TOPIC Ultrafast, element-specific magnetization dynamics of multi-constituent magnetic materials measured by high-harmonic generation 
AREA Microelectronic Technologies & Devices  
SPEAKER Dr Justin M. Shaw, National Institute of Standards and Technology, Boulder, CO 80305 
DATE 8 December 2015, Tuesday 
TIME 11am to 12pm 
VENUE E1-06-04, Engineering Block E1, Faculty of Engineering, NUS  
FEES No Charge 

Studying ultrafast dynamics in magnetic materials provides rich opportunities for a greater fundamental understanding of correlated phenomena in solid-state matter. Progress in this field has grown substantially as a result of the recent emergence of free electron laser (FEL) sources that generate high intensity femtosecond pulses of x-rays. Alternatively, “tabletop” high harmonic generation (HHG) sources produce coherent beams at photon energies spanning the extreme ultraviolet (EUV) to the soft-x-ray regime, with pulse lengths in the femtosecond-to-attosecond range.[1] Such sources combine element-selectivity with femtosecond time resolution, enabling some of the fastest spin dynamics to be captured in complex alloys and multilayer systems. In addition to providing a brief overview of HHG, I will discuss some of our recent work whereby we apply HHG sources to probe femtosecond magnetization dynamics at the M absorption edges of the 3d ferromagnets Ni (68eV) and Fe (55eV) in a transversal magneto-optical Kerr geometry [2]. This capability allowed us to disentangle important microscopic processes that drive magnetization dynamics on femtosecond timescales. I will focus the discussion on two results: (1) Using a pump-probe technique applied to strongly exchange coupled Fe-Ni alloys, we measure the role of the exchange interaction between the Fe and Ni atoms and its effect on magnetization dynamics [3]; and (2) ultra-fast excitation of a trilayer material consisting of an Fe and Ni layer separated by a non-magnetic layer dives a superdiffusive spin current between the two magnetic layers.[4,5] As a consequence of this spin current we are able to generate a significant enhancement of the Fe magnetization. Finally, I will present some recent advances in HHG technology: the generation of intense, coherent circular-polarized light for x-ray magnetic circular dichroism (XMCD) experiments [6], and some recent advances in coherent diffractive imaging (CDI) that will allow for nanoscale resolution of magnetic structure with femtosecond resolution.
[1]T. Popmintchev. Science, 336, 1287 (2012):[2]C. La-O-Vorakiat et. al. Phys. Rev. X. 2, 011005 (2012):[3]S. Mathias et. al. Proc. Natl. Acad. Sci. 109, 4792 (2012):[4]M. Battiato et. al. Phys. Rev. B 86, 024404 (2012):[5]D. Rudolf et. al. Nat. Comm. 3, 1037 (2012):[6] O. Kfir et al., Nat. Photon. 9, 99-105 (2015)

Justin earned bachelor degrees in music composition and materials science engineering at Arizona State University in 1997. He went on to earn a Ph.D. in physics at Arizona State University in 2004. His work at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, began in 2005 when he received a National Research Council (NRC) postdoctoral fellowship before becoming a staff scientist in 2007. During his NRC postdoc at NIST, Justin completed his second Ph.D. in materials science. At NIST, his nanomagnetics research focused on spin dynamics in nanostructures and perpendicular materials.  


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