Ultrafast Demagnetization Dynamics at the M Edges of Magnetic Elements Observed Using a Tabletop High-Harmonic Soft X-Ray Source Chan La-O-Vorakiat, * Mark Siemens, Margaret M. Murnane, and Henry C. Kapteyn Department of Physics and JILA, University of Colorado, Boulder, Colorado 80309-0440, USA Stefan Mathias and Martin Aeschlimann University of Kaiserslautern and Research Center OPTIMAS, 66606, Kaiserslautern, Germany Patrik Grychtol, Roman Adam, and Claus M. Schneider Institute of Solid State Research, IFF-9, Research Center Ju ¨lich, 52425, Ju ¨lich, Germany Justin M. Shaw, Hans Nembach, and T.J. Silva Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado 80305-3328, USA (Received 19 May 2009; published 15 December 2009) We use few-femtosecond soft x-ray pulses from high-harmonic generation to extract element-specific demagnetization dynamics and hysteresis loops of a compound material for the first time. Using a geometry where high-harmonic beams are reflected from a magnetized Permalloy grating, large changes in the reflected intensity of up to 6% at the M absorption edges of Fe and Ni are observed when the magnetization is reversed. A short pump pulse is used to destroy the magnetic alignment, which allows us to measure the fastest, elementally specific demagnetization dynamics, with 55 fs time resolution. The use of high harmonics for probing magnetic materials promises to combine nanometer spatial resolution, elemental specificity, and femtosecond-to-attosecond time resolution, making it possible to address important fundamental questions in magnetism. DOI: 10.1103/PhysRevLett.103.257402 PACS numbers: 78.70.Ck, 75.25.+z, 78.20.Ls The study of magnetism, magnetic materials, and dy- namics in magnetic systems is a topic of fundamental interest in our understanding of correlated systems, as well as being directly relevant to technology and informa- tion storage [1]. In recent years, magnetism at ultrafast time scales has been a topic of increasing interest. A thorough understanding of femtosecond magnetism will address the important questions of how fast the magneti- zation can be reoriented in a material and what physical processes present fundamental limits to this speed. In the spatial domain, magnetism at nanometer length is a topic directly relevant to data storage, since future advances in this technology will require a further reduction in device dimensions to increase the storage density. These consid- erations have motivated a variety of studies using magneto- optic effects in conjunction with ultrafast light pulses to explore these fundamental limits. Magneto-optical dynamic studies currently make use either of visible-wavelength light from ultrafast lasers, or x-rays from large-scale synchrotron x-ray facilities. Ultra- fast lasers produce short pulses ( 30 fs), making possible femtosecond time resolution [2–5], but with a spatial reso- lution that is generally limited by the wavelength of the probe light. X-rays, on the other hand, allow for high spatial resolution and high contrast imaging at the elemen- tal absorption edges of ferromagnetic materials. However, the available time resolution to date is too slow to resolve the fastest dynamics involved in domain reorientation or to illuminate the physics behind the recently observed ultrafast coherent interactions between light and the elec- tron spin system. Because of this, significant efforts have been devoted to using laser pulses to select a short burst (  100 fs) of x rays from synchrotron radiation (called femtosecond-slicing) [6]. However, these experiments are time consuming and challenging, due to the low flux of sliced photons. Continued scientific and technological progress thus requires studies that combine nanometer spatial resolution with femtosecond-to-attosecond time resolution. This is a challenging proposition, but one that can be addressed using newly developed tabletop-scale coherent light sources based on high-harmonic up-conversion (HHG) of a femtosecond laser. HHG is an extreme nonlinear process that produces coherent short wavelength beams with the shortest pulse durations demonstrated to date for any light source—in the 0.1 fs to 10 fs range [7–9]. The generated harmonics extend from 10 eV to greater than 2 keV, and retain the polarization and coherence properties of the driving laser under phase-matched generation conditions. Bright HHG beams with sufficient flux for experimental applications can currently be generated with photon ener- gies of up to 330 eV [9]. Past synchrotron measurements using long duration pulses have shown that the magneti- zation can be probed at the M edges of Fe, Co, and Ni, at photon energies around 55 eV to 65 eV [10–14]. This is an energy range that is easily accessible using HHG. PRL 103, 257402 (2009) PHYSICAL REVIEW LETTERS week ending 18 DECEMBER 2009 0031-9007= 09=103(25)=257402(4) 257402-1 Ó 2009 The American Physical Society