P. Galtier

Structure and magnetism of the Fe/GaAs interface

We study the magnetic properties of Fe thin films epitaxially grown on GaAs (001) for a large range of substrate temperature. Magnetization deficiency has been observed and studied. Its dependence on both thickness and temperature clearly show the existence of a nearly half-magnetized phase at the interface, covered by “as-bulk” Fe. Furthermore, reflection high-energy electron diffraction studies show a transition between two bcc structures with different crystalline parameters. Transmission electron microscopy confirms the formation of this interfacial phase, for which the compound Fe3Ga2−xAsx seems to be the best candidate.

Origin of the uniaxial magnetic anisotropy in Fe films grown by molecular beam epitaxy

Abstract We report the observation of a weak in-plane uniaxial magnetic anisotropy in Fe thin films grown by molecular beam epitaxy onto (001)-oriented MgO substrates. This anisotropy has been observed both in (Fe/Pd) superlattices and in single epitaxial Fe layers within a large range of thicknesses (15–800 A). We find that the uniaxial anisotropy results from the geometry of the deposition system, and in particular from the angle of incidence of the Fe atomic flux during deposition. Detailed magnetic and structural studies of a 800 A thick single Fe layer show that this anisotropy can be explained as a volume effect resulting from the magneto-elastic energy associated with an in-plane tetragonal distortion of the cubic Fe structure, with (a − b)/a = +(1.2 ± 0.3) × 10−3.

Magnetic properties and domain structure of epitaxial (001) Fe/Pd superlattices

Abstract The magnetic properties of well-characterized (001)Fe/Pd superlattices prepared by molecular beam epitaxy are presented. The saturation magnetization is enhanced due to the polarization of the Pd interface, and analysis of hysteresis loops indicate low coercive fields, abrupt magnetic reversals, and ferromagnetic coupling between the Fe layers for all Pd thickness investigated (10–50 A). It is also found that deposition on a stationary substrate can create a weak uniaxial in-plane anisotropy, which, for one of the two easy directions of Fe, causes a spontaneous rotation of the magnetization by 90° at low fields. This effect is clearly demonstrated by optical Kerr-effect imaging of the magnetic domain structure, and can be mistaken for antiferromagnetic coupling with very weak coupling fields. The strength of this uniaxial anisotropy is found to oscillate rapidly with Pd thickness, suggesting that it is very sensitive to the microstructure.

Vibrational spectroscopy of undoped and nitrogen-doped ZnO grown by metalorganic chemical vapor deposition

Raman backscattering and Fourier transform infrared measurements were performed on undoped and nitrogen-doped ZnO. Two broad vibrational modes near 1414 and 1585cm−1 were found that are attributed to the presence of carbon sp2 clusters. In nitrogen-doped ZnO these modes are enhanced. In addition a broad vibrational mode near 2004cm−1 is believed to be due to Zn–H complexes. The incorporation of N results in the formation of cyano radicals NO and NNO complexes effectively lowering the N doping efficiency. Furthermore, infrared measurements revealed the presence of CO2 molecules.

Semiconducting and magnetic properties of Zn1−xMnxO films grown by metalorganic chemical vapor deposition

Zn1−xMnxO diluted magnetic semiconductor single phase thin films have been grown by the MOCVD technique. Depositions have been done on fused silica and (0001)-sapphire substrates. Layers on silica are polycrystalline with [001] preferential orientation, while Zn1−xMnxO films on c-sapphire are highly (0001) textured. Manganese content (x) in the samples varies in 0.00–0.44 range. Change of c lattice parameter follows Vegard’s law. Hall effect measurements show a decrease of mobility with Mn increase and no change of carrier concentration. Optical transmission results present the increase of band gap with manganese incorporation. Superconducting quantum interference device magnetometry shows Curie–Weiss behaviors of the susceptibility with antiferromagnetic coupling with an effective exchange constant of J1∕kB=−16K.

TIME-RESOLVED PHOTOLUMINESCENCE SPECTROSCOPY FROM ERBIUM-DOPED GA0.55AL0.45AS

Time‐resolved photoluminescence (PL) spectroscopy has been performed on erbium‐doped Ga0.55Al0.45As. We have investigated the 4I13/2→4I15/2 optical transition. The measured fluorescence lifetime is around 1.2 ms which is similar to values found for other erbium‐doped III‐V compounds. Studies as a function of temperature indicate nonradiative transitions characterized by thermal activation of 40 meV. With the help of two beam experiments we have confirmed the existence of Auger losses during the PL excitation and a model of the excitation process is proposed that accounts for the experimental results.

Metal organic chemical vapor deposition growth and luminescence of ZnO micro- and nanowires

One dimensional (1D) ZnO heterostructures were deposited on C-plane sapphire using metal organic chemical vapor deposition at atmospheric pressure. Both catalyst-assisted and catalyst-free processes were investigated. In this latter case, growth parameters such as the substrate temperature, the oxygen/zinc ratio, or the carrier gas nature were varied in order to observe the modification of the film morphology. Different 1D shapes were produced, including rods, tubes, and needles, and were observed by scanning electron microscopy. Photoluminescence spectra reveal sharp excitonic transitions, and cathodoluminescence signals recorded along a ZnO conical needle exhibit a blueshift from the base to the top. The vapor-liquid-solid growth of ZnO nanowires using gold droplets was also successful.

Optical studies of erbium excited states in Ga0.55Al0.45As

Photoluminescence experiments have been carried out on erbium doped Ga0.55Al0.45As under continuous wave and pulsed laser excitation. For the first time the emissions arising from the two first Er3+ excited states 4I11/2, 4I13/2 and their temperature dependence have been systematically studied. From these results the nature and the excitation‐deexcitation processes of these photoluminescences are discussed. With the help of these spectroscopic data we give an estimate of the stimulated emission cross section and the laser applications of such rare‐earth doped III‐V materials are examined.

Contribution of current perpendicular to the plane to the giant magnetoresistance of laterally modulated spin values

Giant magnetoresistance (GMR) effects up to 10% have been observed in Co/Cu/FeNi spin valve structures grown onto step bunched vicinal Si(111) substrates misoriented towards [11-2]. The step bunching is activated using a simple thermal treatment which leads to surfaces where terraces alternate with facets at the nanometer scale. GMR of the spin valve structures is investigated with the current applied parallel or perpendicular to the steps. An in-plane uniaxial magnetic anisotropy is induced in each magnetic layer with the easy axis parallel to the steps. This results in square GMR behavior when the field is applied along the easy axis. Specific features observed when the field is applied along the hard axis are also shown to be the consequence of this anisotropy. When the initial misorientation angle of the substrate becomes higher than 4°, we observe an enhancement of the room-temperature GMR when the current is applied perpendicular to the steps. The origin of this enhancement is discussed based on the...

Opposite crystal polarities observed in spontaneous and vapour-liquid-solid grown ZnO nanowires

ZnO one-dimensional nanostructures were synthesized using the vapour-liquid-solid (VLS) process. Highly C-axis oriented ZnO nanowires, exhibiting a gold droplet on their top, were observed by scanning electron microscopy. Growth mechanisms are discussed, and the issue of supersaturation and precipitation of zinc in the gold droplet is debated. Transmission electron microscopy was carried out to investigate the crystalline properties and the polarity of the synthesized structures. In particular, an unexpected result shows that VLS ZnO nanowires grow with O-polarity, whereas spontaneous (catalyst-free) ZnO nanowires grow with Zn-polarity. The process at the origin of this polarity inversion is discussed.