Paul A. Kienzle

Vertically graded anisotropy in Co/Pd multilayers

Author(s): Kirby, Brian J.; Davies, J. E.; Liu, Kai; Watson, S. M.; Zimanyi, G. T.; Shull, R. D.; Kienzle, P. A.; Borchers, J. A. | Abstract: Depth grading of magnetic anisotropy in perpendicular magnetic media has been predicted to reduce thefield required to write data without sacrificing thermal stability. To study this prediction, we have producedCo/Pd multilayers with depth-dependent Co layer thickness. Polarized neutron reflectometry shows that thethickness grading results in a corresponding magnetic anisotropy gradient. Magnetometry reveals that theanisotropy gradient promotes domain nucleation upon magnetization reversal - a clear experimental demonstrationof the effectiveness of graded anisotropy for reducing write field.

Neutron reflectometry studies of the adsorbed structure of the amelogenin, LRAP.

Amelogenins make up over 90% of the protein present during enamel formation and have been demonstrated to be critical in proper enamel development, but the mechanism governing this control is not well understood. Leucine-rich amelogenin peptide (LRAP) is a 59-residue splice variant of amelogenin and contains the charged regions from the full protein thought to control crystal regulation. In this work, we utilized neutron reflectivity (NR) to investigate the structure and orientation of LRAP adsorbed from solutions onto molecularly smooth COOH-terminated self-assembled monolayer (SAM) surfaces. Sedimentation velocity (SV) experiments revealed that LRAP is primarily a monomer in saturated calcium phosphate (SCP) solutions (0.15 M NaCl) at pH 7.4. LRAP adsorbed as ∼32 Å thick layers at ∼70% coverage as determined by NR. Rosetta simulations of the dimensions of LRAP in solution (37 Å diameter) indicate that the NR determined z dimension is consistent with an LRAP monomer. SV experiments and Rosetta simulations show that the LRAP monomer has an extended, asymmetric shape in solution. The NR data suggests that the protein is not completely extended on the surface, having some degree of structure away from the surface. A protein orientation with the C-terminal and inner N-terminal regions (residues ∼8-24) located near the surface is consistent with the higher scattering length density (SLD) found near the surface by NR. This work presents new information on the tertiary and quaternary structure of LRAP in solution and adsorbed onto surfaces. It also presents further evidence that the monomeric species may be an important functional form of amelogenin proteins.

Porous Mg formation upon dehydrogenation of MgH2 thin films

The hydrogenation and dehydrogenation of a thin film of Mg with a Pd cap layer was measured using neutron reflectometry. Upon hydrogenation, (at 373 K and 0.2 MPa H2), the Mg film swelled in the surface normal direction by an amount roughly equal to the difference in volume between MgH2 and Mg. After dehydrogenation (at 343–423 K), the Mg film returned to a composition of Mg but retained the swelled thickness by incorporating voids. The presence of the voids is confirmed by SEM micrographs. The voids may explain some of the changes in absorption kinetics after full cycling of Mg films.

Measurement and modeling of polarized specular neutron reflectivity in large magnetic fields

A procedure is described for polarized neutron reflectometry when the Zeeman corrections are significant, which occurs when both the magnetic anisotropy and the applied magnetic field are significant. Calculations and a recommended procedure for an example system are provided.

Bayesian Method for the Analysis of Diffraction Patterns using BLAND

Rietveld refinement of X-ray and neutron diffraction patterns is routinely used to solve crystal and magnetic structures of organic and inorganic materials over many length scales. Despite its success over the past few decades, conventional Rietveld analysis suffers from tedious iterative methodologies, and the unfortunate consequence of many least-squares algorithms discovering local minima that are not the most accurate solutions. Bayesian methods which allow the explicit encoding of a priori knowledge pose an attractive alternative to this approach by enhancing the ability to determine the correlations between parameters and to provide a more robust method for model selection. Global approaches also avoid the divergences and local minima often encountered by practitioners of the traditional Rietveld technique. The goal of this work is to demonstrate the effectiveness of an automated Bayesian algorithm for Rietveld refinement of neutron diffraction patterns in the solution of crystallographic and magnetic structures. A new software package, BLAND (Bayesian library for analyzing neutron diffraction data), based on the Markov–Chain Monte Carlo minimization routine, is presented. The benefits of such an approach are demonstrated through several examples and compared with traditional refinement techniques.

Magnetic form factor of SrFe 2 As 2 : Neutron diffraction measurements

Neutron diffraction measurements have been carried out to investigate the magnetic form factor of the parent SrFe2As2 system of the iron-based superconductors. The general feature is that the form factor is approximately isotropic in wave vector, indicating that multiple d orbitals of the iron atoms are occupied as expected based on band theory. Inversion of the diffraction data suggests that there is some elongation of the spin density toward the As atoms. We have also extended the diffraction measurements to investigate a possible jump in the c-axis lattice parameter at the structural phase transition but find no detectable change within the experimental uncertainties.

Thickness of the pinned layer as a controlling factor in domain wall formation during training in IrMn-based spin valves

Studies of CoFe-based spin valves with antiferromagnetic IrMn layers as thin as 1.6nm have demonstrated that a domain wall parallel to the surface develops in the pinned layer after training at the magnetoresistance (MR) maximum. To investigate the effects of domain wall formation on the MR, we have studied the depth profile of the vector magnetization in comparable spin valves, with pinned ferromagnetic (FM) layer thicknesses, from 1to15nm, using polarized neutron reflectivity. At the maximum MR achieved after training, the antiparallel magnetization of the pinned layer, in a 2nm sample, is reduced to 5% of its saturation value, suggesting the formation of domain walls perpendicular to the surface. In a 9nm sample, the pinned layer magnetization is instead canted away from the field at the MR maximum. A transition from perpendicular to parallel domain wall formation occurs for pinned layer thicknesses greater than 4nm, and the magnitude of the maximum MR subsequently depends on the type of domain wall th...

Nanolayer Analysis by Neutron Reflectometry

Neutron reflectometry offers unique benefits for nanolayer research in green chemistry by providing accurate depth profiles of thin films and interfaces with subnanometer precision, under in operando condition, of active material in their native environments. Advantages of this technique include sensitivity to light elements, isotopic contrast control (which enables elemental depth profiles or labeling of specific features or reactants of interest), and the ability to penetrate complex and multimodal sample environments. This chapter focuses on the most stringent example in green chemistry, in operando electrochemistry. Because neutron reflectometry is not widely utilized, this chapter provides an introduction, literature review, theoretical basis, practical guides to data collection and analysis, examples, best practices, and future advances, with the goal of making this technique more comprehensible by scientists reviewing or applying neutron reflectometry research results, and making it more accessible to a wider range of experimenters.

Magnetic form factor of SrFe

Neutron diffraction measurements have been carried out to investigate the magnetic form factor of the parent SrFe2As2 system of the iron-based superconductors. The general feature is that the form factor is approximately isotropic in wave vector, indicating that multiple d orbitals of the iron atoms are occupied as expected based on band theory. Inversion of the diffraction data suggests that there is some elongation of the spin density toward the As atoms. We have also extended the diffraction measurements to investigate a possible jump in the c-axis lattice parameter at the structural phase transition but find no detectable change within the experimental uncertainties.

_2

Neutron diffraction measurements have been carried out to investigate the magnetic form factor of the parent SrFe2As2 system of the iron-based superconductors. The general feature is that the form factor is approximately isotropic in wave vector, indicating that multiple d orbitals of the iron atoms are occupied as expected based on band theory. Inversion of the diffraction data suggests that there is some elongation of the spin density toward the As atoms. We have also extended the diffraction measurements to investigate a possible jump in the c-axis lattice parameter at the structural phase transition but find no detectable change within the experimental uncertainties.