Francesco Offi

Time-resolved magnetic domain imaging by x-ray photoemission electron microscopy

X-ray photoemission electron microscopy (X–PEEM) is a powerful imaging technique that can be used to perform element selective magnetic domain imaging on heterogeneous samples with different magnetic layers, like spin valves and tunnel junctions. We have performed nanosecond time-resolved X–PEEM measurements, on the permalloy layer of a Ni80Fe20 (5 nm)/Cu (10 nm)/Co (5 nm) trilayer deposited on Si(111). We used the pump-probe mode, synchronizing a magnetic pulse from a microcoil with the x-ray photon bunches delivered by the BESSY synchrotron in single bunch mode. Images could be acquired during and after the 20 ns long and 80 Oe high field pulses. The nucleation and subsequent growth of reversed domains in the permalloy could be observed, demonstrating the feasibility of element selective and time-resolved domain imaging using X–PEEM.

Experimental setup for high energy photoemission using synchrotron radiation

The instrument VOLPE (volume photoemission from solids) is an experimental setup dedicated to high energy photoemission (PE) experiments. The instrument is equipped with an electrostatic hemispherical spectrometer especially designed to analyze high energy electrons (up to 10 keV) with high resolving power. In order to attain an energy resolution of a few tens of millielectron volts, we designed and constructed a dedicated input lens system, high stability power supplies, and a low dark-count detector and readout electronics. The system has been tested and is now operational on the ID16 beamline at European Synchrotron Radiation Facility, where an optical layout has been developed to perform high energy, high resolution PE experiments. First results show an overall energy resolution (electron + photon) of 71+/-7 meV at 5934 eV. The effective attenuation length of the photoelectrons is estimated to be 5+/-0.5 nm at a kinetic energy of 5 keV

Spectroscopic Proof of the Correlation between Redox-State and Charge-Carrier Transport at the Interface of Resistively Switching Ti/PCMO Devices

By using hard X-ray photoelectron spectroscopy experimentally, proof is provided that resistive switching in Ti/Pr₀.₄₈ Ca₀.₅₂ MnO₃ (PCMO) devices is based on a redox-process that mainly occurs on the Ti-side. The different resistance states are determined by the amount of fully oxidized Ti-ions in the stack, implying a reversible redox-reaction at the interface, which governs the formation and shortening of an insulating tunnel barrier.

High-energy photoemission in silver: resolving d and sp contributions in valence band spectra

We present high-resolution valence band and core level spectra of silver for photoelectron kinetic energies up to 8 keV. At these kinetic energies we estimate a surface contribution of less than 3%. Taking advantage of the favourable sp/d relative cross-sections, a comparison with the calculated density of states is presented. We observe an increasing photoemission intensity when approaching the Fermi level, which we assign to a free-electron-like character in the 5p-band, whereas the principal s-like contribution is located at the bottom of the d-band. The difference between measured and calculated values of the sp/d cross-section ratio is discussed.

Chemical insight into electroforming of resistive switching manganite heterostructures

We have investigated the role of the electroforming process in the establishment of resistive switching behaviour for Pt/Ti/Pr0.5Ca0.5MnO3/SrRuO3 layered heterostructures (Pt/Ti/PCMO/SRO) acting as non-volatile Resistance Random Access Memories (RRAMs). Electron spectroscopy measurements demonstrate that the higher resistance state resulting from electroforming of as-prepared devices is strictly correlated with the oxidation of the top electrode Ti layer through field-induced electromigration of oxygen ions. Conversely, PCMO exhibits oxygen depletion and downward change of the chemical potential for both resistive states. Impedance spectroscopy analysis, supported by the detailed knowledge of these effects, provides an accurate model description of the device resistive behaviour. The main contributions to the change of resistance from the as-prepared (low resistance) to the electroformed (high resistance) states are respectively due to reduced PCMO at the boundary with the Ti electrode and to the formation of an anisotropic n-p junction between the Ti and the PCMO layers.

Imaging microspectroscopy of Ni/Fe/Co/Cu(001) using a photoemission microscope

Abstract The magnetic phases of 0–6 atomic monolayers (ML) Ni/0–14 ML Fe/6 ML Co/Cu(001) trilayer crossed double wedges are studied by the combination of photoelectron emission microscopy and X-ray magnetic circular dichroism spectroscopy at the Fe L 2,3 edges. This microspectroscopic technique allows the extraction of local quantitative magnetic information. The presence of three magnetically different thickness regions of Fe with effective spin moments per atom of 2.5 μ B (below ≈3.5 ML), 0.7 μ B (≈3.5–11 ML), and 2.0 μ B (above ≈11 ML) is confirmed. At 7–9 ML thickness, the value of 0.7 μ B is consistent with a ferromagnetic Fe surface layer on top of non-ferromagnetic layers. The ratio of orbital to effective spin moment varies between 0.05 for very thin Fe films and 0.15 for thicker films, if correction of saturation effects is taken into account. Images of the magnetic circular dichroism asymmetry at the Ni and Co L 3 edges show that at 5.5 ML Fe thickness the Ni and Co magnetizations have opposite orientations, pointing towards antiferromagnetic coupling across the Fe layer.

Microspectroscopic two-dimensional Fermi surface mapping using a photoelectron emission microscope

We demonstrate the use of a photoelectron emission microscope in connection with a retarding field electron energy analyzer for the fast acquisition of two-dimensional momentum resolved photoelectron angular distribution patterns. This opens the possibility to combine spatial, momentum, and energy resolution of photoelectrons within the same instrument. We have applied this to observe the Cu(001) Fermi surface from a selected region of the sample. A well defined bulk Fermi surface is quickly mapped in this way.

Metastable domain structures of ferromagnetic microstructures observed by soft X-ray magnetic circular dichroism microscopy

The benefit of combining soft X-ray magnetic circular dichroism and photoelectron microscopy is demonstrated by applying this combination to the observation of the magnetic domain structures of rectangular microstructures. The size and aspect-ratio dependence of the transformation of the domain structures by magnetic field pulses is investigated. The switching mechanism, which is very important in the application to magnetic storage, is discussed in terms of transformation between saturated and vortex domain structures.

Element-selective mapping of magnetic moments in ultrathin magnetic films using a photoemission microscope

We combine X-ray magnetic circular dichroism (XMCD) and photoelectron emission microscopy to obtain locally resolved magnetic information on a microscopic scale. Scanning the photon energy across elemental absorption edges and recording microscopic images of the local secondary electron intensity for both photon helicities at each photon energy step allows to analyze local XMCD spectra at any position of the imaged area of the sample. With the help of magnetic sum-rules local quantitative information about magnetic moments can be extracted from such microspectroscopic measurements. The full power of XMCD as a spectroscopic tool is so maintained, while microscopic spatial resolution is added.

Quantitative x-ray magnetic circular dichroism microspectroscopy of Fe/Co/Cu(001) using a photoemission microscope

Photoelectron emission microscopy is combined with soft x-ray magnetic circular dichroism (XMCD) absorption spectroscopy to obtain local element-resolved quantitative magnetic properties with microscopic resolution. This is applied to study 0–14 ML Fe wedges with a slope of 0.055 ML/μm on 6 ML Co/Cu(001). Local XMCD spectra at the Fe L2,3 edge confirm the presence of three magnetically different thickness regions of Fe with effective spin moments of 2.5μB (0–4.5 ML), 0.7μB (4.5–11 ML), and 1.8μB (>11 ML). The value of 0.7μB in the second phase is consistent with an fcc Fe phase containing nonferromagnetic layers underneath a ferromagnetic surface.