Yuet-Loy Chan

Collecting photoelectrons with a scanning tunneling microscope nanotip

The collection of photoelectrons excited with a synchrotron via a nanotip placed near the surface of a sample is studied. Simulating the electron trajectory, we found that photoelectrons escaping from the surface are too weak to be the only source of electrons contributing to a photocurrent detected with a scanning tunneling microscope tip, as reported recently. The tunneling of low-energy electrons generated with synchrotron irradiation is suggested as an additional channel contributing to the photocurrent at a small separation between tip and sample. An image based on x-ray absorption is expected to attain a resolution comparable to a topographical image.

Deep Co penetration and spin-polarization of C60 molecules at hybridized Co-C60 interfaces

We used near-edge x-ray absorption fine structure spectroscopy to identify the interplays at organic semiconductor-ferromagnet interfaces. When monitoring the L-edge intensity of 0.36 nm Co depositing on C60 films of various thicknesses, we detected weaker Co signal from structures with larger C60 thicknesses. Having determined that the electron mean escape depth in C60 is 4.9 nm, further model analysis indicates that the decline of spectral intensity is due to deep penetration of Co clusters. Finally, C K-edge spectra reveal clear evidences of orbital hybridization between Co and C60 as well as a visible dichroic effect at 125 K.

Interfacial spectroscopic characterization of organic/ferromagnet hetero-junction of 3,4,9,10-perylene-teracarboxylic dianhydride-based organic spin valves

We report interfacial characterization of 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA)-based organic spin valves (OSV) dusted with a thin layer of partially oxidized alumina at the organic semiconductor (OSC)/ferromagnet (FM) interfaces. Up to 13.5% magnetoresistance is achieved at room temperature. X-ray photoelectron spectroscopy measurements reveal interfacial electronic interaction between PTCDA and FM while the application of a thin alumina layer at the PTCDA/FM interfaces prevents the electronic hybridization and effectively preserves the spin injection into the OSC spacer. This finding demonstrates the critical effect of interfacial structure on magnetotransport behavior in OSV.

Perpendicular magnetic anisotropy of Ni/Cu(001) films with surface passivation

A 13 monolayer Ni film was deposited on Cu(001) and followed by oxygen exposure at 300 K. It was confirmed that the surface contained Ni oxide with an amorphous atomic arrangement. The magnetization is perpendicular after surface oxidation. The coercivity is substantially enhanced without shifting of the hysteresis loop after field cooling. This could indicate that the NiO is randomly antiferromagnetic or that the Neel temperature of the NiO is above 300 K. This surface oxide leads to surface passivation so that the film is magnetically alive under ambient conditions, which could have useful applications in the future.

Direct imaging and spectral identification of the interfaces in organic semiconductor-ferromagnet heterojunction

Using x-ray spectromicroscopy, we studied the interface of pentacene (Pn)-cobalt (Co) heterojunction fabricated by thermal evaporation in an ultra-high vacuum environment. Through element-specific images and absorption spectroscopic analysis, we found evidences suggesting that part of the Co top layer penetrates into Pn film and hybridizes with molecules. As the Curie temperature of infiltrated Co clusters is dimension sensitive, the Co penetration would create an ill-defined interfacial region whose magnetization depends on the temperature and depth of cobalt penetration. The magnetic complexity at Pn/Co interface was found reduced after inserting a thin Cu layer between Pn and Co.

Extending the Control of Antiferromagnetic--Ferromagnetic Exchange Coupling on Perpendicular Magnetization into the Soft Magnetic Regime

Antiferromagnetic–ferromagnetic exchange coupling is known for its pinning effect on the magnetic hard layer through coercivity enhancement or exchange bias. This study reports its effect on controlling perpendicular magnetization in the soft magnetic regime. In a series of Ni78Fe22(Py)/Mn bilayers, we demonstrate that the magnetization, coercivity, and thermal stability of perpendicular anisotropy can be controlled by varying the thicknesses of the Py and Mn layers, based on the competition between the in-plane anisotropy of the Py layer and the out-of-plane-oriented Py–Mn exchange coupling. This offers a new possibility for the design of magnetic free layers in perpendicular-based magnetic logical devices.

Spin alignment of surface oxidized CoxNi1–x/Cu(001)

We investigated the ferromagnetic (FM)/antiferromagnetic (AF) spin alignment of a 13 monolayer oxidized CoxNi1–x/Cu(001) (x = 0 or 0.05) surface by X-ray magnetic circular dichroism and X-ray magnetic linear dichroism photoemission electron microscopy (XMCD-PEEM and XMLD-PEEM). Surface NiO and the underlying Ni (CoxNi1–x) were found to be AF and FM by analyzing the gray scale of XMLD-PEEM and XMCD-PEEM images; this indicates the spin orientation with respect to the polarization of the incident X-ray. We found both collinear and perpendicular alignment of the FM and AF spins. This suggests that the AF NiO is magnetically random, resulting in coercivity enhancement without exchange bias in the surface-oxidized CoxNi1–x/Cu(001) films.