Joel Briones

360° domain wall generation in the soft layer of magnetic tunnel junctions

High spatial resolution x-ray photoemission electron microscopy technique has been used to study the influence of the dipolar coupling taking place between the NiFe and the Co ferromagnetic electrodes of micron sized, elliptical shaped magnetic tunnel junctions. The chemical selectivity of this technique allows us to observe independently the magnetic domain structure in each ferromagnetic electrode. The combination of this powerful imaging technique with micromagnetic simulations allows us to evidence that a 360° domain wall can be stabilized in the NiFe soft layer. In this letter, we discuss the origin and the formation conditions of those 360° domain walls evidenced experimentally and numerically.

Seebeck nanoantennas for solar energy harvesting

We propose a mid-infrared device based on thermocouple optical antennas for light sensing and energy harvesting applications. We numerically demonstrate that antennas are able to generate low-power dc signals by beneficing of the thermoelectric properties of the metals that constitute them. We theoretically evaluate the optical-to-electrical conversion efficiency for harvesting applications and finally discuss strategies to increase its performance. Thermocouple optical antennas therefore open the route toward the design of photovoltaic devices.

Seebeck nanoantennas for the detection and characterization of infrared radiation

Arrays of metallic thermocouples in the shape of spiral nanoantennas are proposed as infrared detectors, which use the thermoelectric properties of the metallic interfaces to generate electrical DC signals. The responsivity of these types of antennas is evaluated from both theoretical and numerical perspectives pointing out its potential as infrared sensors. Moreover, the same structures can be used to characterize the state of polarization of the optical near fields with a spatial resolution comparable to the wavelength.

Magnetic domain wall propagation in a submicron spin-valve stripe: Influence of the pinned layer

The propagation of a domain wall in a submicron ferromagnetic spin-valve stripe is investigated using giant magnetoresistance. A notch in the stripe efficiently traps an injected wall stopping the domain propagation. The authors show that the magnetic field at which the wall is depinned displays a stochastic nature. Moreover, the depinning statistics are significantly different for head-to-head and tail-to-tail domain walls. This is attributed to the dipolar field generated in the vicinity of the notch by the pinned layer of the spin valve.

Local Magnetic Anisotropy Induced by a Nano-Modulated Substrate and Application to Two-Dimensional Magnetic Sensors

Topological modulation of magnetic thin films can induce a magnetic anisotropy of magnetostatic origin. In this paper, we report on the magnetic properties of NiFe layers deposited on wavy Si substrates. Without modulation, the film posses an intrinsic anisotropy. We unambiguously show that patterning the substrate overcomes the intrinsic anisotropy and establishes a new easy axis of magnetization. By choosing the patterning appropriately, orthogonal easy axes can be established at different locations of the same sample. We demonstrate the advantage of this approach in the fabrication of bidimensional magnetic sensors.

Study of InAlAs/InGaAs self-switching diodes for energy harvesting applications

In order to improve the rectification efficiency and current–voltage characteristics of self-switching diodes (SSD) the DC response is analyzed using technology computer aided design (TCAD). It is demonstrated that by varying geometrical parameters of L- and V-shaped SSDs or changing the dielectric permittivity of the trenches, a near zero threshold voltage can obtained, which is essential for energy harvesting applications. The carrier distribution inside the nanochannel is successfully simulated in two-dimensional mode for zero-, reverse-, and forward-bias conditions. This process allows for the evaluation of the effect of the lateral surface-charge on the formation and spatial distribution of the depletion region, in addition to, obtaining information on the physics of the SSD through the propose optimized geometries that were designed for tailoring and matching the desired frequencies of operation. The numerical results showed some insights for the improvement of the rectification efficiency and integration density using parallel SSD arrays.

Determination of the depletion layer width and effects on the formation of double-2DEG in AlGaAs/GaAs heterostructures

In this work, the influence of the surface depletion layer on the formation of a two-dimensional electron gas in AlGaAs/GaAs modulated doped heterostructures is studied. The authors explore a method for estimating the depletion region inside of the GaAs-based heterostructures by using the longitudinal optical and L- amplitude modes observed in Raman spectra, which are supported by the modeling results. The authors found that the position of the topmost doping layer changes the electron distribution in the heterostructure and decreases the influence of the depletion layer. Similar effects are perceived when an optimized solution of (NH4)2SX and isopropanol is employed. The authors present a method to evaluate the formation of a double two-dimensional electron gas in a heterostructure by the adequate use of modulation line in the photoreflectance spectroscopy.

Numerical conversion efficiency of thermally isolated Seebeck nanoantennas

In this letter, we evaluate the conversion efficiency of thermally isolated Seebeck nanoantennas by numerical simulations and discuss their uses and scope for energy harvesting applications. This analysis includes the simple case of titanium-nickel dipoles suspended in air above the substrate by a 200 nm silicon dioxide membrane to isolate the heat dissipation. Results show that substantially thermal gradients are induced along the devices leading to a harvesting efficiency around 10-4 %, 400 % higher than the previously reported Seebeck nanoantennas. In the light of these results, different optimizing strategies should be considered in order to make the Seebeck nanoantennas useful for harvesting applications.

Terahertz harvesting with shape-optimized InAlAs/InGaAs self-switching nanodiodes

In this letter, self-switching nanochannels have been proposed as an enabling technology for energy gathering in the terahertz (THz) regime. Such devices combine their diode-like behavior and high-speed of operation in order to generate DC electrical power from high-frequency signals. By using finite-element simulations, we have improved the sensitivity of L-shaped and V-shaped nanochannels based on InAlAs/InGaAs samples. Since those devices combine geometrical effects with their rectifying properties at zero-bias, we have improved their performance by optimizing their shape. Results show nominal sensitivities at zero-bias in the order of 40 V−1 and 20 V−1, attractive values for harvesting applications with square-law rectifiers.

Efficiency of broadband terahertz rectennas based on self-switching nanodiodes

Abstract. The authors investigate the efficiency of a series of broadband rectennas designed to harvest the free-propagating electromagnetic energy at terahertz frequencies. We analyze by simulations the case of self-complementary square- and Archimedean-spiral antennas coupled to L-shaped self-switching diodes (L-SSDs). First, the geometry (i.e., the width and length of the channel) of the L-SSD was optimized to obtain a remarkable diode-like I–V response. Subsequently, the optimized L-SSD geometry was coupled to both types of spiral antennas and their characteristic impedance was studied. Finally, the energy conversion efficiency was evaluated for both rectenna architectures.