OverviewWe probe & control material properties relevant for information technology applications. The unique x-ray sources at SLAC, the Stanford Synchrotron Radiation Laboratory (SSRL) and the Linac Coherent Light Source (LCLS), provide us with access to the fundamental properties of the electron, its charge and spin, evolving on the femtosecond (fs) time- and nanometer lengthscale. The picture to the right shows the undulator hall of the LCLS the world’s first x-ray free electron laser.

Research Projects & Highlights

The x-ray view of next generation magnetic data storage devicesThe 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.)

Ultrafast manipulation of exchange couplingControl of the magnetic exchange coupling, the strongest force in a magnetic solid, is crucial for developing novel magnetic data storage s t ra teg ies . The figure shows our fi rs t demonstration that the exchange coupling in antiferrimagnetically aligned 3d transition metal - 4f rare earth alloys can be influenced by optical laser excitation. 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 the structure of 80nm small magnetic domains. The figure shows a series of single-shot holograms 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.

Competing electronic order in colossal magneto-resistance materialsMixed-valence manganites are well-known for colossal magnetoresistance (CMR), whereby an applied magnetic field of a few tesla can increase their conductivity by orders of magnitude. Manganites also exhibit phase separation, where incompatible

electronic phases can coexist within a homogeneous material. The origins of these two phenomena, as well as their relationship, are not completely understood, as they must be if manganites are to be used in future oxide-based electronics. We perform complementary imaging a n d s c a t t e r i n g m e a s u r e m e n t s o f t h e ferromagnetic metallic and charge/orbital/antiferromagnetically ordered insulating phases, respectively. The figure illustrates the case of La0.35Pr0.275Ca0.375MnO3 where we found that the glassy nature of the ordered insulating phases is responsible for phase separation (Burkhardt, et al., PRL 108, 237202 (2012)).

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-insu la tor t rans i t ions, h igh- temperature superconductivity and colossal magneto-resistance When metallic electrons localize they often order and distort the lattice. We are disentangling this behavior with time resolved x-ray scattering (Pontius, et al., APL 98, 182504 (2011)). Ultimately we aim at establishing transition-metal oxide electronics based on

The Ultrafast Magnetism Lab - Projects

electric field driven metal-to-insulator phase transitions as one of the most promising avenues towards energy efficient field-effect transistors (see figure).

Magnetism at non-magnetic interfacesAtomic 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.

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.

The Ultrafast Magnetism Lab - Projects

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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.