V. H. Etgens

Room temperature ferromagnetism of Co doped CeO2- δ diluted magnetic oxide : Effect of oxygen and anisotropy

The authors report ferromagnetism at room temperature in Co doped CeO2-delta thin films grown by pulsed laser deposition on SrTiO3 and Si substrates. On SrTiO3 ceria is epitaxied and displays a high crystalline quality. On Si the films are textured with a dominant orientation. While the ferromagnetism is little affected by the amounts of structural defects, it depends sensitively on the oxygen pressure during growth and annealing. This indicates that oxygen vacancies could be involved in the magnetic coupling between Co ions. Furthermore, the epilayers grown on SrTiO3 display a large magnetic anisotropy with an out of plane easy axis. (c) 2007 American Institute of Physics.

Structural, magnetic and spectroscopic study of a diluted magnetic oxide: Co doped CeO2−δ

A structural, spectroscopic and magnetic study of Co doped CeO2−δ diluted magnetic oxide (DMO) thin films grown by pulsed laser deposition is presented. No secondary phase or metallic clusters could be detected. The samples are ferromagnetic at room temperature and epitaxial films display a large magnetic anisotropy with an out-of-plane easy axis. The evolution of the magnetization with temperature did not reveal any superparamagnetic signal related to nanosized clusters, reinforcing the conclusion that the ferromagnetism is intrinsic in this system. The magnetization at saturation has the same magnitude (~1.4–1.5 μB/Co) in epilayers and textured films and shows no clear dependence on the structural quality, contrary to other DMOs. It is also shown that ferromagnetism is not restricted to a particular region of the films, such as the interface. The ferromagnetic signal depends sensitively on the oxygen pressure during growth and post-growth annealing. The valence of Ce and Co was deduced from x-ray spectroscopies, revealing a predominant Co2+ state. The origin of ferromagnetism in Co doped CeO2−δ is discussed in connection with possible charge-compensating defects and existing models describing indirect exchange in DMOs.

Grazing incidence fast atom diffraction: An innovative approach to surface structure analysis

An alternative diffraction technique, based on grazing incidence scattering of high energy atoms, is applied to surface structure determination of crystalline surfaces. This technique, named GIFAD for grazing incidence fast atom diffraction, uses the same geometry as reflection high energy electron diffraction but is less invasive, more surface sensitive, and readily interpretable quantitatively. We present here a demonstration of this approach on a prototypical II–VI compound, ZnSe(001). Besides providing lattice parameter with high accuracy, we show that GIFAD gives straightforward access to the surface valence electron density profile, allowing clear identification of an electron transfer from Zn to Se.

Fe/ZnSe(001) Schottky-barrier height evaluated by photoemission

We present the Schottky-barrier height determination for the Fe/ZnSe(001) system performed by core and valence level photoelectron emission spectroscopy. Above the thickness of 2 ML, the Fe–Fermi level position is stabilized at 1.6 eV above the valence-band maximum of the n-type undoped ZnSe. This corresponds to a Schottky-barrier height value of 1.1 eV. A bulk-like d-band electronic structure could be observed for thickness as thin as 2 ML of Fe.

Dynamic grazing incidence fast atom diffraction during molecular beam epitaxial growth of GaAs

A Grazing Incidence Fast Atom Diffraction (GIFAD) system has been mounted on a commercial molecular beam epitaxy chamber and used to monitor GaAs growth in real-time. In contrast to the conventionally used Reflection High Energy Electron Diffraction, all the GIFAD diffraction orders oscillate in phase, with the change in intensity related to diffuse scattering at step edges. We show that the scattered intensity integrated over the Laue circle is a robust method to monitor the periodic change in surface roughness during layer-by-layer growth, with oscillation phase and amplitude independent of incidence angle and crystal orientation. When there is a change in surface reconstruction at the start of growth, GIFAD intensity oscillations show that there is a corresponding delay in the onset of layer-by-layer growth. In addition, changes in the relative intensity of different diffraction orders have been observed during growth showing that GIFAD has the potential to provide insight into the preferential adatom attachment sites on the surface reconstruction during growth.

Resonant tunnel magnetoresistance in epitaxial metal-semiconductor heterostructures

We report on resonant tunneling magnetoresistance via localized states (LS) through a ZnSe semiconducting barrier which can reverse the sign of the effective spin polarization of tunneling electrons. Experiments performed on Fe/ZnSe/Fe planar junctions have shown that positive, negative, or even its sign-reversible magnetoresistance can be obtained, depending on the bias voltage, the energy of LS in the ZnSe barrier, and spatial symmetry. The averaging of conduction over all LS in a junction under resonant condition is strongly detrimental to the magnetoresistance.

Tuning the period of elastic MnAs/GaAs(001) α−β pattern by Fe deposition

The alpha-beta elastic striped pattern of MnAs/GaAs(001) is shown to be perturbed by Fe overlayer growth. After nanometric Fe thin film deposition, the elastic pattern is still present in the 10-45 degrees C temperature range, but the period of the pattern increases by up to 40% for MnAs epilayers 70 to 400 nm thick. This observation can be interpreted within a simple model calculation of the pattern elastic energy, providing quantitative agreement with the experimental results

The use of thickness graded samples to investigate the elastic to plastic relaxation in ZnSe/GaAs(001)

The strain relaxation process of ZnSe grown by molecular beam epitaxy on GaAs(001) has been studied using high resolution x-ray diffraction. One single sample with a tailored thickness gradient has been used, allowing us to continuously follow the strain in a wide range of thicknesses. The relaxation starts around 1300 A with the coexistence of fully strained and partially relaxed regions of the ZnSe and continues up to 1600 A. These findings can be explained by the formation of misfit dislocations on the top surface with subsequent migration to the ZnSe/GaAs interface.

Growth and magnetic properties of MnAs epitaxied on GaAs(111)B

We have grown MnAs layers on GaAs(111)B by molecular beam epitaxy and studied their surface and epilayer morphology and its interconnection with magnetic properties. The influence of growth variables such as the As/Mn flux ratio and substrate temperature on the properties of the epilayer was studied. These parameters were found to drastically modify the epilayers characteristics, including its structural quality, and also strongly alter its magnetic properties. A more in-depth knowledge of how these parameters affect epilayers would be extremely useful in tailoring epilayers with specific desirable properties. (c) 2006 American Institute of Physics.

The role of magnetoelastic and magnetostrictive energies in the magnetization process of MnAs/GaAs epilayers

In this work we present a detailed study of the anisotropic magnetic behavior of MnAs epilayers grown by molecular beam epitaxy on GaAs(001) and GaAs(111)B substrates. An extended approach of the Stoner-Wohlfarth model is used to simulate magnetic hysteresis loops of MnAs epilayers for temperatures around the magnetostructural phase transition. We demonstrate that magnetoelastic and magnetostrictive energy contributions to the magnetic free energy density are crucial to correctly describe the magnetization of both kinds of MnAs epilayers.