Fang Yuh Lo

Electron spin injection into GaAs from ferromagnetic contacts in remanence

We demonstrate electrical spin injection into a (GaIn)As∕GaAs light-emitting diode from the remanent state of ferromagnetic contacts in perpendicular geometry. Using a Fe∕Tb multilayer structure with perpendicular magnetic anisotropy and a reverse-biased Schottky contact, we achieve a circular polarization degree of the emitted light of 0.75% at 90K.

Electrical detection of photoinduced spins both at room temperature and in remanence

We demonstrate a photodetector with ferromagnetic contacts which can electrically detect the polarization degree of incoming light using spin filtering of photoinduced spin-polarized electron currents. Our structure is a pin diode with a single GaAs quantum well as active region and a Fe∕Tb multilayer on top of a MgO tunnel barrier as n-contact where the spin-polarized electron current is filtered. The photocurrent depends on the magnetization of the contacts and on the polarization of the injected light. We prove that even in remanence and at room temperature the degree of circular polarization of the incident light can be unambiguously determined by the photocurrent intensity.

Room temperature electrical spin injection in remanence

We demonstrate electrical spin injection from ferromagnetic Fe/Tb multilayer structures with remanent perpendicular magnetization into GaAs-based light-emitting diodes at room temperature. Using a reverse-biased Schottky contact and a MgO tunnel contact, respectively, we achieve spin injection at remanence. The maximum degree of circular polarization of the emitted light is 3% at room temperature.

Structural, magnetic, and optical properties of Co- and Gd-implanted ZnO(0001) substrates

ZnO(0001) substrates were ion implanted with 100 keV of Co and 300 keV of Gd at different fluences ranging from 5×1013–1×1015/cm2. The resulting Co:ZnO and Gd:ZnO samples were analyzed with respect to their structural, magnetic, and optical properties. The effect of annealing at 350 °C on the structure and the resulting magnetic and optical properties were investigated as well. For Co:ZnO hardly any changes were observable, neither in the structural nor in the magnetic properties, even though the existence of substitutional Co2+ in the ZnO lattice could be shown by means of low temperature photoluminescence especially for Zn-annealed samples. For the much larger Gd ion the implantation leads to a changed crystal structure, which leads to a ferromagneticlike behavior for higher implantation doses, which could even be enhanced by annealing in vacuum. Ferromagnetic behavior in annealed Gd:ZnO is corroborated by ferromagnetic resonance measurements at low temperatures. The distinct behavior of Gd- and Co-impl...

Magnetic and structural properties of Gd-implanted zinc-blende GaN

Zinc-blende GaN layers grown by molecular beam epitaxy were uniformly focused-ion-beam implanted with 300keV Gd3+ ions for doses ranging from 1×1012to1×1015cm−2, and their structural and magnetic properties were studied. The implanted samples were not subjected to any annealing treatment. Only Gd incorporation into zinc-blende GaN was observed by x-ray diffraction. Magnetic investigations using superconducting quantum interference device magnetometry reveal a (super)paramagneticlike behavior with an ordering temperature around 60K for the sample with the highest implantation dose. Our experimental studies indicate that the spontaneous electric polarization in wurtzite GaN is the crucial mechanism for its ferromagneticlike behavior upon Gd doping.

Spin injection light-emitting diode with vertically magnetized ferromagnetic metal contacts

We analyze the electrical injection of spin-polarized electrons into a (GaIn)As∕GaAs light-emitting diode. Using an Fe∕Tb multilayer structure with perpendicular magnetic anisotropy and a reverse-biased Schottky contact, we demonstrate spin injection even in remanence between 90 and 260K. The maximum degree of circular polarization of the emitted light is 0.75% at 90K.

Slanted n-ZnO/p-GaN nanorod arrays light-emitting diodes grown by oblique-angle deposition

High-efficient ZnO-based nanorod array light-emitting diodes (LEDs) were grown by an oblique-angle deposition scheme. Due to the shadowing effect, the inclined ZnO vapor-flow was selectively deposited on the tip surfaces of pre-fabricated p-GaN nanorod arrays, resulting in the formation of nanosized heterojunctions. The LED architecture composed of the slanted n-ZnO film on p-GaN nanorod arrays exhibits a well-behaving current rectification of junction diode with low turn-on voltage of 4.7 V, and stably emits bluish-white luminescence with dominant peak of 390 nm under the operation of forward injection currents. In general, as the device fabrication does not involve passivation of using a polymer or sophisticated material growth techniques, the revealed scheme might be readily applied on other kinds of nanoscale optoelectronic devices.

Uniaxial magnetic anisotropy in Pd/Fe bilayers on Al2O3 (0001) induced by oblique deposition

This study reports the preparation of self-organized 1-dimensional magnetic structures of Fe on Al2O3 (0001) by oblique deposition. The x-ray diffraction (XRD) results in this study show the preferred (110) texture of the Fe films. XRD and extended x-ray adsorption fine structure measurements indicate larger oblique deposition angle (65°) leads to more disorder in the Fe crystalline structure. After capping with a Pd overlayer, the Pd/Fe/Al2O3 (0001) still exhibits uniaxial magnetic anisotropy induced by the underlying 1-dimensional Fe nanostructure. This uniaxial magnetic anisotropy changes with the variation in Fe thickness and oblique deposition angle. These results clearly indicate the feasibility of manipulating uniaxial magnetic anisotropy and crystalline order through the oblique deposition of magnetic materials.

Magnetism modulation of Fe/ZnO heterostructure by interface oxidation

In this study, the magnetic coercivity (Hc) of Fe/ZnO heterostructure was significantly enhanced by 2–3 times after applying a suitable current. This Hc enhancement originates from the Fe-oxidation at the Fe/ZnO interface induced by direct current heating. Depth-profiling X-ray photoemission spectroscopy analysis confirmed the formation of FeO, Fe3O4, and Fe2O3 close to the interface region, depending on the Fe thickness and annealing process. This study demonstrates that direct current heating can moderately change the local interface oxidation and modulate the magnetic properties. These results clearly reveal the correlation between magnetism and interface properties in the Fe/ZnO heterostructure and provide valuable information for future applications.

Hydrogenation-induced change of magneto optical Kerr effect in Pd/Fe bilayers

The hydrogenation induced change of magneto-optical Kerr effect (MOKE) was studied in n ML Pd/30 ML Fe bilayers on Al2O3(0001). With the increasing of Pd thickness from 3 ML to 60 ML, the MOKE extinction angle was gradually shifted by 0.6° and the enhancement of Kerr intensity reached 35%–40% after exposure to 1 atm hydrogen. The reversibility of this significant change was demonstrated by cyclic desorption and reabsorption of hydrogen. This study reveals the sensitive MO response in the combination of a magnetic Fe thin film with a highly hydrogenated Pd capping layer.