Hans T. Nembach

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.

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

Microwave assisted switching in a Ni81Fe19 ellipsoid

The authors demonstrate the stimulation of the magnetization switching process of a Ni81Fe19 ellipsoid, which is dominated by domain nucleation and propagation, by applying a transverse microwave field. The study of the quasistatic switching behavior under the influence of a microwave field was performed using longitudinal magneto-optic Kerr effect magnetometry. A strong reduction of the coercive field for microwave frequencies between 500 and 900MHz has been observed, which can be attributed to two different mechanisms: microwave stimulated enhancement of domain nucleation and microwave stimulated growth of the reversed domain. The authors prove that heating is not the origin of the reduction of the coercive field.

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...

Near-field microwave microscope measurements to characterize bulk material properties

The authors discuss near-field scanning microwave microscope measurements of the complex permittivity for bulk dielectric (fused silica), semiconductor (silicon), and metal (copper). The authors use these measurements to test existing quasistatic theoretical approach to deembed the bulk material properties from the measured data. The known quasistatic models fit the measured data well with parameters for silicon (es=11.9, σSi=50S∕m) and fused silica (es=3.85, tanδ=1.0×10−4). However, for copper (with σCu=5.67×107S∕m), apart from quasistatic coupling, an additional loss of 12Ω is needed to fit the data.

Precise determination of the spectroscopic g-factor by use of broadband ferromagnetic resonance spectroscopy

We demonstrate that the spectroscopic g-factor can be determined with high precision and accuracy by broadband ferromagnetic resonance measurements and by applying an asymptotic analysis to the data. Spectroscopic data used to determine the g-factor are always obtained over a finite range of frequencies, which can result in significant errors in the fitted values. We show that by applying an asymptotic analysis to broadband datasets, precise values of the intrinsic g-factor can be determined with errors well below 1%, even when the exact form of the Kittel equation (which describes the relationship between the frequency and resonance field) is unknown. We demonstrate this methodology with measured data obtained for sputtered Ni80Fe20 (Permalloy) thin films of varied thicknesses, where we determine the bulk g-factor value to be 2.109 ± 0.003. Such an approach is further validated by application to simulated data that include both noise and an anisotropy that is not included in the Kittel equation that was ...

Damping phenomena in Co90Fe10/Ni multilayers and alloys

We used perpendicular ferromagnetic resonance to measure the damping parameter in Co90Fe10/Ni multilayers over a wide range of layer thicknesses. The magnetic anisotropy within this range varied from in-plane to out-of-plane. We measured (Co90Fe10)xNi1−x alloys of identical thicknesses over the same compositional range of Co90Fe10 and Ni in order to isolate the influence of the multilayer structure. The damping parameter varied from 0.004 to 0.030 and depended only on the relative amounts of Co90Fe10 and Ni and was independent of the magnetic anisotropy and layer structure.