POUVOIRS THERMOÉLECTRIQUES D'ALLIAGES DILUÉS DU NICKEL ET DU COBALT AVEC DES ÉLÉMENTS DE TRANSITION

Les pouvoirs thermoélectriques (P. T. E.) d'alliages dilués du nickel et du cobalt avec des éléments des trois premières séries de transition ont été mesurés dans deux domaines de température : de 1,2 à 4,2 °K et de 77 à 300 °K. Aux basses températures, le P. T. E. mesuré (S0) est proportionnel à la température et généralement, indépendant de la concentration en impuretés ; on détermine ainsi le P. T. E. associé à l'impureté seule : S0/T. Aux températures suffisamment élevées (contribution du "phonon-drag" négligeable) le P. T. E. varie encore linéairement avec la température mais dépend fortement de la concentration. On utilise cette dépendance en concentration à une température donnée (295 °K) pour analyser les données expérimentales dans un modèle de conduction à deux bandes indépendantes sσ (σ = ↑, ↓). Ce modèle néglige toute variation de la structure électronique des impuretés avec la température. On en déduit, les P. T. E. et les résistivités associés à chaque bande sσ à la fois pour la matrice et pour l'impureté. Les résultats obtenus confirment le schéma d'état lié virtuel s"up"d"up" déjà proposé pour ces alliages à partir des mesures d'aimantation et de résistivité.Thermoelectric power (T. E. P.) measurements of dilute alloys of nickel and cobalt with transition elements have been carried out in two temperature ranges : between 1.5 and 4.2 °K, and between 77 and 300 °K. At low temperatures, the measured T. E. P. (S0) is proportionnal to the temperature, and generally independent of the impurity concentration ; the T. E. P. araising from the impurity alone (S0/T) is deduced. At sufficiently high temperatures (phonondrag contribution is negligible) the temperature dependence of T. E. P. is still linear, but depends on concentration. We used this concentration dependence at 295 °K to analyse the experimental data in an independent two-band conduction model, assuming as negligible the variation of impurity electronic structure with temperature. The T. E. P. and resistivities associated with each band "sσ" for the matrix and the impurity are deduced. The obtained results check the previous model and the "s"up"d"down"" virtual bound state scheme

Electronic structure of f.c.c. NiFe alloys, investigated by means of secondary ion emission

The secondary ion emission of f.c.c. Ni1-cFec alloys (0 0.2. The concentration dependence of the iron ionization coefficient is well described by a simple local environment model where the magnetic state of an iron atom changes from strong ferromagnetic to polarized paramagnetic if surrounded by at least six iron nearest neighbours. Consequently the occurrence at the alloy Fermi level of a high density of states for the majority spin band is attributed to the existence of these paramagnetic iron atoms and not to a bulk transition of the alloy band structure to a weak ferromagnetic state.On étudie l'émission ionique secondaire des alliages Ni1-cFe c cubiques à faces centrées (0 0,2 sur les atomes de fer. La dépendance en concentration de la constante d'ionisation du fer est bien décrite par un simple modèle d'environnement local dans lequel l'état magnétique du fer change d'un état de ferromagnétisme fort à un état paramagnétique polarisé lorsqu'il est entouré d'au moins 6 plus proches voisins de fer. Conséquemment, on attribue l'apparition au niveau de Fermi d'une densité d'états élevée pour la direction de spin majoritaire à l'existence des atomes de fer paramagnétiques et non à une transition globale de l'alliage à un état de ferromagnétisme faible

XAFS Measurements on Co-K and Pt-LIII Edges in (111) CoPt3 Films

XAFS measurements at the Co-K edge and Pt-LIII edge were performed in an (111) epitaxial CoPt3 film with strong perpendicular magnetic anisotropy. The analysis of XAFS data obtained using in-plane and out-of-plane polarization show the existence of an anisotropic local order characterized by preferential heteroatomic pairs out of the film plane balanced with preferential homoatomic pairs in the (111) plane. The isotropic local order found in a magnetically isotropic CoPt3 film from Pt-LIII edge data should be confirmed by XAFS measurements on Co-edge

XAFS measurements on Co-K and Pt -L(III) edges in (111) CoPt3 films

XAFS measurements at the Co-K edge and Pt-LIII edge were performed in an (111) epitaxial CoPt3 film with strong perpendicular magnetic anisotropy. The analysis of XAFS data obtained using in-plane and out-of-plane polarization show the existence of an anisotropic local order characterized by preferential heteroatomic pairs out of the film plane balanced with preferential homoatomic pairs in the (111) plane. The isotropic local order found in a magnetically isotropic CoPt3 film from Pt-LIII edge data should be confirmed by XAFS measurements on Co-edge

Influence of magnetism on chemical order in a FeAl19.5 at %\mathsf{_{19.5~at\,\%}} single crystal. High temperature measurements of neutron diffuse scattering

The influence of magnetism on atomic short range order is shown in a single crystalline FeAl$_{19.5~{\rm at}\,\%}$ alloy in the disordered phase. The short range order has been measured in the paramagnetic state between 973 and 1 573 K on D7 at ILL through neutron diffuse scattering in the [110] plane. The effective pair interaction potentials $V_{1}$, $V_{2}$, $V_{3}$ and $V_{5}$ are calculated through an inverse cluster variation method. The results are compared to those obtained through X-ray diffuse scattering on the same alloy frozen in the ferromagnetic order state at 772 K. The effect of magnetic coupling on the atomic order is the most important on the 1st effective potential, which is smaller in the ferromagnetic phase than in the paramagnetic one by about a factor of 2.Nous montrons l'influence du magnétisme sur l'ordre atomique à courte distance dans un monocristal FeAl$_{19.5~{\rm at}\,\%}$ désordonné. L'ordre local a été mesuré dans le plan [110] dans l'état paramagnétique entre 973 et 1 573 K par diffusion diffuse de neutrons sur D7 de l'ILL. Les potentiels d'interaction de paires $V_{1}$, $V_{2}$, $V_{3}$ et $V_{5}$ ont été calculés par une methode de variation des amas inverses. Les résultats sont comparés à ceux que nous avions obtenus sur le même échantillon, trempé depuis 772 K dans la phase ferromagnétique, par diffusion diffuse de rayons X. L'effet du couplage magnétique sur l'ordre atomique est surtout visible sur le premier potentiel de paires effectif qui est diminué d'un facteur 2 dans la région ferromagnétique par rapport à la région paramagnétique

Lattice dynamics and migration enthalpies in CoPt3 and FePd

The frequencies of the normal modes of vibration of CoPt3 and FePd single crystals have been measured using inelastic neutron scattering. The measurements were performed in the L1(2) ordered phase (at 300 and 930 K) for CoPt3 and in the L1(0) ordered phase (at 300 and 860 K) for FePd. Dispersion curves were also measured in the fcc disordered states, at 1060 and 1020 K for CoPt3 and FePd, respectively. The activation enthalpy of atomic migration has been evaluated from the phonon density of states by applying Schober's model [H. R. Schober , J. Phys.: Condens. Matter 4, 9321 (1992)] and its extension to the L1(2) ordered structure. The phonon properties of FePd3 reported in the literature have been analyzed similarly and are compared with the results for CoPt3 and FePd. The contribution of the long-range order to the migration enthalpy estimated in the present analyses agrees well in magnitude with the previous evaluation by Monte Carlo simulation for alloys of the fcc, L1(2), and L1(0) structures

Structural origin of magnetic anisotropy in Co-Pt alloy films probed by polarized XAFS

Polarized X-ray absorption fine structure (XAFS) measurements at the Co K and Pt L3 edges show that the perpendicular magnetic anisotropy found in epitaxial fcc CoPt3 (111) films stems from the existence of anisotropic local ordering. Such ordering, induced during the codeposition process and dependent on the growth temperature, is characterized by preferential CoCo pairs in the film plane, balanced by preferential CoPt pairs out of the plane, resulting from some Co 2D-segregation. Polarized XAFS at the Pt edge reveals similar anisotropic local ordering in epitaxial hcp Co3Pt (0001) films exhibiting a larger magnetocrystalline anisotropy compared to that of bulk hcp Co. Besides, a polarization dependence of the Co XANES profile is observed only for the Co3Pt films exhibiting hcp symmetry