M.C. Cadeville

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.

Structural order related to the magnetic anisotropy in epitaxial (111) CoPt3 alloy films

Abstract (111) Co x Pt 1− x epitaxial films (0.25 ≤ x ≤ 0.33) grown on a (0001) Ru buffer at temperatures ranging from 500 to 800 K were investigated. The dependence of the perpendicular magnetic anisotropy on the growth temperature is clearly related to the structural state of the alloy films. The largest perpendicular anisotropy found in a film grown at 690 K is correlated with the existence of a compressive strain resulting from preferential heteroatomic correlations oriented along the growth direction [111], driven by both Pt surface segregation and associated dominant surface diffusion. By contrast, the observation of L1 2 long-range chemical ordering in a film grown at 800 K, favoured by an increase of volume diffusion, is accompanied by the disappearance of perpendicular anisotropy.

Magnetism and spatial order in Ni-Pt and Co-Pt alloys

Abstract The interplay between magnetism and spatial order has been investigated in Ni-Pt and Co-Pt systems. Equilibrium chemical and magnetic phase diagrams have been determined. The effect of ordering on magnetic properties has been measured. Strong independence of magnetism and spatial ordering is observed.

High T-dependence of local order and its coupling with magnetism in iron based alloys

Abstract NMR and neutron diffuse scattering experiments show that positron correlations can be either enhanced (FeCo) or depressed (FeV) by the presence of magnetic interactions. The experimental results are explained using an Ising-Heisenberg Hamiltonian and are in qualitative agreement with the pair interactions we obtained theoretically using the static approximation.

Observation of an ordered new compound Co1−xRux prepared by MBE on a Ru buffer layer

Abstract Co 1− x Ru x (x ≈ 21 and 25 at%) thin layers (35 nm) have been grown by molecular beam epitaxy on a Ru [00.2] buffer layer on mica substrates at different temperatures. The alloys grow epitaxially in the hexagonal compact structure with a columnar microstructure. Some long range order, corresponding to a concentration modulation along the growth axis, has been observed in the layers deposited at low temperature ( T T = 820 K) the diffusion of the buffer layer Ru in the edges of the columns isolates them magnetically and is responsible of the large coercitive field (≈ 3 kOe) measured in the polar geometry for the sample prepared at the higher ( T = 820 K) deposition temperature.

Thermodynamic, electronic and magnetic properties of intermetallic compounds through statistical models

Local and average electronic and magnetic properties of transition metal alloys are strongly correlated to the distribution of atoms on the lattice sites. The ability of some systems to form long range ordered structures at low temperature allows to discuss their properties in term of well identified occupation operators as those related to long range order (LRO) parameters. We show that using theoretical determinations of these LRO parameters through statistical models like the cluster variation method (CVM) developed to simulate the experimental phase diagrams, we are able to reproduce a lot of physical properties. In this paper we focus on two points: (i) a comparison between CVM results and an experimental determination of the LRO parameter by NMR at 59Co in a CoPt3 compound, and (ii) the modelling of the resistivity of ferromagnetic and paramagnetic intermetallic compounds belonging to Co-Pt, Ni-Pt and Fe-Al systems. All experiments were performed on samples in identified thermodynamic states, implying that kinetic effects are thoroughly taken into account.

Respective Roles of Surface, Grain Boundary and Volume Diffusions in Driving Structural, Microstructural and Magnetic Properties of MBE Alloy Thin Films

The co-deposition of two metals using a molecular beam epitaxy (MBE) technique at various growth temperatures (T G ) yields single-crystal alloy thin films with a columnar microstructure whose structural and magnetic properties can be different from those of the corresponding equilibrium bulk alloys. A general overview of results obtained in hcp Co-Ru and hcp or fcc Co-Pt thin films grown on a hcp (0002) substrate will be presented. Around the A 3 B composition (A = Co, B = Ru or Pt) the films display a composition modulation along the growth direction whose amplitude is strongly dependent of TG passing through a maximum at respectively 600 and 650 K. This long range order (LRO) that does not exist in equilibrium phases is explained as resulting from the competition between two phenomena occurring simultaneously during the growth process: a surface effect driven by surface interactions and surface diffusion that tends to enrich the surface layer in one element (Co, Ru or Pt segregation) and a bulk effect driven by bulk interactions and bulk diffusion that tends to restore the bulk equilibrium phase when the bulk diffusion becomes efficient during the growth time. A thermally activated model that takes into account both surface and bulk diffusion during the growth time reproduces quite satisfactorily the T G dependencies of the LRO in Co 3 Ru and Co 3 Pt as well as that of the uniaxial magnetic anisotropy in CoPt 3 . At high T G ( > 800 K) or after ex situ anneals, the diffusion of the Ru buffer through the grain boundaries of the columnar microstructure, that occurs before the inside grain diffusion, isolates the columns magnetically and is thought to be responsible of the large observed coercive field and of a substantial modification of the magnetic domain shapes.

CVM and inverse CVM: Examples in Transition-Metal Alloys and Compounds

Some applications of the Cluster Variation Method (CVM) in analysing or interpreting experimental results are presented and discussed. The first example is the experimental determination of the effective pair interactions in alloys from neutron diffuse scattering experiment using the inverse CVM in both direct and reciprocal spaces. Results in Fe80V20 and Fe80V20 single crystals are presented. Another example is the modelling of the temperature dependence of the resistivity in ferromagnetic and paramagnetic compounds belonging to Co-Pt, Ni-Pt and Fe-Al systems. This approach uses the chemical (η) and magnetic (σ) long-range order parameter values deduced from the CVM simulations of the corresponding phase diagrams.

High resolution transmission electron microscopy studies of Co/Ru bilayers

Abstract Two bilayers, [Co 1.2 nm Ru 3.5 nm ] × 2 and [Co 4 nm Ru 3.5 nm ] × 2, are studied by high resolution transmission electron microscopy in plan-view and cross-sectional orientations, focusing on the structural variation of the Co at interfaces. We simulate the observed contrast, introducing a hexagonal lattice of misfit dislocations on the (0001) interface.