Pd Multilayers Research Articles

The influence of a copper spacer on the magnetic properties of Pd/Co/Cu/Pd multilayers with perpendicular anisotropy

The potential for technological applications of systems presenting perpendicular magnetic anisotropy is undeniable. With the advent of spintronics and the possibility to explore spin textures, such as skyrmions, in devices, it becomes fundamental to understand how to finely tune the magnetic properties of nanometric systems. In multilayered thin films, characteristics such as domain structure, saturation magnetization and perpendicular anisotropy may be tailored by modifying the interfaces. In the present study, the impact of a copper spacer of varying thickness on the magnetic properties of Pd/Co/Cu/Pd multilayers is investigated. The evolution of magnetization curves, domain patterns and magnetization reversal processes in terms of the spacers thickness are analyzed.

A Study on the Interfacial Reactions between Gallium and Cu/Ni/Au(Pd) Multilayer Metallization.

This research introduces low-temperature soldering of Ga with practical metallization structures, namely, Cu/Ni/Pd and Cu/Ni/Au, applied to contemporary microelectronic packages. Through these multilayer configurations, the study investigates the stability of the Ni diffusion barrier by examining changes in the interfacial microstructure as Ni is consumed. The interfacial reactions are conducted across a temperature spectrum of 160, 200, 240, and 280 °C, with reaction durations ranging from 30 to 270 min. Valuable insights for low-temperature soldering with Ga are extracted from the data. At lower reaction temperatures, the presence of Ga-rich intermetallic compounds (IMCs), specifically GaxNi (x = 89 to 95 at%), on the Ga7Ni3 layer is notably confirmed. As the reaction temperature and duration increase, the gradual consumption of the Ni layer occurs. This gives rise to the formation of Ga-Cu IMCs, specifically CuGa2 and γ3-Cu9Ga4, beneath the Ga-Ni IMC layer. Concurrently, the gap between the Ga-Ni and Ga-Cu IMC layers widens, allowing molten Ga to infiltrate. The rate of Ga7Ni3 growth follows a time exponent ranging approximately from 1.1 to 1.7. This highlights the significant influence of interface reaction-controlled kinetics on Ga7Ni3 IMC growth. The activation energy for Ga7Ni3 growth is determined to be 61.5 kJ/mol. The growth of Ga7Ni3 is believed to be primarily driven by the diffusion of Ga atoms along grain boundaries, with the porous microstructure inherent in the Ga7Ni3 layer providing additional diffusion pathways.

Temperature-dependent Scaling Behavior of Magnetic Hysteresis Loop of CoFeB/Pd Multilayer Film

We report our experimental investigation on the temperature-dependent hysteresis loops and magnetic domain behaviors of CoFeB (0.4 nm)/Pd (1 nm) multilayer on polyimide substrate with a perpendicular magnetic anisotropy, by means of magneto-optical Kerr microscopy. Hysteresis characteristics such as saturation magnetization, coercivity, and hysteresis loop area with respect to the temperature have been analyzed, where powerlaw scaling behaviors are observed with microscopic magnetic domain patterns mediated by numerous nucleation sites.

Magnetic anisotropy in the exchange-biased laser-patterned thin Co/CoO films

Materials exhibiting an exchange bias effect are a class of magnetic systems that have a wide range of possible technological applications e.g. in sensors, read heads, and spintronic devices. In this study, we demonstrate the effect of laser interference patterning on the magnetic properties of Pd/CoO/Co/Pd multilayers. Laser patterning creates arrays of well-ordered stripes, rectangles, and squares on the substrate surface. We found that the laser treatment caused magnetic softening of the structure edges while the centers of the objects remained unchanged and exhibited the exchange bias effect. In this study we focused on the shape and configurational magnetic anisotropies induced by patterning and showed that the magnetic properties varied depending on the angle at which the external magnetic field was applied with respect to the pattern geometry.

FORC analysis of nanopatterned vs unpatterned films: Coercivity and switching mechanisms

We have studied the use of self-assembled block copolymers to pattern multilayers of Co and Pd on silicon wafers. Stacks ranging from four to twelve bilayers of Co (0.3 nm)/Pd (0.8 nm) were sputtered onto Ta/Pd seed layers and capped with 3 nm of Ta and were found to have perpendicular magnetic anisotropy as-deposited. The block copolymer polystyrene-block-poly(ferrocenyl dimethylsilane) (PS-b-PFS) was dissolved in toluene and spun onto the wafers. The polymers were phase-separated by heat treatment, leaving self-assembled PFS spheres embedded in PS, which was removed by oxygen-plasma ashing. The PFS spheres were then used as masks to ion-mill the Co/Pd multilayers into nanopillars. To study the effect of etch time and etch angle on the coercivity distribution, we synthesized samples in a Design of Experiments-(DoE)- in these two factors. Scanning electron micrographs showed nanopillars ranging from 15 to 30 nm in diameter, depending primarily on etch time. M-H loops measured on both patterned and unpatterned wafers showed an increase of up to 130% in overall coercivity upon patterning. First Order Reversal Curves (FORC) were measured, and the resulting FORC distributions displayed using a smoothing program (FORCinel) and one that can display the raw data without smoothing (FORC+). We find that FORC+ reveals information about fine-scale structure and switching mechanism that cannot be seen in the smoothed display.

Effect of increasing interaction interface on perpendicular magnetic anisotropy of Pd/Co2MnSi/Pd multilayers

Interfacial interaction induced perpendicular magnetic anisotropy (PMA) is one of the research hotspots of magnetic recording thin films. The PMA of Pd/Co2MnSi(CMS)/Pd multilayers can be improved by increasing the interfaces of heavy metal/ferromagnetic (HM/FM). In order to study the effect of adding HM/FM interface on the PMA of multilayers, a Pd layer was inserted into the CMS of Pd/CMS/Pd film to form Pd/CMS(t/2)/Pd/CMS(t/2)/Pd multilayer structure with quadruple Pd/CMS interface, and the [Pd/CMS(t/2)]N was inserted into the Pd/CMS(t/2)/Pd film to form Pd/CMS(t/2)[Pd/CMS(t/2)]N/Pd multilayer structure. The results showed that the PMA and MS of Pd/CMS(t/2)/Pd/CMS(t/2)/Pd multilayers were enhanced. The maximum Keff was 52.3% higher than that of the Pd/CMS(t)/Pd structure. The increase of MS can effectively reduce the switching current density of the magnetic tunnel junction, and then reduce the energy consumption of magnetic record writing. For the Pd/CMS(3.5)/[Pd(0.8)/CMS(3.5)]N/Pd multilayers, the PMA peaked at N = 1. With the increase of N, the PMA decreases and gradually changes from PMA to IMA. This indicates that the increase of more Pd/CMS interfaces does not increase the PMA further. In addition, when N = 3 and 4, the expression of Keff values is inconsistent with the corresponding hysteresis loops, and the exchange bias effect of in-plane hysteresis loops appears at N = 4 and N = 5.

Water structure at the multilayers of palladium deposited at nanostructured Au electrodes

In situ ATR-SEIRAS experiments were carried out to describe water structure at the surface of multilayers of Pd modified nanopatterned gold film electrodes in contact with aqueous 0.1 M HClO 4 solution. The results show that the surface water becomes more ordered when the thickness of the Pd film increases. The “ice-like water” band becomes stronger and for the electrode with 5 ML Pd@Au, the ratio of “ice-like water” to “liquid-like water” becomes larger than that in bulk water. Concomitantly, the population of “monomers” or small water clusters diminishes. All surface water bands increase when hydrogen adsorption takes place. The bending band corresponding to “monomers” or “multimers” water is observed chiefly at the potentials of hydrogen adsorption.

Perpendicular magnetic anisotropy of Pd/Co2 MnSi/NiFe2O4/Pd multilayers on F-mica substrates

Pd/Co2MnSi (CMS)/NiFe2O4 (NFO)/Pd multilayers were fabricated on F-mica substrate by magnetron sputtering. The best PMA performance of the multilayer structure Pd(3 nm)/CMS(5 nm)/NFO(0.8 nm)/Pd(3 nm) was obtained by adjusting the thickness of the CMS and NFO layers. F-mica substrate has a flatter surface than glass and Si/SiO2 substrate. The magnetic anisotropy energy density (K eff) of the sample deposited on F-mica substrates is 0.6711 Merg/cm3 (1 erg=10−7 J), which is about 30% higher than that of the multilayer films deposited on glass (0.475 Merg/cm3) and Si/SiO2 (0.511 Merg/cm3) substrates, and the R Hall and H C are also significantly increased. In this study, the NFO layer prepared by sputtering in the high purity Ar environment was exposed to the high purity O2 atmosphere for 5 min, which can effectively eliminate the oxygen loss and oxygen vacancy in NFO, ensuring enough Co–O orbital hybridization at the interface of CMS/NFO, and thus effectively improve the sample PMA.

Methods to induce perpendicular magnetic anisotropy in full-Heusler Co2FeSi thin layers in a magnetic tunnel junction structure

In this study, to obtain perpendicular magnetic tunnel junctions (p-MTJs) using half-metallic ferromagnets (HMFs), several methods were developed to induce perpendicular magnetic anisotropy (PMA) in full-Heusler Co2FeSi (CFS) alloy thin layers in an MTJ multilayer composed of a layered CFS/MgO/CFS structure. Oxygen exposure at 2.0 Pa for 10 min after deposition of the bottom CFS layer was effective for obtaining PMA in the CFS layer. One of the reasons for the PMA is the formation of nearly ideal CFS/MgO interfaces due to oxygen exposure before the deposition of the MgO layer. The annealing process was effective for obtaining PMA in the top CFS layer capped with a Pd layer. PMA was clearly observed in the top CFS layer of a Cr(40 nm)/Pd(50 nm)/bottom CFS(0.6 nm)/MgO(2.0 nm)/top CFS(0.6 nm)/ Pd(10 nm) multilayer, where the top CFS and Pd thin films were deposited at RT and subsequently annealed at 300°C. In addition to the continuous layer growth of the films, the crystalline orientation alignment at the top CFS/Pd interface probably attributes to the origin of PMA at the top CFS layer.

CoFeSiB–Pd multilayers and co-deposited alloy films exhibiting perpendicular magnetic anisotropies after heat treatment up to 500 °C

Ferromagnetic layers exhibiting perpendicular magnetic anisotropy (PMA) are essential components in magnetic random access memory cells. Various Co- and Fe-based multilayers (MLs) have been considered because the magnetic properties of MLs can be easily controlled by changing the ferromagnetic and nonmagnetic layer thicknesses and number of bilayers. However, MLs generally suffer from the loss of PMA after high-temperature heat treatments (beyond approximately 400 °C) during device processing, which are associated with back-end-of-line device fabrication. Moreover, ML deposition may limit the throughput of the film mass production. Here, we investigate the PMA stability of sputter-deposited CoFeSiB/Pd ML as well as CoFeSiB–Pd co-deposited alloy (CA) films exposed to various annealing temperatures. Although the ML films exhibited PMA, regardless of the annealing temperature up to 500 °C, the CA films started to exhibit strong PMA after 400 °C, which was maintained up to 500 °C. To understand the magnetic property changes, we performed microstructural analyses. The ML films in the as-deposited state exhibited well-defined layered microstructures and evolved into polycrystalline alloys at 500 °C. In contrast, the CA films in the as-deposited state exhibited an amorphous microstructure but transformed into polycrystalline alloys at 500 °C, similar to their ML counterparts. Both Co and Pd, the major constituents of the films, underwent interdiffusion and formed a Co–Pd ordered phase, which contributed to the appearance of PMA.

Multipoint Hydrogen Sensing of Hetero-Core Fiber SPR Tip Sensors With Pseudorandom Noise Code Correlation Reflectometry

In this paper, we describe multipoint hydrogen sensing in real time using surface plasmon resonance (SPR) fiber optic hydrogen tip sensors with Au/Ta2O5/Pd multilayers film combined with a time-domain interrogating system based on pseudorandom noise code correlation reflectometry. This method uses the correlation between a launched pseudorandom noise code signal and its reflection enabling simultaneous measurement at many points along the optical fiber in real time with a high signal-to-noise ratio. In a light intensity-based experiment with an 850-nm LED, the tested three hetero-core optical fiber hydrogen tip sensors proved to be reproducible responses with and without hydrogen absorption, with showing the similar time response and sensitivity compared with those of our previously reported sensor. Multipoint hydrogen detection was demonstrated using four SPR hydrogen tip sensors. Our experimental results showed that four sensors using the 25 nm Au/60 nm Ta2O5/5 nm Pd multilayer film exhibited a response time of $\sim 25$ s for a 4% hydrogen concentration. In addition, it was found that all sensors can detect the hydrogen concentration with sufficient sensitivities in real time.

Electronic Structure of Pd Multilayers on Re(0001): The Role of Charge Transfer

Understanding the origin of the properties of metal-supported metal thin films is important for the rational design of bimetallic catalysts and other applications, but it is generally difficult to separate effects related to strain from those arising from interface interactions. Here we use density functional (DFT) theory to examine the structure and electronic behavior of few-layer palladium films on the rhenium (0001) surface, where there is negligible interfacial strain and therefore other effects can be isolated. Our DFT calculations predict stacking sequences and interlayer separations in excellent agreement with quantitative low-energy electron diffraction experiments. By theoretically simulating the Pd core-level X-ray photoemission spectra (XPS) of the films, we are able to interpret and assign the basic features of both low-resolution and high-resolution XPS measurements. The core levels at the interface shift to more negative energies, rigidly following the shifts in the same direction of the valence d-band center. We demonstrate that the valence band shift at the interface is caused by charge transfer from Re to Pd, which occurs mainly to valence states of hybridized s-p character rather than to the Pd d-band. Since the d-band filling is roughly constant, there is a correlation between the d-band center shift and its bandwidth. The resulting effect of this charge transfer on the valence d-band is thus analogous to the application of a lateral compressive strain on the adlayers. Our analysis suggests that charge transfer should be considered when describing the origin of core and valence band shifts in other metal / metal adlayer systems.

Multi-point detection of hydrogen using the hetero-core structured optical fiber hydrogen tip sensors and Pseudorandom Noise code correlation reflectometry

In this paper, the multi-point hydrogen detection system based on the combination of the hetero-core optical fiber SPR hydrogen tip sensor and interrogator by pseudorandom noise (PN) code correlation reflectometry has been developed. In a light intensity-based experiment with an LED operating at 850 nm, it has been presented that a transmitted loss change of 0.32dB was induced with a response time of 25 s for 4% H₂ in N₂ in the case of the 25-nm Au, 60-nm Ta₂O 5 , and 5-nm Pd multi-layers film. The proposed sensor characteristic shows excellent reproducibility in terms of loss level and time response for the in- and out- H₂ action. In addition, in the experiment for multi-point hydrogen detection, all sensors show the real-time response for 4% hydrogen adding with reproducible working. As a result, the real-time multi-point hydrogen detection could be realized by means of the combination of interrogating system and hetero-core optical fiber SPR hydrogen tip sensors.

Surface plasmon resonance-based fiber-optic hydrogen gas sensor utilizing palladium supported zinc oxide multilayers and their nanocomposite.

We analyze surface plasmon resonance-based fiber-optic sensor for sensing of small concentrations of hydrogen gas in the visible region of the electromagnetic spectrum. One of the two probes considered has multilayers of zinc oxide (ZnO) and palladium (Pd) while the other has layer of their composite over a silver coated unclad core of the fiber. The analysis is carried out for different volume fractions of palladium nanoparticles dispersed in zinc oxide host material in the nanocomposite layer. For the analysis, a Maxwell-Garnett model is adopted for calculating the dielectric function of a ZnO:Pd nanocomposite having nanoparticles of dimensions smaller than the wavelength of radiation used. The effects of the volume fraction of the nanoparticles in the nanocomposite and the thickness of the nanocomposite layer on the figure of merit of the sensor have been studied. The film thickness of the layer and the volume fraction of nanoparticles in the ZnO:Pd nanocomposite layer have been optimized to achieve the maximum value of the figure of merit of the sensor. It has been found that the figure of merit of the sensing probe coated with ZnO:Pd nanocomposite is more than twofold of the sensing probe coated with multilayers of Pd and ZnO over a silver coated unclad core of the fiber; hence, the sensor with a nanocomposite layer works better than that with multilayers of zinc oxide and palladium. The sensor can be used for online monitoring and remote sensing of hydrogen gas.

Hydrogen evolution reaction on palladium multilayers deposited on Au(111): a theoretical approach.

We have investigated the electrocatalytic properties of multilayers of Pd epitaxially deposited on Au(111). In contrast to the numerous previous works in this area, we have focused on the kinetics of the electrochemical step for hydrogen adsorption (Volmer reaction) and determined its energies of activation. We have used a combination of density functional theory calculations and our own theory of electrocatalysis, which allows us to investigate the systems in an electrochemical environment. Contrary to our previous work with a submonolayer of Pd in Au(111), the activation barrier for the hydrogen adsorption process from proton is very low or almost zero for all bimetallic systems investigated. It is about 0.2 eV for pure Pd(111). In the case of two layers of Pd on Au(111) containing absorbed hydrogen in the subsurface, the adsorption free energy is less negative and the barrier lower than for the other investigated systems. This is in agreement with experimental data that shows a larger activity for hydrogen oxidation with hydride Pd systems.

The effect of microstructure on the hydrogenation of Mg/Fe thin film multilayers

Nanoconfined magnesium hydride can be simultaneously protected and thermodynamically destabilized when interfaced with materials such as Ti and Fe. We study the hydrogenation of thin layers of Mg (<14 nm) nanoconfined in one dimension within thin film Fe/Mg/Fe/Pd multilayers by the optical technique Hydrogenography. The hydrogenation of nanosized magnesium layers in Fe/Mg/Fe multilayers surprisingly shows the presence of multiple plateau pressures, whose nature is thickness dependent. In contrast, hydrogen desorption occurs via a single plateau which does not depend on the Mg layer thickness. From structural and morphological analyses with X-ray diffraction/reflectometry and cross-section TEM, we find that the Mg layer roughness is large when deposited on Fe and furthermore contains high-angle grain boundaries (GB's). When grown on Ti, the Mg layer roughness is low and no high-angle GB's are detected. From a Ti/Mg/Fe multilayer, in which the Mg layer is flat and has little or no GB's, we conclude that MgH2 is indeed destabilized by the interface with Fe. In this case, both the ab- and desorption plateau pressures are increased by a factor two compared to the hydrogenation of Mg within Ti/Mg/Ti multilayers. We hypothesize that the GB's in the Fe/Mg/Fe multilayer act as diffusion pathways for Pd, which is known to greatly alter the hydrogenation behavior of Mg when the two materials share an interface.

Surface structures of In-Pd intermetallic compounds. II. A theoretical study.

The (110) surface of the InPd intermetallic compound and the In-Pd surface alloy properties are investigated in the framework of the density functional theory, within the projector augmented plane-wave method. Surface segregation is calculated to be energetically unfavorable at stoichiometric InPd(110) surfaces, while indium antisites are shown to segregate to the surface in off-stoichiometric InPd(110) systems. Concerning surface alloys obtained by burying In-doped Pd layers in Pd(111), we demonstrated that the most stable ones are those presenting atomic indium concentrations below 50 at. % (11 at. %, 25 at. %, 33 at. %). According to our calculations, the In-doped Pd layers with concentration above or equal to 50% lead to In-doped Pd multilayers, each presenting an atomic indium concentration below 50 at. %. Alloying and segregation effects in InPd intermetallic compound and In-Pd surface alloys clearly agree with the larger bonding strength of In-Pd (-0.44 eV) compared to In-In (-0.29 eV) and Pd-Pd (-0.31 eV).

A hydrogen curing effect on surface plasmon resonance fiber optic hydrogen sensors using an annealed Au/Ta₂O₅/Pd multi-layers film.

In this paper, a response time of the surface plasmon resonance fiber optic hydrogen sensor has successfully improved with keeping sensor sensitivity high by means of hydrogen curing (immersing) process of annealed Au/Ta2O5/ Pd multi-layers film. The hydrogen curing effect on the response time and sensitivity has been experimentally revealed by changing the annealing temperatures of 400, 600, 800°C and through observing the optical loss change in the H2 curing process. When the 25-nm Au/60-nm Ta2O5/10-nm Pd multi-layers film annealed at 600°C is cured with 4% H2/N2 mixture, it is found that a lot of nano-sized cracks were produced on the Pd surface. After H2 curing process, the response time is improved to be 8 s, which is two times faster than previous reported one in the case of the 25-nm Au/60-nm Ta2O5/3-nm Pd multi-layers film with keeping the sensor sensitivity of 0.27 dB for 4% hydrogen adding. Discussions most likely responsible for this effect are given by introducing the α-β transition Pd structure in the H2 curing process.

Effects of surface charge doping on magnetic anisotropy in capping 3d–5d(4d) multilayers deposited on highly polarizable substrates

We present a systematic study regarding the influence of surface charge doping on the magnetic anisotropy (MA) of capping 3d–5d/(4d) multilayers deposited on highly polarizable substrates such as Pt or Pd. The effects on the MA are investigated within a first-principles approach with a focus on Fe–Pt and Fe–Pd multilayers. For both layered systems, it is found that the MA is strongly altered upon charge doping, exhibiting different behaviour, however. A remarkable enhancement on the MA along with magnetization reversal is observed in Fe–Pt multilayers. In contrast, Fe–Pd multilayers have small MA energies as a consequence of their reduced orbital moments. Some MA features observed in the multilayers can be related with variations in the spin and orbital moment of the magnetic layers. Insights of the origin of the MA behaviour are investigated from a local perspective by analysing the d-orbital resolved local density of states in the framework of the second-order perturbation theory. Finally, the relation between the MA and orbital moments is discussed in terms of Bruno's formula.

Optimization of Tunneling Magnetoresistance in Perpendicular Magnetic Tunnel Junctions With Co|Pd Reference Layers

We report on the optimization of Co|Pd-multilayer-based reference layers in magnetic tunnel junctions with perpendicular magnetic anisotropy. By inserting a thin Ta-spacer layer between the CoFeB interfacial layer and Co|Pd multilayer, a high tunneling magnetoresistance up to 98.5% can be achieved. Electron energy loss spectroscopy and synchrotron X-ray diffraction studies show that the high magnetoresistance is related primarily to the suppression of Pd diffusion and secondarily to improved CoFeB texture during annealing.