S. A. Morton

Observation of current-driven oscillatory domain wall motion in Ni80Fe20/Co bilayer nanowire

Direct observation of current-driven oscillatory domain wall motion above the Walker breakdown by x-ray magnetic circular dichroism in photoemission electron microscopy is reported in Ni80Fe20/Co nanowire, showing micrometer-scale displacement at ∼13u2009MHz. We identify two key factors that enhance the oscillatory motion: (i) increase of the hard-axis magnetic anisotropy field value |H⊥| and (ii) increase of the ratio between non-adiabatic spin-transfer parameter to the Gilbert damping coefficient, β/α, which is required to be larger than 1. These findings point to an important route to tune the long-scale oscillatory domain wall motion using appropriate geometry and materials.

Magnetocrystalline Anisotropy of Magnetic Grains in Co80Pt20:Oxide Thin Films Probed by X-ray Magnetic Circular Dichroism

Magnetocrystalline Anisotropy of Magnetic Grains in Co 80 Pt 20 :Oxide Thin Films Probed by X-ray Magnetic Circular Dichroism W. Zhang 1 , S. A. Morton 2 , P. K. J. Wong 1, * , X. F. Hu 1 , E. Arenholz 2 , B. Lu 3 , T. Y. Cheng 1 , Y. B. Xu 1, † , and G. van der Laan 4 Spintronics and Nanodevice Laboratory, Department of Electronics, University of York, York YO10 5DD, UK Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Seagate Technology, 47010 Kato Road, Fremont, CA 94538, US Diamond Light Source, Chilton, Didcot OX11 0DE, UK Abstract Using angle-dependent X-ray magnetic circular dichroism we have measured magnetic hysteresis loops at the CoL 2,3 edges of oxide-doped Co 80 Pt 20 thin films. The magnetocrystalline anisotropy energy (MAE) of the Co atoms, which is the main source of the magnetocrystalline anisotropy of the CoPt magnetic grains, has beendetermined directly from these element-specific hysteresis loops. When the oxide volume fraction (OVF) is increased from 16.6% to 20.7%, the Co MAE has been found to decrease from 0.117 meV/atom to 0.076 meV/atom.While a larger OVF helps to achieve a smaller grain size, it reduces the magnetocrystalline anisotropyas demonstrated unambiguously from the direct Co MAE measurements.Our results suggest that thoseCo 80 Pt 20 :oxide films with OVF between 19.1% and20.7%aresuitable candidates for high-density magnetic recording. Present address: MESA+ Institute of Nanotechnology, University of Twente, P. O. Box 217, 7500 AE Enschede, The Netherlands. Corresponding author: yx2@ohm.york.ac.uk

The developmental long trace profiler (DLTP) optimized for metrology of side-facing optics at the ALS

The autocollimator and moveable pentaprism based DLTP [NIM A 616 (2010) 212-223], a low-budget, NOM-like profiler at the Advanced Light Source (ALS), has been upgraded to provide fast, highly accurate surface slope metrology for long, side-facing, x-ray optics. This instrument arrangement decreases sensitivity to environmental conditions and removes the gravity effect on mirror shape. We provide design details of an affordable base tool, including clean-room environmental arrangements in the new ALS X-ray Optics Laboratory with advanced temperature stabilization and turbulence reduction, that yield measurements in under 8 hours with accuracy better than 30 nanoradians (rms) for super polished,190 mm flat optics, limited mainly by residual temporal instability of the experimental set-up. The upgraded DLTP has been calibrated for highly curved x-ray optics, allowing same day measurements of a 15 m ROC sphere with accuracy of better than 100 nanoradians (rms). The developed calibration procedure is discussed in detail. We propose this specific 15 m ROC sphere for use as a round-robin calibration test optic.

Magnetic circular dichroism in Co1- xPtx nanowire bundles at the Co L2,3 edges

Co1−xPtx nanowire arrays embedded in anodic aluminum oxide templates were fabricated by an electrochemical route. X-ray magnetic circular dichroism (XMCD) measurements were conducted at the Co L2,3 edges on samples with an array of nanowire bundles at the substrate surfaces. The ratios between the orbital and the spin moments R were estimated to be about 0.19 (±0.03) and 0.07 (±0.03) for 20nm Co and 14nm Co91Pt9 nanowires, respectively. A strong temperature dependence of the R value was observed. The x-ray absorption spectra also indicated the presence of CoO, which did not contribute to the XMCD signals.

Discontinuous properties of current-induced magnetic domain wall depinning

The current-induced motion of magnetic domain walls (DWs) confined to nanostructures is of great interest for fundamental studies as well as for technological applications in spintronic devices. Here, we present magnetic images showing the depinning properties of pulse-current-driven domain walls in well-shaped Permalloy nanowires obtained using photoemission electron microscopy combined with x-ray magnetic circular dichroism. In the vicinity of the threshold current density (Jth = 4.2 × 1011u2005A.m−2) for the DW motion, discontinuous DW depinning and motion have been observed as a sequence of “Barkhausen jumps”. A one-dimensional analytical model with a piecewise parabolic pinning potential has been introduced to reproduce the DW hopping between two nearest neighbour sites, which reveals the dynamical nature of the current-driven DW motion in the depinning regime.

Focused ion beam patterned Fe thin films: A study by selective area Stokes polarimetry and soft x-ray microscopy

A combination of analytical techniques, with special emphasis on selective area Stokes polarimetry, has been used to explore the structural properties and magnetic behavior of focused ion beam patterned Fe thin films under controlled Ga+ ion irradiation. Ion irradiation at doses ranging from 7.7u2009×u20091015 to 5.2u2009×u20091016 Ga ions cm−2 did not noticeably alter the chemical properties of the Fe, but changes to the film structure and increased coercivity were observed even after the lowest doses. Magnetic transmission x-ray microscopy provided detailed information about the magnetization reversal process occurring within a patterned area of film comprising both Fe and Fe-Ga regions, and clearly showed domain wall pinning around the magnetically harder Fe-Ga.

Recent Major Improvements to the ALS Sector 5 MacromolecularCrystallography Beamlines

Although the Advanced Light Source (ALS) was initially conceived primarily as a low-energy (1.9-GeV) third-generation source of vacuum ultraviolet (VUV) and soft X-ray radiation, it was realized very early in the development of the facility that a multipole wiggler source coupled with high-quality (brightness-preserving) optics would result in a beamline whose performance across the optimal energy range (5–15 keV) for macromolecular crystallography (MX) would be comparable to, or even exceed, that of many existing crystallography beamlines at higher-energy facilities. Hence, starting in 1996, a suite of three beamlines, branching off a single wiggler source, was constructed, which together formed the ALS Macromolecular Crystallography Facility [1]. From the outset, this facility was designed to cater equally to the needs of both academic and industrial users, with a heavy emphasis placed on the development and introduction of high-throughput crystallographic tools, techniques, and facilities, including large-area CCD detectors, robotic sample-handling and automounting facilities [2], a service crystallography program, and a tightly integrated, centralized, and highly automated beamline control environment for users.

Microscopic origin of the reduced magnetocrystalline anisotropy with increasing oxide contenct in Co80Pt20:oxide thin films

Angle-dependent x-ray magnetic circular dichroism at the Co L2.3 edges has been utilized to systematically study Co80Pt20u2009:u2009WO3 perpendicular magnetic recording thin films, in which the magnetocrystalline anisotropy significantly drops as the oxide volume fraction increases. The microscopic origin of this phenomenon in the studied films can be mainly attributed to an increase in orbital moment normal to the grain–oxide interface, with increasing oxide volume fraction, which arises from a more pronounced effect of symmetry breaking at the grain–oxide interface in smaller grains.

The Berkeley Center for Structural Biology at the Advanced Light Source

The Berkeley Center for Structural Biology (BCSB) operates and develops a suite of protein crystallography beamlines at the Advanced Light Source (ALS) located at Lawrence Berkeley National Laboratory (LBNL). Although the ALS was conceived as a low-energy (1.9-GeV), third-generation synchrotron source of vacuum ultraviolet (VUV) and soft X-ray radiation, it was realized during the development of the facility in the mid-1990s that a multipole wiggler coupled with brightness-preserving optics would result in a beamline whose performance in the energy range of 5 to 15 keV would be sufficient for most protein crystallographic experiments. Later, the hard X-ray capabilities of the ALS were expanded by the addition of three superconducting bending magnets, resulting in additional protein crystallography facilities at the ALS [1].

Direct magnetic imaging of domain wall manipulation in necked Permalloy wire using XPEEM

This paper presents the study using X-ray photoemission electron microscopy (XPEEM) of a simple and well defined single-layer necked Permalloy nanowire, which provides an artificial pinning site for domain wall. The XPEEM images provide direct evidence of the formation of the head-to-head or tail-to-tail domain wall by controlling the magnetization process.