S. T. Lim

Two-dimensional electron gas in Zn-polar ZnMgO/ZnO heterostructure grown by metal-organic vapor phase epitaxy

This work was supported by the Singapore Agency for Science, Technology, and Research A*STAR, under SERC Grant No. 0521260095 and Infuse Project No. I01-0911-06.

Optical, magnetic, and transport behaviors of Ge1−xMnxTe ferromagnetic semiconductors grown by molecular-beam epitaxy

The optical, magnetic, and transport behaviors of Ge1−xMnxTe (x=0.24 and 0.55) grown by solid-source molecular-beam epitaxy are investigated. X-ray diffraction shows that Ge1−xMnxTe crystallizes in rocksalt structure. The temperature-dependent magnetization (M-T) for x=0.55 sample gives a Curie paramagnetic temperature of θp∼180 K, which is consistent with the temperature-dependent resistivity ρ(T) measurement. Anomalous Hall effect is clearly observed in the samples and can be attributed to extrinsic skew scattering based on the scaling relationship of ρxy∝ρxx1.06. The magnetoresistance of Ge1−xMnxTe is isotropic and displays a clear hysterestic loop at low temperature, which resembles that of giant-magnetoresistance granular system in solids.

Exchange interaction and Curie temperature in Ge1−xMnxTe ferromagnetic semiconductors

We present the magnetotransport studies of Ge1−xMnxTe ferromagnetic semiconductor under hydrostatic pressure. The investigation of the normal and Hall resistivities provide an insight to the dependence of carrier concentration, mobility, and magnetic properties on pressure. Our results reveal that the application of pressure changes the band structure, which can be explained by a two valence band model. We observe the enhancement and reduction of Curie temperature within a pressure range of 0–24 kbar. Analysis within the framework of the Ruderman–Kittel–Kasuya–Yosida model allows us to identify the factors in controlling the Tc, in which the exchange interaction plays a predominant role in the formation of ferromagnetic phase.

Spin-polarized Wide Electron Slabs in Functionally Graded Polar Oxide Heterostructures

We report on the high mobility wide electron slabs with enhanced correlation effects by tailoring the polarization effects in a functionally graded ZnMgO/ZnO heterostructures. The characteristics of three-dimensional (3D) spreading electrons are evidenced by the capacitance-voltage profiling and the quantization of 3D Fermi surface in magneto-transport measurements. Despite the weak spin-orbit interaction, such electron slabs are spin-polarized with a large zero-field spin splitting energy, which is induced by the carrier-mediated ferromagnetism. Our results suggest that the vast majority of electrons are localized at the surface magnetic moment which does not allow spin manipulations, and only in the region visited by the itinerant carriers that the ferromagnetic exchange interactions via coupling to the surface local moments contribute to the spin transport. The host ferromagnetism is likely due to the formation of Zn cation vacancies on the surface regime induced by the stabilization mechanism and strain-relaxation in ZnMgO polar ionic surface.

Magnetism and magnetotransport studies in Ge0.9Mn0.1Te

We have investigated the magnetic and magnetotransport properties of Ge1-xMnxTe (x = 0.1) grown by molecular-beam epitaxy. Our results show that the sample exhibits two ferromagnetic transition temperatures at TC = 34 K and TC* = 100 K. We infer that Tc is a long-range ferromagnetic ordering in view of sufficient carriers generating uniform ferromagnetism, whereas TC* is a short-range ferromagnetic ordering due to ferromagnetic clusters. The temperature dependence of the resistivity ρ(T) curve exhibits a shallow minimum near TC. The upturn of ρ(T) toward the low temperature (T   TC), the phonon scattering dominates.

Effect of hydrostatic pressure in degenerate Ge1-xMnxTe

We utilize the effect of hydrostatic pressure to investigate the magnetotransport properties of degenerate p-Ge1−xMnxTe (x=0.10) ferromagnetic semiconductor. The Curie temperature was found to increase with pressure as 0.27 K/kbar, which can be understood on the basis of the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction mechanism. For sufficiently high carrier concentration of po∼1021 cm−3, both the light holes from the L valence band and the heavy holes from the Σ valence band contribute to the RKKY interaction. Additionally, a negative magnetoresistance is observed at low temperature and is found to decrease with pressure.

Weak localization and antilocalization of hole carriers in degenerate p-Ge1−xMnxTe

We have performed magnetotransport studies on p-type Ge0.7Mn0.3Te under hydrostatic pressure. The magnetoresistance (MR) is characterized by both positive and negative contributions, which can be described by the antilocalization and weak localization models, respectively. We report the temperature and pressure dependence of the spin-orbit, elastic and inelastic scattering times, as well as the coherence length in Ge0.7Mn0.3Te. The spin-orbit scattering time is found to be independent of pressure and temperature and it dominates over the inelastic scattering time leading to the observed positive MR. The phase coherent length is correlated to the inelastic scattering which is predominately due to electron-electron scattering.

THE GD CONCENTRATION DEPENDENCE OF THE MAGNETIC PROPERTIES OF ROOM TEMPERATURE FERROMAGNETIC ZNO:GD SEMICONDUCTOR

We present structural and magnetic properties of ZnO films doped with rare-earth Gd ions at various concentrations, achieved by ion implantation technique and pulsed laser deposition (PLD) on ZnO (0001) single crystals and sapphire. X-ray diffraction shows Gd doping in ZnO and crystal quality degrades with increasing Gd concentration. Magnetization as a function of temperature revealed a positive magnetization at 305 K and a concave trend was observed for all samples for both implanted and PLD grown samples. The highest saturation magnetization was achieved for Gd concentration of x = 7.9 % (0.7 µB/Gd) and at dosage of 9.0 × 1015cm-2 (2.6 µB/Gd) for the PLD grown and implanted samples, respectively. We believe that the Gd ion solubility limit in the implanted and PLD grown samples, of around 3% and 7.9%, respectively, are reached.

Landing zone skew angle, long term stiction, and contamination build up on slider surfaces of disk drives

This work presents results of experimental investigation on the relationship between landing zone skew angle, long term stiction, and contamination build up on slider surface caused by slider-disk interaction in the landing zone. Results indicate that larger skew angle in the landing zone will lead to higher long-term stiction force and increases the likelihood of linelike contamination build up on slider surface.

Magneto-transport study on the two dimensional electron gas in ZnMgO/ZnO heterostructure grown by MOVPE

Magneto-transport studies on ZnMgO/ZnO heterostructures grown by MOVPE technique have been performed. The features of spin-polarization of 2DEG and intrinsic ferromagnetism of heterostructure were observed at 1.5 K and the related physical mechanisms are discussed.