Spin-polarized Photoemission Spectroscopy Research Articles

Co/Ni multilayers for spintronics: High spin polarization and tunable magnetic anisotropy

In this paper we analyze in details the electronic properties of (Co/Ni) multilayers, a model system for spintronics devices. We use magneto-optical Kerr (MOKE), spin-polarized photoemission spectroscopy (SRPES), x-ray magnetic circular dichroism (XMCD) and anomalous surface diffraction experiments to investigate the electronic properties and perpendicular magnetic anisotropy (PMA) in [Co(x)/Ni(y)] single-crystalline stacks grown by molecular beam epitaxy.

Oxygen-induced immediate onset of the antiferromagnetic stacking in thin Cr films on Fe(001)

We investigated the magnetic coupling of ultra-thin Cr films grown at 600 K on a Fe(001)-p(1 × 1)O substrate by means of spin-polarized photoemission spectroscopy. Our findings show that the expected antiferromagnetic stacking of the magnetization in Cr(001) layers occurs right from the first atomic layer at the Cr/Fe interface. This is at variance with all previous observations in similar systems, prepared in oxygen-free conditions, which always reported on a delayed onset of the magnetic oscillations due to the occurrence of significant chemical alloying at the interface, which is substantially absent in our preparation.

Magnetic coupling of Fe-porphyrin molecules adsorbed on clean andc(2×2) oxygen-reconstructed Co(100) investigated by spin-polarized photoemission spectroscopy

The spin-polarized electronic structure of iron octaethylporphyrin (FeOEP) molecules adsorbed on a pristine and on a $c$($2\ifmmode\times\else\texttimes\fi{}2$) oxygen-reconstructed Co(100) surface has been analyzed by means of spin-polarized photoemission spectroscopy (SPPES) and first-principles density functional theory with the on-site Coulomb repulsion $U$ term ($\mathrm{DFT}+U$) calculations with and without Van der Waals corrections. The aim is to examine the magnetic exchange mechanism between the FeOEP molecules and the Co(100) substrate in the presence or absence of the oxygen mediator. The results demonstrate that the magnetic coupling from the ferromagnetic substrate to the adsorbed FeOEP molecules is ferromagnetic, whereas, the coupling is antiferromagnetic for the FeOEP on the $c$($2\ifmmode\times\else\texttimes\fi{}2$)O/Co(100) system. Spin-resolved partial densities of states extracted from ab initio $\mathrm{DFT}+U$ modeling are in fairly good comparison with the electronic spectral densities seen in angle-integrated SPPES energy dispersion curves for submonolayer coverages of FeOEP. Through combined analysis of these spectra and theoretical results, we determine that hybridization of 2$p$ orbitals of N and O with Co 3$d$ orbitals facilitates indirect magnetic exchange interactions between Fe and Co, whereas, a direct Fe-Co interaction involving the Fe ${d}_{{z}^{2}}$ orbital is also found for FeOEP on Co. It is observed through SPPES that the spin polarization of the photoemission-visible molecular overlayers decreases to zero as coverage is increased beyond the submonolayer regime, indicating that only interfacial magnetic coupling is at work. Microspot low-energy electron diffraction and low-energy electron microscopy were performed to characterize the physical order of the molecular coverage, revealing that FeOEP structural domains are orders of magnitude greater in size on $c$($2\ifmmode\times\else\texttimes\fi{}2$)O/Co(100) than on clean Co(100), which coincides with reduced scattering from the disorder and sharper features seen in SPPES.

Construction and performance of an elliptically polarized undulator of type Apple-II

Ten years ago, an elliptically polarized undulator (EPU) with a periodic length of 56 mm was constructed at the Taiwan Light Source. It was the longest EPU at that time. An electron beam of energy 1.5 GeV and current 300 mA passes through the undulator gap and radiates variously polarized light in the soft X-ray spectral domain. This EPU was extensively used in a wide range of research fields, including inelastic scattering, spin-polarized photoemission spectroscopy, photoelectron emission microscopy, and soft X-ray scattering. In response to increased demand by users with differing experimental requirements and applications, the construction of an EPU with a period of length 46 mm is under way. This investigation describes many aspects of the magnetic design, the structural engineering, and the control system. As magnetic technology is undergoing dramatic advances, advanced mechanical devices, and mechanisms have been included in the new EPU design; this design and its differences from the earlier design are summarized here.

A new spin-polarized photoemission spectrometer with very high efficiency and energy resolution

A new spin- and angle-resolved photoemission spectrometer was developed adopting the very-low-energy-electron-diffraction (VLEED)-type spin polarimeter. The Fe(001)p(1x1)-O film grown on MgO(001) crystal for the VLEED target yields significantly high spin-resolving power, the effective Sherman function of 0.40+/-0.02, with long lifetime and stability compared to the conventional Fe(001) target. Under the favor of high resolving power, approximately 100 times higher efficiency than that of conventional Mott-type spin polarimeter, the figure of merit of 1.9+/-0.2x10(-2) was achieved. Owing to this high efficiency, high-energy resolution can be realized with this new spin-polarized photoemission spectrometer. The simplified ways of target preparation and revitalization make the VLEED spin polarimeter much more convenient and feasible for the spin-polarized photoemission spectroscopy.

The effective spin dependent Debye temperature of Gd(0001)

The collective vibrational motions along the surface normal direction of the expansively strained Gd(0001) surface has been investigated. The effective Debye temperature, indicative of the dynamic motion of lattice normal to the surface, is 137±35 K from X-ray photoemission. With angle-resolved spin polarized photoemission spectroscopy, the Debye temperature is determined to be 79±35 K in spin majority, and 125±35 K for spin minority. Residual spin-mixing contributions to bands with Stoner like behavior as well as contributions from the spin dependent plasmon modes for Gd(0001) are implicated.

Band structure of strained Gd(0001) films

The electronic structure of strained and unstrained Gd(0001) surfaces has been studied both theoretically and experimentally with spin-polarized photoemission spectroscopy and spin-polarized inverse photoemission spectroscopy. Good agreement between calculated surface bands and surface-induced features of the spectra provides the basis for a more detailed explanation of the origin of the spin-polarized bands than was previously possible. It has been found that observed relaxation of the expansively strained in-plane crystal lattice constant, of Gd(0001) on Mo(112), significantly affects the electronic structure of the surface.

Fe3O4(111)/Fe(110)magnetic bilayer: Electronic and magnetic properties at the surface and interface

The surface and interface properties of the magnetic bilayer ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)/\mathrm{F}\mathrm{e}(110)$ have been studied by low-energy electron diffraction, spin-polarized photoemission spectroscopy, and magnetic linear dichroism. The electronic structure at the surface region of the oxide departs considerably from the one of the underlying bulk layer. Correspondingly, the surface spin polarization is found to be lower than expected for pure ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$ The magnetic bilayer coupling is driven by the formation of a specific ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)/\mathrm{F}\mathrm{e}(110)$ interface. The same interface is also formed when an Fe metal overlayer is deposited on the oxide surface.

Strain and strain relief in Gd(0001) films on Mo(112)

The electronic structure of strained and unstrained Gd(0001) has been studied with spin-polarized photoemission spectroscopy and spin-polarized inverse photoemission spectroscopy. In this work, we observed that relaxation of the expansively strained in-plane crystal lattice constant, of Gd(0001) on Mo(112), significantly diminishes the differences in the electronic structure from that observed for Gd(0001) grown on W(110). The defects that are incorporated in the Gd films, with increasing film thickness, lead to an in-plane lattice relaxation. Such thickness dependent strain relief results a loss of net polarization for Gd(0001) grown on Mo(112) compared to the relatively unstrained Gd(0001) films grown on W(110).

New opportunities in spin-polarized photoemission spectroscopy (invited)

New opportunities for probing magnetism and magnetic materials are emerging as a result of improved electron-spin detectors, broad advances in materials synthesis and characterization techniques, and refinements in spin-sensitive spectroscopic techniques. A multi-investigator group is currently commissioning a spin-polarized photoemission beamline at the National Synchrotron Light Source that has been designed to synthesize and study magnetic materials. The beamline exploits the significant increase in synchrotron radiation flux that is emitted from a magnetic insertion device. Key issues and new opportunities for advancing the understanding of magnetism and magnetic materials using this facility are addressed.

Design of NSLS undulator beamline U5 for spin-polarized photoemission spectroscopy

Beamline U5 of the vacuum ultraviolet storage ring at the National Synchrotron Light Source is being set up to conduct spin-polarized angle-resolved photoemission experiments. Ray tracing analysis including a realistic characterization of the undulator source is used to optimize the optical configuration of a 3-m toroidal grating monochromator, and to predict the monochromator performance.

The electron spin in photoemission spectroscopy and in the optical orientation technique

Recent progress in spin-polarized photoemission spectroscopy and in the optical orientation technique is shown to allownovel insights in quite different questions, like the 50 year old controversy itinerant vs. localized description of 3d magnets at high temperatures or band hybridization in semiconductors of diamond structure. The spin-polarized electron states and their changes upon magnetic phase transitions are investigated by using linearly polarized synchroton radiation from the new German dedicated storage ring BESSY and high angle- and energy resolution, while the degree of mixing (via spin-orbit interaction) of electrons states of different orbital symmetry is investigated by optical pumping using circularly polarized low energy photons. We finally discuss the potential of these techniques for the physics of interfaces especially in view of recent developments in the field of photon sources (e.g. ondulators) as well as materials preparation and characterization (e.g. epitaxy). In perspective it seems appropriate to compare the present experiment using dedicated synchrotron radiation at BESSY,with its degree of sophistication, to the one involving scattering of polarized neutrons in the late fifties and early sixties using the first ‘dedicated’ research reactors.