Kuan-Li Yu

Anomalous grazing-incidence small-angle X-ray scattering investigation on the surface morphology of an FePt magnetic nanoparticle monolayer on functional modulated substrates

The structural stability and coalescence of self-assembled FePt monolayer nanoparticles on functional substrates with/without an Au overlayer during annealing were studied. From X-ray diffraction and the anomalous grazing-incidence small-angle X-ray scattering techniques, the nanoparticles were found to be intact under the annealing process when a 5–10 nm overlayer of Au was deposited on top of FePt monolayer nanoparticles. From the simulation result, the particle size 4.5 ± 0.5 nm is typically unchanged, but the distance between particles is reduced from 7.5 ± 1.5 nm to 5.5 ± 1.1 nm after annealing. The results suggest that the 5 nm Au coverlayer is an effective diffusing barrier layer to prevent the FePt nanoparticles from sintering during the annealing process.

Structure and magnetism in Co/Pt multilayers

The structure of [Co(tCo)/Pt(10 A)]30 multilayers has been studied by x-ray absorption spectroscopy and x-ray diffraction using in-plane and out- of-plane polarized synchrotron radiation. The x-ray absorption near-edge spectroscopy shows that the Co layer is like a fcc pseudomorphic structure for tCo less than 4 A. For tCo=10 A, the spectra in both directions are separated and look closer to the bulk hcp Co. The extended x-ray absorption fine structure studies reveal that the Co–Pt interface of the multilayers fitted with a sharp boundary better than an interdiffusion model. The in-plane x-ray diffraction shows that the Pt layer in the multilayer possesses a 2%–3.5% compressible strain. Although the saturation magnetization Ms does not depend on tCo or interfacial roughness in any simple form, the Ms values scale quite linearly with the in-plane Pt strain. We conclude that the interfacial strain is important for the perpendicular magnetization in the Co/Pt multilayers.

Polarized neutron and X-ray reflectivity study of the structure and exchange coupling of permalloy(Ni80Fe20)/Cr/permalloy trilayers

The magnetic coupling between two permalloy layers with a Cr separating layer was studied using LMOKE, polarized neutron reflectivity and XRD. The coupling between two permalloy layers is antiferromagnetic-like when the Cr thickness is near 2.0 nm along the easy axis. A strong biquadratic coupling term has been found from the polarized neutron reflectivity study. The strong biquadratic coupling may be caused by the rough interface between the permalloy and Cr layers. The missing GMR effect of this system might be due to this strong biquadratic coupling of permalloy layers.

A COMPARISON BETWEEN THE ATOMIC FORCE MICROSCOPY AND X-RAY REFLECTIVITY ON THE CHARACTERIZATION OF SURFACE ROUGHNESS

Atomic force microscopy and X-ray reflectivity methods are used to characterize a surface morphology which includes the information of rms roughness, roughness exponent, and the height–height correlation length. Two major reasons to interpret the discrepancy of rms roughness data measured by AFM and X-ray reflectivity are (1) the bandpass of power spectra density is different and (2) the X-ray reflectivity probes the high density buried layer.

X-ray absorption spectroscopy study of Co structure in the epitaxial Co/Pt multilayers on Al2O3(11-20) substrates.

In this study, five epitaxial [Co(t nm)/Pt(1 nm)]30, multilayer samples (t=0.16-1.07 nm) were studied using polarized X-ray absorption spectroscopy method. These samples were prepared on Mo(110)/ Al2O3(11-20) substrates by MBE technique. The results show that the Co layer is more like an fcc pseudomorphic structure for the Co thickness of less than 0.3 nm. For Co layer thickness of 1 nm, the first shell distance is 0.25 nm, which is very close to the Co-Co distance of bulk hcp Co. On the other hand, for Co layer of less than 0.3 nm, the in plane first shell distance is expanded by 4% and most of the neighboring atoms are Pt atoms. The fitting results of the Co/Pt multilayers seem to support a sharp boundary model rather than an interdiffusion model.

Grazing incidence small-angle X-ray scattering and diffraction study of the resistance of Mo implanted Si wafer after annealing

Grazing-incidence small-angle X-ray scattering and X-ray diffraction were used to probe the particle size and distribution of MoSi2 in a Mo ion implanted Si(100) wafer at 1073 K after different annealing times. The sheet resistance of the sample decreases at the beginning of annealing due to the recovery of defects and then increases due to the growth of larger and separated particles. The particle size of MoSi2 determined by the grazing incidence small angle X-ray scattering is larger than that determined by the in-plane wide angle X-ray diffraction, which might indicate that the MoSi2 particle consists of a larger amount of disordered components or is a cluster of several smaller well-ordered grains.

Study of the interface roughness using polarized neutron and X-ray reflectivity on MBE permalloy thin films

Abstract Polarized neutron and X-ray reflectivity methods have been used to study the permalloy epitaxial thin films on sapphire with Cr as a buffer layer. The reflectivity result indicates that the magnetic interface is smoother than the chemical interface. It is possible that the magnetic moment of the intrusive Ni or Fe atoms across the interface can be assimilated by the near-by Cr atoms or the other way around.