Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)(13) clusters with Mn(2+) ions governs the Mn(2+) incorporation during the nucleation stage. This highly efficient Mn(2+) doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of approximately 600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s-d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.

The ferromagnetic semiconductor (Ga,Mn)As has emerged as the most studied material for prototype applications in semiconductor spintronics. Because ferromagnetism in (Ga,Mn)As is hole-mediated, the nature of the hole states has direct and crucial bearing on its Curie temperature T(C). It is vigorously debated, however, whether holes in (Ga,Mn)As reside in the valence band or in an impurity band. Here we combine results of channelling experiments, which measure the concentrations both of Mn ions and of holes relevant to the ferromagnetic order, with magnetization, transport, and magneto-optical data to address this issue. Taken together, these measurements provide strong evidence that it is the location of the Fermi level within the impurity band that determines T(C) through determining the degree of hole localization. This finding differs drastically from the often accepted view that T(C) is controlled by valence band holes, thus opening new avenues for achieving higher values of T(C).

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We discuss magnetic anisotropy parameters of ferromagnetic body-centered cubic (bcc) Fe films grown by molecular beam epitaxy (MBE) on (001) substrates of face-centered cubic (fcc) GaAs, ZnSe, and Ge. High-quality MBE growth of these metal/semiconductor combinations is made possible by the fortuitous atomic relationship between the bcc Fe and the underlying fcc semiconductor surfaces, resulting in excellent lattice match. Magnetization measurements by superconducting quantum interference device (SQUID) indicate that the Fe films grown on (001) GaAs surfaces are characterized by a very strong uniaxial in-plane anisotropy; those grown on (001) Ge surfaces have a fully cubic anisotropy; and Fe films grown on ZnSe represent an intermediate case between the preceding two combinations. Ferromagnetic resonance measurements carried out on these three systems provide a strikingly clear quantitative picture of the anisotropy parameters, in excellent agreement with the SQUID results. Based on these results, we propose that the observed anisotropy of cubic Fe films grown in this way results from the surface reconstruction of the specific semiconductor substrate on which the Fe film is deposited. These results suggest that, by controlling surface reconstruction of the substrate during the MBE growth, one may be able to engineer the magnetic anisotropy in Fe, and possibly also in other MBE-grown ferromagnetic films.

GaAs/Fe/Au core-shell nanowires were grown on GaAs(111)B substrates by molecular beam epitaxy. Scanning electron microscopy images show that the Fe shell has successfully coated the sidewalls of GaAs nanowires. Magnetic anisotropy of GaAs/Fe core-shell nanowires was studied by ferromagnetic resonance and by superconducting quantum interference device magnetometer. The authors’ results show that the magnetic anisotropy of this novel core-shell nanowire system cannot be simply described by any known theory, as revealed by attempts to use micromagnetic simulation using the Object Oriented MicroMagnetic Framework. The observed features thus suggest the existence of a domain structure that is specific to this new system

Interlayer exchange coupling (IEC) between two Ga 0.95 Mn 0.05 As layers separated by Be-doped GaAs spacers was investigated using magnetometry and neutron scattering measurements, which indicated the presence of robust antiferromagnetic IEC under certain conditions. We argue that the observed behavior arises from a competition between the IEC field and magnetocrystalline anisotropy fields intrinsic to GaMnAs layers. We estimate the magnitude of the IEC field and show how it decays with increasing temperature.

The magnetic nanoparticles of cobalt-and nickel-iron oxide have been extensive interest due to their superparamagnetic properties and their potential applications in many fields. The iron, cobalt and nickel can stay in many oxidation states and are easily oxidized especially in ambient air therefore the composition and oxidation states of these oxides can be unintentionally modified. Usually, the composition and oxidation states in these magnetic nanoparticles are determined by various experimental techniques required a sample in solid phase. This may lead the nanoparticles to directly contact with air and change the state. In this study, the magnetic nanoparticles in colloidal phase with concentration of 24mg/ml, derived from co-precipitation process, were directly injected to liquid cell for X-ray absorption near-edge structure (XANES) measurement. The iron-, cobalt-or nickel-iron oxide nanoparticles were prepared by dissolving CoCl2/FeCl3 or NiCl2/FeCl3, respectively, in deionized water with various atomic ratios. The average iron oxide nanoparticle size obtained by dynamic light scattering is about 4.2 nm with polydispersity of 0.987. Spherical shape with some stabilizer layer was observed by transmission electron microscope. The iron content in various composition nanoparticles was estimated in liquid phase by the ratio between the Fe edge peak and Co or Ni edge peak. The oxidation states of metal ions were also derived from the linear fitting of standard compounds at particular oxidation states. The shifts of peak positions were examined to indicate the variation of oxidation state as well.

Magnetization measurements on a series of Fe films grown by molecular beam epitaxy on GaAs (001) substrates and capped with a thin Au layer reveal interesting exchange bias (EB) properties at low temperatures. The observed exchange bias decreases rapidly with increasing temperature, and completely disappears above 30 K. While the Fe samples were not grown with an intentionally deposited antiferromagnetic (AFM) layer, X-ray reflectometry, X-ray absorption near-edge spectroscopy carried out near the L-edge of Fe, and comparison with similar Fe/GaAs samples capped with Al, which do not show exchange bias, suggest that the exchange bias in the GaAs/Fe/Au multilayers is caused by an AFM Fe oxide at the Fe/Au interface formed by penetration of oxygen through the Au capping layer. The observed exchange bias is accompanied by a strikingly asymmetric magnetization reversal of the Fe films occurring when the magnetic field is applied at angles away from the easy axis of the film. The observed asymmetry can be interpreted in terms of a competition between cubic, uniaxial, and unidirectional magnetic anisotropy characteristic of the exchange-biased Fe film.