C.-C. Kao

Precessional dynamics of elemental moments in a ferromagnetic alloy

We demonstrate an element-specific measurement of magnetization precession in a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19. Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short magnetic field pulses, is used to measure free magnetization oscillations up to 2.6 GHz with elemental specificity and a rotational resolution of < 2 deg. Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin film are found to precess together at all frequencies, coupled in phase within instrumental resolution of 90 ps.

Exploring magnetic roughness in CoFe thin films

The behavior of chemical and magnetic interfaces is explored using diffuse x-ray resonant magnetic scattering (XRMS) for CoFe thin films with varying interfacial roughnesses. A comparison of the chemical versus magnetic interfaces shows distinct differences in the behavior of these two related interfaces as the chemical roughness is increased. Such changes appear to be correlated with the behavior of the magnetic hysteresis of the interface, measured by tracking the diffuse XRMS intensity as a function of applied magnetic field.

Weakly coupled motion of individual layers in ferromagnetic resonance

We demonstrate a layer- and time-resolved measurement of ferromagnetic resonance (FMR) in a Ni{sub 81}Fe{sub 19}/Cu/Co{sub 93}Zr{sub 7} trilayer structure. Time-resolved x-ray magnetic circular dichroism has been developed in transmission, with resonant field excitation at a FMR frequency of 2.3 GHz. Small-angle (to 0.2 deg), time-domain magnetization precession could be observed directly, and resolved to individual layers through elemental contrast at Ni, Fe, and Co edges. The phase sensitivity allowed direct measurement of relative phase lags in the precessional oscillations of individual elements and layers. A weak ferromagnetic coupling, difficult to ascertain in conventional FMR measurements, is revealed in the phase and amplitude response of individual layers across resonance.

X-Ray Spectrometry of Copper: New Results on an Old Subject

We review recent, and some less recent, measurements of several emission spectra of copper. The results are discussed with special emphasis on elucidating the structure of the Kα1,2 and Kβ1,3 diagram lines and their underlying transitions. These lines are found to contain ≈30 % contribution from 3d spectator hole transitions. Other multielectronic transitions, the 2p spectator hole (satellites) and 1s spectator hole (hypersatellites) transitions were also measured. They are discussed paying special attention to the evolution of the lineshapes and intensities from the excitation threshold to saturation. Trends in the measured quantities depending on the spectator hole’s shell and subshell are also discussed.

Interaction effects within Langmuir layers and three-dimensional arrays of ε-Co nanoparticles

Magnetostatic interactions are studied in two types of nanoparticle assemblies. Highly ordered monolayers of e-Co nanoparticles are prepared from Langmuir films and self-assembled three-dimensional (3D) arrays formed by slow precipitation of colloidal dispersions of same nanoparticles. The structure of assemblies is found from electron microscopy and small-angle x-ray scattering. The differences in magnetic behavior are interpreted in terms of the particle moment, interparticle separation, and coordination number. Preferential in-plane magnetization is observed due to interactions within a monolayer. 3D arrays show stronger interactions compared to monolayers due to an increased number of nearest neighbors.

Structural ordering effects in Fe nanoparticle two- and three-dimensional arrays

Two- and three-dimensional arrays were prepared by self-assembly of iron nanoparticles with similar magnetic moments and interparticle separations, and characterized both magnetically and structurally. The rapid magnetization decay in the three-dimensional (3D) arrays suggests a relaxation mechanism than has been previously reported, perhaps associated with the existence of domain walls within large structurally ordered regions. Small angle x-ray scattering indicates the presence of such regions in the 3D arrays.

SOFT X-RAY RESONANT MAGNETIC REFLECTIVITY STUDY OF THIN FILMS AND MULTILAYERS

Soft x-ray resonant magnetic reflectivity measurements on thin films and multilayers in a transverse geometry using linear polarized photons are presented. Magneto-optic calculations taking into account the layer roughness allows us to reproduce all the experimental features of the angular and energy reflectivity curves as well as the asymmetry ratio in both cases. Application to FexMn1−x alloy films epitaxially grown on Ir(001) brings more insights on the magnetic transition occurring at x=0.75.

Resonant magnetic scattering in soft X-ray range using multilayers

Abstract X-ray resonant magnetic scattering experiments using linearly polarised light have been performed at the L 2,3 absorption edges of Co in a magnetically coupled Cu/Co multilayer. Taking advantage of the dependence of the scattering intensity on the magnetisation direction, we succeed in bringing out a purely magnetic peak due to the antiferromagnetic coupling between successive Co layers. Then for a ferromagnetically coupled multilayer, large changes in the scattering intensity of Bragg peaks of the multilayer (up to 45%) have been observed upon reversal of the direction of magnetisation in the multilayer.

Small-gap insertion-device development at the National Synchrotron Light Source--performance of the new X13 mini-gap undulator.

The National Synchrotron Light Source (NSLS) 2.8 GeV electron storage ring continues to set high standards in insertion-device research and development. The Chasman-Green NSLS lattice design provides for dispersion-free long straight sections in addition to a very small vertical beta function. As the electron beam size is proportional to the square root of this function, a program to exploit this feature was undertaken more than a decade ago by implementing short-period small-gap insertion devices in the NSLS storage ring. The possibility of utilizing existing moderate-energy synchrotron radiation electron storage rings to produce high-brightness photon beams into the harder X-ray region have been realised using in-vacuum undulators. In this article the operation of a 1.25 cm-period mini-gap undulator, operating down to a gap of 3.3 mm within the NSLS X13 straight section, is reported. It is the brightest source of hard X-rays in the energy range approximately 3.7-16 keV at the NSLS, and replaces an in-vacuum undulator which had a more limited tunability.

The role of Ta and Pt in segregation within Co‐Cr‐Ta and Co‐Cr‐Pt thin film magnetic recording media

Polarization dependent extended x‐ray absorption fine structure (PD‐EXAFS) and magnetic circular dichroism (MCD) measurements of CoCrTa and CoCrPt films, sputter deposited at varying substrate temperatures, were performed to investigate the average local structure and chemistry about the Ta, Pt, and Co atoms and the average magnetic moment of the Co and Cr atoms within these films. Results from the MCD measurements indicate the average net magnetic moment of the Cr atoms is opposite in direction and five percent in amplitude relative to the Co moments. Inspection of the Fourier transforms of the XAFS data from these samples shows an increase in structural disorder around the Ta and Pt atoms with increasing substrate deposition temperature. A further comparison between the Ta and Pt edge EXAFS results show that the temperature‐dependent increase in structural disorder is greater around the Ta atoms in the CoCrTa system than it is around the Pt atoms in the CoCrPt system.