Justin M. Shaw

Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current

Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferro- or antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers.

Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics

Circularly-polarized extreme UV and X-ray radiation provides valuable access to the structural, electronic and magnetic properties of materials. To date, this capability was available only at large-scale X-ray facilities such as synchrotrons. Here we demonstrate the first bright, phase-matched, extreme UV circularly-polarized high harmonics and use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to the linearly polarized high harmonic sources that have been used very successfully for ultrafast element-selective magneto-optic experiments. This work thus represents a critical advance that makes possible element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution, using tabletop-scale setups.

Probing the timescale of the exchange interaction in a ferromagnetic alloy

The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.

Spin-transfer dynamics in spin valves with out-of-plane magnetized CoNi free layers

We have measured spin-transfer-induced dynamics in magnetic nanocontact devices having a perpendicularly magnetized Co/Ni free layer and an in-plane magnetized CoFe fixed layer. The frequencies and powers of the excitations agree well with the predictions of the single-domain model and indicate that the excited dynamics correspond to precessional orbits with angles ranging from zero to 90° as the applied current is increased at a fixed field. From measurements of the onset current as a function of applied field strength we estimate the magnitude of the spin torque asymmetry parameter ??1.5. By combining these with spin torque ferromagnetic resonance measurements, we also estimate the spin-wave radiation loss in these devices.

Magnetic nanostructures for advanced technologies: fabrication, metrology and challenges

Magnetic nanostructures are an integral part to many state-of-the-art and emerging technologies. However, the complete path from parts (the nanostructures) to the manufacturing of the end products is not always obvious to students of magnetism. The paper follows this path of the magnetic nanostructure, and explains some of the steps along the way: What are the technologies that employ magnetic nanostructures? How are these nanostructures made? What is the physics behind the functional parts? How are the magnetic properties measured? Finally, we present, in our view, a list of challenges hindering progress in these technologies.

Origins of switching field distributions in perpendicular magnetic nanodot arrays

We studied the reversal properties of perpendicularly magnetized Co∕Pd nanodots from 100to50nm in diameter fabricated using electron beam lithography. Polycrystalline Co∕Pd multilayers show considerable differences in the switching field distribution (SFD) depending on the seed layer used. With a Ta seed layer, we reduced the SFD to approximately 5% of the average switching field. To rule out effects of grain boundaries, we also fabricated nanodot arrays from epitaxial Co∕Pd superlattices. Although significant improvement in SFDs are obtained using epitaxial superlattices, our results indicate that grain boundary variation within nanodots is not the primary origin of SFD broadening that occurs with nanopatterning.

Linear relation between Heisenberg exchange and interfacial Dzyaloshinskii-Moriya interaction in metal films

Hans T. Nembach, Justin M. Shaw, Mathias Weiler*, Emilie Jué and Thomas J. Silva Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado 80305, USA *present address: Walther-Meißner-Institut, 85748 Garching, Germany Linear relation between Heisenberg exchange and interfaci l Dz loshinskii–Moriya i teraction in metal films SUPPLEMENTARY INFORMATION DOI: 10.1038/NPHYS3418

Ultra-low magnetic damping of a metallic ferromagnet

Materials with low magnetic damping are important for a range of applications but are typically insulating, which limits their use. Thanks to a unique feature of the band structure, similar levels of damping can now be achieved in a metallic alloy.

Spin transport parameters in metallic multilayers determined by ferromagnetic resonance measurements of spin-pumping

We measured spin-transport in nonferromagnetic (NM) metallic multilayers from the contribution to damping due to spin pumping from a ferromagnetic Co90Fe10 thin film. The multilayer stack consisted of NM1/NM2/Co90Fe10(2 nm)/NM2/NM3 with varying NM materials and thicknesses. Using conventional theory for one-dimensional diffusive spin transport in metals, we show that the effective damping due to spin pumping can be strongly affected by the spin transport properties of each NM in the multilayer, which permits the use of damping measurements to accurately determine the spin transport properties of the various NM layers in the full five-layer stack. We find that due to its high electrical resistivity, amorphous Ta is a poor spin conductor, in spite of a short spin-diffusion length of 1.0 nm, and that Pt is an excellent spin conductor by virtue of its low electrical resistivity and a spin diffusion length of only 0.5 nm. Spin Hall effect measurements may have underestimated the spin Hall angle in Pt by assumi...

Infrared spectroscopic analysis of an ordered Si/SiO2 interface

Infrared spectroscopy is used to compare the Si/SiO2 interfaces created by thermal oxidation of a standard Si(100) substrate and of an ordered, (1×1) Si(100) substrate. The thermal oxides (approximately 25 A) examined in this study are etched in dilute hydrofluoric acid and the resulting films analyzed spectroscopically. The behavior of the dominant optical phonon modes as a function of film thickness provides strong evidence that the ordered Si(100) substrate provides a template for an Si/SiO2 interface with a higher degree of homogeneity in the Si–O bonding environment of the intervening substoichiometric SiOx layer than does the standard Si(100) substrate.