bands at the ferromagnet/insulator interface. Our results are in qualitative agreement with numerical calculations.

cond-mat.mes-hall↗

arXiv2012

Broadband probing magnetization dynamics of the coupled vortex state permalloy layers in nanopillars

Broadband magnetization response of coupled vortex state magnetic dots in layered nanopillars was explored as a function of in-plane magnetic field and interlayer separation. For dipolarly coupled circular Py(25 nm)/Cu(20 nm)/Py(25 nm) nanopillars of 600 nm diameter, a small in-plane field splits the eigenfrequencies of azimuthal spin wave modes inducing an abrupt transition between in-phase and out-of-phase kinds of the low-lying coupled spin wave modes. The critical field for this splitting is determined by antiparallel chiralities of the vortices in the layers. Qualitatively similar (although more gradual) changes occur also in the exchange coupled Py(25 nm)/Cu(1 nm)/Py(25 nm) tri-layer nanopillars. These findings are in qualitative agreement with micromagnetic dynamic simulations.

cond-mat.mes-hall↗

arXiv2008

Vortex dynamics in confined stratified conditions

We report on linear spin dynamics in the vortex state of the Permalloy dots subjected to stratified (magnetic) field. We demonstrate experimentally and by simulations the existence of two distinct dynamic regimes corresponding to the vortex stable and metastable states. Breaking cylindrical symmetry leads to unexpected eigenmodes frequency splitting in the stable state and appearance of new eigenmodes in the metastable state above the vortex nucleation field. Dynamic response in the metastable state strongly depends on relative orientation of the external rf pumping and the bias magnetic fields. These findings may be relevant for different vortex states in confined and stratified conditions.

cond-mat.stat-mech↗