Optimized Spintronic Terahertz Emitters Based on Epitaxial Grown Fe/Pt Layer Structures

We report on generation of pulsed broadband terahertz radiation utilizing the inverse spin Hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on nonlinear generation methods.Comment: 10 pages, 7 figure

A Survey for Alkaloids in Hawaiian Plants, III

Volume: 16Start Page: 63End Page: 6

Repensar la alfabetización a partir de la multimodalidad: Aproximaciones interdisciplinarias y multiculturales de la comunicación

El Proyecto de Investigación 12.2 propone una indagación estrecha de la articulación entre los campos de la comunicación, las artes visuales y performativas y la educación, con el fin gestar un aporte hacia una concepción amplia de la alfabetización, entendida como un proceso multimodal, situado, que se desarrolla y reconfigura durante toda la vida en distintos ámbitos de práctica, y en el que el pensamiento y la actividad exploratorias ocupan un lugar destacado. Se presentan tres ejes de análisis interrelacionados que focalizan en la experimentación, la multimodalidad y la dinámica dialógica, abordados desde distintas manifestaciones discursivas tales como la historieta, el humor gráfico, la música, el dibujo, definiciones conceptuales, muestras y experimentaciones científicas, puestas en juego por niños/as, adolescentes, adultos e instituciones.Research Project 12.2 proposes a close investigation of the articulation between the fields of communication, visual and performative arts and education, in order to create a contribution towards a broad conception of literacy, understood as a multimodal process, situated, which is developed and reconfigured throughout life in different fields of practice, and in which exploratory thought and activity occupy a prominent place. Three interrelated axes of analysis are presented that focus on experimentation, multimodality and dialogic dynamics, approached from different discursive manifestations such as comics, graphic humor, music, drawing, conceptual definitions, samples and scientific experiments, put into game for children, adolescents, adults and institutions.O Projeto de Pesquisa 12.2 propõe uma investigação atenta da articulação entre os campos da comunicação, artes visuais e performativas e educação, a fim de contribuir para uma concepção ampla de letramento, entendida como um processo multimodal, situada, que se desenvolve e se reconfigura ao longo da vida em diferentes campos de prática, e na qual o pensamento e a atividade exploratória ocupam lugar de destaque. São apresentados três eixos de análise inter-relacionados que enfocam a experimentação, a multimodalidade e a dinâmica dialógica, abordadas a partir de diferentes manifestações discursivas como quadrinhos, humor gráfico, música, desenho, definições conceituais, samples e experimentos científicos, colocados em jogo para crianças, adolescentes, adultos e instituições

Modification of spintronic terahertz emitter performance through defect engineering

Spintronic ferromagnetic/non-magnetic heterostructures are novel sources for the generation of THz radiation based on spin-to-charge conversion in the layers. The key technological and scientific challenge of THz spintronic emitters is to increase their intensity and frequency bandwidth. Our work reveals the factors to engineer spintronic Terahertz generation by introducing the scattering lifetime and the interface transmission for spin polarized, non-equilibrium electrons. We clarify the influence of the electron-defect scattering lifetime on the spectral shape and the interface transmission on the THz amplitude, and how this is linked to structural defects of bilayer emitters. The results of our study define a roadmap of the properties of emitted as well as detected THz-pulse shapes and spectra that is essential for future applications of metallic spintronic THz emitters.Comment: 33 pages, 13 figure

Coherent and incoherent magnons induced by strong ultrafast demagnetization in thin permalloy films

Understanding spin dynamics on femto- and picosecond timescales offers new opportunities for faster and more efficient spintronic devices. Here, we experimentally investigate the coherent spin dynamics after ultrashort laser excitation by time-resolved magneto optical Kerr effect (TR-MOKE) in thin Ni80Fe20 films. We provide a detailed study of the magnetic field and pump fluence dependence of the coherent precessional dynamics. We show that the coherent precession lifetime increases with the applied external magnetic field which cannot be understood by viscous Gilbert damping of the coherent magnons. Instead, it can be explained by nonlinear magnon interactions and by the change in the fraction of incoherent magnons. This interpretation is in agreement with the observed trends of the coherent magnon amplitude and lifetime as a function of the exciting laser fluence. Our results provide a new insight into the magnetization relaxation processes in ferromagnetic thin films, which is of great importance for further spintronic applications.Comment: 8 pages, 7 figure

Efficient Terahertz Generation Using Fe/Pt Spintronic Emitters Pumped at Different Wavelengths

Recent studies in spintronics have highlighted ultrathin magnetic metallic multilayers as a novel and very promising class of broadband terahertz radiation sources. Such spintronic multilayers consist of ferromagnetic (FM) and non-magnetic (NM) thin films. When triggered by ultrafast laser pulses, they generate pulsed THz radiation due to the inverse spin-Hall effect, a mechanism that converts optically driven spin currents from the magnetized FM layer into transient transverse charge currents in the NM layer, resulting in THz emission. As THz emitters, FM/NM multilayers have been intensively investigated so far only at 800-nm excitation wavelength using femtosecond Ti:sapphire lasers. In this work, we demonstrate that an optimized spintronic bilayer structure of 2-nm Fe and 3-nm Pt grown on 500 {\mu}m MgO substrate is just as effective as a THz radiation source when excited either at {\lambda} = 800 nm or at {\lambda} = 1550 nm by ultrafast laser pulses from a fs fiber laser (pulse width close to 100 fs, repetition rate around 100 MHz). Even with low incident power levels, the Fe/Pt spintronic emitter exhibits efficient generation of THz radiation at both excitation wavelengths. The efficient THz emitter operation at 1550 nm facilitates the integration of such spintronic emitters in THz systems driven by relatively low cost and compact fs fiber lasers without the need for frequency conversion

Competing signatures of intersite and interlayer spin transfer in the ultrafast magnetization dynamics

Optically driven intersite and interlayer spin transfer are individually known as the fastest processes for manipulating the spin order of magnetic materials on the sub 100 fs time scale. However, their competing influence on the ultrafast magnetization dynamics remains unexplored. In our work, we show that optically induced intersite spin transfer (also known as OISTR) dominates the ultrafast magnetization dynamics of ferromagnetic alloys such as Permalloy (Ni80Fe20) only in the absence of interlayer spin transfer into a substrate. Once interlayer spin transfer is possible, the influence of OISTR is significantly reduced and interlayer spin transfer dominates the ultrafast magnetization dynamics. This provides a new approach to control the magnetization dynamics of alloys on extremely short time scales by fine-tuning the interlayer spin transfer

THz emission from Fe/Pt spintronic emitters with L10_{0}-FePt alloyed interface

Recent developments in nanomagnetism and spintronics have enabled the use of ultrafast spin physics for terahertz (THz) emission. Spintronic THz emitters, consisting of ferromagnetic FM / non-magnetic (NM) thin film heterostructures, have demonstrated impressive properties for the use in THz spectroscopy and have great potential in scientific and industrial applications. In this work, we focus on the impact of the FM/NM interface on the THz emission by investigating Fe/Pt bilayers with engineered interfaces. In particular, we intentionally modify the Fe/Pt interface by inserting an ordered L1$_{0}$-FePt alloy interlayer. Subsequently, we establish that a Fe/L1$_{0}$-FePt (2\,nm)/Pt configuration is significantly superior to a Fe/Pt bilayer structure, regarding THz emission amplitude. The latter depends on the extent of alloying on either side of the interface. The unique trilayer structure opens new perspectives in terms of material choices for the next generation of spintronic THz emitters

An observational test for correlations between cosmic rays and magnetic fields

We derive the magnitude of fluctuations in total synchrotron intensity in the Milky Way and M33, from both observations and theory under various assumption about the relation between cosmic rays and interstellar magnetic fields. Given the relative magnitude of the fluctuations in the Galactic magnetic field (the ratio of the rms fluctuations to the mean magnetic field strength) suggested by Faraday rotation and synchrotron polarization, the observations are inconsistent with local energy equipartition between cosmic rays and magnetic fields. Our analysis of relative synchrotron intensity fluctuations indicates that the distribution of cosmic rays is nearly uniform at the scales of the order of and exceeding 100\p, in contrast to strong fluctuations in the interstellar magnetic field at those scales. A conservative upper limit on the ratio of the the fluctuation magnitude in the cosmic ray number density to its mean value is 0.2--0.4 at scales of order 100\,pc. Our results are consistent with a mild anticorrelation between cosmic-ray and magnetic energy densities at these scales, in both the Milky Way and M33. Energy equipartition between cosmic rays and magnetic fields may still hold, but at scales exceeding 1\,kpc. Therefore, we suggest that equipartition estimates be applied to the observed synchrotron intensity smoothed to a linear scale of kiloparsec order (in spiral galaxies) to obtain the cosmic ray distribution and a large-scale magnetic field. Then the resulting cosmic ray distribution can be used to derive the fluctuating magnetic field strength from the data at the original resolution. The resulting random magnetic field is likely to be significantly stronger than existing estimates.Comment: submitted to MNRA