"Ultrafast and very small: Discover nanoscale magnetism with picosecond time resolution using X-rays"

Hendrik Ohldag
SLAC & Stanford
Wednesday, February 22, 2017
4:00 pm
NS2 1201

Today’s magnetic device technology is based on complex magnetic alloys or multilayers that are patterned on the nanoscale and operate at GHz frequencies.  To better understand the behavior of such devices one needs an experimental approach that is capable to detect magnetization with nanometer and picosecond sensitivity. In addition, since a device contains different magnetic elements, a technique is needed that provides element specific information about not only ferromagnetic but antiferromagnetic materials as well. Synchrotron based x-ray microscopy provides exactly these capabilities, because a synchrotron produces tunable and fully polarized x-rays with energies between several tens of eV up to tens of keV. The interaction of tunable x-rays with matter is element specific, allowing us to separately address different elements in a device. The polarization dependence or dichroism of the x-ray interaction provides a path to measure a ferromagnetic moment and measure its orientation or determine the orientation of the spin axis in an antiferromagnet.  The wavelength of x-rays is of the order of nanometer, which enables microscopy with nanometer spatial resolution. And finally, a synchrotron is a pulsed x-ray source, with a pulse length of tens of picosecond, which enables us to study magnetization dynamics with a time resolution given by the x-ray pulse length in a pump-probe fashion.

The goal of this talk is to present an introduction into the field and explain the capabilities of synchrotron based x-ray microscopy to a diverse audience, which is becoming a tool that is available at every synchrotron. The general introduction will be followed by a set of examples that will depend on the audience and range from properties of magnetic materials in rocks and meteorites over magnetic inclusions in magnetic oxides and interfacial magnetism in magnetic multilayer to the dynamics of nanostructured due to field, current pulses as well as microwave excitations.


Dr. Hendrik Ohldag received his Ph.D in Experimental Physics from the University of Düsseldorf (Germany) in 2002. He joined the Stanford Synchrotron Radiation Light Source (SSRL) in 1999 as a Research Assistant as part of his Ph.D. research. After a Postdoc at SSRL he became a permanent member of the research staff at SSRL in 2005. Between 1999 and 2002 he was a visiting researcher at the Advanced Light Source (ALS) and he is a visiting researcher at New York University (NYU) since 2014. Dr. Ohldag has been awarded the David. A Shirley Award at the Advanced Light Source in 2006 for “Outstanding contribution in using photoemission electron microscopy for the study of magnetic materials”. He is a senior member of the IEEE magnetics society and the chair of the Magnetic Interfaces and Nanoscale Device Division of the American Vacuum Society. Dr. Ohldag has authored or co-authored over 50 peer-reviewed papers and book chapters that are altogether cited more than 2500 times. He has also participated in the organization of 25 international conferences and workshops. His research focuses on the use of x-ray microscopy to study the dynamic and static properties of complex magnetic materials.


Ruqian Wu