Showing papers by "Sarnjeet S. Dhesi published in 2011"

Clocking the Melting Transition of Charge and Lattice Order in 1T−TaS2 with Ultrafast Extreme-Ultraviolet Angle-Resolved Photoemission Spectroscopy

Abstract: We use time- and angle-resolved photoemission spectroscopy with sub-30-fs extreme-ultraviolet pulses to map the time- and momentum-dependent electronic structure of photoexcited 1T-TaS2. This compound is a two-dimensional Mott insulator with charge-density wave ordering. Charge order, evidenced by splitting between occupied subbands at the Brillouin zone boundary, melts well before the lattice responds. This challenges the view of a charge-density wave caused by electron-phonon coupling and Fermi-surface nesting alone, and suggests that electronic correlations play a key role in driving charge order.

Driving magnetic order in a manganite by ultrafast lattice excitation

Abstract: Femtosecond midinfrared pulses are used to directly excite the lattice of the single-layer manganite La0.5Sr1.5MnO4. Magnetic and orbital orders, as measured by femtosecond resonant soft x-ray diffraction with an x-ray free-electron laser, are reduced within a few picoseconds. This effect is interpreted as a displacive exchange quench, a prompt shift in the equilibrium value of the magnetic- and orbital-order parameters after the lattice has been distorted. Control of magnetism through ultrafast lattice excitation may be of use for high-speed optomagnetism.

Photoinduced Melting of Antiferromagnetic Order in La0.5Sr1.5MnO4 Measured Using Ultrafast Resonant Soft X-Ray Diffraction

Abstract: We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La0:5Sr1:5MnO4 after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a ð 1 ; 1 ; 1 Þ diffraction peak. On the other hand, the ð 1 ; 1 ; 0Þ diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.

Magnetic imaging by x-ray holography using extended references

Abstract: We demonstrate magnetic lensless imaging by Fourier transform holography using extended references. A narrow slit milled through an opaque gold mask is used as a holographic reference and magnetic contrast is obtained by x-ray magnetic circular dichroism. We present images of magnetic domains in a Co/Pt multilayer thin film with perpendicular magnetic anisotropy. This technique holds advantages over standard Fourier transform holography, where small holes are used to define the reference beam. An increased intensity through the extended reference reduces the counting time to record the farfield diffraction pattern. Additionally it was found that manufacturing narrow slits is less technologically demanding than the same procedure for holes. We achieve a spatial resolution of ∼30 nm, which was found to be limited by the sample period of the chosen experimental setup.

Anisotropic x-ray magnetic linear dichroism and spectromicroscopy of interfacial Co/NiO(001)

Abstract: Photoemission electron microscopy (PEEM) with linearly polarized x rays is used to determine the orientation of antiferromagnetic domains by monitoring the relative peak intensities at the $3d$ transition metal ${L}_{2}$ absorption edge. In such an analysis it is necessary to take into account the orientations of the x-ray polarization $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and magnetization $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ with respect to the crystalline axes. We address this problem by presenting a general expression of the angular dependence for both x-ray absorption (XA) spectroscopy and x-ray magnetic linear dichroism (XMLD) for arbitrary directions of $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ in the (001) cubic plane. In cubic symmetry the angular-dependent XMLD is a linear combination of two spectra with different photon energy dependencies. As a result the intensity maxima of the XA and XMLD are no longer found for $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ parallel to $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ and their directions will vary with photon energy. When $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ or $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ is along a high-symmetry axis there is only one spectrum. The angular-dependent XMLD can be separated into an isotropic part, which is symmetric around $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$, and an anisotropic part, which depends on the orientation of the crystal axes. The anisotropic part has maximal intensity when $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ have equal but opposite angles with respect to the [100] direction. The Ni${}^{2+}$ ${L}_{2}$ edge has the peculiarity that the isotropic part vanishes, which has the interesting consequence that the maximum in the XMLD intensity is obtained not only for $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [100] but also for ($\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [110], $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [1$\mathrm{1\ifmmode \bar{}\else \={}\fi{}}$0]). We apply the angular-dependent theory to determine the spin orientation near the Co/NiO(100) interface. The PEEM images show that the ferromagnetic Co spins and antiferromagnetic NiO spins are aligned perpendicular to each other. By rotating the sample with respect to the linear x-ray polarization we furthermore find that the perpendicular coupling with the ferromagnetic Co layer at the interface causes a canting of the antiferromagnetic Ni moments. This shows that taking into account the angular dependence of the XMLD in the detailed analysis of PEEM images can lead to an accurate retrieval of the spin axes of the antiferromagnetic domains.

Orbital moment anisotropy of Pt/Co/AlO x heterostructures with strong Rashba interaction

Abstract: We study the anisotropy of the spin and orbital magnetization of ultrathin Co layers characterized by structure inversion asymmetry, namely Pt/Co/AlO${}_{x}$ trilayers with Co thicknesses between 0.6 and 2 nm. We use x-ray magnetic circular dichroism (XMCD) to probe the Co layer along two orthogonal measurement geometries. By combining transverse and collinear XMCD, we achieve a vector measurement of the Co spin and orbital magnetic moments. We find an enhanced and anisotropic orbital magnetic moment localized at the Co interface and verify the connection between the Co orbital moment anisotropy and the macroscopic magnetic anisotropy of the trilayer. These results provide a link between diverse interfacial phenomena originating from spin-orbit coupling, such as the Rashba effect and perpendicular magnetic anisotropy. Using simple considerations derived from tight-binding models, we show that the Rashba constant is proportional to the ratio between out-of-plane and in-plane orbital magnetic moments.

High-precision soft x-ray polarimeter at Diamond Light Source

Abstract: The development and performance of a high-precision polarimeter for the polarization analysis in the soft x-ray region is presented. This versatile, high-vacuum compatible instrument is supported on a hexapod to simplify the alignment with a resolution less than 5 μrad, and can be moved with its own independent control system easily between different beamlines and synchrotron facilities. The polarimeter can also be used for the characterization of reflection and transmission properties of optical elements. A W/B4C multilayer phase retarder was used to characterize the polarization state up to 1200 eV. A fast and accurate alignment procedure was developed, and complete polarization analysis of the APPLE II undulator at 712 eV has been performed.

Exchange-spring-driven spin flop in an ErFe 2 /YFe 2 multilayer studied by x-ray magnetic circular dichroism

Abstract: X-ray magnetic circular dichroism at the Er ${M}_{4,5}$ edge is used to study the switching behavior of the hard ErFe${}_{2}$ layers in an epitaxial [ErFe${}_{2}$(70 \AA{})/YFe${}_{2}$(150 \AA{})] \ifmmode\times\else\texttimes\fi{} 25 exchange-spring superlattice. Magnetic hysteresis loops for the Er magnetization, at temperatures $T$ 200 K, reveal a single irreversible switch between a vertical exchange spring and its reversed state. Experiments at $T$ g 200 K reveal a crossover to a regime with two irreversible switching processes. Computational modeling for this system gives good agreement with the experiment, revealing that the observed high-temperature switching behavior is due to an exchange-spring-driven spin-flop-like transition. In contrast to the conventional spin-flop transition in an antiferromagnet, the increase in anisotropy energy of the hard magnetic ErFe${}_{2}$ layers and Fe-Fe exchange energy is overcome by a decrease in overall Zeeman energy. Computational studies also reveal two types of transitions between vertical exchange-spring and spin-flop states with first-order and second-order character.