Soft X-ray Diffraction Research Articles

Competing Ferri- and Antiferromagnetic Phases in Geometrically FrustratedLuFe2O4

We present a detailed study of magnetism in LuFe(2)O(4), combining magnetization measurements with neutron and soft x-ray diffraction. The magnetic phase diagram in the vicinity of T(N) involves a metamagnetic transition separating an antiferro- and a ferrimagnetic phase. For both phases the spin structure is refined by neutron diffraction. Observed diffuse magnetic scattering far above T(N) is explained in terms of near degeneracy of the magnetic phases.

Resonant soft x-ray scattering from stepped surfaces of SrTiO3

We studied the resonant diffraction signal from stepped surfaces of SrTiO3 at the Ti 2p → 3d (L2,3) resonance in comparison with x-ray absorption (XAS) and specular reflectivity data. The steps on the surface form an artificial superstructure suitable as a model system for resonant soft x-ray diffraction. A small step density on the surface is sufficient to produce a well defined diffraction peak. We determined the optical parameters of the sample across the resonance and found that the differences between the energy dependence of the x-ray absorption signal, the specular reflectivity and the step-related peak reflect the different quantities probed in these signals. When recorded at low incidence or detection angles, XAS and specular reflectivity spectra are strongly distorted by the changes of the angle of total reflection with energy. The resonant diffraction spectrum is less affected and can be used as a spectroscopic probe even in less favorable geometries.

Comparison of a CCD and an APS for soft X-ray diffraction

We compare a new CMOS Active Pixel Sensor (APS) to a Princeton Instruments PIXIS-XO: 2048B Charge Coupled Device (CCD) with soft X-rays tested in a synchrotron beam line at the Diamond Light Source (DLS). Despite CCDs being established in the field of scientific imaging, APS are an innovative technology that offers advantages over CCDs. These include faster readout, higher operational temperature, in-pixel electronics for advanced image processing and reduced manufacturing cost.The APS employed was the Vanilla sensor designed by the MI3 collaboration and funded by an RCUK Basic technology grant. This sensor has 520 x 520 square pixels, of size 25 μm on each side. The sensor can operate at a full frame readout of up to 20 Hz. The sensor had been back-thinned, to the epitaxial layer. This was the first time that a back-thinned APS had been demonstrated at a beam line at DLS.In the synchrotron experiment soft X-rays with an energy of approximately 708 eV were used to produce a diffraction pattern from a permalloy sample. The pattern was imaged at a range of integration times with both sensors. The CCD had to be operated at a temperature of -55°C whereas the Vanilla was operated over a temperature range from 20°C to -10°C. We show that the APS detector can operate with frame rates up to two hundred times faster than the CCD, without excessive degradation of image quality. The signal to noise of the APS is shown to be the same as that of the CCD at identical integration times and the response is shown to be linear, with no charge blooming effects.The experiment has allowed a direct comparison of back thinned APS and CCDs in a real soft x-ray synchrotron experiment.

Magnetic structure and electric field effects in multiferroic YMn2O5

The magnetic structure of multiferroic RMn${}_{2}$O${}_{5}$ (R $=$ Y, Er) has been investigated by means of resonant soft x-ray diffraction. Energy, temperature, and azimuthal angle scans were performed in addition to reciprocal space maps on the magnetic reflection in the different magnetic phases of YMn${}_{2}$O${}_{5}$. We also investigated the orbital magnetic moment at the oxygen K-edge for RMn${}_{2}$O${}_{5}$ with both, R $=$ Y and R $=$ Er compositions. These moments reflect the strong hybridization between Mn 3$d$ and oxygen 2$p$ states. Experiments with applied electric fields are additionally presented, showing that the helical component of the magnetic structure in the CM phase of YMn${}_{2}$O${}_{5}$ can be reversed by the application of an electric field. However, the incommensurate magnetic reflection in the high-temperature phase is unaffected. Interestingly, this is observed only in the presence of a small electrical current, indicative of a current-induced/-enhanced switching of magnetic domains.

Origin of the anomalous low-temperature phase transition in BaVS3

The origin of the anomalous electronic phase transition (${T}_{X}\ensuremath{\approx}30$ K) caused by a single-electron-occupied ${t}_{2g}$ orbital in quasi-one-dimensional BaVS${}_{3}$ is studied by means of resonant soft x-ray diffraction taken at the V $L$${}_{2,3}$ absorption edges. The energy and azimuthal angle dependence of the observed $(0\phantom{\rule{4pt}{0ex}}k\phantom{\rule{4pt}{0ex}}l)$ reflection can be explained by the presence of at least two electronically different V contributions. Moreover, analysis of the scattered x-ray polarization indicates that the observed reflection is purely magnetic in origin, which discourages models with orbital and charge modifications occurring at ${T}_{X}$. The dependence of the reflection on left or right circular polarized incoming x rays indicates that the incommensurability of the reflection is connected to a helical component of the magnetic structure. These results, together with those from neutron scattering, are discussed in terms of the different models proposed for the electronic and magnetic ground state.

A setup for probing ultra-short soft X-ray diffraction by means of curved multilayer structures

We propose an experimental setup allowing for measurement of the whole diffraction curve of a Bragg peak by single pulse exposure where a bended sample is illuminated by a set of parallel pencil beams under locally different angles of incidence. The feasibility is demonstrated probing the 1st order Bragg peak of Ru/B 4C multilayers for photon energies close to Boron K-edge. The evaluated optical parameters recorded from bent sample under fixed sample setting equals those obtained from a flat sample using angular dispersive recording. Subsequently our scheme is appropriate for solid state experiment using at high intense femtosecond pulses provided by free-electron laser sources.

Photoinduced Melting of Antiferromagnetic Order inLa0.5Sr1.5MnO4Measured Using Ultrafast Resonant Soft X-Ray Diffraction

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La(0.5)Sr(1.5)MnO(4) after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand, the (1/4,1/4,0) 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.

Time-resolved resonant soft x-ray diffraction with free-electron lasers: Femtosecond dynamics across the Verwey transition in magnetite

Resonant soft x-ray diffraction (RSXD) with femtosecond (fs) time resolution is a powerful tool for disentangling the interplay between different degrees of freedom in strongly correlated electron materials. It allows addressing the coupling of particular degrees of freedom upon an external selective perturbation, e.g., by an optical or infrared laser pulse. Here, we report a time-resolved RSXD experiment from the prototypical correlated electron material magnetite using soft x-ray pulses from the free-electron laser FLASH in Hamburg. We observe ultrafast melting of the charge-orbital order leading to the formation of a transient phase, which has not been observed in equilibrium.

Analytic theory of soft x-ray diffraction by lamellar multilayer gratings

An analytic theory describing soft x-ray diffraction by Lamellar Multilayer Gratings (LMG) has been developed. The theory is derived from a coupled waves approach for LMGs operating in the single-order regime, where an incident plane wave can only excite a single diffraction order. The results from calculations based on these very simple analytic expressions are demonstrated to be in excellent agreement with those obtained using the rigorous coupled-waves approach. The conditions for maximum reflectivity and diffraction efficiency are deduced and discussed. A brief investigation into p-polarized radiation diffraction is also performed.

Anti-contamination device for cryogenic soft X-ray diffraction microscopy

Cryogenic microscopy allows one to view frozen hydrated biological and soft matter specimens with good structural preservation and a high degree of stability against radiation damage. We describe a liquid nitrogen-cooled anti-contamination device for cryogenic X-ray diffraction microscopy. The anti-contaminator greatly reduces the buildup of ice layers on the specimen due to condensation of residual water vapor in the experimental vacuum chamber. We show by coherent X-ray diffraction measurements that this leads to fivefold reduction of background scattering, which is important for far-field X-ray diffraction microscopy of biological specimens.

Intrinsic and extrinsic x-ray absorption effects in soft x-ray diffraction from the superstructure in magnetite

We studied the ($00\frac{1}{2}$) diffraction peak in the low-temperature phase of magnetite (${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$) using resonant soft x-ray diffraction at the $\mathrm{Fe}\ensuremath{-}{L}_{2,3}$ and $\mathrm{O}\ensuremath{-}K$ resonances. We studied both molecular-beam-epitaxy grown thin films and in situ cleaved single crystals. From the comparison, we have been able to determine quantitatively the contribution of intrinsic absorption effects, thereby arriving at a consistent result for the ($00\frac{1}{2}$) diffraction peak spectrum. Our data also allow for the identification of extrinsic effects, e.g., for a detailed modeling of the spectra in case a dead surface layer is present that is only absorbing photons but does not contribute to the scattering signal.

Depth-resolved magnetic structure across the ferromagnetic to helical-antiferromagnetic phase transition in Dy/W(110)

We investigated the magnetic depth profile during the transition from the ferromagnetic (FM) to the helical-antiferromagnetic (AFM) phase in Dy-metal films grown on W(110). To this purpose, we used the strongly varying photon penetration depth across the $\text{Dy}\text{ }{M}_{5}$ absorption threshold in resonant magnetic soft x-ray diffraction. Unlike in earlier studies on Dy films with two equivalent interfaces, we find a gradual growth of the AFM part of the film with temperature. The near-surface magnetic structure does not become FM and appears to serve as seed layer for the growth of the AFM order.

Cycloidal Order of4fMoments as a Probe of Chiral Domains inDyMnO3

Using soft x-ray diffraction at the Dy-M₅ resonance, pronounced circular dichroism in the ferroelectric phase of DyMnO₃ is observed in connection with sizable b and c components of the Dy-4f magnetic moments. This provides strong evidence for cycloidal order of the 4f moments, corroborating that inversion-symmetry breaking in this material is not accomplished by the Mn spins alone. The 4f circular dichroism allows us to image multiferroic domains that are imprinted on the surface of DyMnO₃ using the local charging by the x-ray beam via the photoelectric effect.

Ultrafast dynamics of antiferromagnetic order studied by femtosecond resonant soft x-ray diffraction

Femtosecond (fs) soft x-ray diffraction at the Eu-M5 resonance was employed to study the dynamics of antiferromagnetic (AFM) order in a thin film of the magnetic semiconductor EuTe after fs laser excitation. The AFM Bragg peak intensity displays an ultrafast decay with an upper limit for the time constant of (700±200) fs followed by fast recovery with (47±10) ps. Reciprocal space scans across the diffraction peak could be recorded with picosecond and fs time resolution, demonstrating that even at synchrotron slicing sources ultrafast studies of spatial magnetic correlations are feasible. The results show that fs dynamics of materials with complex order parameters can be addressed.

RASOR: An advanced instrument for soft x-ray reflectivity and diffraction

We report the design and construction of a novel soft x-ray diffractometer installed at Diamond Light Source. The beamline endstation RASOR is constructed for general users and designed primarily for the study of single crystal diffraction and thin film reflectivity. The instrument is comprised of a limited three circle (theta, 2theta, and chi) diffractometer with an additional removable rotation (phi) stage. It is equipped with a liquid helium cryostat, and post-scatter polarization analysis. Motorized motions are provided for the precise positioning of the sample onto the diffractometer center of rotation, and for positioning the center of rotation onto the x-ray beam. The functions of the instrument have been tested at Diamond Light Source, and initial test measurements are provided, demonstrating the potential of the instrument.

Circularly polarized soft x-ray diffraction study of helical magnetism in hexaferrite

Magnetic spiral structures can exhibit ferroelectric moments as recently demonstrated in various multiferroic materials. In such cases the helicity of the magnetic spiral is directly correlated with the direction of the ferroelectric moment and measurement of the helicity of magnetic structures is of current interest. Soft x-ray resonant diffraction is particularly advantageous because it combines element selectivity with a large magnetic cross-section. We calculate the polarization dependence of the resonant magnetic x-ray cross-section (electric dipole transition) for the basal plane magnetic spiral in hexaferrite ${\text{Ba}}_{0.8}{\text{Sr}}_{1.2}{\text{Zn}}_{2}{\text{Fe}}_{12}{\text{O}}_{22}$ and deduce its domain population using circular polarized incident radiation. We demonstrate there is a direct correlation between the diffracted radiation and the helicity of the magnetic spiral.

Soft x-ray diffraction from lattice constrained orbital order in Pr(Sr0.1Ca0.9)2Mn2O7

Controlling orbital occupancy is a fundamental prerequisite for orbitronics. It has been shown in the orthorhombic bilayer manganite Pr(Sr0.1Ca0.9)2Mn2O7 that the direction of orbital order stripes can be influenced by controlling temperature or through inducing strain in the material. In this paper we have used resonant soft x-ray diffraction at the Mn L-edge to confirm the rotation of the orbital direction TOO2 and furthermore prove that there is no change in the occupied orbital type, however the orbital rotation causes a switch from 3x2−r2 to 3y2−r2 on a single site. We find that unlike the tetragonal bilayer manganites, where an onset of A-type AFM quenches the orbital order, no such effect is found on the orbital order below TN.

Advanced resonant soft x-ray diffraction to study ordering phenomena in magnetic materials

Here we present recent advances in the resonant soft x-ray diffraction technique and present a review of recent experiments in the field. Our review is focused on the study of charge and orbital phenomena occurring in manganites systems. It is shown how resonant diffraction can access directly the 3d shell of Mn by tuning the x-ray energy to the Mn L2,3edges. Similarly, by tuning the energy to the oxygen K edge, empty oxygen 2p states can be probed. It is shown how the technique probes corresponding atomic multipoles, the monopole (charge), the magnetic dipole, and the orbital (electronic quadrupole) of the open valence shell.

Measuring the Structure of Epitaxially Assembled Block Copolymer Domains with Soft X-ray Diffraction

The size, shape, and interface structure of poly(styrene-b-methyl methacrylate) block copolymer domains assembled on an epitaxial template are characterized with soft X-ray diffraction. The domain size and shape are deformed when the dimensions of the epitaxial template are incommensurate with the equilibrium dimensions of the block copolymer, producing sidewall angles in the range of 1−2° (±0.2°). The average width of the copolymer interface is (4.9 ± 0.1) nm. Comparison with mean-field theoretic predictions for the structure of block copolymer interfaces suggests a low-frequency variance in the copolymer interface position of 1.2 nm2, or a low-frequency line-edge roughness of approximately 3 nm.

Soft X-ray diffraction microscopy of a frozen hydrated yeast cell.

We report the first image of an intact, frozen hydrated eukaryotic cell using x-ray diffraction microscopy, or coherent x-ray diffraction imaging. By plunge freezing the specimen in liquid ethane and maintaining it below -170 degrees C, artifacts due to dehydration, ice crystallization, and radiation damage are greatly reduced. In this example, coherent diffraction data using 520 eV x rays were recorded and reconstructed to reveal a budding yeast cell at a resolution better than 25 nm. This demonstration represents an important step towards high resolution imaging of cells in their natural, hydrated state, without limitations imposed by x-ray optics.