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Overview Research Projects & Highlights the X-Ray View of Next The Ultrafast Magnetism Lab - Projects Overview We are interested in probing & controlling material properties that are governed on the nanometer (nm) lengthscale by the fundamental properties of the electron, its charge and spin, evolving on the femtosecond (fs) timescale. Access to the ultra- fast time and ultra-small length scales is enabled by synchrotron radiation such as the Stanford Synchrotron Radiation Laboratory (SSRL) and since 2010 by the intense, coherent fs soft x-ray pulses at the Linac Coherent Light Source (LCLS), the worlds first x-ray free electron laser. The use of these unique x-ray tools requires a highly collaborative and interdepartmental approach. Research Projects & Highlights The x-ray view of next generation magnetic data storage devices The rapidly shrinking bit dimensions of next generation data storage devices require to ‘soften’ the magnetization before magnetic fields can switch the bit magnetization. This is done by heating the magnetic bit with a tiny laser integrated into read/write head. We studied the process of laser heating on its fundamental femtosecond time scale and found that the orbital motion of spins around the nuclei is affected much earlier that the spin orientation (Boeglin et al., Nature 465, 458 (2010). Once the bits are hot coherent x-ray scattering studies indicate that ‘spin memory’ is retained differently for smaller magnetic structures (Wu, et al., APL 99, 252505 (2011)). (picture courtesy of G. Bertero Western Digital Co.) Magnetic order out of chaos The so-called heat assisted magnetic recording requires the generation of magnetic order out of a chaotic, demagnetized state by cooling down the bit in an applied magnetic field. Bringing together 3d transition metals (Fe) and 4f rare earth elements (Gd) in a single alloy allows us to switch the sample magnetization with only optical laser pulses. Our results published in Nature (Radu, et al., Nature 472, 205 (2011)) demonstrated a novel transient ferromagnetic state responsible for magnetic switching. Our recent LCLS experiments identified the ultrafast flow of angular momentum to be the driving force for emergent magnetic order far from equilibrium. The Ultrafast Magnetism Lab - Projects Making of the femtosecond movie 80 fs short, fully coherent x-ray pulses from the LCLS can capture 80nm magnetic domains. The figure shows a series of single-shot holographic images that capture the true magnetic structure. Longer pulses capture the ultimate speed for writing magnetic information. This paves the way for making the ultra-fast movie and directing electrons & spins in future magnetic data storage devices (Wang, et al., PRL 108, 267403 (2012)). The basis of such studies is the development of novel holographic imaging techniques using coherent x-rays from the Stanford Synchrotron Radiation Laboratory. When does an insulator become a metal? Strongly correlated electron materials display a dazzling interplay between electronic and lattice degrees of freedom giving rise to spectacular functional properties such as metal-insulator transitions, high-temperature superconductivity and colossal magneto-resistance When metallic electrons localize they often order and distort the lattice. We are disentangling this behavior for manganites in thermal equilibrium using magnetic imaging and x-ray scattering techniques (Burkhardt, et al., PRL accepted). Time resolved x-ray scattering (Pontius, et al., APL 98, 182504 (2011)) at LCLS allows us to answer the ‘chicken & egg’ question about the driving force behind metal to insulator transitions such as the Verwey transition in magnetite. Magnetism at non-magnetic interfaces Atomic engineering of interfaces can lead to surprising new properties (Bert et al., Nature Communications 3 922 (2012)). In heterostructures of LaAlO3 (LAO) and SrTiO3 (STO), two nonmagnetic insulators, ferromagnetic patches have been imaged with micro-squid microscopy (see image taken from Kalinsky, et al., arXiv: 1201.1063v1). The fact that the ferromagnetism appears in isolated patches whose density varies greatly between samples strongly suggests that disorder or local strain induce magnetism in a population of the interface carriers. We recently succeeded in imaging these ferromagnetic patches using X-ray Photo-Electron Emission Microscopy (X-PEEM). This opens the door for element- specific investigations of the origin of this interface phenomenon and their response to external stimuli such as pulsed electric fields. Figure 1. Scanning SQUID images of the ferromagnetic landscape with increasing LAO thickness. (a-d) Typical scanning SQUID images of the LAO/STO surface, showing no ferromagnetic patches for (a) annealed STO and (b) 2 uc (unit cell) of LAO, and ferromagnetic patches for (c) 5 uc and (d) 10 uc of LAO, taken at 4 K. (e-f) Sketch of magnetic imaging of a ferromagnetic patch with scanning SQUID. (e) Sketch of a SQUID probe with a 3 µm pickup loop near the surface of the sample. (f) Sketch of field lines from a ferromagnetic patch (conceptually shown here as a small bar magnet) captured in the pickup loop. (g) Image (data) of the flux through the pickup loop as the SQUID is scanned over a typical ferromagnetic patch. (h) Calculated image for an in-plane dipole whose moment is 7x107 Bohr magnetons and azimuthal angle -20o, as determined by fitting the data in (g) to a point dipole model. 14 The Ultrafast Magnetism Lab - Projects Electrons & spins in strong electric fields It is generally assumed that spins react only to magnetic fields. Bits are switched by magnetic fields from read/write heads hovering only tens of nm above the hard disk drives in our computers. However, if switching spins with electric fields were possible completely novel applications could arise. We investigate the magnetic response to intense, ultrashort electric field pulses using table top fs lasers (see image) and using the electromagnetic field surrounding relativistic electron bunches from the FACET user facility. Towards computing with spin waves Based on our experience in imaging current induced magnetic switching in nanopilars (Bernstein, et al., PRB 83, 180410 (2011)) we are investigating the spin waves emitted by these devices. Implementing spin-wave transistors (see schematic) and other logic elements could lead to low-power information processing applications based on spin waves (magnons) in the emerging field of magnonics..
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