V. Pierron-Bohnes

Room-temperature ferromagnetism in Zn1−xCoxO magnetic semiconductors prepared by sputtering

We have used magnetron cosputtering to grow Zn1−xCoxO magnetic dilute semiconductors. The growth has been performed on SiO2∕Si and Al2O3(0001) substrates. The Co concentration has been varied between 0.1 and 0.25 and the substrate temperature between room temperature and 600°C. X-ray diffraction analysis has shown that for the films grown on Si substrates the structural quality of the film is improved by increasing the growth temperature and/or postgrowth annealing. The films are textured with c axis of the wurtzite structure along the growth direction. However, for the films grown on Al2O3 substrate quasi-epitaxial films have been obtained for 600°C substrate temperature. Magnetization measurements have shown that the ferromagnetism is directly correlated to the structural quality and appears by increasing the growth temperature and/or postgrowth annealing. Moreover, for the highly textured film a clear magnetic perpendicular anisotropy has been evidenced with the easy magnetization axis along the growth...

Evidence of superparamagnetic co clusters in pulsed laser deposition-grown Zn0.9Co0.1O thin films using atom probe tomography.

Nanosized Co clusters (of about 3 nm size) were unambiguously identified in Co-doped ZnO thin films by atom probe tomography. These clusters are directly correlated to the superparamagnetic relaxation observed by ZFC/FC magnetization measurements. These analyses provide strong evidence that the room-temperature ferromagnetism observed in the magnetization curves cannot be attributed to the observed Co clusters. Because there is no experimental evidence of the presence of other secondary phases, our results reinforce the assumption of a defect-induced ferromagnetism in Co-doped ZnO diluted magnetic semiconductors.

Growth conditions to optimize chemical order and magnetic properties in molecular-beam-epitaxy-grown CoPt/MgO(001) thin films

CoPt films grown on MgO(001) substrates by molecular-beam epitaxy are expected to develop a very large uniaxial magnetocrystalline anisotropy and a high coercivity when the [001] single variant L10 structure develops along the growth direction. The present study investigates the growth conditions that provide the best structural quality of the films and their related magnetic properties. The quality of the substrate surface, the thickness, and the texture of the Pt buffer layer, the presence or absence of a Cr seed layer, are found to be determinants for the fraction of well-oriented grains in the films, whereas the growth temperature mainly pilots the degree of L10 order η. The uniaxial magnetocrystalline anisotropy energy of the [001] grains (Ku001) is maximum at the stoichiometric composition and shows a linear increase with η. The coercive fields and the magnetic domain size are discussed in relation to the microstructure, the degree of L10 order and the magnetic microscopic constants of the films.

Effect of ion irradiation on the structural and magnetic properties of sputtered CoPt alloy

Abstract We are interested in studying thin layers of CoPt alloy prepared by co-deposition sputtering. The CoPt layers are deposited on a MgO(001) substrate with a Pt(001) buffer layer. We obtain a layer with the L1 0 tetragonal structure, ordered in the growth direction. This equiatomic L1 0 phase is a “natural” multilayer which consists in a stacking along the (001) direction of pure Co and pure Pt monolayers. In this case, we get an easy magnetization direction perpendicular to the layer plane, which is the required configuration for the perpendicular magneto-optic recording. The order state of these thin films has been modified locally by a low energy ion irradiation. The structural and magnetic properties have changed jointly: with a 40-keV energy and a 4×10 16 -ions/cm 2 flux, the chemical order decreases (the long-range-order (LRO) parameter changed from S =0.80 to S =0.50), the coercive field diminished from 0.090 T down to 0.052 T. Such results show that it is possible to locally change the magnetization easy axis, which is promising for applications.

Lattice dynamics and migration enthalpies in iron-rich Fe - Al alloys and ordered and B2 compounds

The phonon dispersion curves of iron-rich Fe - Al alloys have been studied using inelastic neutron scattering. Measurements were performed on three single crystals at three temperatures, 300 K, 800 K and 1250 K, corresponding to the bcc solid solution for x = 0.2 and to three different states of order , B2 and A2 for x = 0.25 and 0.30. The migration enthalpies are deduced from the phonon density of states through a model proposed by Schober. They decrease with increasing temperature and have a minimum value at the composition. A comparison of the present determination of in Fe - Al with similar -measurements on Fe - Si confirms the contribution of the effective pair interaction energies to the migration enthalpy, as described in models of Monte Carlo simulations of ordering kinetics in B2 and compounds. In the phase, the three separate determinations of (present work), of the formation enthalpy of the vacancies , and of the activation enthalpy of the ordering kinetics , which are all average quantities, satisfy the addition law: .

Magnetic patterning using ion irradiation for highly ordered CoPt alloys with perpendicular anisotropy

We used a combination of ion irradiation and e-beam lithography to magnetically pattern an ordered CoPt alloy with strong perpendicular magnetic anisotropy. Ion irradiation disorders the alloy and strongly reduces the magnetic anisotropy. Magnetic force microscopy showed a regular array of 1μm2 square dots with perpendicular anisotropy separated by 1 μm large ranges with in-plane anisotropy. This is further confirmed by magnetic measurements, which showed that arrays protected by a 200 nm Pt layer present the same coercive field and the same perpendicular anisotropy as before irradiation. This is promising for applications in magnetic recording technologies.

Ordering kinetics in Ni3Al by molecular dynamics

We present molecular dynamics simulations of ordering kinetics in the Ni3Al alloy performed within the embedded atom method (EAM) as a scheme for interatomic potentials. The simulation cell containing 1372 atoms was initially perfectly L12-long-range ordered. After having artificially created one vacancy by removing at random one Ni-atom, the dynamics of the system was simulated at constant temperature and pressure. Atoms are found to migrate predominantly via jumps to nearest-neighbour (nn) vacancies. The number of antisites is low in comparison to the total number of atomic jumps. Therefore most of the jumps are ineffective for ordering kinetics causing only temporary change of the chemical order. Accordingly, the jumps creating antisites are most often followed by reverse jumps. Antisite defects are created as nn antisite pairs. This result is in agreement with predictions based on a model originally formulated within Monte Carlo simulations. 2002 Published by Elsevier Science B.V.

RECIPROCAL-SPACE ANALYSIS OF SHORT-RANGE-ORDER INTENSITIES BY THE CLUSTER-VARIATION METHOD

A reciprocal space formulation of the Cluster Variation is used in order to extract effective pair interactions in alloys from experimental short-range order diffuse intensities. The method is applied to the analysis of the short range order contribution to the neutron diffuse scattering of a

Growth of Co/Ru strained superlattices

Fe-19.5 {\%} Al

Magnetic switching field distribution of patterned CoPt dots

single crystal. A detailed comparison with real space methods is carried out for three different levels of approximations. For the highest level of approximation used in this study, effective pair interactions up to fifth neighbors are obtained.