W. P. Beyermann

High-mobility Sb-doped p-type ZnO by molecular-beam epitaxy

Reproducible Sb-doped p-type ZnO films were grown on n-Si (100) by electron-cyclotron-resonance-assisted molecular-beam epitaxy. The existence of Sb in ZnO:Sb films was confirmed by low-temperature photoluminescence measurements. An acceptor-bound exciton (A°X) emission was observed at 3.358 eV at 8 K. The acceptor energy level of the Sb dopant is estimated to be 0.2 eV above the valence band. Temperature-dependent Hall measurements were performed on Sb-doped ZnO films. At room temperature, one Sb-doped ZnO sample exhibited a low resistivity of 0.2Ωcm, high hole concentration of 1.7×1018cm−3 and high mobility of 20.0cm2∕Vs. This study suggests that Sb is an excellent dopant for reliable and reproducible p-type ZnO fabrication.

p-type ZnO films with solid-source phosphorus doping by molecular-beam epitaxy

Phosphorus-doped p-type ZnO films were grown on r-plane sapphire substrates using molecular-beam epitaxy with a solid-source GaP effusion cell. X-ray diffraction spectra and reflection high-energy electron diffraction patterns indicate that high-quality single crystalline (112¯0) ZnO films were obtained. Hall and resistivity measurements show that the phosphorus-doped ZnO films have high hole concentrations and low resistivities at room temperature. Photoluminescence (PL) measurements at 8 K reveal a dominant acceptor-bound exciton emission with an energy of 3.317 eV. The acceptor energy level of the phosphorus dopant is estimated to be 0.18 eV above the valence band from PL spectra, which is also consistent with the temperature dependence of PL measurements.

Electron concentration dependent magnetization and magnetic anisotropy in ZnO:Mn thin films

Well-above room temperature and electron concentration dependent ferromagnetism was observed in n-type ZnO:Mn films, indicating long-range ferromagnetic order. Magnetic anisotropy was also observed in these ZnO:Mn films, which is another indication for intrinsic ferromagnetism. The electron-mediated ferromagnetism in n-type ZnO:Mn contradicts the existing theory that the magnetic exchange in ZnO:Mn materials is mediated by holes.

Electronic properties of

We report on the electronic properties of , which crystallizes in a hexagonal structure with space group . Anomalies in the temperature dependences of the transport, magnetic and thermal properties indicate that two magnetic transitions occur at and , the lower transition being of antiferromagnetic type. The assertion that the ground state of is antiferromagnetic is corroborated by magnetization measurements, which reveal a slight S shape in the M versus B curve with no indication of saturation in fields up to 18 T at 2.2 K. The electrical resistivity increases with lowering temperature, reaching a value of at 0.26 K. Magnetoresistance studies at low temperatures show that a large part of the zero-field resistance is due to antiferromagnetic correlations, and a resistivity drop of is observed upon the application of a field of 18 T at 2 K. Another striking feature is the enhanced Sommerfeld coefficient of , extracted from extrapolation of the specific-heat data to T = 0 K, which we report here for the first time. Therefore, can be described as a moderately enhanced heavy-fermion system.

Magnetotransport studies of a single nickel nanowire

The diameter dependent magnetoresistance of a single Ni nanowire (i.e. 200 and 30 nm diameter) bridging microfabricated Ni electrodes was investigated at temperatures ranging from 10 to 300 K. Overall, the magnetotransport behaviour of single Ni nanowires was similar to the results for thin films, except that the magnitudes of transverse magnetoresistance and longitudinal magnetoresistance differed from the values for thin films when the diameter of the nanowire was small. The difference might be attributed to a change in the stress anisotropy of the nanowire from a tensile stress induced by a thermal expansion mismatch between the nanowire and the Si substrate or different magnetocrystalline anisotropy. Based on the angle dependent switching field, the magnetization reversal mechanism of 30 and 200 nm nanowires at 10 K was determined to be the curling mode, and the size of the curling nucleation is smaller than the diameter of the nanowire.

Electron carrier concentration dependent magnetization and transport properties in ZnO:Co diluted magnetic semiconductor thin films

Diluted magnetic semiconducting ZnO:Co thin films with above room-temperature TC were prepared. Transmission electron microscopy and x-ray diffraction studies indicate the ZnO:Co thin films are free of secondary phases. The magnetization of the ZnO:Co thin films shows a free electron carrier concentration dependence, which increases dramatically when the free electron carrier concentration exceeds ∼1019 cm−3, indicating a carrier-mediated mechanism for ferromagnetism. The anomalous Hall effect is observed in the ZnO:Co thin films. The anomalous Hall coefficient and its dependence on longitudinal resistivity were analyzed. The presence of a side-jump contribution further supports an intrinsic origin for ferromagnetism in ZnO:Co thin films. These observations together with the magnetic anisotropy and magnetoresistance results support an intrinsic carrier-mediated mechanism for ferromagnetic exchange in ZnO:Co diluted magnetic semiconductor materials.

Microstructure and transport properties of ZnO: Mn diluted magnetic semiconductor thin films

Microstructural studies using transmission electron microscopy were performed on a ZnO:Mn diluted magnetic semiconductor thin film. The high-resolution imaging and electron diffraction reveal that the ZnO:Mn thin film has a high structural quality and is free of clustering/segregated phases. High-angle annular dark field imaging and x-ray diffraction patterns further support the absence of phase segregation in the film. Magnetotransport was studied on the ZnO:Mn samples, and from these measurements, the temperature dependence of the resistivity and magnetoresistance, electron carrier concentration, and anomalous Hall coefficient of the sample is discussed. The anomalous Hall coefficient depends on the resistivity, and from this relation, the presence of the quadratic dependence term supports the intrinsic spin-obit origin of the anomalous Hall effect in the ZnO:Mn thin film.

Synthesis and magnetotransport studies of single nickel-rich NiFe nanowire

One of the main concerns in the preparation of alloy nanowires is the ability to synthesize compositionally uniform nanowires along the axis. Since most of the conventional mild acidic permalloy (Ni80Fe20) electroplating baths consist of an extremely low concentration of Fe ions compared with Ni ions, the electrodeposition of iron is controlled by mass transfer, which leads to a significant change in the composition along the axis of the nanowire. To overcome this obstacle, we developed a new acidic chloride electrolyte with a high concentration of Fe and Ni ions to electrodeposit homogeneous nanowires. After synthesizing nanowires, the temperature dependent magneto- and electro-transport properties of individual nanowires were investigated. The temperature coefficient of resistance of a nanowire is much lower than the bulk counterpart, which might be attributed to a higher residual resistivity. The magnetoresistance shows a typical anisotropic magnetoresistance behaviour where the maximum anisotropic magnetoresistance ratio decreased with increasing temperature. The angular dependence of the magnetization switching field indicated that curling is the magnetization reversal mode at all temperatures.

Exchange bias in large three dimensional iron oxide nanocomposites

A processing method is presented for the production of macroscopic nanocomposites that display antiferromagnetic/ferrimagnetic (AFM/fM) coupling. The technique takes advantage of the metastability of iron oxide phases and the fast densification of nanocrystalline powders. The total processing time is under 500s. It is possible to manipulate the composition of fM and AFM phases with processing temperature. The relatively high density of AFM/fM boundaries produces an exchange bias caused by coupling at the interfaces. The magnitude of the exchange field (Hex) is affected the composition as well as the grain size; the smaller grain size samples have the highest Hex.

Coherent magnetic semiconductor nanodot arrays

In searching appropriate candidates of magnetic semiconductors compatible with mainstream Si technology for future spintronic devices, extensive attention has been focused on Mn-doped Ge magnetic semiconductors. Up to now, lack of reliable methods to obtain high-quality MnGe nanostructures with a desired shape and a good controllability has been a barrier to make these materials practically applicable for spintronic devices. Here, we report, for the first time, an innovative growth approach to produce self-assembled and coherent magnetic MnGe nanodot arrays with an excellent reproducibility. Magnetotransport experiments reveal that the nanodot arrays possess giant magneto-resistance associated with geometrical effects. The discovery of the MnGe nanodot arrays paves the way towards next-generation high-density magnetic memories and spintronic devices with low-power dissipation.