Research frontiers in magnetic materials at soft X-ray synchrotron radiation facilities

Journal of Physics: Condensed Matter, 2005

A new ultrahigh-resolution photoemission electron microscope called PEEM3 is being developed at the advanced light source (ALS). An electron mirror combined with a sophisticated magnetic beam separator is used to provide simultaneous correction of spherical and chromatic aberrations. Installed on an elliptically polarized undulator beamline, PEEM3 will be operated with very high spatial resolution and high flux to study the composition, structure, electric and magnetic properties of complex materials.

Physical Review Letters, 2008

The magnetic profile across the interface of a perpendicular exchange coupled [NiO/CoO]3/Pt-Co/Pt(111) system is investigated. The magneto-optic Kerr effect reveals a strong coupling between the antiferromagnetic (AFM) oxide and the ferromagnetic (FM) Pt-Co layer, by an increasing coercivity and a rotation of the easy magnetization axis of the FM layer along the AFM spins. Soft x-ray resonant magnetic reflectivity is used to probe the spatial distribution of the out-of-plane magnetization inside the oxide above its ordering temperature. It extends over 1 nm and exhibits a change of sign.

Physical Review B, 2002

A 10 nm Fe/5 nm NiO/10 nm Co polycrystalline trilayer prepared by magnetron sputtering has been investigated by resonant magnetic reflectivity of linearly polarized soft x rays ͑transversal magneto-optical Kerr effect geometry͒ at the L 3 edges of Fe, Ni, and Co. The exchange coupling in the trilayer, introduced during synthesis, has been altered by cooling through the Néel temperature T N of NiO at three different cooling fields H FC . We show that the chemical structure at the interfaces, the grain size and the magnetic properties of the trilayer are governed by factors such as successive heat treatment, magnetic field applied during synthesis and cooling through T N of NiO. Clear evidence for oxidation of Fe and Co states has been obtained, suggestive for the formation of mixed oxides at the Fe/NiO and NiO/Co interfaces. Oxidized Fe and Co are found to carry a net magnetic moment, which couples to the moments of the metallic states. We find that the coupling between oxidized and metallic Co spins is ferromagnetic, whereas the coupling between oxidized and metallic Fe spins is antiferromagnetic. We observe the presence of two types of excess Ni spins. A rigid excess Ni magnetic moment is induced at the interface during synthesis as follows from the strong asymmetry of the x-ray Kerr loops of the oxidized Co spins at H FC ϭ0 Oe and metallic Co spins at H FC ϭ240 Oe. Heat treatment leads to appearence of a soft excess Ni magnetic moment, which is ferromagnetically coupled to the metallic Fe and Co spins and exhibits a conventional hysteresis loop. Competing interactions lead to frustrated magnetization at the NiO/Fe interface and cause a tilt of the Fe spins away from the direction of applied magnetic field. This manifests itself in the presence of characteristic humps in the Fe Kerr loops.

Journal of Physics: Conference Series, 2010

A continuously working arc cluster ion source was used to prepare mass-filtered Fe nanoparticles with a mean size of 6 to 10 nm. Their structure was determined by HRTEM. The nanoparticles were deposited into an Al matrix and additionally on a W(110) substrate for comparison. Within the matrix they maintain their spherical shape and do not show any magnetic anisotropic effect. For uncapped Fe nanoparticles on a tungsten surface, a flattening is observed by STM and the nanoparticles exhibit a distinct magnetic anisotropy with the easy magnetization axis being in the surface plane. Correlating both investigations we conclude that this observation is due to shape and interface-induced contributions to the magnetic anisotropy.

Physical Review B, 2005

Exchange-coupled Fe/ FeSn 2 ͑001͒ bilayer systems consisting of a polycrystalline ferromagnetic Fe layer grown on an epitaxial antiferromagnetic FeSn 2 layer have been prepared by molecular beam epitaxy and investigated by 57 Fe conversion electron Mössbauer spectroscopy and superconducting quantum interference device magnetometry. The systems show a significant exchange bias effect at low temperatures. Tracer layers of 57 Fe ͑in the Fe layer͒ and 57 FeSn 2 ͑in the FeSn 2 layer͒ have been placed in the samples in order to probe the spontaneous spin orientation at different distances from the Fe/ FeSn 2 interface. The Fe spins in the ferromagnetic layer are preferentially oriented in the interfacial plane. In as-prepared samples the presence of the Fe top layer induces a striking out-of-plane component of the interfacial Fe spins in the antiferromagnetic FeSn 2 film. This perpendicular component decreases in magnitude at a larger distance from the interface. A reorientation transition from out-of-plane toward in-plane spin orientation was observed in the interfacial FeSn 2 layer with increasing age of the sample. This effect is correlated with an increased magnitude of the exchange bias field for the aged samples.

Physical Review B, 2003

We present resonant coherent small-angle scattering from sub-mum magnetic domains in an in-line holography geometry. By varying the polarization of the incident soft x rays the interference between the reference wave propagating from a pinhole and scattered object wave can be switched ``on'' or ``off.'' The reference-object interference contribution in the scattering can be isolated by combining measurements with right and left circular polarizations. The implications for spatial and temporal interference experiments of magnetic specimen such as holography, lensless imaging, or photon correlation spectroscopy are discussed.

Physical Review B, 2009

The element-specific and shell-specific magnetism of Eu in the filled skutterudite ferrimagnet Eu 0.95 Fe 4 Sb 12 has been investigated using Eu M 4,5 -and Eu L 2,3 -edge x-ray magnetic circular dichroism ͑XMCD͒ spectroscopy as a function of temperature. Eu L 3 -edge x-ray absorption spectroscopy shows that Eu is mostly in the divalent state. Eu M 5 -edge x-ray absorption spectroscopy, measured by electron yield, shows nearly equal fractions of Eu 2+ and Eu 3+ states because it probes a significant portion of the surface volume which is dominated by the Eu 3+ impurity state. Sum-rule analysis of the Eu M 4,5 -edge XMCD spectrum measured at 4.9 K yielded the 4f spin moment of ͑7.15Ϯ 0.3͒ B per Eu 2+ ion. By comparing the Eu L 2,3 -edge XMCD spectrum in the ferrimagnetic state of Eu 0.95 Fe 4 Sb 12 to that of a divalent Eu reference compound, the clathrate Eu 8 Ga 16 Ge 30 , we show that the 5d spin polarization of Eu in the skutterudite is strongly enhanced by the exchange coupling with the 3d band electrons of Fe, which were shown to have an ordered moment of −0.21 B / Fe in our earlier Fe L 2,3 XMCD measurements. The temperature dependence of the magnetic order parameter, determined from the Eu L 3 -edge XMCD intensity, yields a mean-field-like exponent ͑ϳ0.52͒ in the skutterudite and a three-dimensional Heisenberg-type ͑ϳ0.36͒ exponent in the clathrate.

Physical Review Letters, 2007

We combine x-ray magnetic circular dichroism spectroscopy at Fe L 2;3 edges, at Eu M 4;5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu 0:95 Fe 4 Sb 12 is a ferrimagnet in which the Fe 3d moment and the Eu 2 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L 3 edge XAS, we find that about 13% of the Eu have a formal valence of 3 . We ascribe the origin of ferrimagnetism at a relatively high transition temperature T C of 85 K in Eu 0:95 Fe 4 Sb 12 to f-electron interaction with the nearly ferromagnetic Fe 4 Sb 12 2:2ÿ host lattice.

Physical Review B, 2003

It is shown that the spin-polarized electronic structure of antiferromagnetic ͑AFM͒ materials can be investigated on an element-selective level using a versatile, novel x-ray magneto-optical spectroscopy in reflection. We demonstrate this with spectra observed at the Ni L 3 absorption edge of two buried exchange-biased microstructures of current technological interest: NiO/Co and NiMn/Co, which contain the insulating antiferromagnet NiO, and the metallic antiferromagnet NiMn, respectively. The measured spectrum provides information about the exchange-split d density of states of the AFM atom.

Physical Review B, 2004

We present resonantly enhanced longitudinal magneto-optical Kerr rotation and ellipticity spectra measured across the 2p absorption edges of amorphous Fe, Co, and Ni. The Kerr spectra are acquired by a complete polarization analysis of linearly polarized synchrotron radiation upon reflection from the ferromagnetic films. We observe large Kerr rotation angles of up to Ϯ24°, more than two orders of magnitude larger than detected in the visible energy range. Spectral contributions stemming from atomic transitions could be separated from interference effects by simulations based on the optical constants, which have been determined through independent experiments. For grazing incidence, we show theoretically and experimentally that the longitudinal Kerr rotation and ellipticity spectra are related to transversal Kerr effect spectra and x-ray magnetic circular dichroism reflection spectra, respectively.

Physical Review B, 2002

A systematic study of the specular reflectivity using circularly and linearly polarized radiation on magnetic materials is presented. Within a frame work based on the complete reflection matrix and the Stokes parameter formalism, the reflectance is modeled as a function of the polarization state of the incoming light for the longitudinal and the transversal magnetization. The nonmagnetic reflection coefficients and their individual magnetic contributions are determined from resonant magnetic reflectivity experiments of polarized soft x rays across the Fe-2p absorption edge on a ferromagnetic Fe/C multilayer. Exploiting tunable undulator radiation the absolute reflectance is investigated as a function of the degree of circular or linear polarization, photon energy, angle of incidence, and magnetic-field direction. As predicted by the developed formalism the corresponding magnetic asymmetry parameters depend nonlinearly on the degree of polarization at large angles of incidence due to the influence of the polarizing power of the sample. In the longitudinal geometry using circularly polarized light, the magnetic contribution is directly related to the magnetic contribution of the optical constants, which have independently been determined by Faraday measurements on an identical transmission sample.

Physical Review B, 2004

We investigate the magnetization reversal in ͓Co͑4 Å͒ /Pt͑7 Å͔͒ 50 multilayers with perpendicular magnetic anisotropy both macroscopically by the first-order reversal curve (FORC) technique and microscopically by transmission x-ray microscopy (TXRM) and resonant x-ray small angle scattering (SAS). In particular, we focus on the nucleation and saturation processes. The onset of magnetization reversal is dominated by irreversible processes corresponding to the avalanchelike propagation of one-dimensional stripe domains that originate from earlier nucleated zero-dimensional bubble domains. In a second stage we observe mainly reversible behavior where the overall domain topology is preserved. Finally another irreversible process brings the sample to negative saturation, corresponding to the contraction and annihilation of domains. Interestingly, even well beyond the apparent major-loop saturation field, the FORC diagram exhibits pronounced irreversible switching and thus provides a direct measure of the true (and significantly higher) saturation field. TXRM and SAS measurements reveal on a microscopic level that some residual bubble domains with negligible moments still persist for fields well above the apparent saturation field. These residual domains, if unannihilated, significantly alter the subsequent magnetization reversal.

Synchrotron Radiation News, 2001

Review of Scientific Instruments, 2002

To use the unique element-specific nature of polarized x-ray techniques to study a wide variety of problems related to magnetic materials, we have developed a dual-branch sector that simultaneously provides both hard and soft x-ray capabilities. This facility, which is located in sector 4, is equipped with two different insertion devices providing photons in both the intermediate ͑0.5-3 keV͒ and hard x-ray regions ͑3-100 keV͒. This facility is designed to allow the simultaneous branching of two undulator beams generated in the same straight section of the ring.

Reviews of Modern Physics, 2006

Nanomagnetism is the discipline dealing with magnetic phenomena specific to structures having dimensions in the submicron range. This Colloquium addresses the challenges and scientific problems in this emerging area, including its fabrication strategies, and describes experiments that explore new spin-related behaviors in metallic systems as well as theoretical efforts to understand the observed phenomena. As a subfield of nanoscience, nanomagnetism shares many of the same basic organizing principles such as geometric confinement, physical proximity, and chemical self-organization. These principles are illustrated by means of several examples drawn from the quests for ultrastrong permanent magnets, ultra-high-density magnetic recording media, and nanobiomagnetic sensing strategies. As a final example showing the synergetic relationships to other fields of science, this Colloquium discusses the manipulation of viruses to fabricate magnetic nanoparticles.

Physical Review B, 2009

Years of intensive work on perovskite manganites has led to a detailed understanding of the phenomena that emerge from competition between the electronic and lattice degrees of freedom in these correlated electron systems. It is well understood that the related cobaltites provide an additional spin-state degree of freedom. Here, we use the magnetic properties of a particular cobaltite, Pr 1−x Sr x CoO 3 , to demonstrate the vital role played by a further ingredient often negligible in manganites; magnetocrystalline anisotropy. Pr 1−x Sr x CoO 3 exhibits an anomalous "double magnetic transition" that cannot be ascribed to a spin-state transition or the usual charge/orbital/antiferromagnetic ordering and has thus far evaded explanation. We show that this is actually due to a coupled structural/magnetocrystalline anisotropy transition driven, in this case, by PrO hybridization. The results point to the existence of a distinct class of phenomena in the cobaltites due to the unique interplay between structure and magnetic anisotropy.

Journal of Applied Physics, 2006

Condensed Matter

Polarization selection of the reflected radiation has been employed in Mössbauer reflectivity measurements with a synchrotron Mössbauer source (SMS). The polarization of resonantly scattered radiation differs from the polarization of an incident wave so the Mössbauer reflectivity contains a scattering component with 90° rotated polarization relative to the π-polarization of the SMS for some hyperfine transitions. We have shown that the selection of this rotated π→σ component from total reflectivity gives an unusual angular dependence of reflectivity characterized by a peak near the critical angle of the total external reflection. In the case of collinear antiferromagnetic interlayer ordering, the “magnetic” maxima on the reflectivity angular curve are formed practically only by radiation with this rotated polarization. The first experiment on Mössbauer reflectivity with a selection of the rotated polarization discovers the predicted peak near the critical angle. The measurement of t...

Reviews of Modern Physics, 2004

Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

Review of Scientific Instruments, 2021

Diagnosing free electron laser (FEL) polarization is critical for polarization-modulated research such as x-ray FEL diffraction imaging and probing material magnetism. In an electron time-of-flight (eTOF) polarimeter, the flight time and angular distribution of photoelectrons were designed based on x-ray polarimetry for on-site diagnosis. However, the transverse position of x-ray FEL pulses introduces error into the measured photoelectron angular distribution. This work, thus, proposes a method of compensating transverse position jitters for the polarization by the eTOF polarimeter itself without an external x-ray beam-position monitor. A comprehensive numerical model is developed to demonstrate the feasibility of the compensation method, and the results reveal that a spatial resolution of 20 μm and a polarity improved by 0.02 are possible with fully polarized FEL pulses. The impact of FEL pulses and a method to calibrate their linearity are also discussed.

Journal of Physics: Condensed Matter, 2012

Much interest is being devoted to designing systems where magnetic and ferroelectric orders coexist (multiferroics), and where the presence of magnetoelectric coupling could enable the electrostatic control of magnetism in the solid state. In particular, proximity effects can be tailored to design novel electronic structures with enhanced magnetoelectric couplings in composite heterostructures [1]. A striking example of this approach is our recent demonstration of a large, charge-mediated, magnetoelectric coupling in epitaxial PZT/LSMO heterostructures [2], which explores the sensitivity of the magnetic properties of the doped manganites to charge. Through magnetic, electric, structural and spectroscopic characterization, we demonstrate that the magnetoelectric coupling in PZT/LSMO heterostructures is electronic in origin, and results from the modulation in the valency of the Mn upon switching the PZT ferroelectric polarization [3]. In particular, we conclude that the interfacial spin ordering is modified upon charge doping, which explains the large magnetoelectric response found in this system [4]. This ability to control spin via electric fields opens a new pathway for the development of novel spin-based technologies.

Physical Review B, 2004

The structure of Co epitaxial films deposited on Pt(001) has been investigated with scanning tunnelling microscopy and surface x-ray diffraction. The Co films grow in a layer-by-layer mode up to the highest examined thickness ͑ϳ30 Å͒. Their crystalline structure is a tetragonal distortion of the bcc structure in pseudomorphic epitaxy with the substrate. It consists of a −1.8% in-plane compressive strain and 5.1% outof-plane tensile strain. A similar pseudomorphic structure is also observed in Pt/ Co/ Pt͑001͒ trilayers. The magnetic properties of the films have been investigated by transverse magneto-optical Kerr effect and magnetic-resonant surface x-ray diffraction. The films exhibit in-plane magnetic anisotropy easy axes within the thickness range of 5-30 Å. The magnetization curves of Co and interfacial Pt are similar, indicating ferromagnetic coupling.

Physical Review B, 2004

We present absolute values for the complete set of magneto-optical constants around the Gd 4,5 and Fe L 2,3 dipole resonances as obtained from measurement of the polarization dependent photoabsorption cross sections and Kramers-Kronig transformation. The results are verified by comparing the resulting resonant scattering factors with the resonant magnetic scattering from a stripe domain lattice, showing an excellent agreement for both the circular and linear dichroic contributions.

Review of Scientific Instruments

The scope of this paper is to outline the main marks and performances of the MagneDyn beamline, which was designed and built to perform ultrafast magnetodynamic studies in solids. Open to users since 2019, MagneDyn operates with variable circular and linear polarized femtosecond pulses delivered by the externally laser-seeded FERMI free-electron laser (FEL). The very high degree of polarization, the high pulse-to-pulse stability, and the photon energy tunability in the 50–300 eV range allow performing advanced time-resolved magnetic dichroic experiments at the K-edge of light elements, e.g., carbon and at the M- and N-edge of the 3d-transition-metals and rare earth elements, respectively. To this end, two experimental end-stations are available. The first is equipped with an in situ dedicated electromagnet, a cryostat, and an extreme ultraviolet Wollaston-like polarimeter. The second, designed for carry-in user instruments, hosts also a spectrometer for pump-probe resonant x-ray emi...

Physical Review B

The Fe L-edge resonant magneto-optical Kerr effect of a buried Fe nanofilm was investigated by rotatinganalyzer ellipsometry and the results were compared with those from three theoretical simulations. The reversal of the Kerr rotation angle θ K between the L 3 and L 2 edges, observed in the experiment, was consistent with classical electromagnetic simulation using empirical optical constants. The spectral θ K feature was reproduced by the first-principles calculation of the KKR-Green's function method on the itinerant electronic system. The demonstration indicates that spectra of the L-edge resonant MOKE can be understood in terms of both the macroscopic and microscopic pictures.