Shu Kin Lok

MBE-Grown Fe Magnetic Quantum Dots in ZnS Matrix

A multilayer magnetic quantum dot sample containing 5 layers of Fe quantum dots (QDs) embedded in six layers of ZnS spacer was grown by molecular beam epitaxy (MBE). High-resolution transmission electron microscopy (HRTEM) observations reveal that the Fe QDs are single crystalline with spherical shape of diameters around 3 to 4 nm and area density of 1.5times1012 cm-2. Its zero-field cooled (ZFC) and field cooled (FC) curves measured at low field (100 Oe) show the magnetic relaxation effect with a blocking temperature around 26 K. The hysteresis loop measured at 5 K shows a coercivity of 83 Oe, confirming the slow relaxation process and coercivity enhancement attributed to the nanoparticle nature of the sample

Molecular beam epitaxy-grown wurtzite MgS thin films for solar-blind ultra-violet detection

Molecular beam epitaxy grown MgS on GaAs(111)B substrate was resulted in wurtzite phase, as demonstrated by detailed structural characterizations. Phenomenological arguments were used to account for why wurtzite phase is preferred over zincblende phase or its most stable rocksalt phase. Results of photoresponse and reflectance measurements performed on wurtzite MgS photodiodes suggest a direct bandgap at around 5.1u2009eV. Their response peaks at 245u2009nm with quantum efficiency of 9.9% and enjoys rejection of more than three orders at 320u2009nm and close to five orders at longer wavelengths, proving the photodiodes highly competitive in solar-blind ultraviolet detection.

Factors affecting the shape of MBE-grown laterally aligned Fe nanowires.

Various microstructural and chemical analysis techniques were applied to study two types (type-A and B) of self-assembled laterally aligned Fe nanowires (NWs) fabricated by molecular beam epitaxy on a ZnS buffer layer. The formation of the three-dimensional shapes of these NWs was found to be driven by the principle of surface energy minimization. We have provided phenomenological models to address the factors affecting the observed topological shape of these NWs, including the role of the lattice relationship between the Fe NWs and the underlying buffer layer, growth temperature, Fe nominal coverage and substrate orientation. Magnetic hysteresis measurements were performed at different temperature, demonstrating the Fe NWs possess a coercivity about 30 times larger than that of a Fe thin film. The observed gradual magnetization reversal indicates the magnetization process is accomplished by the rotation of magnetic moments within a single domain.

Competitive antiferromagnetic and ferromagnetic coupling in a CrSe/Fe/GaAs(111)B structure

A novel transition from the negative exchange bias (NEB) to the positive exchange bias (PEB), due to coexistent and competitive ferromagnetic and antiferromagnetic coupling in a molecular-beam-expitaxy-grown CrSe/Fe/GaAs(111)B structure, is described. The source of the unusual PEB effect was found to originate from the interface at the hetero-junction of Fe/GaAs(111)B while the NEB effect resulted from the CrSe/Fe bilayer structure. Phenomenological models are presented to explain the mechanism of the PEB effect and the varying asymmetry of the magnetic hysteresis of this structure as a function of the measured temperature.

Molecular-beam-epitaxy-grown CrSe∕Fe bilayer on GaAs(100) substrate

A novel CrSe∕Fe bilayer structure has been fabricated on a GaAs (100) substrate by the molecular beam epitaxy technique. Microstructural characterizations have revealed that the Fe layer is a single-crystalline bcc structure with the orientation relationship of (100)Fe‖(100)GaAs, while the top CrSe layer shows four preferred hexagonal domains with their c axis each along one of the four upward-pointing ⟨111⟩ directions of the underlying Fe lattice. The magnetic hysteresis loops of this bilayer structure measured by a superconducting quantum interference device magnetometer demonstrate a strong exchange bias effect with a negative exchange bias field as high as −48.4Oe at 5K. The magnetization reversal process shows an abrupt transition nature at temperature from 5to300K. An enhancement of the coercivity not accompanied by the exchange bias field was observed at temperature higher than and well above the blocking temperature. We have interpreted these observations based on the well-established exchange spr...