Hans Nembach (SIMES Seminar)

Date(s) - Feb 29 2016
2:00 PM - 3:00 PM

Sycamore Room, Building 40, room 195


Magnetization Dynamics in Nanostructures and Thin Films

Hans T. Nembach

Electromagnetics Division, National Institute of Standards and Technology and JILA, University of Colorado, Boulder


Abstract: In the first part of my talk I will discuss the outstanding question in the broad field of spin dynamics with ferromagnets whether the damping of gyromagnetic precession is in actuality subject to finite size effects at the nanometer length scale. We demonstrate that the effective damping in nanomagnets depends strongly on the excited spin-wave mode and on the size of the nanomagnet. The damping constant a is a critical parameter for spintronics devices, e.g., spin-torque-transfer magnetic random access memory (STT-MRAM). Optical measurements of the magnetization dynamics are particularly challenging when the diffraction-limited laser spot is much larger than the size of the nanomagnet. We developed a novel heterodyne magneto-optical microwave microscope (H-MOMM) to measure ferromagnetic resonance in individual, well-separated nanomagnets by use of heterodyne detection of magneto-optical signals at microwave frequencies. The experimental results are in good agreement with calculations based on the theory of dissipative transverse spin-currents internal to a conductive magnetic film, where the spin-currents are proportional to the spatial curvature of the excited mode [1], [2].

In the second part I will demonstrate how the Dzyaloshinskii-Moriya Interaction (DMI) can be directly determined from the frequency of propagating spin-waves. The DMI has recently attracted great interest as it is the origin of many chiral phenomena including chiral domain-walls and skyrmions. We quantified the DMI induced frequency-shift with Brillouin-Light-Scattering spectroscopy (BLS) in a series of Ni80Fe20/Pt samples for a range of Ni80Fe20 thicknesses. It has been predicted earlier by Fert [3] and Moriya [4] that for metallic oxides and magnetic spin-glasses the DMI and the Heisenberg exchange are proportional to each other. We determined the Heisenberg exchange for our samples from temperature dependent SQUID measurements and found that the proportionality holds also for the interfacial DMI in our sample system [5].


Bio: Hans T. Nembach is a Senior Research Associate at JILA, University of Colorado and Research Associate at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. He received his PhD in physics from the Technical University Kaiserslautern, Germany in 2006, where he worked in the group of Prof. Burkhard Hillebrands. He began work at NIST in 2006 under the auspices of a DAAD postdoctoral fellowship. In 2015 he received the NIST Physical Measurement Laboratory Distinguished Associate Award. His research interests are magnetization dynamics in thin films, multilayers and magnetic nanostructures.


[1] H.T. Nembach, J.M. Shaw, C.T. Boone and T.J. Silva, Physical Review Letters, 110, 117201 (2013), Highlight in Nature Nanotechnology 8, 227 (2013)

[2] Y. Tserkovnyak, E. M. Hankiewicz, and G. Vignale, Physical Review B, vol. 79, 094415 (2009).
[3] A. Fert, A. , Mater. Sci. Forum 59&60, 439 (1990)

[4] Moriya, T. , Phys. Rev. 120, 91–98 (1960).

[5] H. T. Nembach, J.M. Shaw, E. Jue, T.J. Silva, Nature Physics, 11, 825 (2015)