Antonio Lara

Localized domain-wall excitations in patterned magnetic dots probed by broadband ferromagnetic resonance

We investigate the magnetization dynamics in circular Permalloy dots with spatially separated magnetic vortices interconnected by domain walls (double vortex state). We identify a novel type of quasi one-dimensional (1D) localized spin wave modes confined along the domain walls, connecting each of two vortex cores with two edge half-antivortices. Variation of the mode eigenfrequencies with the dot sizes is in quantitative agreement with the developed model, which considers a dipolar origin of the localized 1D spin waves or so-called Winter’s magnons [J. M. Winter, Phys. Rev. 124, 452 (1961)]. These spin waves are analogous to the displacement waves of strings and could be excited in a wide class of patterned magnetic nanostructures possessing domain walls, namely in triangular, square, circular, or elliptic soft magnetic dots.

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.

Low frequency noise due to magnetic inhomogeneities in submicron FeCoB/MgO/FeCoB magnetic tunnel junctions

We report on room temperature low frequency noise due to magnetic inhomogeneities/domain walls (MI/DWs) in elliptic submicron FeCoB/MgO/FeCoB magnetic tunnel junctions with an area between 0.0245 and 0.0675 μm2. In the smaller area junctions we found an unexpected random telegraph noise (RTN1), deeply in the parallel state, possibly due to stray field induced MI/DWs in the hard layer. The second noise source (RTN2) is observed in the antiparallel state for the largest junctions. Strong asymmetry of RTN2 and of related resistance steps with current indicate spin torque acting on the MI/DWs in the soft layer at current densities below 5 × 105 A/cm2.

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.

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 (Co) disks with two and three linearly arranged nanoholes directed at 45° and 135° 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.

Information processing in patterned 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 significant progress, the available magnonic devices are not broadband yet and have restricted capabilities to redirect spin waves. Here we propose a breakthrough approach to spin wave manipulation in patterned magnetic nanostructures with unmatched characteristics, which exploits a spin wave analogue 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 a broad of logic devices such as splitters, interferometers, or edge spin wave transistors with unprecedented characteristics and a potentially strong impact on information technologies.Low dissipation data processing with spins is one of the promising directions for future information and communication technologies. Despite a significant 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. Recently there has been an increasing interest in the use of spin waves for information transmission and processing.1–5,7, 8, 28 In contrast to electronics, the information encoded in the amplitude and phase of spin waves eliminates the contribution of charge dissipation. As a consequence, spin waves (SWs) propagate coherently through large distances even at room temperature. Contrary to the classic electromagnetic waves, a relatively small group velocity reduces the SWs wavelength down to the nanoscale. This opens new possibilities for a higher integration density in spin-wave logic devices through the effective manipulation of SWs propagating through periodic interferences or arrays of scatterers. The related research field of magnonics has been reviewed recently.1,9–11 Gaining an effective control over SWs through their redirection,12–14 multiplexing15 or data processing4 is a very active research area nowadays. The global aim is the creation of SW based logic devices. One of the most important features for future spin wave logics is the broadband capability of its building blocks. Also crucial is SW guidance, including the ability to split spin waves and/or redirect them at large effective angles. So far, the basic building blocks for SW transmission, redirection and control have consisted in straight or curved ferromagnetic strips.12,14,15 Such elements transmit information via SW modes which have a restricted frequency range (typically below 10 GHz) and are essentially suppressed after changing the direction of propagation up to 90 degrees or by increasing the drive frequency.12,14,15 This strongly limits the development of redirectional SW logic devices with broadband microwave capabilities. One way to circumvent the problem is reprogrammable magnonic wave guides made out of dipole interacting elements with controlled switching of their local magnetization.16 A fundamentally different approach would be to create low dimensional SW logics with spin waves propagating along artificial or natural magnetic domain walls. Predicted by Winter17 SWs propagating along domain walls (DWs) have been identified in a number of recent experimental magnetization configurations in patterned films, involving the domain walls connecting magnetic vortices and edge magnetic charges.18–21 The main drawback of the classic DW magnonics is its relatively low frequency of operation and certain difficulties to redirect SWs. This work explores edge spin waves (E-SWs) propagating along edge domain walls (E-DWs) localized at magnetic nanostructure edges (triangular dot, see Fig. 1) and displaying a number of remarkable and unprecedented features. Besides the broadband character (with the capability to process information at frequencies above 10 GHz) E-SWs (i) can split easily comparing to the conventional spin waves, (ii) could be redirected at angles up to 120 degrees and (iii) show interference phenomena providing the possibility to create phase controlled nanoscale microwave emitters. The proposed approach based on E-SWs could constitute a fruitful platform for the creation of low dissipation, ultrafast E-SW information processing devices such as spin wave diodes, multiplexors or phase detectors. An effective incorporation of the spin waves in data processing devices requires their transmission and direction control using more complicated structures than just straight or curved micro waveguides currently among the basic elements of 1 ar X iv :1 70 5. 10 53 6v 2 [ co nd -m at .m es -h al l] 3 1 M ay 2 01 7 magnonic waveguides.3,4 The capability of changing the SW direction opens perspectives for the creation of novel logic elements based on the SW interference.14

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.

Reduction of Microwave Loss by Mobile Fluxons in Grooved Nb Films

In the mixed state of type II superconductors penetrated by an external magnetic field in the form of a lattice of Abrikosov vortices the dc resistance is known to increase with increasing velocity of the vortex lattice. Accordingly, vortex pinning sites impeding the vortex motion are widely used to preserve the low-dissipative response of the system. Here, while subjecting superconducting Nb films with nanogrooves to a combination of dc and ac current stimuli and tuning the number of mobile and pinned vortices by varying the magnetic field around the so-called matching values, we observe a completely opposite effect. Namely, the vortex-related microwave excess loss for mobile vortices becomes smaller than for pinned vortices in a certain range of power levels at ac current frequencies above 100\,MHz. While a theoretical description of the observed effect is yet to be elaborated we interpret our findings in terms of a competition of the effective cooling of the system by the quasiparticles leaving the vortex cores with the conventional Joule heating caused by the current flow.

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.

Transient lateral photovoltaic effect in patterned ferromagnetic metal-oxide-semiconductor films

The time dependent transient lateral photovoltaic effect (T-LPE) has been studied with microsecond time resolution and with chopping frequencies in the kHz range, in lithographically patterned 21 nm thick, 5, 10 and 20 micron wide and 1500 micron long Co lines grown over naturally passivated p-type Si (100). We have observed a nearly linear dependence of the LPE transient response with the laser spot position. An unusual T-LPE dynamic response with a sign change in the laser-off stage has also been corroborated by numerical simulations. A qualitative explanation suggests a modification of the drift-diffusion model by including the influence of a local inductance. In addition, influence of anisotropic magnetoresistance of the Co line structure on dynamic response on T-LPE has been investigated. Specifically, we have experimentally investigated influence of the direction of the external magnetic field respect to the drift velocity of the photogenerated carriers on the T-LPE. We have observed notable dependence of the T-LPE on the magnetic field in the small field range (below 100 Oe), compatible with anisotropic magnetoresistance values. The strong influence of the magnetization alignment on the dynamic response of photogenerated carriers has been also observed through a phase sensitive lock-in experiment. These findings indicate that the microstructuring of the ferromagnetic line based position sensitive detectors (PSD) could improve their space-time resolution and add capability of magnetic field tuning of the main PSD characteristics.