Kathryn L. Krycka

Magnetic-crystallographic phase diagram of the superconducting parent compound Fe1+xTe

Through neutron diffraction experiments, including spin-polarized measurements, we find a collinear incommensurate spin-density wave with propagation vector k= [0.4481(4)012] at base temperature in the superconducting parent compound Fe1+xTe. This critical concentration of interstitial iron corresponds to x?12% and leads to crystallographic phase separation at base temperature. The spin-density wave is short-range ordered with a correlation length of 22(3) A, and as the ordering temperature is approached its propagation vector decreases linearly in the H direction and becomes long-range ordered. Upon further populating the interstitial iron site, the spin-density wave gives way to an incommensurate helical ordering with propagation vector k= [0.3855(2)012] at base temperature. For a sample with x?9(1)%, we also find an incommensurate spin-density wave that competes with the bicollinear commensurate ordering close to the Neel point. The shifting of spectral weight between competing magnetic orderings observed in several samples is supporting evidence for the phase separation being electronic in nature, and hence leads to crystallographic phase separation around the critical interstitial iron concentration of 12%. With results from both powder and single crystal samples, we construct a magnetic-crystallographic phase diagram of Fe1+xTe for 5%

Multicore Magnetic Nanoparticles for Magnetic Particle Imaging

Biocompatible magnetic nanoparticles are interesting tracers for diagnostic imaging techniques, including magnetic resonance imaging and magnetic particle imaging (MPI). Here, we will present our studies of the physical and especially magnetic properties of dextran coated multicore magnetic iron oxide nanoparticles, with promising high MPI signals revealed by magnetic particle spectroscopy (MPS) measurements. The Nanomag-MIP particles with a hydrodynamic diameter of 106 nm show an increase of the MPS amplitude by a factor of about two at the 3rd harmonic, as compared to Resovist. In particular, the signal improves progressively with the order of the harmonic, a prerequisite for better spatial resolution. To understand this behavior, we investigated the samples using quasistatic magnetization measurements yielding bimodal size distributions for both systems, and magnetorelaxometry providing the mean effective anisotropy constant. The mean effective magnetic diameter of the dominant larger size mode is 19 nm with a dispersion parameter of σ = 0.3 for Nanomag-MIP, and 22 nm with σ = 0.25 for Resovist. However, about 80% of the magnetic nanoparticles of Nanomag-MIP belong to this larger size mode whereas in Resovist only 30% do. The remaining Resovist particles are in the range of 5 nm, and, in practice, do not contribute to the MPI signal.

Noncollinear spin-density-wave antiferromagnetism in FeAs

The nature of the magnetism in the simplest iron arsenide is of fundamental importance in understanding the interplay between localized and itinerant magnetism and superconductivity. We present the magnetic structure of the itinerant monoarsenide FeAs with the B31 structure. Powder neutron diffraction confirms incommensurate modulated magnetism with wave vector q=(0.395±0.001)c* at 4 K, but can not distinguish between a simple spiral and a collinear spin-density-wave structure. Polarized single-crystal diffraction confirms that the structure is best described as a noncollinear spin-density wave arising from a combination of itinerant and localized behavior with spin amplitude along the b-axis direction being (15±5)% larger than in the a direction. Furthermore, the propagation vector is temperature dependent, and the magnetization near the critical point indicates a two-dimensional Heisenberg system. The magnetic structures of closely related systems are discussed and compared to that of FeAs. © 2011 American Physical Society.

Resolving Material-Specific Structures within Fe3O4|γ-Mn2O3 Core|Shell Nanoparticles Using Anomalous Small-Angle X-ray Scattering

Here it is demonstrated that multiple-energy, anomalous small-angle X-ray scattering (ASAXS) provides significant enhancement in sensitivity to internal material boundaries of layered nanoparticles compared with the traditional modeling of a single scattering energy, even for cases in which high scattering contrast naturally exists. Specifically, the material-specific structure of monodispersed Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles is determined, and the contribution of each component to the total scattering profile is identified with unprecedented clarity. We show that Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles with a diameter of 8.2 ± 0.2 nm consist of a core with a composition near Fe₃O₄ surrounded by a (Mn(x)Fe(1-x))₃O₄ shell with a graded composition, ranging from x ≈ 0.40 at the inner shell toward x ≈ 0.46 at the surface. Evaluation of the scattering contribution arising from the interference between material-specific layers additionally reveals the presence of Fe₃O₄ cores without a coating shell. Finally, it is found that the material-specific scattering profile shapes and chemical compositions extracted by this method are independent of the original input chemical compositions used in the analysis, revealing multiple-energy ASAXS as a powerful tool for determining internal nanostructured morphology even if the exact composition of the individual layers is not known a priori.

Polarization-analyzed small-angle neutron scattering. II. Mathematical angular analysis

Polarization-analyzed small-angle neutron scattering (SANS) is a powerful tool for the study of magnetic morphology with directional sensitivity. Building upon polarized scattering theory, this article presents simplified procedures for the reduction of longitudinally polarized SANS into terms of the three mutually orthogonal magnetic scattering contributions plus a structural contribution. Special emphasis is given to the treatment of anisotropic systems. The meaning and significance of scattering interferences between nuclear and magnetic scattering and between the scattering from magnetic moments projected onto distinct orthogonal axes are discussed in detail. Concise tables summarize the algorithms derived for the most commonly encountered conditions. These tables are designed to be used as a reference in the challenging task of extracting the full wealth of information available from polarization-analyzed SANS.

Polarization-analyzed small-angle neutron scattering. I. Polarized data reduction using Pol-Corr

Pol-Corr is a free computer program that corrects for the neutron polarization inefficiencies that are characteristic of polarization-analyzed small-angle neutron scattering experiments, namely those inefficiencies associated with a static neutron polarizer, a neutron spin flipper, beam depolarization and a time-varying neutron spin analyzer. The software is designed to interface directly with small-angle neutron scattering data acquired at the NIST Center for Neutron Research, but the algorithms are generally applicable and can be readily adapted for other data formats. The explicit neutron measurements required to characterize each polarizing element are derived, and these become the input parameters for Pol-Corr.

Internal magnetic structure of dextran coated magnetite nanoparticles in solution using small angle neutron scattering with polarization analysis

For many applications, the internal magnetic domain structure of magnetic nanoparticles may play a critical role in the determination of their collective magnetic properties. Here we utilize polarization analyzed small angle neutron scattering (PASANS) to study the individual magnetic morphologies of an interacting aqueous Fe3O4 nanoparticle system. Our results demonstrate that the total magnetic moment of the colloid is randomized, as expected in low fields, while the nuclear structure is anisotropic. Model fits indicate that the magnetic domains within the nanoparticle core at 1.5 mT have dimensions that approximate those of the structural grains perpendicular to the field, but the domains extend over multiple grains along the field direction. The asymmetry in the magnetic domain formation in weak fields undoubtedly contributes to the magnetic anisotropy and thus to the enhanced heating reported for hyperthermia applications of these systems.

Correlating material-specific layers and magnetic distributions within onion-like Fe3O4/MnO/γ-Mn2O3 core/shell nanoparticles

The magnetic responses of two nanoparticle systems comprised of Fe3O4/γ-Mn2O3 (soft ferrimagnetic, FM/hard FM) and Fe3O4/MnO/γ-Mn2O3 (soft FM/antiferromagnetic, AFM/hard FM) are compared, where the MnO serves to physically decouple the FM layers. Variation in the temperature and applied field allows for Small Angle Neutron Scattering (SANS) measurements of the magnetic moments both parallel and perpendicular to an applied field. Data for the bilayer particle indicate that the graded ferrimagnetic layers are coupled and respond to the field as a single unit. For the trilayer nanoparticles, magnetometry suggests a Curie temperature (TC) ≈ 40 K for the outer γ-Mn2O3 component, yet SANS reveals an increase in the magnetization associated with outer layer that is perpendicular to the applied field above TC during magnetic reversal. This result suggests that the γ-Mn2O3 magnetically reorients relative to the applied field as the temperature is increased above 40 K.

3He spin filter based polarized neutron capability at the NIST Center for Neutron Research

A 3He neutron spin filter (NSF) program for polarized neutron scattering was launched in 2006 as part of the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) Expansion Initiative. The goal of the project was to enhance the NCNR polarized neutron measurement capabilities. Benefitting from more than a decades development of spin-exchange optical pumping (SEOP) at NIST, we planned to employ SEOP based 3He neutron spin filters for the polarized neutron scattering community. These 3He NSF devices were planned for use on different classes of polarized neutron instrumentation at the NCNR, including triple-axis spectrometers (TAS), small-angle neutron scattering instruments (SANS), reflectometers, and wide-angle polarization analysis. Among them, the BT-7 thermal TAS, NG-3 SANS, and MAGIK reflectometer have already been in the user program for routine polarized beam experiments. Wide-angle polarization analysis on Multi-Axis Crystal Spectrometer (MACS) has been developed for user experiments. We describe briefly the SEOP systems dedicated for polarized beam experiments and polarizing neutron development for each instrument class. We summarize the current status and polarized neutronic performance for each instrument. We present a 3He NSF hardware and software interface to allow for synchronization of 3He polarization inversion (neutron spin flipping) and free-induction decay (FID) nuclear magnetic resonance (NMR) measurements with neutron data collection.

Magnetic properties of GaAs/Fe core/shell nanowires

We describe the magnetic properties of nanoscale Fe shells grown on GaAs nanowires (NWs) by molecular beam epitaxy. The ferromagnetic character of these tubular Fe shells has been confirmed by dc magnetization measurements, and is further studied by ferromagnetic resonance (FMR), small-angle neutron scattering (SANS), and off-axis electron holography (EH).