Co-Pt Systems Research Articles

PARTICLE MORPHOLOGY AND PHASE COMPOSITIONS OF COPT AND COPD NANOSTRUCTURED SYSTEMS IN THE REGIONS RICH IN PRECIOUS METALS

The morphology of nanoparticles, structural-phase properties, and phase transformations that occur during heating in the nanostructured CoPt and CoPd systems rich in precious metals, obtained by co-reduction of aqueous precursor solutions by hydrazine hydrate, were studied by X-ray diffraction and X-ray structural analysis, small-angle X-ray scattering, and high-resolution transmission electron microscopy. The results obtained are of interest for the development of the fundamentals of materials science of bimetallic nanoalloys, and provide a key to understand the processes of the formation of FePt, CoPt and FePd nanoalloys while heating intermetallic compounds with a highly ordered structure L10, which possess record-setting magnetic and magneto-optical properties.

Solid-state synthesis, rotatable magnetic anisotropy and characterization of Co1-xPtx phases in 50Pt/50fccCo(001) and 32Pt/68fccCo(001) thin films

We reported the phase formation sequences in 50Pt/50fcc-Co(001) and 32Pt/68fcc-Co(001) thin films after annealing up to 850 °C. In both cases, the ordered L10 phase formed first on the Pt/Co interface at ∼400 °C and as the annealing temperature increased the L10 phase transformed into the chemically disordered fcc A1 phase in 50Pt/50fcc-Co(001) at 750 °C and in 32Pt/68fcc-Co(001) films at 550 °C. Based on the analysis of solid-state reactions in thin films, a phase transition at ∼ 400 °C is predicted in Co-Pt systems with a 32–72% Pt composition. Torque measurements of the 50Pt/50fcc-Co(001) samples showed that the rotatable magnetic anisotropy coexisted with the three variants of L10 in a temperature range of 400–750 °C. An analysis of the torque curves revealed that the L10 films consist of a soft magnetic layer epitaxially intergrown to the substrate MgO(001) and a top layer having rotatable magnetic anisotropy. It showed that the magnetically hard properties of L10 films are associated with a rotatable magnetic anisotropy layer. A model of rotatable magnetic anisotropy is reasoned, which is founded on some identical mechanisms of rotatable magnetic anisotropy and magnetic-field-induced strains, explaining the ferromagnetic shape-memory effect in Heusler alloys. Our results suggested that the rotatable magnetic anisotropy phenomena may have an important role in the origin of perpendicular anisotropy in hard magnetic L10 structures.

The electrochemical synthesis and investigation of nanostructured Fe-Pt and Co-Pt systems

By means cyclic and anodic linear sweep/stripping voltammetry methods the electrochemical behavior of bimetallic electrolytic nanoalloys and nanostructured powders Fe–Pt, Co–Pt was investigated in acidic (chloride, sulfate, tartrate) and ammonia buffer media with the glassy carbon electrode. The conditions (supporting electrolytes, concentrations of metal ions, voltammetric parameters) of the electrochemical synthesis and voltammetric characterization of the binary systems were established.

Structural, electronic and magnetic properties of MxPt1-X, (M= Co, Ni and V) binary alloys

The structural, optical and magnetic properties of ordered MxPt1-x (M = Co, Ni and V) binary alloys have been investigated using Vienna ab initio Simulation Package (VASP) within the framework of Density Functional Theory (DFT) and the Generalized Gradient Approximation (GGA). Ab initio calculations have been performed to obtain the most stable structure for each of the three binary systems. In addition, the optical and electrical properties such as electronic band structure, density of states and partial density of states of MxPt1-x binary alloys have been investigated. Specifically, total energy minimization has been performed to calculate the equilibrium in-plane, ao, out-of-plane, co, and volume, Vo, structural lattice parameters of MxPt1-x binary alloys. We found that ao, co and Vo for CoPt, NiPt and VPt3 equal to (ao = 3.806 A, co = 3.707 A and Vo = 53.7 A3) (ao = 3.84 A, co = 3.62 A and Vo = 53.64 A3) and (ao = 3.88 A, co = 7.88 A and Vo = 118.71 A3) respectively. Furthermore, the magneto-crystalline anisotropy energies (MAE) have been calculated to get a deeper insight into magnetic characteristics of the MxPt1- x binary alloys. We found that MAE values for CoPt, NiPt and VPt3 binaries are equal to 1.60, 0.231 and 0.0116 meV/unit cell respectively. These MAE values correspond to magneto-crystalline anisotropy constant K values equal to 4.8 ×107, 6.9 ×106 and 1.46 × 105 erg/cm3. The obtained results reveal that CoPt and NiPt binary systems exhibit attractive optical and magnetic properties, which make both systems potential candidates for magneto-optical and optical-electronic devices. Our results are in good agreement with the previous experimental and theoretical findings.

Magnetic Properties of Strained L10-ordered FePt and CoPt: An ab initio Study

Using ab initio calculations, the effects of uniaxial, biaxial, and hydrostatic strains on the magnetocrystalline anisotropy of L10-orderd FePt and CoPt alloys were systematically investigated. Interestingly, the rates and the signs of magnetocrystalline anisotropy changes of FePt and CoPt were determined by the directions and dimensions of strains. The calculation results are consistent with the previous experimental observations and are expected to provide directions to tailor magnetic properties of various types of L10-ordered FePt and CoPt systems.

Co-Pt and Fe-Pt bimetallic nanoparticles in a carbon matrix are prepared by a plasma arc process

Nanoparticle ensembles in Co-Pt and Fe-Pt systems embedded in a carbon matrix are prepared by a plasma-arc method. Two-phase samples based on disordered FCC solid solutions are prepared in both systems. In the Co-Pt system, the solid solujtions are mixed within one ensemble; in the Fe-Pt system, they are spatially separated. A magnetization hysteresis curve from Co-Pt nanoparticles (3–12 nm) fully coincides with a magnetization curve from a single-phase disordered solid solution Co0.50Pt0.50 (7–9 nm) prepared at 325°C by thermolysis of the precursor double complex salt (DCS). Metal ratios, solid solution structures, and particle sizes being the same, the magnetic response is dictated exclusively by the amount of a magnetic phase of mixed and interacting particles and is independent of the distribution of the magnetic phase over the solid solution phases.

Structural, magnetic, and defect properties of Co-Pt-type magnetic-storage alloys: Density-functional theory study of thermal processing effects

Using an optimized-basis Korringa-Kohn-Rostoker-coherent-potential approximation method, we calculate formation enthalpies $\ensuremath{\Delta}{E}_{f}$, structural, and magnetic properties of paramagnetic (PM) and ferromagnetic, disordered A1 and ordered $\text{L}{1}_{0}$ CoPt, FePd, and FePt systems that are of interest for high-density magnetic-recording media. To address processing effects, we focus on the point defects that dictate thermal properties and planar defects (e.g., $c$ domain and antiphase boundaries) which can serve as pinning centers for magnetic domains and affect storage properties. We determine bulk Curie $({T}_{c})$ and order-disorder $({T}_{\text{o-d}})$ transition temperatures within 4% of observed values, and estimates for nanoparticles. Planar-defect energies ${\ensuremath{\gamma}}_{x}^{hkl}$ show that the favorable $[hkl]$ defects depend on processing conditions as ${T}_{c}l{T}_{\text{o-d}}$ in these alloys, and the PM defects agree with those observed. We correlate all properties with the electronic structure.

Formation of nanosized bimetallic particles based on noble metals

Methods for the preparation of nanosized alloys from noble metals by decomposition of monomolecular precursors are described, and the results of these studies are reported. Other aspects of the synthesis procedure and properties of bimetallic nanostructures are also considered. Thermolysis of noble metal compounds led to the formation of an ultradisperse powder of their alloys. This method affords catalysts with a uniform distribution of active particles. Nanosized particles of a number of alloys in hydrogen and inert media were prepared. The optimum parameters of the reduction of complex compounds (gas medium, thermolysis temperature, heating rate, and annealing time) were determined to obtain ultradisperse powders with the required particle size, phase composition, and structure. For Co-Pt and Pt-Pd systems, bimetallic catalysts deposited on γ-Al2O3 and Sibunit can be obtained; these catalysts show higher activity in selective oxidation of CO compared with monometallic catalysts.

Influence of composition, many-body effects, spin-orbit coupling, and disorder on magnetism of Co-Pt solid-state systems

Systematic trends in the magnetism of Co-Pt solid-state systems are explored by studying the effects of composition, electron correlations, spin-orbit coupling (SOC), and long-range order. Ab initio fully relativistic calculations were performed for bulk Co, a substitutional Pt impurity in Co, ${\text{Co}}_{3}\text{Pt}$, CoPt, ${\text{CoPt}}_{3}$, and a substitutional Co impurity in Pt. Many-body effects beyond the local spin-density approximation (LSDA) were included via the dynamical mean-field theory (DMFT); a comparison with results based on the Brooks orbital-polarization (OP) scheme was also made. The disorder was treated within the coherent-potential approximation. We found that while the spin magnetic moments ${\ensuremath{\mu}}_{\text{spin}}$ at the Co atoms monotonously increase with increasing Pt concentration, the orbital magnetic moments ${\ensuremath{\mu}}_{\text{orb}}$ do not follow the trend of ${\ensuremath{\mu}}_{\text{spin}}$. Magnetic moments ${\ensuremath{\mu}}_{\text{spin}}$ and ${\ensuremath{\mu}}_{\text{orb}}$ at Pt atoms do not depend on Pt concentration monotonously. Most of these trends can be understood in terms of hybridization and site-dependent SOC. The OP scheme of Brooks corrects the deficiencies of a pure LSDA only if the Pt concentration is low. The $\text{LSDA}+\text{DMFT}$ scheme provides ${\ensuremath{\mu}}_{\text{orb}}/{\ensuremath{\mu}}_{\text{spin}}$ ratios for Co atoms which agree with experiment even for high Pt content. Introducing disorder leads to an enhancement of ${\ensuremath{\mu}}_{\text{spin}}$ at Co atoms and to a suppression of ${\ensuremath{\mu}}_{\text{spin}}$ at Pt atoms. The ${\ensuremath{\mu}}_{\text{orb}}$ at Co atoms does not exhibit any systematic dependence on the degree of order, while ${\ensuremath{\mu}}_{\text{orb}}$ at Pt atoms decreases with increasing disorder for all Pt concentrations.

Comparative study on alloy cluster formation in Co-Al and Co-Pt systems

The formation of alloy clusters using a plasma-gas-aggregation technique is described for Co-Al and Co-Pt systems. This method employs two separate elemental sputtering sources and a growth chamber. Metallic vapors generated were cooled rapidly in an Ar atmosphere, and grown into alloy clusters. The composition of the clusters was controlled by adjusting the ratio of the applied sputtering power. We found that B2-CoAl clusters of about 12 nm in diameter were formed for a composition range wider than that predicted by the Co-Al phase diagram, and that high-temperature fcc-CoPt clusters were formed in the Co-Pt system. These findings suggest the nonequilibrium nature of the cluster formation. The size distribution of the clusters is highly monodispersive and does not follow commonly observed log-normal distribution. These results were discussed from the viewpoint of simple gas dynamics. We concluded that monomer absorption with discrete residence time is the dominant mechanism for monodispersive alloy cluster formation, and that the contrasting thermodynamical features between the Co-Al and Co-Pt systems are at the cause of the observed difference in average cluster size.

Light ion irradiation of Co/Pt systems: Structural origin of the decrease in magnetic anisotropy

We study the structural properties of Pt/Co/Pt systems submitted to ${\mathrm{He}}^{+}$ ion irradiation, in order to understand why the magnetic anisotropy can be decreased in a controlled way. It is shown by grazing x-ray reflectometry that the irradiation-induced Pt and Co atom displacements can be largely accounted for by a simple ballistic recoil mechanism model. Our results indicate that even in these nm-thick films, irradiation may affect the upper and lower interfaces differently. Specifically, the upper Co interface undergoes short-range mixing, resulting in roughness, whereas the lower Co interface mostly evolves by longer-range mixing, leading to alloy formation. Irradiation also releases strain in these Co-Pt systems, but has no chemical ordering effect. Together with slow asymmetric interface roughening, the cobalt tensile strain relaxation at low fluences accounts for the magnetic anisotropy decrease. The type of analysis we propose could be useful to understand why other magnetic properties, such as interlayer exchange coupling, can be controlled by light ion irradiation.

Magnetic hardening and coercivity mechanisms in L1 0 ordered FePd ferromagnets

The tetragonal L1 0 family of ferromagnets including the Co-Pt, Fe-Pt, Fe-Pd and Mn-Al alloy systems exhibits high, uniaxial magnetocrystalline anisotropies with first anisotropy constants K 1 ~ 10 7–10 8 ergs/cm 3. The domain parameters of these materials are similar to the rare earth permanent magnets and the defect structure is dominated by planar faults such as APB's, stacking faults, and twins. In this study the micromagnetic analysis of Kronmüller has been applied to elucidate the mechanism of coercivity controlling the hysteresis of the polytwinned L1 0 FePd equiatomic alloy and the results are consistent with pinning control involving atomically thick, planar defects. Also, Lorentz microscopy has been used to observe the interaction of magnetic domain walls with the planar faults characteristic of these materials. Finally, a thermomechanical treatment involving concomitant ordering and recrystallization is shown to eliminate the well-known polytwinned structures which generally evolve during the formation of the L1 0 phase leading to enhanced coercivities. The fundamental structure-property relationships involved are discussed.

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.

Magnetic and Chemical Ordering in the NiPt and the CoPt Systems

AbstractThe interplay between atomic ordering and magnetism in the NiPt and the CoPt systems is investigated theoretically and experimentally. The complete equilibrium phase diagrams are calculated within the framework of statistical models and are compared with experimental data. Experimental results for the Curie temperature, magnetization and paramagnetic susceptibility in ordered and disordered states are satisfactorily reproduced by a Hamiltonian that contains chemical and short-range order dependent magnetic interactions.

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

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.