Shannon Watson

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.

Direct Observation of Magnetic Gradient in Co/Pd Pressure-Graded Media

Magnetometry and neutron scattering have been used to study the magnetic properties of pressure-graded Co/Pd multilayers. The grading of the multilayer structure was done by varying the deposition pressure during sputtering of the samples. Magnetic depth profiling by polarized neutron reflectometry directly shows that for pressure-graded samples, the magnetization changes significantly from one pressure region to the next, while control samples sputtered at uniform pressure exhibit essentially uniform magnetic depth profiles. Complementary magnetometry results suggest that the observed graded magnetic profiles are due in part to a decrease in saturation magnetization for regions deposited at progressively higher pressure. Increased deposition pressure is shown to increase coercivity, and for graded samples, the absence of discrete steps in the hysteresis loops implies exchange coupling among regions deposited at different pressures.

Polarized 3He cell development and application at NIST

Over a period of well over a decade, a large number and variety of polarized 3He cells for neutron applications have been developed and tested at the U.S. National Institute of Standards and Technology (NIST). These cells have primarily been employed for spin-exchange optical pumping (SEOP), and applied to neutron scattering and fundamental neutron physics. We describe the procedures we have employed for producing these cells, as well as their characteristics and applications. Whereas our best results are for cells blown from boron-free aluminosilicate glass, we summarize results with a range of other methods. We discuss our recent work on SEOP cells for wide angle neutron polarization analysis.

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.

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.

Internal magnetic structure of magnetite nanoparticles at low temperature

Small-angle neutron scattering with polarization analysis reveals that Fe3O4 nanoparticles with 90 A diameters have ferrimagnetic moments significantly reduced from that of bulk Fe3O4 at 10 K, nominal saturation. Combined with previous results for an equivalent applied field at 200 K, a core-disordered shell picture of a spatially reduced ferrimagnetic core emerges, even well below the bulk blocking temperature. Zero-field cooling suggests that this magnetic morphology may be intrinsic to the nanoparticle, rather than field induced, at 10 K.

Relationship between tunnel magnetoresistance and magnetic layer structure in EuO-based tunnel junctions investigated using polarized neutron reflectivity

This paper presents a study of the depth-dependent magnetic structure of a EuO magnetic tunnel junction having a Gd electrode, Si∕Cu∕EuO∕Gd∕Al. Related samples that are patterned exhibit large tunneling magnetoresistance as high as 280%. Though Gd has a much higher coercivity than EuO in bulk, magnetometry reveals no “steps” in the hysteresis loop as expected for a true antiparallel alignment of the EuO and Gd layer magnetizations. Using polarized neutron reflectometry to measure the structural and field-dependent magnetic depth profile at 5K, we determine that the Gd and EuO layers have similar coercivities and that the Gd layer exhibits an anomalously small magnetization at all fields. Polarized neutron reflectometry results also suggest that the chemical density of the Gd layer is not that of bulk Gd. The differences of the structural and magnetic behavior of the Gd layer relative to bulk may be the key in optimizing the tunnel magnetoresistance in these samples.

Recent advancements of wide-angle polarization analysis with 3He neutron spin filters

Wide-angle polarization analysis with polarized 3He based neutron spin filters (NSFs) has recently been employed on the Multi-Axis Crystal Spectrometer (MACS) at the National Institute of Standards and Technology Center for Neutron Research (NCNR). Over the past several years, the apparatus has undergone many upgrades to address the fundamental requirements for wide angle polarization analysis using spin exchange optical pumping based 3He NSFs. In this paper, we report substantial improvements in the on-beam-line performance of the apparatus and progress toward routine user capability. We discuss new standard samples used for 3He NSF characterization and the flipping ratio measurement on MACS. We further discuss the management of stray magnetic fields produced by operation of superconducting magnets on the MACS instrument, which can significantly reduce the 3He polarization relaxation time. Finally, we present the results of recent development of horseshoe-shaped wide angle cells.

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...

Direct observation of the Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point

Spontaneous symmetry-breaking quantum phase transitions play an essential role in condensed matter physics. The collective excitations in the broken-symmetry phase near the quantum critical point can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless Nambu