Pd Multilayers Research Articles

First-principles calculations of the magnetic anisotropic constants of Co–Pd multilayers: Effect of stacking faults

Using first-principles density functional theory calculations with spin-orbit coupling, we systematically investigate the magnetic anisotropic energy (MAE) of ConPdm (n+m=5) magnetic multilayers. We consider the influences of the relative atomic weight of Co, wCo, stacking fault, and external stress on the MAE. We find that out-of-plane lattice constant, saturation magnetization, and magnetic moments are almost linearly correlated with wCo. The magnetic anisotropic constant (MAC) curve of ConPdm without stacking fault shows a near-linear dependence on wCo that agrees with our derived effective MAC Kueff which includes shape, magneto-crystalline, and magneto-elastic contributions. We also show that the contributions from Pd layers to both the total magnetic moments and magnetic anisotropy are significant. The stress anisotropy due to the substrate has a weak effect on the MAC. However the stacking fault has a strong effect on the MAC. When the Co layer is thin, a Co–Pd interface without stacking fault is necessary for higher Kueff. However, when the Co layer is thick, creating stacking faults inside the Co region may produce a larger Kueff. Our study suggests the ways to increase the perpendicular magnetic anisotropy in Co–Pd multilayer systems and subsequently leads to the development of novel magnetic recording devices.

Determination of spin pumping as a source of linewidth in sputtered Co90Fe10/Pd multilayers by use of broadband ferromagnetic resonance spectroscopy

We performed a systematic study of damping in Co${}_{90}$Fe${}_{10}$/Pd multilayers by use of broadband (1--60 GHz) ferromagnetic resonance (FMR) spectroscopy in the perpendicular geometry. The data were fitted with the conventional Landau-Lifshitz equation in conjunction with an inhomogeneous contribution to linewidth $\ensuremath{\Delta}$${H}_{0}$. Samples were prepared with net perpendicular anisotropy field values ranging from \ensuremath{-}0.5 to $+$1.2 T. $\ensuremath{\Delta}$${H}_{0}$ shows a dependence on the perpendicular anisotropy, though the Landau-Lifshitz damping parameter $\ensuremath{\alpha}$, which ranged from 0.016 to 0.04, exhibits no trend as a function of anisotropy. We explain the wide variation of $\ensuremath{\alpha}$ as a result of spin pumping from Co${}_{90}$Fe${}_{10}$ into adjacent nonmagnetic layers. We use a quantitative model for spin pumping that includes the intrinsic spin-mixing conductance at the Co${}_{90}$Fe${}_{10}$/Pd interface and the spin-diffusion length of Pd, which were experimentally measured at room temperature to be (1.07 $\ifmmode\pm\else\textpm\fi{}$ 0.13) $\ifmmode\times\else\texttimes\fi{}$ 10${}^{19}$ m${}^{\ensuremath{-}2}$ and 8.6 $\ifmmode\pm\else\textpm\fi{}$ 1.0 nm, respectively. We quantitatively show how $\ensuremath{\alpha}$ is enhanced by spin pumping through an FMR investigation of individual Pd/CoFe/Pd, and Pd/CoFe/Pd/CoFe/Pd layer structures.

Observation of Magnetic Compton Profile of Interface Controlled Co/Pd Multilayer

We compare two Co/Pd multilayers with correspondingly smooth and rough interfaces. The first is a Co (1.5 nm)/Pd (2.6 nm) multilayer with a smooth interface deposited by the MBE technique, and the second is a Co (1.6 nm)/Pd (4.0 nm) multilayer with a rough interface deposited by the sputter technique. Both multilayers have almost the same perpendicular magnetic anisotropy energy, 1.15 Merg/cc for the Co (1.5 nm)/Pd (2.6 nm) multilayer and 1.20 Merg/cc for the Co (1.6 nm)/Pd (4.0 nm) multilayer, respectively. The symmetry of the wave function, which is measured using the magnetic Compton profile, is almost the same for both multilayers. This suggests that the smooth interface controls the wave function and enhances the PMA energy even if the Co/Pd multilayer has a thinner Pd layer.

Time dependence of interlayer coupling in Pd(50 Å)/Co(tCo Å)/Pd(54 Å)/Co(tCo Å)/Pd(50 Å) multilayer with perpendicular anisotropy

Time-dependent ferromagnetic (FM)-antiferromagnetic (AFM) interlayer coupling transition has been found in Pd(50 Å)/Co(tCo Å)/Pd(54 Å)/Co(tCo Å)/Pd(50 Å) multilayer. During exposure to the air, saturation magnetic moment and coercivity both drop with time; moreover, a shift of the minor loop center for AFM coupling also decreases with time. During exposure of the sample to the air, as one possibility, the observed time-dependent FM-AFM interlayer coupling transition and decrease of AFM coupling strength may result from absorption of gaseous elements, which can induce reduction of Pd-polarization and a decrease of saturation magnetic moment; another possibility may be the alloying of the Co/Pd interface.

Construction of multilayers of bare and Pd modified gold nanoclusters and their electrocatalytic properties for oxygen reduction

Multilayers of gold nanoclusters (GNCs) coated with a thin Pd layer were constructed using GNCs modified with self-assembled monolayers (SAMs) of mercaptoundecanoic acid and a polyallylamine hydrochloride (PAH) multilayer assembly, which has been reported to act as a three-dimensional electrode. SAMs were removed from GNCs by electrochemical anodic decomposition and then a small amount of Pd was electrochemically deposited on the GNCs. The kinetics of the oxygen reduction reaction (ORR) on the Pd modified GNC/PAH multilayer assembly was studied using a rotating disk electrode, and a significant increase in the ORR rate was observed after Pd deposition. Electrocatalytic activities in alkaline and acidic solutions were compared both for the GNC multilayer electrode and Pd modified GNC electrode.

Magnetization reversal assisted by the inverse piezoelectric effect in Co-Fe-B/ferroelectric multilayers

We investigate magnetization reversal assisted by electric fields in a Co${}_{40}$Fe${}_{40}$B${}_{20}/$Co$/$Pd multilayer with soft perpendicular anisotropy deposited onto an epitaxial piezoelectric layer. Both the nucleation and depinning magnetic fields show a strong reduction of up to 30$%$ under the application of an electric field. These results show good prospects for assisting the switching processes in ultra-low-power spintronic devices.

Electrochemical properties of palladium adlayers on Pt(1 1 0) substrates

Palladium deposition on Pt(110) electrodes was studied. Differently from Pt(111) and Pt(100), first and further layers cannot be distinguished in this case only through the voltammetric behavior of hydrogen and anion adsorption. The potential of zero total charge (pztc) was determined as a function of the amount of deposited Pd using CO charge displacement experiments and voltammetric curves. The variation of the voltammetric charge due to hydrogen and anion adsorption has been followed during Pd deposition. The voltammetric charge between 0.06 and 0.4V first decreases to a minimum, then increases and finally becomes stable, suggesting that the surface is finally covered with a Pd multilayer. The pztc and CO oxidation charge show similar behavior. CO oxidation, NO reduction and Cu UPD were used as probes to monitor Pd coverage. The potential of CO oxidation increases with Pd coverage while the potential of NO reduction decreases. Using the information obtained from Cu UDP and FTIR experiments it has been possible to determine when the first Pd single layer was completed and when a second (further) layer(s) starts to grow.

Electrochemical behaviors of nanoporous coordination polymer multilayers in hexacyanoferrate solution

Coordination polymers (CPs) can encapsulate small molecules within their nanosized coordination cages to form hybrid supramolecular materials. We reported here assembly of Pd(II)-mediated CP multilayers on the substrate surfaces by layer-by-layer method with six kinds of multidentate ligands as linkers, which included 4,4′-bipyridyl (BPy), binaphthylbis(amidopyridyl) (BNBAPy), tris(4-pyridyl)-1,3,5-triazine (TPyTa), tetra(4-pyridyl)porphyrin (TPyP), ligand-like complex of Fe(pyterpy) 2 (pyterpy: 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine), and poly(4-vinylpyridine) (PVP). The multilayer assembly process was characterized by using UV–vis absorption spectra and X-ray photoelectron spectra. Electrochemical behaviors of the as-prepared CP-modified electrodes were investigated in hexacyanoferrate solution, which revealed well-reversible redox waves of Fe(CN) 6 3−/4− ions for the electrode covered with the Pd–TPyTa or Pd–Fe(pyterpy) 2, while irreversible ones for the electrode covered with the Pd–BPy or Pd–PVP. The results indicated that Fe(CN) 6 3−/4− ions could reach electrode surface across the CP multilayers of TPyTa or Pd–Fe(pyterpy) 2, which was attributed to the fact that their coordination cages were large enough for the Fe(CN) 6 3−/4− ions to penetrate through. Charge-transfer diffusion coefficient ( D) was calculated based on chronocoulomograms of the CP multilayer-modified electrodes. D was in the range from 0.34 × 10 −10 to 1.3 × 10 −10 cm 2/s for the electrodes modified by four layers of Pd–TPyP, Pd–BNBAPy, Pd–TPyTa and Pd–Fe(pyterpy) 2, respectively. The present methodology provides a facile way to construct three-dimensional supramolecular materials on the solid surfaces as well as to study the charge-transfer behaviors in artificial membranes.

Perpendicular magnetic tunnel junctions using Co-based multilayers

CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) free and reference layers composed of Co/M (where M=Pd or Ni) multilayers have been optimized for high PMA and high tunneling magnetoresistance (TMR). The effects of Co thickness, Pd thickness, and the number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]n multilayer films have been studied for both free and reference layers. The damping parameter α of CoFeB capped multilayers was determined using broadband ferromagnetic resonance. The transport properties of the patterned MTJ stacks were measured from 10 to 400 K. A maximum TMR of 10% at 10 K (5%–10% at 300 K) was obtained for these perpendicular MTJs, regardless of whether or not they were magnetically annealed for MgO–CoFeB crystallization. This indicates that the fcc-bcc-fcc transitions from the fcc multilayers to the bcc CoFeB/MgO/CoFeB do not promote the “giant MgO TMR effect” caused by symmetry filtering.

Nitrate reduction on Pt single crystals with Pd multilayer

Nitrate reduction on palladium multilayers deposited on platinum single crystal electrodes was studied by cyclic voltammetry and FTIR spectroscopy in acid and alkaline media. The results are compared with those obtained with bulk palladium single crystals. The reaction is sensitive to the electrode surface structure, the reactivity depending on the solution pH. In acid solution nitrate was reduced at potentials below the potential of zero total charge (pztc), when the electrode is negatively charged. Competition between nitrate, hydrogen and anion adsorption and NO formation and accumulation at the surface are proposed as the main reasons for the slow reaction rate. On the bulk palladium single crystal electrodes, NO formation leads to a fast blockage of the surface resulting in a very low activity for nitrate reduction. In alkaline solution, nitrate is reduced at more positive potentials with significantly higher current being measured on the Pd multilayer on Pt(1 0 0) electrode.

Magnetic strip patterns induced by focused ion beam irradiation

Focused ion beam exposure was used to locally alter the magnetic properties of a continuous Co∕Pd multilayer film with perpendicular magnetic anisotropy. The saturation magnetization, coercivity, and magnetic anisotropy of the films can be tuned by Ga irradiation depending on exposure dose. As a result, a periodic strip pattern consisting of 80nm wide exposed strips which are magnetically soft, separated by 170nm wide magnetically hard, unexposed areas was created. Due to strong magnetostatic coupling between the strips, a number of magnetic domain configurations could be stabilized and these have been observed by magnetic force microscopy and magneto-optic Kerr effect measurements. The magnetic domain configurations and their reversal behavior were investigated by micromagnetic simulations as a function of exposure dose and strip period.

Low temperature vacuum annealing study of (Co∕Pd)n magnetic multilayers

The correlation between the magnetic properties and the microstructural and chemical composition modifications of Co∕Pd magnetic multilayers upon annealing in ultrahigh vacuum at 250°C is presented. Magnetic characterization using magnetic sample magnetometer shows the vertical magnetic anisotropy increase and the switching field distribution decrease in the annealed samples. The larger values of magnetic anisotropy in the annealed samples are further shown using the magnetic force microscopy of the ac demagnetized states in the Co∕Pd multilayer films. X-ray diffraction rocking curves show an improvement in the texture and the initial magnetization curve slopes indicate the decreases in defect densities. Overall, vacuum annealing under optimal conditions improves the magnetic properties of Co∕Pd multilayers for applications in ultrahigh density magnetic recording.

Study of high magnetic anisotropy Co∕Pd multilayers for patterned media

We have studied the perpendicular magnetic anisotropy of Co∕Pd multilayers for ultrahigh recording density patterned media. By using Pt as an underlayer, the magnetic anisotropy enhances significantly, which is considered to be due to a magnetostriction by inducing a tensile strain into the Co∕Pd multilayers. A patterned medium using the Co∕Pd multilayers has been successfully fabricated with the diameter of the dots and the distance between the center of the dots measuring 20 and 30nm, respectively. The coercivity of the nanopatterned medium is 9.5kOe.

Effects of Ta seed layer and annealing on magnetoresistance in CoFe∕Pd-based pseudo-spin-valves with perpendicular anisotropy

We have studied the switching characteristics and magnetoresistance of pseudo-spin-valves with perpendicular anisotropy based on CoFe∕Pd multilayers. In unpatterned thin films without exchange biasing, a maximum current-in-plane giant magnetoresistance of 7% was achieved, the highest reported to date in perpendicular pseudo-spin-valves. A Ta seed layer and the fcc (111) orientation of Pd was shown to be important in order to achieve good perpendicular anisotropy and sharp switching behavior. The improvement in perpendicular anisotropy and decay in magnetoresistance upon postdeposition annealing have been attributed to the formation of CoPd alloys at the CoFe∕Pd interfaces.

Hybrid recording on bit-patterned media using a near-field optical head

We have demonstrated hybrid recording on magnetic bit-patterned media by a near-field optical head with plasmon probe. The near-field plasmon probe nanobeak that uses a triangular metallic plate with a three-dimensionally tapered apex can provide a highly efficient optical spot as small as 16 nm × 20 nm. The bit-patterned media consist of a Co (0.3 nm)/Pd (0.7 nm) multilayer of 20-nm-thick magnetic nanodots each with a diameter of 20 nm and a period of 30 nm. We confirmed magnetic single-dot recording on the Co/Pd bit-patterned media. Hybrid recording simulation results suggest that hybrid recording on bit-patterned media can achieve significant data-density recording, even when the magnetic dot size is less than 5 nm. For practical use, we made a prototype of nanodot circularly aligned Co/Pd bit-patterned media, and also demonstrated light delivery method using either solid immersion optics or an optical fiber.

Optimization of magnetic properties of Co∕Pd multilayers by applying a large persistent biaxial stress

We report about optimization of the effective perpendicular magnetic anisotropy (Keff) of [Pd(0.98nm)∕Co(tnm)]30 multilayers by applying an in-plane biaxial tensile stress by loading the Ta substrate with hydrogen. The stress induced anisotropy enhances the Keff, which in turn increases the coercive field and the squareness of the perpendicular magnetization curves. A quantitative analysis of the data using a simple model yields magnetostriction constants for the [Pd(0.98nm)∕Co(tnm)]30 multilayers which are in good agreement with the literature values. We conclude that the Keff can be significantly enhanced by applying persistent in-plane biaxial tensile stress.

Thermally activated magnetization reversal behavior of uniaxial ferromagnetic thin films in the microsecond to second time regime

We investigate the thermally activated magnetization reversal of Co∕Pd multilayers with uniaxial perpendicular anisotropy using a magneto-optical Kerr effect microscope capable of direct domain observation in a wide time range of microsecond to second. It is found that the energy barrier in the thermally activated process is nonlinearly dependent on the applied field, irrespective of the difference in the domain evolution processes of wall motion, dendrite growth, and nucleation. These results are well explained by a theoretical model considering a ferromagnetic thin film with uniaxial perpendicular anisotropy.

He + ion irradiation study of continuous and patterned Co∕Pd multilayers

Ion irradiation of continuous and patterned (Co∕Pd)n magnetic multilayer films has been studied as a mean to control magnetic anisotropy as well as to evaluate possible ion irradiation damage involved in ion-beam proximity lithography patterning. The coercivity of patterned medium was found to decrease from 11kOe for as patterned samples to 0.3kOe for samples with 800μC∕cm2 ion irradiation. Remnant squareness of the patterned samples remained essentially unchanged. As the number of bilayers increases in the sample, the effects vary, suggesting that several mechanisms of damage occur. Significantly, for typical irradiation doses used in ion-beam proximity lithography, no measurable alteration of magnetic properties was observed.

Origins of switching field distributions in perpendicular magnetic nanodot arrays

We studied the reversal properties of perpendicularly magnetized Co∕Pd nanodots from 100to50nm in diameter fabricated using electron beam lithography. Polycrystalline Co∕Pd multilayers show considerable differences in the switching field distribution (SFD) depending on the seed layer used. With a Ta seed layer, we reduced the SFD to approximately 5% of the average switching field. To rule out effects of grain boundaries, we also fabricated nanodot arrays from epitaxial Co∕Pd superlattices. Although significant improvement in SFDs are obtained using epitaxial superlattices, our results indicate that grain boundary variation within nanodots is not the primary origin of SFD broadening that occurs with nanopatterning.

Effects of interlayer exchange coupling on transport properties of Co∕Pd metallic multilayers with perpendicular magnetic anisotropy

Ru-mediated interlayer exchange coupling (IEC) was introduced in Co∕Pd(111) metallic multilayers with perpendicular magnetic anisotropy. The magnetic and electronic transport properties were investigated. The magnetically controlled Co∕Pd(111) multilayers exhibited unique resistivity changes. This is quite common in perpendicular antiferromagnetically (AF) coupled systems where the AF coupled multilayer stacks switch separately during field reversal. However, we found that even within the stacks, the individual ferromagnetic layers switch separately, such that the AF configurations are present during reversal even within the same multilayer stack. On considering the magnetic reversal and transport properties of the Co∕Pd multilayers, the IEC was found to cause antiferromagnetic alignment inside a multilayer stack that was sandwiched between two Ru layers. This phenomenon is mainly due to the thicker Pd films between the Co layers that allow more decoupling of individual Co layers than those employed in most of the previous studies.