Ir Alloy Research Articles

Selectivity of Ni-based surface alloys toward hydrazine adsorption: A DFT study with van der Waals interactions

We use dispersion corrected DFT calculations (DFT+D3) to investigate the selectivity of Ni-based surface alloys toward hydrazine adsorption. A series of Ni–M (M=Fe, Pt, Ir, Pd and Rh) alloy films were investigated, namely Ni15/M1/Ni(111), Ni14/M2/Ni(111), Ni12/M4/Ni(111) and Ni8/M8/Ni(111). Our results show that the doped atoms of Ir, Rh and Fe provide stronger adsorption sites than the Ni atom on the Ni(111) surface, while the doped atoms of Pt and Pd provide weaker adsorption sites. By analyzing the most favorable adsorption of hydrazine on Ni–M alloy surfaces we found that Ni8Fe8/Ni(111), Ni8Rh8/Ni(111), Ni15Ir1/Ni(111) and Ni14Ir2/Ni(111) present enhanced adsorption properties if compared to the pure Ni(111) surface, and seem to be better candidates for hydrazine catalysis, which are in agreement with that found by experiments. The correlation between d-band center position and adsorption energies of top modes in the Ni or doped atom has been calculated at DFT+D3 level to provide further insight into the Ni-based surface alloy properties for hydrazine adsorption.

One pot synthesis of octahedral {111} CuIr gradient alloy nanocrystals with a Cu-rich core and an Ir-rich surface and their usage as efficient water splitting catalyst

A facile one pot synthetic route has been developed to obtain octahedral CuIr nanocrystals with Ir-rich {111} facets. Co-decomposition of Cu and Ir precursors in the presence of CTAC (cetyltrimethylammonium chloride) forms CuIr nanocrystals with an Ir-rich {111} facet. A mechanistic study reveals that the surface Cu atoms of the initially formed octahedral-shaped Cu-rich Cu–Ir alloy nanoparticles are replaced by Ir atoms via a galvanic replacement reaction. The formation of the Ir phase is rather slow, leading to the impermeable nature of the Ir shell, which does not allow the dissolution of the Cu phase. The CuIr nanocrystals with an Ir-rich shell show high catalytic activity toward oxygen evolution reaction.

Total Hydrogenation of Furfural and 5-Hydroxymethylfurfural over Supported Pd–Ir Alloy Catalyst

Hydrogenation of aqueous furfural was conducted with SiO2-suported palladium-based bimetallic catalysts. The combination of palladium and iridium gave the best performance for the total hydrogenation to tetrahydrofurfuryl alcohol. Higher H2 pressure and lower reaction temperature were advantageous to suppress side reactions. The synergy between Pd and Ir in the hydrogenation catalysis is most remarkable for substituted furans as substrates. Furfural was first converted into furfuryl alcohol, which was further converted to tetrahydrofurfuryl alcohol. A small amount of tetrahydrofurfural was formed in the first step (∼20% selectivity), and the subsequent hydrogenation of tetrahydrofurfural was much slower. The combined yield of tetrahydrofurfuryl alcohol and tetrahydrofurfural reached 98%. The yield of tetrahydrofurfuryl alcohol reached 94% with larger amount of catalyst. Total hydrogenation of 5-hydroxymethylfurfural was also possible using Pd–Ir/SiO2 catalyst. The performance of Pd–Ir/SiO2 catalyst was sl...

Screening iridium-based bimetallic alloys as catalysts for direct ethanol fuel cells

Current ethanol oxidation catalysts in direct ethanol fuel cells (typically platinum-based) suffer from low conversion and are susceptible to CO poisoning. We therefore determined to find viable alternative catalysts for ethanol oxidation based on iridium using density functional theory to model bimetallic alloy (111) surfaces. Iridium was alloyed with another transition metal (from groups 4 through 11), M, in an overlayer (one layer of metal M on top of bulk iridium) or subsurface configuration (M inserted under the first layer of iridium). Segregation energies were calculated and the subsurface configuration was found to be most stable configuration in the majority of alloy cases under vacuum conditions. Select alloys (including Ir–Pt and Ir–Rh) showed a reversal of preferred alloy state (overlayer or underlayer) upon adsorption of CO. CO adsorption was modeled and lower CO adsorption energies were found for many alloys compared to pure Pt, indicating less CO poisoning. Complete oxidation of ethanol is typically limited by the breaking of strong CC bonds, so any catalyst must lower the barriers for CC bond breaking. Activation energies for CC bond breaking (by considering a representative intermediate molecule CHCO) were lowered for the vast majority of the alloys used in an underlayer structure, reinforcing the significance of the underlayer structures or “subsurface” alloys. Finally, we found, based on CO adsorption energy, activation energy for CC breakage reaction, and metal cost, three important catalyst descriptors, a number of promising catalysts for the ethanol oxidation reaction, such as alloys with M=Zr, Nb, Mo, W, Ru, Os, or Re. Experimental verification (cyclic voltammetry and chronoamperometry for ethanol oxidation) on Ir, Ir–Ru, and Ir–Rh catalysts confirmed the superiority of these Ir-based catalysts compared to Pt. Our results demonstrate that alloys of Ir show promise as more affordable substitutes for platinum group metals.

Pd–Ir alloy as an anode material for borohydride oxidation

A Pd–Ir alloy (1:1) coated on microfibrous carbon (11 μm diameter) supported on a titanium plate was evaluated as an electrode for the anodic oxidation of borohydride. The hydrogen generated, due to the parallel reaction of borohydride hydrolysis, was measured during the electrolysis obtaining less than 0.1 cm3 min−1 H2 between −1 and 0 V vs. Hg/HgO (−0.86 and 0.14 V vs. SHE), while the current densities for the oxidation of borohydride were up to 367 mA cm−2 in 0.5 mol dm−3 NaBH4 + 3 mol dm−3 NaOH. The low rate of hydrogen generation suggests that Pd–Ir could be a promising catalyst for borohydride oxidation. However, higher rates of hydrogen were generated at the open circuit potential, which is inconvenient in the direct borohydride fuel cell. Cyclic voltammetry allowed analysis of the oxidation peaks due to the borohydride oxidation. To obtain a further understanding of the borohydride oxidation mechanism at Pd–Ir electrodes, density functional theory (DFT) was used to examine the reaction mechanism at Pd2–Ir1(111) and Pd2–Ir2(111) surfaces. The competition between borohydride oxidation and hydrogen evolution on the Pd–Ir alloys is compared with that on pure Pd(111), suggesting that the presence of Ir favors borohydride oxidation rather than hydrogen evolution.

Alloying effects on the hydrogen-storage capability of Pd–TM–H (TM = Cu, Au, Pt, Ir) systems

Pressure–composition isotherms and the magnetic susceptibilities of Pd–TM–H (TM=Cu, Au, Pt, and Ir) systems were measured at ambient temperature, and the effects of alloying between Pd and transition metals on the hydrogen storage capability of these Pd–TM alloys were investigated by considering their electronic band structures. All of the magnetic susceptibilities for the Pd–TM–H systems decreased linearly with hydrogen uptake. For the Pd–Cu alloy, the magnetic susceptibility was nearly zero at the terminal composition of hydrogen in the plateau region obtained from the pressure–composition isotherm, and the terminal composition decreased with increasing Cu substitution. These results indicated that the hydrogen-storage capability was proportional to the amount of unoccupied d states in the electronic band structure of the Pd–Cu alloy. The Pd–Au–H system exhibited substantially the same behavior as the Pd–Cu–H system. For the Pd–Pt and Pd–Ir alloys, the magnetic susceptibility at the terminal composition of hydrogen in the plateau exhibited a finite positive value, indicating that the unoccupied d states in the Pd–Pt and Pd–Ir alloys were not filled when the maximum quantity of hydrogen was stored in the alloys. These finite magnetic susceptibilities at the terminal composition of hydrogen in the plateau region were explained by the structural modification of the unoccupied d states in the electronic band structures due to alloying.

Microstructure and oxidation behaviour of Ir-rich Ir–Al binary alloys

In the current study, alloys of Ir–11Al, Ir–23Al, Ir–30Al, Ir–41Al and Ir–45Al (at.%) were prepared to investigate the microstructure and oxidation behaviour of Ir-rich Ir–Al alloys. Ir(Al)ss and/or β-IrAl intermetallic phases were found to exist in the prepared alloys. During isothermal oxidation at 1100°C, the Ir(Al)ss and β-IrAl individually changed to porous and dense Al2O3. The microstructure of the oxide scale formed on Ir–23Al was similar to that of its former alloy which possessed a dendrite-like configuration. It was found that the mass change of Ir–45Al followed a parabolic law, showing the best oxidation resistance among the Ir–Al alloys.

Ir promotion of TiO2 supported Au catalysts for selective hydrogenation of cinnamaldehyde

We synthesized Ir-on-Au (Au–Ir) nanoparticles (NPs) using 8–9nm Au NPs, which had a partial coverage of Ir. Both the studied systems allowed the obtainment of cinnamyl alcohol with high selectivities (>83%). However, Au–Ir/TiO2 delivered a hydrogenation rate 5 times higher than that of Au/TiO2. The deposition of Ir onto the surface of Au and the presence of surface Au–Ir alloy were confirmed by UV–vis and HRTEM respectively. Moreover. Moreover, the strongly shifted XPS binding energy implyed the electron transfer from Ir to Au, which was believed to be responsible for the enhanced H2 activation capacity of Au.

Remarkable durability of Pt–Ir alloy catalysts supported on graphitic carbon nanocages

Low durability is the major challenge hindering the large-scale implementation of proton exchange membrane fuel cell (PEMFC) technology. To improve the electrochemical activity and durability of conventional Pt catalyst used in PEMFCs, we prepare Pt–Ir alloy catalysts supported on homemade graphitic carbon nanocages (CNCs). Results from cyclic voltammetry (CV) and linear sweep voltammetry (LSV) show that the alloy catalyst with an atomic Pt/Ir ratio of 1:1 supported on CNCs with a high degree of graphitization exhibits an excellent electrocatalytic activity. Furthermore, this alloy catalyst shows a drastic improvement in durability and stability over the unalloyed Pt catalyst supported on the same CNCs and the catalyst from a commercial source (Johnson Matthey Co.). It is found that the concurrent uses of well-graphitized CNCs as the support material and Ir as the alloying element are critical for the observed improvement. Such a Pt–Ir alloy catalyst provides a new replacement for the conventional Pt catalyst with not only a drastic improvement in durability but also a reduction in Pt usage by 50% as well.

Effect of reduction temperature on Ru–Ir/ZnO catalyst for selective hydrogenation of crotonaldehyde

A Ru–Ir/ZnO catalyst with metal loadings of 3% Ru and 3% Ir was reduced at different temperatures (150 to 400°C) and tested for vapor-phase selective hydrogenation of crotonaldehyde at 80°C. It was found that with increasing reduction temperature, the crotonaldehyde conversion over the catalysts first increased and then decreased. A conversion of 93.5% and the selectivity to crotyl alcohol of 86.6% was observed after 10h reaction on a Ru–Ir/ZnO catalyst reduced at 200°C. Various characterizations such as X-ray photon spectroscopy (XPS) and ammonia temperature-programmed desorption (NH3-TPD) results demonstrated that moderate interaction between the CO bond and the M0 (M=Ru, Ir or Ru–Ir alloy) due to the proper charge density of M0, and surface acidity of the catalysts played decisive roles in the enhanced activity and selectivity obtained on the catalyst. In addition, the deactivation of the catalyst was due to the carbon deposit (including organic compounds) on the catalyst surface, as evidenced by Raman spectroscopy and temperature-programmed oxidation over the spent catalyst. Also, the strong adsorption of CO on the catalyst surface generated by a decarbonylation reaction could be another reason for catalyst deactivation, as evidenced by a CO poisoning experiment.

Platinum Group Element Geochemistry of Sulfide-Rich Horizons in the Ural-Alaskan-Type Ultramafic Complex of Duke Island, Southeastern Alaska

The Duke Island Ultramafic Complex in southeastern Alaska is a Ural-Alaskan–type complex that hosts sulfide-rich horizons primarily in the olivine clinopyroxenite unit. Although placer deposits hosting platinum group elements (PGEs) have been found in association with Ural-Alaskan–type complexes in several locations around the world, these ultramafic complexes have traditionally been viewed as sulfide-poor systems and therefore unlikely hosts of economic Ni-Cu sulfide deposits. Sulfide mineralization in the Duke Island Complex consists of massive, net-textured, and disseminated assemblages that formed as a result of the interaction between magma and sulfidic-graphitic country rocks. The sulfide mineralization at Duke Island is characterized by strong depletion in Ir, Os, and Ru (IPGEs), and relative enrichment in Pt, Pd, and Cu. The sulfide-bearing olivine clinopyroxenites formed from a derivative magma produced as a result of the fractional crystallization of olivine and chromite. Although IPGEs may be compatible within olivine and chromite, fractional crystallization of up to 20% cannot explain the observed IPGE depletion. We propose that early crystallization of laurite and Ir alloys accompanied the accumulation of olivine and chromite. R-factor modeling indicates that much of the inter-element variation in the Sulfide mineralization can be attributed to variations in the degree of equilibration between Sulfide and silicate liquids. Mixing between monosulfide solid solution and fractionated Sulfide liquid may also have contributed to the variability. The modeling suggests that the pre-sulfide saturation concentrations of Os, Pt, and Pd in the magmas were approximately 0.03, 2, and 3 ppb, respectively. The concentration of Pd is near that of primitive mantle and the low Pt concentration can be explained by the crystallization of Pt alloy with early-formed dunites and chromitites. This is consistent with models for Pt-rich placers in the Urals that have been derived from chromitites and dunites. The relative enrichments in Pd and Cu are the results of fractional crystallization before Sulfide saturation was achieved. Taken together, the Ni-Cu-PGE elemental characteristics at the Duke Island Complex are similar to those shown by flood basalt-related Sulfide deposits and Cu-rich Sulfide veins studied in different locations around the world.

Using Multilayer Precursors to Create Nanoporous Gold and Nanoporous Iridium Thin Films with Layered Architecture

Although dealloying is commonly used to fabricate nanoporous (np) metals, with precursor alloy composition and dealloying conditions determining the length scale of ligaments and pores, the technique typically allows limited control over ligament/pore size and design of np geometry. This paper presents a novel approach to creating np metal films with layered structure through the film thickness. This method is demonstrated for Au and Ir alloy films and can successfully control ligament size, spacing, and even orientation.

Performance and characterization of rhenium-modified Rh–Ir alloy catalyst for one-pot conversion of furfural into 1,5-pentanediol

Rh–Ir–ReOx/SiO2 catalyst with ReOx-modified Ir–Rh alloy particles can convert furfural to 1,5-PeD in high yield.

Compositional variations of chromite and solid inclusions in ophiolitic chromitites from the southeastern Turkey: Implications for chromitite genesis

Chromitites, associated with upper mantle peridotites of ophiolites from southeastern Turkey, are lenticular bodies or veinlets made up of massive, banded and disseminated chromite. Chromite shows a wide range of Cr# [Cr/(Cr+Al)] varying between 0.42 and 0.81, and Mg# [Mg/(Mg+Fe2+)] of 0.45 to 0.75. Geochemically, they are divided into two compositional groups, such as high-Cr (Cr#≥0.7) and high-Al (Cr#<0.6). The contents of the platinum-group elements (PGE) in high-Cr chromitites range between 42 and 348ppb (average 158ppb), whereas high-Al ones are represented by a lower total PGE, ranging between 70 and 94ppb (average 84ppb). However, two high-Cr chromitite samples showed significant enrichments especially of Ir-subgroup of PGE (IPGE: Os, Ir, Ru) with total PGE contents reaching up to 2.7ppm. The chromitites mostly contain tiny inclusions (<5μm) of platinum-group minerals (PGM). The most abundant PGM is laurite which is rarely associated with irarsite±Os–Ir alloys; these minerals in particular are more abundant in high-Cr chromitites, especially in PGE enriched chromitites. The primary base-metal sulfide (BMS) inclusions consist of millerite and to a lesser extent pentlandite, and very rarely pyrite. Violarite, polydymite and heazlewoodite form the secondary BMS phases found mostly in altered silicate matrix. Both PGM and BMS are frequently associated with the silicates: amphibole, olivine and clinopyroxene. Textural relationships and the composition of PGM and BMS inclusions suggest that they crystallized at varying temperature (1300°C–1000°C) and sulfur fugacity conditions, before or during the event of crystallization of chromite. The high-Cr chromitites crystallized in equilibrium with boninite melt, probably originated in a supra-subduction zone (SSZ), while the high-Al chromitites crystallized in equilibrium with middle oceanic ridge basalt (MORB) type melts. The presence of amphiboles in high-Al chromitites suggests hydrous parental melts that could be produced, most probably, in back-arc tectonic setting.

Determination of the Frumkin and Temkin Adsorption Isotherms of Underpotentially Deposited Hydrogen at Pt Group Metal Interfaces Using the Standard Gibbs Energy of Adsorption and Correlation Constants

At Pt(111), Pt(100), Pt, and Rh interfaces, the Frumkin adsorption isotherm of underpotentially deposited hydrogen (UPD H) and related electrode kinetic data are determined using the standard Gibbs energy of adsorption. The Temkin adsorption isotherm of UPD H correlating with the Frumkin adsorption isotherm of UPD H is readily determined using the correlation constants between the Temkin and Frumkin or Langmuir adsorption isotherms. At the Pt(111), Pt(100), Pt, and Rh interfaces, the lateral repulsive interaction between the UPD H species is interpreted using the interaction parameter for the Frumkin adsorption isotherm. The lateral repulsive interaction between the UPD H species at the Pt(111), Pt(100), Pt, and Rh interfaces is significantly different from the lateral attractive interaction between the overpotentially deposited hydrogen (OPD H) species at Pt, Ir, and Pt-Ir alloy interfaces.

Mechanical properties at 2223 K and oxidation behavior of Ir alloys

Compressive strengths of twelve Ir-1at%X binary alloys were evaluated at 2223 K to understand the strengthening effects of alloying elements. The alloying elements were selected from group 4 to group 10 in the periodic table. The maximum strength of about 65 MPa was obtained in Ir–Hf and Ir–Zr alloys. The effect of adding multiple alloying elements on strength was also investigated. Isothermal oxidation behavior was investigated at 1373 K for Ir alloys including Nb and/or Hf. The weight of all the tested alloys decreased linearly during the oxidation test, indicating that Ir oxide evaporated during oxidation.

Genesis and tectonic implications of podiform chromitites in the metamorphosed ultramafic massif of Dobromirtsi (Bulgaria)

Podiform chromitites occur in the meta-dunite horizon of the Dobromirtsi ultramafic massif, in the Central Rhodope metamorphic core complex (southern Bulgaria). Although these were deformed and metamorphosed together with their host rocks several chromite cores from massive chromitite samples still preserve compositions unaffected by metamorphism – at least in terms of major elements. In situ laser ablation ICP-MS analyses of these chromite cores show a bimodality in the composition of chromite between high-Cr and high-Al {Cr#=[Cr/(Cr+Al)], atomic ratio=0.74–0.55} with Mg# between 0.69 and 0.60 [Mg#=Mg/(Mg+Fe2+), atomic ratio]. These compositions of chromite and the whole rock enrichment of Os–Ir–Ru relative to Pt–Pd in the chromitites resemble that reported for podiform chromitites in ophiolites elsewhere.Magmatic platinum-group minerals (PGM), including laurite (RuS2), Os–Ir alloys and sulfarsenides (irarsite, IrAsS), exhibit a wide range of 187Os/188Os (from 0.1097 to 0.1272) whereas 187Re/188Os is nearly zero. The distribution of the Os model ages calculated for these PGM ranges between 0.13 and 2.6Ga, with age peaks at 0.25, 0.4, 0.7, 2.1 and 2.6Ga. Two xenocrystic zircons were identified in the chromitites and yield concordant U–Pb age of 2257±80Ma (2σ) and 1952±82Ma (2σ). The oldest zircon exhibits a depleted-mantle Hf model age (TDM) of 2.77Ga and a “crustal” Hf model age (TCDM) of 3.1Ga, whereas the youngest has a TDM of 2.21Ga and a TCDM of 2.35Ga.We interpret the coexistence of both types of chromitites in a horizon of meta-dunites as the result of their precipitation from fractionating island arc tholeiite melts. The first reactions produced dunite; later the batches of magma were isolated from reaction with the peridotite wall-rocks and progressively fractionated in a network of dunite channels developed in the upper mantle beneath the back-arc spreading centre. The Os and Hf model ages suggest that the ultramafic protolith of the Dobromirtsi ophiolite was derived from an original Archean mantle at least 3Ga old, which underlay a continental crust and was reworked in the Paleo-proterozoic and possibly even younger times.

Partitioning of platinum-group elements and Au between sulfide liquid and basalt and the origins of mantle-crust fractionation of the chalcophile elements

The partitioning of platinum-group elements (PGE; Os, Ir, Ru, Rh, Pt, and Pd) and Au between sulfide melt and silicate melt (i.e., DPGEsul) exerts a critical control on the PGE composition of the Earth’s crust and mantle, but previous estimates have been plagued by experimental uncertainties and vary through several orders of magnitude. Here we present direct experimental measurements of DPGEsul, based on in situ microanalysis of the sulfide and silicate melt, with values ranging from ∼4×105 (Ru) to ∼2–3×106 (Ir, Pt). Our measurements of DPGEsul are >100 times larger than previous results but smaller than anticipated based on comparison of alloy solubilities in sulfide melts and S-free silicate melts. The presence of S in the silicate melt greatly increases alloy solubility. We use our new set of partition coefficients to develop a fully constrained model of PGE behavior during melting which accurately predicts the abundances of PGE in mantle-derived magmas and their restites, including mid-ocean ridge basalts, continental picrites, and the parental magmas of the Bushveld Complex of South Africa. Our model constrains mid-ocean ridge basalt (MORB) to be the products of pooled low and high degree fractional melts. Within-plate picrites are pooled products of larger degrees of fractional melting in columnar melting regimes. A significant control on PGE fractionation in mantle-derived magmas is exerted by residual alloy or platinum group minerals in their source. At low pressures (e.g., MORB genesis) the mantle residual to partial melting retains primitive mantle inter-element ratios and abundances of PGE until sulfide has been completely dissolved but then evolves to extremely high Pt/Pd and low Pd/Ir because Pt and Ir alloys form in the restite. During melting at high pressure to form picrites or komatiites Ir alloy appears as a restite phase but Pt alloy is not stable due to the large effect of pressure on fS2, and of temperature on fO2 along an internal oxygen buffer, which causes large increases in alloy solubility. The magmas parental to the Bushveld Complex of South Africa appear, at least in part, to be partial melts of mantle that has previously been melted to the point of total sulfide exhaustion at low pressure, closely resembling mantle xenoliths of the Kaapvaal craton. Using the new extremely large DPGEsul the world-class Merensky Reef and UG2 Pt deposits of the Bushveld Complex can readily be modeled as the result of sulfide saturation due to mixing of magmas with unremarkable PGE contents, obviating the need to postulate anomalously PGE-rich parent magmas or hydrothermal inputs to the deposits.

Structural and catalytic properties of supported Ni–Ir alloy catalysts for H2 generation via hydrous hydrazine decomposition

Iridium modified Ni/Al2O3 catalysts were prepared using Ni–Al hydrotalcite as precursor and tested in the selective decomposition of hydrous hydrazine for H2 generation. Compared to Ni/Al2O3, NiIrx/Al2O3 catalysts exhibited an increase of the reaction rate up to 6-fold and the H2 selectivity from 93% to above 99%. The XRD and EXAFS analysis confirmed that Ir and Ni were present in the form of Ni–Ir alloy; and the H2 differential heat of adsorption is appreciably lowered with increasing the amount of Ir in Ni–Ir alloy. High-temperature reduction, e.g. 700°C, induced the migration of Ir atoms into the Ni bulk and produced a Ni-skin surface. This structural modification is associated with a strongly decrease of activity and selectivity. EXAFS measurements were also performed on different noble metals (NM, Ir, Pt, and Au) modified Ni/Al2O3 catalysts in order to verify the extent of NM-Ni bimetallic bond formation, which was found to correlate with the catalytic performance.

Ag-related alloy formation and magnetic phases for Ag/Co/Ir(111) ultrathin films

The Kerr intensity versus the Ag thickness for Ag grown on the top of Co/Ir(111) exhibits an oscillating behavior with a period around one monolayer which should be due to the morphological change related electronic structure differences of the Ag layer. From systematical investigations of Ag/Co/Ir(111) films with the Co layer thinner than 4monolayers at temperatures below 900K, a magnetic phase diagram has been established. As the annealing temperature increases for Ag/Co/Ir(111) films, enhancements of the coercive force occur in both the polar and longitudinal configurations due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy. The disappearance of ferromagnetism is mainly attributed to the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of the thickness of magnetic layers, and the intermixing of Ag and Co at the Ag/Co interface.