Search

The electron transfer of a well-defined single-crystalline organic P–N interface of the epitaxial layer of C60 on the surface of a single-crystalline pentacene (Pen-SC) was observed using time-reso...

Organic antiambipolar transistors (AATs) have partially overlapped p-n junctions. At room temperature, this p-n junction induces a negative differential transconductance in an AAT. However, the detailed carrier-transport mechanism remains unclear. Herein, an operando photoemission electron microscopy is used to tackle this issue owing to the technique's ability to visualize conductive electrons in real time during transistor operation. Notably, it is observed that when the AAT is on, a depletion layer forms at the lateral p-n junction. The visualized depletion layer shows that both p- and n-type channels have pinch-off states in the gate voltage range when the AAT is in on state. The steep potential gradient at the lateral p-n interface enhances the electron conduction from n-type to p-type semiconductor. Another significant finding is that most electrons are considered to recombine with the accumulated holes in the p-type semiconductor, affording the reduction of photoemission intensity by ≈80%. This technique provides a thorough understanding of carrier transport in AATs, further improving the device performance.

International audience; Mott insulators display puzzling insulator to metal transitions under electric field. We study here the Mott insulator GaV4S8 using time-resolved Photoemission Electron Microcopy and reveal unusually long carrier lifetimes, supporting the mechanism of electronic avalanche.

Twisted multilayer graphene (tMLG) present electronic properties that depend on the relative misalignment and interaction between layers. These interactions affect the band structures and the carrier dynamics upon photonic excitation. These structures are being under scrutiny and recent work high-lighted the strong potential they offer for optoelectronic devices. However, the ultrafast carrier dynamics is still at an early stage, often due to the instrumental limitations. Here, we investigated the carrier dynamics by femtosecond photoemission electron microscopy of chemical vapor deposited (CVD) twisted graphene super lattices presenting different interlayer rotation angles. The photo-generated carrier lifetimes in these selected regions show a longer lifetime compared to monolayer graphene (1 ML). This observation is assigned to the presence of band gap and sub bands in the trilayer graphene and has been supported by DFT calculations.

Hexagonal boron nitride (hBN) has become a prominent material for nanophotonic and quantum technology studies. Its wide bandgap can accommodate room temperature radiative optical transitions originating from defect states in different atomic structures. Here, we report the engineering of visible defects in chemical vapor deposited monolayer hBN irradiated by femtosecond pulses (0.1–0.5 W) at room temperature. Photoluminescence and transient photoluminescence measurements reveal the presence of a sharp emission at 630 nm with a time constant of approximately 3 ns randomly distributed around the irradiated region. We imaged the distribution of the photogenerated electrons by time-resolved photoemission electron microscopy in the picosecond and nanosecond timescales with 100 nm spatial resolution. We determined the precise location of the defects within the region of interest corresponding to an optical transition between 1.95 eV and 3.90 eV above the valence band maximum of hBN, ascribed to NBVN color centers.

When the order of magnetic spins is locally different, it is the so-called magnetic domains. Domain structures can be randomly formed, or it can be also formed by the competition of magnetic energies such as magnetic dipole, exchange, anisotropy, and Zeeman energies.

The present status and the recent progress of pump-probe time-resolved photoemission electron microscopy at SPring-8 are introduced. Combining the variety of bunch operation modes available at the SPring-8 storage ring with ultra-short excitation sources including pulsed magnetic fields, electric fields and lasers, the element-specific time evolution of materials in response to the excitation can be observed with spatial and temporal resolutions of (50–300) nm and (40–100) ps, respectively, with repetition frequencies of up to 42 MHz. By using the magnetic circular dichroism effect, the domain motion of sub-micron sized magnetic areas can be observed. The time evolution of electronic structures in local areas can also be studied. The experimental setups and representative activities are described.

A combination of X-Ray Magnetic Circular Dichroism and PhotoEmission Electron Microscopy (XMCD-PEEM) was applied to the magnetic domain analysis of Nd-Fe-B sintered magnets. The XMCD-PEEM high-resolution images revealed both the magnetic domain structures and the microstructural morphologies. In the thermally demagnetized state, each grain in a polycrystalline sample exhibits a multidomain structure, which is magnetically coupled across grain boundaries. After the DC field-demagnetization, it changed to a single domain structure. The magnetization vector in each surface grain reversed to the negative direction during the field-demagnetization procedure because of the small coercivity in the surface region. In the present study, we observed this surface domain reversal for the first time by means of XMCDPEEM imaging method, which is important in order to understand the surface phenomena of Nd-Fe-B magnets. [doi:10.2320/matertrans.MRA2008141]

The recombination dynamics of photogenerated carriers on temporal and spatial scales of 100 fs and 100 nm, respectively, on an inhomogeneous GaAs surface with structural defects was investigated using time-resolved photoemission electron microscopy with femtosecond laser pulses. The observed photocarrier lifetimes (τ) ranged from subpicoseconds to picoseconds at isolated structural defects and were inversely proportional to the photoemission intensity at each defect. We concluded that τ corresponds to the carrier trapping time to midgap defect states and estimated the density of the midgap states at each defect on the basis of the relation between τ and the photoemission intensity.

The magnetization reversal of the magnetically soft permalloy ( Fe 20 Ni 80 ) layer in a spin-valve-like FeNi/Cu/Co trilayer was studied using photoelectron emission microscopy combined with X-ray magnetic circular dichroism, which allows to image the magnetic domain structure in an element-selective way. Nanosecond-short magnetic field pulses with amplitudes between 3.3 and 16.3 mT were applied one by one to reverse the magnetization of the FeNi layer. Images of the magnetic domain structure were taken after application of each pulse, and the mobility of magnetic domain wall motion in the FeNi layer was deduced. The reversal mechanism of the FeNi layer was found to depend on the applied pulse length, amplitude, and on the energy and direction of the coupling between the two ferromagnetic layers.

The complex magnetic properties of Fe films epitaxially grown on Cu(0 0 1) have been discussed in relation to their atomic structure. We have studied the Fe films on Cu(0 0 1) by a new direct method for three-dimensional (3D) atomic structure analysis, so-called “stereoscopic photography”. The forward-focusing peaks in the photoelectron angular distribution pattern excited by the circularly polarized light rotate around the light axis in either clockwise or counterclockwise direction depending on the light helicity. By using a display-type spherical mirror analyzer for this phenomenon, we can obtain stereoscopic photographs of atomic structure. The photographs revealed that the iron structure changes from bcc to fcc and almost bcc structure with increasing iron film thickness.

We have performed magnetic domain imaging with spatial, temporal, and layer resolution using x-ray photoelectron emission microscopy. The element selectivity of x-ray magnetic circular dichroism allows the magnetization dynamics of the different magnetic layers in spin-valve-like FeNi/Cu/Co trilayers to be studied separately, using the time structure of synchrotron radiation. The unique possibilities of this technique have been used to study the influence of the intrinsic magnetic properties of the different layers on the magnetization dynamics and the interlayer magnetic coupling.

(2002). Layer‐resolved magnetic domain imaging using X‐ray photoelectron emission microscopy. Synchrotron Radiation News: Vol. 15, No. 6, pp. 12-16.

The micromagnetic properties of a Ni film in a Cu/Ni crossed-wedge on Cu(0 0 1) are investigated by a combination of photoelectron emission microscopy and X-ray magnetic circular dichroism. Two spin-reorientation transitions (SRT) from in-plane to perpendicular to the film plane and back to in-plane were observed with increasing Ni thickness. Whereas no clear Cu thickness dependence of the former SRT has been observed, the latter strongly depends on Cu thickness, and is shifted to thinner Ni thicknesses by increasing Cu overlayer thickness. Furthermore, the size and the shape of the ferromagnetic domain structure in the perpendicular magnetization region also depend on Cu thickness. The average domain size decreases with increasing Ni thickness near the second SRT line. It is observed for the first time that these domain structure changes also depend on Cu overlayer thickness.

We studied magnetism and morphology of metastable bcc Co nanostructures on Au(001) by x-ray magnetic circular dichroism in combination with scanning tunneling microscopy and photoelectron emission microscopy. While room-temperature deposition of Co onto Au(001) leads to the formation of bcc Co thin films with pure in-plane magnetization, postannealing of these thin films at 500 K drastically changes the morphology to bcc Co islands embedded in Au. In accordance with this morphological change, we find that an out-of-plane magnetization emerges additionally and the coexistence of in-plane and out-of-plane remanent magnetizations is observed for the islands. The nanostructure-size dependence of magnetic moments suggests that the magnetization easy axis of the island changes from the in-plane to the out-of-plane direction with decreasing nanostructure size. Such a spin reorientation transition is likely due to the increased fractional population of rim atoms generating the out-of-plane magnetization in the smaller islands. The observed out-of-plane remanent magnetization of the smaller islands indicates their large magnetic anisotropy.

We constructed an instrument for time-resolved photoemission electron microscopy (TR-PEEM) utilizing femtosecond (fs) laser pulses to visualize the dynamics of photogenerated electrons in semiconductors on ultrasmall and ultrafast scales. The spatial distribution of the excited electrons and their relaxation and/or recombination processes were imaged by the proposed TR-PEEM method with a spatial resolution about 100 nm and an ultrafast temporal resolution defined by the cross-correlation of the fs laser pulses (240 fs). A direct observation of the dynamical behavior of electrons on higher resistivity samples, such as semiconductors, by TR-PEEM has still been facing difficulties because of space and/or sample charging effects originating from the high photon flux of the ultrashort pulsed laser utilized for the photoemission process. Here, a regenerative amplified fs laser with a widely tunable repetition rate has been utilized, and with careful optimization of laser parameters, such as fluence and repetition rate, and consideration for carrier lifetimes, the electron dynamics in semiconductors were visualized. For demonstrating our newly developed TR-PEEM method, the photogenerated carrier lifetimes around a nanoscale defect on a GaAs surface were observed. The obtained lifetimes were on a sub-picosecond time scale, which is much shorter than the lifetimes of carriers observed in the non-defective surrounding regions. Our findings are consistent with the fact that structural defects induce mid-gap states in the forbidden band, and that the electrons captured in these states promptly relax into the ground state.

Circular dichroism has been measured in the photoelectron diffraction of W 4f photoelectrons from W(1 1 0)1×1 clean surface. The forward-focusing peaks along symmetric axes in diffraction pattern rotate azimuthally in either counterclockwise or clockwise direction depending on light helicity around the incident light axis. Using the formula suggested by Daimon et al. [Jpn. J. Appl. Phys. 32 (1993) L1480] substituting measured rotation angles for the [1 0 0] and [1 1 1] peaks, we evaluated effective atomic distance between an emitter and a scatterer, after calculating effective magnetic quantum number as a function of the polar angle between directions of the incident light and the photoelectron. The results of the effective atomic distance longer than the nearest-neighbor distance for each forward-focusing direction suggest partial contribution to the forward-focusing peak by the scatterers behind the nearest neighbor, within a single scattering theory.

A new type of display-type spherical mirror analyzer has been constructed at BL25SU of SPring-8 to study atomic arrangement on surface and spin arrangement on magnetic surface, and to analyze the atomic orbital of the electrons constituting the valence band and the other interesting electron spectroscopy. The advantage of this analyzer is that the emitted photoelectrons are converged to a focal point with keeping both kinetic energy and momentum. Hence we can realize energy and momentum analyses at once. To perform these studies accurately, the display-type spherical mirror analyzer should satisfy the following specifications such that the energy resolution should be higher than 1% of the pass-energy and the angular resolution should be higher than 1° at least. The improvement was estimated by LEED and photoelectron diffraction of Si(0 0 1). Finally, we could confirm that the energy resolution is 0.2% of the pass energy and the angular resolution is 0.6°. We conclude that the new analyzer satisfy the above requirement.

Circular dichroism has been measured in the photoelectron diffraction of bulk W 4f photoelectrons from the W(110)(1×1) clean surface. The forward focusing peaks along the symmetric axis in the diffraction pattern showed an azimuthal rotation similar to those reported in a prior experiment on Si(001) and chemically shifted W 4f photoelectrons from the W(110)(1×1)-O surface. The emission angle dependence of the rotation angles has been measured and analyzed for the first time and the angles observed are in good agreement with those calculated using the formula Δ ϕ=m/kR sin 2θ derived previously by Daimon et al. [Jpn. J. Phys.32, L1480 (1993)] considering the angular dependence of m. This property gives a basis for the analysis of structure or various magnetic and electronic properties on surfaces.

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.

A determination of the three-dimensional spin directions in all types of domain walls (DWs) of antiferromagnetic NiO has been successfully performed by photoemission electron microscopy combined with x-ray magnetic linear dichroism (XMLD), both for $s$- and $p$-polarized light. By comparing the azimuthal angle dependence of the XMLD contrast in the DWs with cluster model calculations which include the crystal symmetry and full-multiplet splitting, we determine the spin structures in the ${001}$ T walls, ${011}$ T walls, 120\ifmmode^\circ\else\textdegree\fi{} S walls, and 180\ifmmode^\circ\else\textdegree\fi{} S walls. In some cases, distinct S walls are not formed between two adjacent S domains, and the spin direction changes gradually over a wide range of the S domain structures. In the S walls, the spin direction is parallel to the magnetic easy ${111}$ plane. These spin configurations arise from the large difference in anisotropy energy between the in-plane and out-of-plane directions. Unexpectedly large widths in the several hundred nanometer range were observed for all the DWs. This also shows that NiO has a small magnetocrystalline anisotropy energy. Together with Monte Carlo simulation results, the qualitative phenomena concerning the wall energies are discussed. We further investigated the difference in wall energy between the ${001}$ T wall and the ${011}$ T wall. From the Monte Carlo simulation and an experimental study of heating effects, it is revealed that the ${001}$ T wall energy is smaller than the ${011}$ T wall energy.

We present a microscopic investigation of the temperature dependence of stripe domains in perpendicularly magnetized Ni films on Cu(001) using photoelectron emission microscopy in combination with x-ray magnetic circular dichroism (XMCD) in the resonant absorption of soft x rays. When the temperature approaches the Curie temperature of the system, the average width of the observed stripe domains is reduced along with the XMCD contrast. In addition, the domains become mobile. A quantitative analysis of the temperature-dependent motion of the domains yields an exponential behavior of the domain mobility with temperature, pointing toward thermally activated processes.

A femtosecond pulsed laser system has been installed at the BL25SU soft x-ray beamline at SPring-8 for time-resolved pump-probe experiments with synchronization of the laser pulses to the circularly polarized x-ray pulses. There are four different apparatuses situated at the beamline; for photoemission spectroscopy, two-dimensional display photoelectron diffraction, x-ray magnetic circular dichroism (XMCD) with electromagnetic coils, and photoelectron emission microscopy (PEEM). All four can be used for time-resolved experiments, and preliminary investigations have been carried out using the PEEM apparatus to observe magnetization dynamics in combination with XMCD. In this article, we describe the details of the stroboscopic pump-probe XMCD-PEEM experiment, and present preliminary data. The repetition rate of the laser pulses is set using a pulse selector to match the single bunches of SPring-8's hybrid filling pattern, which consists of several single bunches and a continuous bunch train. Electrons ejected during the bunch train, which do not provide time-resolved signal, are eliminated by periodically reducing the channel plate voltage using a custom-built power supply. The pulsed laser is used to create 300 ps long magnetic field pulses, which cause magnetic excitations in micron-sized magnetic elements which contain magnetic vortex structures. The observed frequency of the motion is consistent with previously reported observations and simulations.

International audience; We have used time-resolved x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM) to investigate the layer-resolved microscopic magnetization reversal in FeNi/X/Co (with X = Cu, Al2O3) trilayer systems. These measurements were performed in pump-probe mode, synchronizing magnetic pulses with synchrotron x-ray pulses. The good magnetic contrast observed for most samples reveals that in many cases the magnetization reversal is reproducible. We have used the measurements to obtain domain wall propagation speeds as a function of applied magnetic field, and to investigate the influence of domain wall interactions on the magnetic switching.

We present an x-ray spectromicroscopic investigation of single-crystalline magnetic $\mathrm{Fe}\mathrm{Mn}∕\mathrm{Co}$ bilayers on Cu(001), using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism (XMLD) at the Co and Fe ${L}_{3}$ absorption edges in combination with photoelectron emission microscopy. Using the magnetic coupling between the ferromagnetic Co layer and the antiferromagnetic FeMn layer, we are able to produce magnetic domains with two different crystallographic orientations of the magnetic easy axis within the same sample at the same time. We find a huge difference in the XMLD contrast between the two types of magnetic domains, which we discuss in terms of intrinsic magnetocrystalline anisotropy of XMLD of the Co layer. We also demonstrate that due to the high sensitivity of the method, the small number of induced ferromagnetic Fe moments at the FeMn-Co interface is sufficient to obtain magnetic contrast from XMLD in a metallic system.

The status of the soft x‐ray beamline BL25SU at SPring‐8 is reported. The beamline performance for circular dichroism measurements using the helicity switching of helical undulator radiation is focused on. Light spots on a sample of the opposite helicities have been observed with a photoelectron emission microscope. The vertical profiles of the spots have almost the same line shape and a positional difference of ∼ 0.5 μm. The horizontal spot positions coincide, but the horizontal spot widths are different by ∼ 7 %. The energy difference of the monochromatized light between the opposite helicities has been estimated to be less than or comparable to 1 meV in the Ne 1s absorption region. The kick angle of the kicker magnets has been increased from 0.2 to 0.3 mrad to avoid penetration of the higher harmonics.

We have studied the magnetization reversal dynamics of FeNi/Al$_2$O$_3$/Co magnetic tunnel junctions deposited on step-bunched Si substrates using magneto-optical Kerr effect and time-resolved x-ray photoelectron emission microscopy combined with x-ray magnetic circular dichroism (XMCD-PEEM). Different reversal mechanisms have been found depending on the substrate miscut angle. Larger terraces (smaller miscut angles) lead to a higher nucleation density and stronger domain wall pinning. The width of domain walls with respect to the size of the terraces seems to play an important role in the reversal. We used the element selectivity of XMCD-PEEM to reveal the strong influence of the stray field of domain walls in the hard magnetic layer on the magnetic switching of the soft magnetic layer.

The influence of magnetic anisotropy on nanosecond magnetization reversal in coupled $\mathrm{FeNi}∕\mathrm{Cu}∕\mathrm{Co}$ trilayers was studied using a photoelectron emission microscope combined with x-ray magnetic circular dicroism. In quasi-isotropic samples the reversal of the soft FeNi layer is determined by domain-wall pinning that leads to the formation of small and irregular domains. In samples with uniaxial magnetic anisotropy, the domains are larger and the influence of local interlayer coupling dominates the domain structure and the reversal of the FeNi layer.

We have studied the magnetization reversal dynamics of spin valves and magnetic tunnel junctions deposited on step bunched silicon substrates with a strong topological modulation. Our measurements show that the magnetization reversal is dominated by domain wall propagation at low field sweep rates and nucleation processes at high sweep rates. The magnetostatic orange peel coupling present in quasi-static conditions between the magnetic layers disappears when switching by nucleation becomes dominant. Micromagnetic simulations show that this phenomenon can be explained taking into account the modulated topology of the substrate.

The magnetization reversal dynamics of an Fe20Ni80∕Cu∕Co spin valve is investigated on the nanosecond time scale by magnetic microscopy with time and layer resolution. It is found that the speed by which micron-sized magnetic domains in the magnetically soft Fe20Ni80 layer are expanded by external field pulses exhibits a dependence on the change in domain-wall length, and on the coupling to the local magnetization direction of the Co layer.

Using photoemission electron microscopy in combination with x-ray magnetic circular dichroism, element selective magnetic domain images have been obtained from single-crystalline Co/FeMn and FeMn/Co bilayers epitaxially grown on a Cu(001) single crystal. The contact with ferromagnetic Co leads to the observation of a net magnetic moment in Fe and Mn, independently of the paramagnetic or antiferromagnetic state of the FeMn thin films. Only a small fraction of this moment might mediate the magnetic interaction at the interface, and thus be responsible for the exchange bias effect.

The magnetic coupling between epitaxial single-crystalline Co and FeMn layers on Cu(001) was investigated by element-resolved magnetic circular dichroism domain imaging using a photoelectron emission microscope. As-grown Co domain patterns reveal the presence of many small domains in the antiferromagnet. The coupling of the Co layer is found to be along $〈100〉$ crystallographic directions. This is discussed in terms of a $45\ifmmode^\circ\else\textdegree\fi{}$ coupling due to frustrations at topological $90\ifmmode^\circ\else\textdegree\fi{}$ domains in the FeMn layer. Coercivity oscillations as a function of FeMn thickness with atomic monolayer period support the importance of such step-induced domains in the coupling.

Examples of the application of photoelectron emission microscopy (PEEM) combined with synchrotron radiation excitation are reviewed. In particular, the combination study of PEEM and X-ray magnetic dichroism spectroscopy is very valuable for magnetic domain imaging. According to the demands of the industry, such as the production of magnetic recording media, the application method becomes very important. In this review, several examples of studies using magnetic circular and/or linear dichroism spectroscopy for ferro- and antiferromagnetic materials are introduced. In addition, pump–probe time-resolved PEEM is introduced. These methods allow us to measure the dynamical motion of micro-magnetic structures in mesoscopic size magnets in response to the field pulse in the sub-nanosecond range.

The domain structure of antiferromagnetic (AFM) NiO(100) has been investigated by nonmagnetic x-ray linear dichroism (XLD) at the O $K$ edge using photoemission electron microscopy and linearly polarized x-ray light. The evolution of XLD image contrast as the function of the angle between the crystal orientation and the incident $s$- or $p$-polarized light is clearly observed. The angular dependence of the XLD contrast is in good agreement with the calculated x-ray absorption cross section of O 1$s$ to 2$p$ orbital that is deformed anisotropically by the strong hybridization with Ni 3$d$ orbitals. This agreement strongly supports the conclusion that the observed XLD contrast reflects the twin-domain ($T$-domain) structure of the NiO crystal. By comparing the experimental data with the calculation, it is possible to assign each domain contrast to a specific $T$-domain in the quantitative manner. The proposed simple and clear way of $T$-domain assignment will be useful for investigating the magnetic domain structures of NiO and other oxide AFM materials such as FeO, CoO, and CuO, and will be important for understanding the magnetic properties of AFM materials, such as the exchange bias effect.

Three dimensional spin configuration in an exchange coupled Fe/NiO(100) system is investigated using a combination of photoemission electron microscopy and magnetic linear- and circular dichroism spectroscopy in the soft-x-ray range. By comparing the experimental image contrasts with a cluster model calculation including the crystal symmetry, we have successfully determined the spin configuration of the bulk antiferromagnetic NiO substrate. The easy axes of the outermost ferromagnetic Fe film and the ferromagnetic interface corresponding to the three kinds of spin axes of the antiferromagnetic NiO substrate have been assigned. We also observe that the magnetic field amplitude required for magnetization reversal depends on these initial easy axes directions. These findings are strongly related to the mechanism of exchange bias phenomena.

We have succeeded in developing a method for photoemission electron microscopy (PEEM) on fully magnetized ferromagnetic bulk samples and have applied this technique to Dy-doped Nd-Fe-B permanent magnets. Remanence magnetization of the sample was approximately 1.2 T, and its dimension was 3 × 3 × 3 mm3. By utilizing a yoke as an absorber of the stray magnetic field from the sample, we can obtain well-focused PEEM images of magnetized samples. We have observed not only chemical distributions to visualize Dy-rich and Dy-poor areas but also magnetic domains by x-ray magnetic circular dichroism. The formation of reversed magnetic domains is strongly suppressed at room temperature by Dy-doping. As the temperature of the Dy-doped sample is raised, starting from room temperature, the reversed magnetic domains first grow along the magnetization easy axis. Next, above approximately 80°C, the shapes of reversed domains start to expand in the direction perpendicular to the easy axis. Above approximately 85°C, the reversed domains cover more than half of the field of view of 30 µm. More importantly, the reversed magnetic domains tend to nucleate or extend in Dy-poor regions. We discuss the relationship between the chemical distribution and the magnetic domain structure.

The dynamics of magnetostatically coupled vortices in a pair of ferromagnetic micron-sized disks has been observed in real space by pump–probe time-resolved photoemission electron microscopy. It is found that the dynamics of paired vortices is affected by a magnetic dipolar interaction. The displacement of a vortex core from its equilibrium position in the paired vortices during the supply of a magnetic field pulse is smaller than that in the isolated vortex, which is in qualitative agreement with micromagnetic simulation results. Unexpectedly, two eigenfrequencies of the paired vortices are different from each other. When the separation distance between the paired disks is shorter, the difference between the two eigenfrequencies is larger.

Spin domains in bulk NiO have been observed by photoemission electron microscopy combined with X-ray magnetic linear dichroism (XMLD) using s - and p -polarized light. The azimuthal angle dependence of domain contrast at Ni L 2 -edge XMLD has been analyzed with a NiO 6 cluster model taking into account crystal symmetry and multiplet splitting. The complete assignment of spin domains has been achieved. The results evoke the reconsideration of the domain assignment and XMLD studies based on the existing atomic multiplet theory.

In order to clarify the correlation between the exchange bias strength and magnetic domain structure in ferromagnetic/antiferromagnetic (AF) bilayers, ferromagnetic domain structure was observed at Co-L3 edge for polycrystalline Mn–Ir/Co–Fe bilayers by using the x-ray magnetic circular dichroism photoelectron emission microscopy technique. From the observation for four different samples prepared with the respective thermal annealing procedures, a positive correlation was found between the strength of exchange bias field and the ferromagnetic domain size. Within the framework of single spin ensemble model, it was indicated that the larger size of AF domain, which means small distribution of fixing direction of AF spins, can induce the larger exchange bias field.

The search for materials that show a phase transition triggered by weak external stimulation of light is an important and attractive target for photonic and materials science today. My talk reviews experimental evidence indicating that photo-injected local excitation can really trigger an electron-lattice coupled cooperative phenomenon named as photo-induced phase transition (PIPT: Photo-domino effect) in wide class of materials based on the dynamical x-ray technique combined with the ultrafast spectroscopy. Obtained results also demonstrate that ‘hidden phase’ in strongly correlated systems which never appears in the phase diagram under thermo-equilibrium condition plays an essential role for the gigantic photo-response of the strongly correlated electron systems.