Quantitative depth profiling of an alternating Pt/Co multilayer and a Pt-Co alloy multilayer by SIMS using a Buckminsterfullerene (C60) source

A Buckminsterfullerene ion beam has been applied to the depth profiling of an alternating pure Pt and pure Co multilayer. Quantitative depth profiling was performed by secondary ion mass spectrometry (SIMS) with C₆₀ ions using Pt-Co alloy films with different compositions. Relative sensitivity factors (RSFs) derived from a Pt₃₉Co₆₁ alloy film were used to convert an original depth profile to a composition depth profile. A severe interface artifact observed in the depth profile of a Pt/Co multilayer was quantitatively correlated with a gradual variation of matrix composition through the Pt/Co and Co/Pt interfaces by comparison with the depth profiling of an alloy multilayer film. Moreover, the interface artifact could be compensated by conversion of the profile to a composition profile using the same RSFs. The depth resolutions of a Pt/Co multilayer derived from the composition depth profile were much larger than the apparent interface widths measured from the original depth profile due to the nonlinear relationship between the Co and Pt ion intensities and their compositions.     

Solid-state reaction in Ti/Ni multilayers studied by magneto-optical and optical spectroscopies, and X-ray diffraction

Comparative study of the solid-state reaction (SSR) in a series of Ti/Ni multilayered films (MLF) with a bilayer period of 0.65-22.2 nm and a constant Ti to Ni sublayer thickness ratio has been performed by using the experimental and computer-simulated magneto-optical (MO) and optical spectroscopies as well as X-ray diffraction (XRD). It was shown that alloyed-like regions in an amorphous structure is spontaneously formed near the interfaces between pure elements during the film deposition. The thickness of this region was estimated as 2-3.8 nm on the basis of the MO and optical studies. The SSR in the Ti/Ni MLF caused by an annealing at 580 K for 60 min increases the thickness of these interfacial amorphous regions. It was shown that SSR takes place mainly in the Ti/Ni MLF with relatively “thick” sublayers. The existence of a threshold nominal Ni-sublayer thickness for observing the equatorial Kerr effect of about 3.0 and 4.5 nm for the as-deposited and annealed Ti/Ni MLF, respectively, is explained by formation of the nonmagnetic alloyed regions between pure components during the film deposition as a result of the SSR. For the case of Ti/Ni MLF, the MO and optical approaches turn out to be more sensitive in determining the thickness of the reacted zone, while XRD is more useful for the structural analysis. It was also shown that the very thin nonreacted Ni sublayers have different MO properties (and hence electronic structure) from the bulk. Received 2 May 2001 and Received in final form 21 November 2001     

Thermo-stimulated surface segregation in the ordering alloy Pt80Co20(1 1 1): Experiment and modeling

In this paper, a method of Ionization Spectroscopy (IS) is proposed for the non-destructive layer-by-layer analysis of the elemental composition of a solid surface. Using ionization energy loss spectra, a layer-by-layer concentration profile of the Pt₈₀Co₂₀(1 1 1) alloy surface is obtained for different annealing temperatures. For the disordered Pt₈₀Co₂₀(1 1 1) at room temperature, the first atomic layer consists of pure Pt with damped oscillations in the deeper layers. Heating the sample reduces the oscillations. However, at a temperature of 823 K, a sandwich-like structure of the type Pt/Co/Pt was found in the first three atomic layers. For the ordered state the first atomic layer also consists of pure Pt with bulk concentration in other layers. LEED analysis shows a p(2 × 2) superstructure for the surface of the ordered Pt₈₀Co₂₀(1 1 1) alloy. The segregation behavior in this alloy is further studied by Monte Carlo (MC) simulations combined with the Constant Bond Energy (CBE) model. The results of the MC simulations agree well with the experiments at the higher temperatures, both for the surface composition and the concentration depth profile. At lower temperatures, some discrepancies exist between the MC results and the measured concentration profile.     

The effect of gold on platinum oxidation in homogeneous Au-Pt electrocatalysts

Oxidation of Au-Pt thin films was carried out in ambient air at room temperature and characterized by X-ray photoelectron spectroscopy. The homogeneous films were prepared by RF co-sputtering with concentrations varying from Au₉Pt₉₁ to Au₈₉Pt₁₁ and compared to pure Pt and Au thin films. Spectral deconvolution of the Au 4f and Pt 4f core levels revealed linear peak shifts for both the Au-Au and Pt-Pt bonding components as a function of alloy mixture and metallic component peak asymmetry that remained constant for all alloy stoichiometries. The predominant oxidation products were PtO and PtO₂ and were characterized by stable core level binding energies for all films. A gradual decline in the Pt-O_x products and corresponding levels of elemental oxygen was observed with increasing Au content but was similar in proportion to the metallic Pt components. Based on these results, variations in Pt oxide phases and/or concentration do not appear to contribute to enhanced electrocatalytic activity for oxygen reduction observed for the intermediate alloy stoichiometries.     

Density functional study of small cobalt-platinum nanoalloy clusters

The structural, energetic, electronic and magnetic properties of small bimetallic Co_nPt_m (n+m≤5) nanoalloy clusters are investigated by density functional theory within the generalized gradient approximation. A plausible candidate for the ground state isomer and the other possible local minima, binding energies, relative stabilities, magnetic moments, the highest occupied and the lowest unoccupied molecular orbital energy gaps have been calculated. It is found as a general trend that average binding energies of Co-Pt bimetallic clusters increase with Pt doping. Planar structures of pure Co clusters become 3D with the addition of Pt atoms. CoPt₂, Co₂Pt₂, Co₃Pt₂, and CoPt₄ nanoalloys are identified as the most stable species since they have higher second finite difference in energy than the others. Pt doping decreases the total spin magnetic moment gradually. A rule for the prediction of the total spin moments of small Co-Pt bimetallic clusters is derived.     

Alloy formation of Co/Ni/Pt(1 1 1)

The initial growth and alloy formation of ultrathin Co films deposited on 1 ML Ni/Pt(1 1 1) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). A sequence of samples of d_Co Co/1 ML Ni/Pt(1 1 1) (d_Co = 1, 2, and 3 ML) were prepared at room temperature, and then heated up to investigate the diffusion process. The Co and Ni atoms intermix at lower annealing temperature, and Co-Ni intermixing layer diffuses into the Pt substrate to form Ni-Co-Pt alloys at higher annealing temperature. The diffusion temperatures are Co coverage dependent. The evolution of UPS with annealing temperatures also shows the formation of surface alloys. Some interesting LEED patterns of 1 ML Co/1 ML Ni/Pt(1 1 1) show the formation of ordered alloys at different annealing temperature ranges. Further studies in the Curie temperature and concentration analysis, show that the ordered alloys corresponding to different LEED patterns are Ni_xCo_1−xPt and Ni_xCo_1−xPt₃. The relationship between the interface structure and magnetic properties was investigated.     

Effects of soft layer softness on the magnetic properties of perpendicular exchange-coupled nanocomposite films

The anisotropy of the soft layer in the Co_100−xPt_x/Co₇₁Pt₂₉ (x=0, 7 and 17) perpendicular exchange-coupled composite (ECC) films was varied by changing the Pt content. The effects of soft layer softness (thickness and anisotropy) on the coercivity and magnetization reversal mechanisms of ECC were studied. Results showed that both remanence ratio (M_r/M_s) and coercivity of the ECC films reduced with an increase in soft layer thickness. However, the rate of coercivity reduction reduced when soft layer anisotropy was increased simultaneously. This was confirmed by the following facts. For the ECC with Co soft layer, the magnetization reversal mechanism within the ECC grains changed from coherent rotation to domain wall motion when soft layer thickness was changed from 2 to 15 nm. The impact of soft layer thickness on the magnetization reversals of the ECC grains reduced with an increase in soft layer anisotropy. On the other hand, the change of soft layer easy axis direction could possibly change the reversal mechanism of the ECC grains. The above experimental results showed that the coercivity of ECC film was controlled by the reversal mechanism inside the ECC grains.     

Perpendicular magnetic anisotropy in Co50Pt50 and Co50Pt50 ? x Pd x films

Atomic ordering and its effect on the perpendicular magnetic anisotropy and coercive force in Co₅₀Pt₅₀ and Co₅₀Pt_{50 − x} Pd _x (x = 3–10 at %) alloy films are investigated. The dependence of the coercive force on the film thickness is studied. It is shown that thin films of the ordered alloys become magnetically uniaxial with the easy magnetic axis normal to the plane of the film. Such films can be used for magnetic and thermomagnetic data recording and storage.     

DOMAIN STRUCTURES AND ANISOTROPY IN Pt1-xCux/Co MULTILAYER THIN FILMS

The domain structures of Pt_1-xCu_x/Co multilayer films in as-grown and remanent states have been investigated by using magnetic force microscope. The magnetic domain patterns are strongly influenced by the Cu concentration. For pure Pt/Co multilayer in as-grown state, its domain pattern is depicted as island-like one; however, with the Cu doped in Pt spacer layers, the doman patterns become dot-like, but the island-like domain pattern appears again for the Cu concentration of 14 at%. The domain patterns variation can be attributed to the change of effective perpendicular anisotropy K_ueff due to the effect of Cu atom doped in the Pt spacer layers. Besides, the domain structures of Pt/Co multilayer films in remanent state after the application of various perpendicular magnetic fields have also been studied.     

Perpendicular magnetic anisotropy and magnetic proximity effect in Pt1−δFeδ/Co multilayer films

The perpendicular magnetic anisotropy (PMA) and magnetization in Pt_1−δFe_δ/Co (δ=0, 0.017, 0.04 and 0.06) multilayer films have been investigated. It is found that, on adding a small amount of Fe into the Pt layers, Pt/Co multilayer films maintain well-defined PMA at both 5 and 300 K along with significantly enhanced magnetization even at room temperature, which is far greater than the Curie temperature of Pt_1−δFe_δ dilute alloys. Further study demonstrates that the large enhancement of the magnetization in the Fe doped Pt/Co multilayers at 300 K arises from the bulk moment of the Pt_1−δFe_δ layers at the interface region, where the ferromagnetic order persists up to room temperature due to the strengthened exchange interactions between Fe atoms via strongly polarized Pt near the Pt_1−δFe_δ/Co interfaces. For the Pt_0.96Fe_0.04/Pt multilayers, the magnetically ordered region in each Pt_0.96Fe_0.04 layer extends over at least 10 Å from the interface at room temperature.     

Effect of the CrW sublayer on the structure and magnetic properties of thin FePt films

Multilayer Fe₅₅Pt₄₅(20 nm)/Pt(5 nm)/Cr_{100 ? x} W _x (80 nm)/glass structures, in which the Fe₅₅Pt₄₅ magnetic film has a face-centered tetragonal (FCT) structure of the L1₀ phase with the (001) texture, have been prepared using magnetron sputtering. The microstructure and texture of the FePt films have been studied as functions of the W content in the Cr_{100 ? x} W _x sublayer, where 0 < x < 25. It has been established that an increase in the W ion concentration leads to the formation of the (200) texture in the Cr_{100 ? x} sublayer and to an increase in the Cr lattice constant. This is accompanied by a decrease in the temperature at which the facecentered cubic phase transforms into the FCT phase of the FePt films as a result of the increase in tensile stresses along the a axis. It has been found that the coercivity of FePt films deposited on CrW substrates increases with increasing W content in the Cr_{100 ? x} W _x sublayer because the CrW alloy thus formed precludes diffusion between the FePt film and the CrW sublayer. An additional 5-nm-thick intermediate Pt layer is also deposited to suppress diffusion between the FePt and CrW layers. As a result, the highly textured FePt(001) films intended for ultra-high density magnetic information recording are deposited on a substrate heated to a temperature of 400°C and the Cr₈₅W1₁₅ sublayer.     

Structural and magnetic properties of Pt in Co/Pt multilayers

[Co(30 Å)/Pt(x Å)]₂₀ multilayers with the Pt layer thicknesses varying from 5 Å to 20 Å were characterized structurally by high angle X-ray diffraction, X-ray reflectivity, X-ray absorption spectroscopy and magnetically by X-ray magnetic circular dichroism. It is found that the structure and magnetic properties of Pt have a strong correlation with the Pt layer thickness. The 20 Å thickness Pt layer is not almost influenced by the adjacent Co layer and the nearest neighbors are dominated by Pt-Pt shells. With decreasing Pt layer thickness, the nearest neighbors are gradually dominated by Pt-Co shells and the Pt-Co intermixing regions also remarkable increase at the interfaces, especially for the 5 Å thickness Pt layer. The orbital and spin magnetic moments as well as the ratio m_orb/m_spin all decrease systematically with increasing Pt layer thickness, indicating that the interface atoms are polarized by direct Pt-Co hybridization, but that the adjacent layers are polarized by Pt-Pt interactions.     

Combinatorial screening of fuel cell cathode catalyst compositions

An improved fuel cell cathode catalyst composition was pursued by fabricating and screening thin film combinatorial libraries. Results from the Pt-Ru, Pt-Co-Ti, Pt-Co-Cu and Pt-Co-Cr systems are reported. The discrete composition combinatorial libraries were fabricated by plasma sputtering through shadow masks. Each combinatorial library was tested by cyclic voltammetry in a multichannel electrochemical cell. Compositions were ranked based on the onset potential of the oxygen reduction reaction. Several compositions exhibited better onset potentials than pure Pt. The optimum composition from the Pt-Co-Ti system was Pt₄₄Ti₁₂Co₄₄ but showed signs of corrosion after prolonged testing. A wide range of Pt-Co-Cu compositions also outperformed Pt initially, but ultimately failed due to poor corrosion resistance. Among all of the compositions that were screened, the best performance was demonstrated by Pt₂₈Co₃₆Cr₃₆, with an onset potential 107 mV higher than pure Pt and no sign of corrosion.     

Ion beam induced atomic transport in bilayer systems

Atomic transport in ion beam mixed Co/Pt and Pd/Au bilayer systems have been studied from the shifts of maker layers in Rutherford backscattering spectroscopy. Thin layers (1 nm) of marker (Pd for Co/Pt and Ni for Pd/Au) were embedded as markers at each interfaces. 80 keV Ar⁺ was used to irradiate the marker samples at the temperature range between 90 and 600 K. The Co/Pt system shows isotropic atomic transport (J_Co/J_Pt∼1.1) at low temperatures and anisotropic atomic transport (J_Co/J_Pt∼5.0) at high temperatures. Meanwhile, the Pd/Au system shows near isotropic atomic transport (J_Pd/J_Au∼1.2) at all temperatures examined. These results were discussed in terms of the activation energies for the normal impurity diffusion, cohesive energy difference, and the vacancy migration energy. Atomic transport in thermal spike regime is closely related with the activation energy for normal impurity diffusion. In radiation enhanced diffusion regime, the cohesive energy and/or the vacancy migration energy plays a dominant role for the atomic transport.     

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. Received: 29 April 1998 / Revised: 27 July 1998 / Accepted: 31 August 1998     

(Pt3Co)1-xNix合金薄膜的结构、磁性和磁光特性研究

采用直流溅射的方法,在Pt₃Co合金靶上加贴Ni片,把（Pt₃Co)_1-xNi_x合金薄膜中非磁性原子与磁性原子的比率从3：1调制到1：1.通过高角X射线衍射谱和磁性的研究,发现<111>织构至少在0.33—0.50的Ni成分范围内不是引起垂直磁各向异性的主要原因.在此基础上研究了(Pt₃Co)_1-xNi_x合金薄膜的磁光特性.研究表明,适量Ni合金化（x=0.43)可以获得较低的(Pt₃Co)_1-xNi_x合金薄膜居里温度,同时保持较高的垂直磁各向异性、较大的矫顽力和蓝波长下较高的Kerr转角,从而为PtCoNi合金薄膜的实用化提供了实验参考. 关键词：     

Direct observation of dendritic domain growth in perpendicular magnetic anisotropy CoFe/Pt multilayers

We present the experimental results on thermally activated magnetization reversal for [Co_0.9Fe_0.1(5.0 Å)/Pt(20 Å)]₄ multilayer. Direct domain observations show that magnetization reversal is initiated with rare nucleation and followed by dendritic growth of domain walls. Based on macroscopic magnetic parameters from experimental data, the dendritic domain growth mode is qualitatively interpreted by Monte Carlo simulations in terms of a simple uniaxial magnetic anisotropy model. Moreover, both time evolution of domain growth observation and magnetic relaxation measurements reveal that CoFe/Pt multilayer has a relatively large activation volume compared with Co/Pt multilayers.     

Spectroscopic ellipsometric evidence of the solid-state reactions in Ni/Si multilayered films,induced by ion-beam mixing and thermal annealing

Solid-state reactions, induced by ion-beam mixing (IBM) and thermal annealing, in Ni/Si multilayered films (MLF) with an overall stoichiometry of Ni2Si, NiSi and NiSi2, while keeping the nominal thickness of Ni sublayer constant (3.0 nm), were studied by using spectroscopic ellipsometry as well as X-ray diffraction (XRD). The mixing was performed with Ar+ ions of an energy of 80 keV and a dose of 1.5× 1016 Ar+/cm2. Unlike the results of our previous study on Fe/Si MLF [Y.V. Kudryavtsev et al., Phys. Rev. B 65, 104417 (2002)], it was shown that an amorphous phase of NiSi in the B20 phase was formed during deposition independent of the overall stoichiometry of MLF, i.e., the nominal thickness of Si sublayer. IBM leads to some structural changes in the Ni/Si MLF, which cannot be detected by XRD but are confidently recognized by optical tools. A thermal annealing at 673 K of the Ni/Si MLF with an overall stoichiometry of NiSi and NiSi2 causes formation of the -NiSi phase. The first trace of NiSi2 phase on the background of the -NiSi phase was detected by XRD after an annealing at 1073 K, while, according to the optical results, NiSi2 turned out to be the dominant phase for the annealed Ni/Si MLF with an overall stoichiometry of NiSi2.     

Interface characterization of a [Pt/Co)]3/IrMn multilayer by laser-assisted tomographic atom probe

A sputtered Ta_3 nm/[[Pt_2 nm/Co_0.4 nm)]₃/IrMn_7 nm]₇/Pt_10 nm multilayer has been analyzed by laser-assisted tomographic atom probe, allowing to perform three-dimensional reconstructions of the layers and to determine their chemical composition at the atomic scale. From the concentration profiles, we show that the Co layer in contact with IrMn is strongly mixed. The Co/IrMn and IrMn/Pt interfaces are thus non-symmetric, the Co/IrMn interface being more diffuse than the IrMn/Pt one. This study demonstrates that the LATAP technique is extremely well suited for atomic scale structural and chemical characterizations of magnetic multilayers in relationship with their magnetic properties.     

Structural and magnetic properties of CoPt clusters

The geometrical structures, electronic and magnetic properties of Co_n − xPt_x (n=2–13,38,55$n=2\text{–}13,38,55$) alloy clusters have been systematically investigated by using the density functional theory within the generalized gradient approximation (DFT–GGA). It is found that CoPt alloy clusters adopt the structures of corresponding monatomic Co clusters, where Pt atoms localize at the surface sites and tend to bond together forming a Pt exterior shell. The ferromagnetic coupling between atoms is determined in CoPt clusters, and the Co local magnetic moments can be enhanced by the increase of Pt concentration.