Laser Induced Magnetization Reversal for Detection in Optical Interconnects
The use of optical interconnects has become a front runner to replace more traditional (usually Cu based) electrical interconnects in many modern devices. One of the major drawbacks of optical interconnects is overcoming the need for photodetectors and (power hungry) amplifiers at the receiver. Such detection is in most cases performed by CMOS circuits or direct band gap semiconductors. As part of a collaboration lead by engineers at Perdue University, IN, USA a new use of ultrafast heat induced switching, has been proposed as a means of using optical signals directly with standard CMOS circuits.
Schematic view of focusing of the laser beam on the detecting MTJ.
The data is transmitted using femtosecond laser pulses that induce magnetisation reversal in a magnetic tunnel junction (MTJ) in the receiver. The proposed scheme offers almost a 40% energy improvement over current technology and speeds of up to 5 GBits/sec for a single link. The preprint of the article can be found on arXiv.
Quantitative Characterization of Nanoscale Polycrystalline Magnets with Electron Magnetic Circular Dichroism
Complex materials with one or more magnetic species are of particular interest within the FemtoSpin project. As we stimulate these materials on faster timescales, it has been shown that the behavior of the individual species can be very different. To measure the different magnetic elements is a challenging field of experimental physics often requiring large-scale synchrotron facilities. Another challenge for the measurement of magnetic properties is their observation at high spatial resolution. In a recent publication in Nature Communications, a team involving Uppsala University have demonstrated a new technique to measure the spin and orbital angular momentum at resolutions of only a few nanometers.
Schematics of the proposed scanning-mode measurement of EMCD. (a) Schematic drawing of the experimental setup and the data obtained (ADF: annular dark field, PL: projector lens). The detector aperture is placed at the PL cross-over position. In the present STEM mode, the PL cross-over position is on the diffraction plane. (b) ATEM image of the investigated polycrystalline iron film. Scale bar, 50nm. (c) Calculated EMCD signal intensity distribution of a polycrystalline iron film in the diffraction plane. The highlighted area indicates the measured area covered by the detector entrance aperture. The detector entrance aperture (solid circle) is located at the position of 0.4 g(110) away from the origin, and its diameter is 0.5 g(110). The white broken circle represents the possible aperture centre positions in the diffraction plane and blue broken circle corresponds to g(110) ring position for comparison. Scale bar, 2nm-1. The minimum (black) and maximum (white) EMCD values range from -3 to +3%.
The method is based on the electron magnetic circular dichroism (EMCD), in which electrons are transmitted though a magnetic sample in a transmission electron microscope (TEM). Electron energy loss spectroscopy (EELS) measured at core levels can then be employed to extract element-selective magnetic information. This method was outlined in 2003 by Hébert and Schattschneider (Ultramicropscopy 96, 463-468 (2003)) but until now quantitative information has not been possible to extract because of the inherently low signal strength. The new approach was stimulated from simulated distribution of dichroic signals (one such example was performed as part of a collaboration with Uppsala University) that suggested the EMCD is present almost everywhere in the diffraction plane. This means that by, rather than optimizing the signal-to-noise ration in a fixed geometry, the group was able to collect a large number of independent spectra and apply a new statistical technique to overcome the low signal strength restrictions of the technique. This significant step forward provides the ability to determine local magnetic moments on the nanometer scale, even for polycrystalline materials.
Since the discovery by Ostler et al. of a purely thermally induced magnetisation switching (TIMS) in GdFeCo, there has been much effort to identify the cause of this unexpected phenomenon. While several works have studied the macroscopic relaxation behaviour (Mentink et al., Phys. Rev. Lett. 108, 057202 (2012). Atxitia et al., Phys. Rev. B 87, 224417 (2013)), there has been little headway made in finding the material origins of the switching. In our new work “Two-magnon bound state causes ultrafast thermally induced magnetisation switching” published in the open access journal Scientific Reports we have found, through simulation and described with a combination of theoretical approaches, that the switching is caused by angular momentum transfer from a two magnon bound state which exists in this class of ferrimagnetic materials. Specifically, within GdFeCo we have shown that the amorphous properties of the material affect the switching behaviour because the antiferromagnetic interactions which couple the rare-earth and transition metal species have a large effect only at the interfaces of Gd clusters within the FeCo background. Our work provides a new insight into the switching which is induced by femtosecond laser pulses and gives new directions for experimentalists to focus their research.
A new article has been presented in Physical Review B by the group of Peter Oppeneer. This computational study investigates electron spin-flip scattering induced by the electron-phonon interaction in the transition-metal ferromagnets bcc Fe, fcc Co and fcc Ni. Elliot-Yafet spin-flip scattering is computed from first principles, by employing a generalized spin-flip Eliashberg function as well as ab-initio computed phonon dispersions. Aiming at investigating the amount of electron-phonon mediated demagnetization in femtosecond laser-excited ferromagnets, the formalism is extended to treat laser-created thermalized as well as nonequilibrium, nonthermal hot electron distributions. Non-thermal distributions are found to lead to a stronger demagnetization rate than hot, thermalized distributions, yet their demagnetizing effect is not enough to explain the experimentally observed demagnetization occurring in the sub-picosecond regime.
Full details can be found at the APS website via the link here.
A low-energy theory for magnetic interactions between electrons in the multi-band Hubbard model in non-equilibrium conditions has been formulated. By introducing a time-dependent electric field it is possible to simulate laser-induced spin dynamics. Expressions for dynamical exchange parameters in terms of non-equilibrium Green's functions and self-energies are obtained via time-dependent dynamical mean-field theory. The analysis shows a new type of magnetic interaction, a so-called "twist exchange", which formally resembles Dzyaloshinskii-Moriya coupling, but is not due to spin-orbit interactions, but is due to an effective three-spin interaction.
Full details can be found at the Elsevier website via the link here.
Last months edition of Nature Nanotechnology sees the latest advancement in the use of femtosecond lasers to drive spin currents. Kampfrath et al. demonstrate both numerically and experimentally that it is possible to drive spins from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contact-less amperemeter based on the inverse spin Hall effect, which converts the spin flow into a terahertz electromagnetic pulse.
The full details can be found at the Nature Nanotechnology website via the link here.
New Paper On All-Optical Switching from Radboud University, Nijmegen
The latest installment in the story of all-optical switching (AOS) was published last week in the APS journal, Physical Review B: Rapid Communications by Savoni et al. These latest results compare experiments with theory and investigate the switching efficiency of GdFeCo microstructures of different sizes. Using magnetization-sensitive microscopy the dynamics of the different microstructures were studied.
The results show that smaller structures require less energy to induce switching, an important realization for any potential technology. Results show a large decrease in the required energy density, around 60% of that required to switch the thin film equivalent. Savioni et al. predict that as little as 10fJ might be required to switchg a 20 x 20 nm2. Such an energy would be a few orders of magnitude smaller than that used in current magnetoresistive random access memory (MRAM) technology, and even compares favourably with the low energy requirements for magnetic semiconductor switching.
The full details can be found at the APS website via the link here.