Amorphous CoZr Research Articles

Thermal desorption behaviors of helium in Zr-Co films prepared by sputtering deposition method

The amorphous ZrCo films on Mo substrates with different 4He content were prepared through direct current (DC) magnetron sputtering deposition method. The atomic compositions of Zr, Co and He in Zr-Co films were measured using Rutherford backscattering (RBS) and elastic recoil detection analysis (ERDA), respectively. The helium desorption behaviors in the films were investigated. Microstructural changes during THDS measurement were analyzed with grazing incidence X-ray diffraction (GIXRD). It was found that ZrCo2 phase, an important disproportionation product in ZrCo hydriding-dehydriding process, was formed during the THDS experiment in a rising temperature range of 300 K–1323 K. The helium desorption signals can be evidently divided into three main zones and represent different trapping states. Synchronous desorption of hydrogen and helium was observed.

Properties of amorphous CoZr(RE) (RE=Gd, Sm, Dy) films with various uniaxial anisotropies, prepared by a new process

If a DC magnetic field is applied parallel to the plane of amorphous CoZr(RE) thin films during sputter depositing, a uniaxial anisotropy is formed the direction of which depends upon the choice of RE substituted and its concentration. When RE=Gd a perpendicular anisotropy K p forms over a large concentration range, a spin reorientation process being at the origin of the process. A well-defined K p is developed also in CoFeZrGd and CoZrGdSm films. CoZrGdDy films exhibit simultaneously a perpendicular and an in-plane uniaxial anisotropy. The related magnetization process and domain structures are quite peculiar.

Phase separation in amorphous Zr-Co and Zr-Fe alloys

Abstract Amorphous Zr-Co and Zr-Fe alloys were prepared by melt spinning and co-puttering and investigated by electromotive force measurements. The experiments show constant chemical potentials in different concentration ranges, which are indicative of the presence of two amorphous phases in both alloy systems. The thermodynamic data are analysed using the calculation-ofphase-diagrams method. In amorphous Zr-Co a two-phase region is found for compositions with between 63 and 77 at.% Co. In Zr-Fe the metastable equilibrium between two amorphous phases extends from 37 to 62 at.% Fe. The calculation of the enthalpies of mixing and the Gibbs energies shows that, in both systems, one amorphous phase has its maximum stability at about equiatomic composition, whereas a second minimum of the Gibbs energy is located at Zrpoor concentrations. The different two-phase range of amorphous Zr-Co compared with that of Zr-Fe is explained by the relative stabilities of the chemical order in the amorphous phases, which diff...

Soft a-CoZr-based spin valves (abstract)

The materials options to combine high giant magnetoresistance (GMR) ratios with soft magnetic properties have been relatively few, with NiFe providing low-field switching in the great majority of studies. These NiFe layers tend to be hardened appreciably when directly exchange coupled to Co to provide higher GMR ratios, as in (Ta)/NiFe/Co/Cu/Co/NiFe/FeMn spin valves. We propose magnetically soft, amorphous CoZr as a replacement couple to Co in these and other structures. While the high resistivity of amorphous films discourages their use as a single switching element, it favors their use as the drive layer for Co in a spin valve. Our transport simulations, using the Camley–Barnas model modified by Dieny,1 indicate that GMR ratios fall off much less steeply with increasing a-CoZr drive layer thickness than with NiFe drive layer thickness. Since lower coercivities are expected for higher fractions of the drive material, CoZr-based spin valves show promise in combining soft properties and high GMR ratios. We have demonstrated the combination of magnetic softness and high GMR ratio in our own CoZr-based spin valves. A free layer coercivity of 7.2 Oe and GMR ratio of 6.4% were achieved in a nominal structure of NiO(400)/Co(50)/Cu(20)/Co(25)/CoZr(200AA). Differential coercivity structures were deposited as well, using Cr underlayers to induce hardness of the bottom Co. An identical “active layer” structure of the form SiO2/Cr/Co(50)/Cu(20)/Co(25)/CoZr(200AA) demonstrates GMR ratios as high as 4.9%; removal of the interfacial Co layer reduces the GMR ratio to 2.8%, in accordance with our expectations. Discussion of optimization and detailed structural analysis will be presented in the full paper.

Giant magnetoresistance effect in CoZr/Cu/Co spin-valve films (abstract)

A high sensitivity of giant magnetoresistance (GMR) has been observed recently from soft magnetic layers such as NiFe, NiFeCo, and FeCoB.12 Amorphous CoZr alloys present ultrasoft properties compared to NiFe.3 GMR has been investigated for amorphous CoZr/Cu/Co thin films grown by rf diode sputtering using a target consisting of a Co disk partially covered with a Zr foil. The influence of the argon pressure on Cu layer deposition, Cu thickness, and Zr content on magnetic and transport properties was analyzed. The highest value of transverse GMR obtained along the easy axis is 3.6% and the MR curve was saturated in a magnetic field of 100 Oe at room temperature. GMR shows scaling behavior with the sample composition. Very high sensitivity, around 1–2%/Oe was observed in a CoZr (3 nm)/Cu (3 nm)/Co (2 nm) sandwich. This study shows a large dependence of GMR on Cu thickness and the maximum of magnetoresistance strongly depending on the Ar pressure which modifies the interface roughness. The Zr content also influences the magnetotransport properties (ΔR/R and ΔR/RΔH). The difference in coercivity between soft magnetic CoZr and hard magnetic Co layers induces antiferromagnetic alignment. Therefore a high MR ratio and field sensitivity are achieved by improving the magnetic properties of the CoZr layer.

Influence of Composition and Plastic Deformation on the Hydrogen Internal Friction Peak in Amorphous Co-Zr Alloys

Vibrating-reed measurements of the hydrogen internal friction peak were performed on a series of Co-Zr glasses ranging from Co 55 Zr 45 to Co 24 Zr 76 . Variations in peak shape and growth behaviour (on increasing the H concentration) were observed for different alloy compositions, together with an increase of the average activation energy with cobalt concentration. These findings can be explained by the interstitial reorientation mechanism, assuming reorientation jumps of the H atoms between Zr 4 tetrahedra; the activation energy increase then reflects the increase of the average Zr-Zr nearest neighbour distance with Co concentration known for this system. In Co 55 Zr 45 , two peaks were detected under certain H-charging conditions, possibly as a result of a chemical decomposition. After cold-rolling, additional IF peaks appear superimposed on the hydrogen peak (with large scatter), but can be distinguished as separate processes by a different annealing behaviour.

Crystallization of near-eutectic Co-Zr alloys and the formation and fast decay of a bcc Co 92Zr 8 solid solution

Crystallization of amorphous Co-Zr is studied for compositions ranging from 7 to 14 at.% Zr using time-resolved small and large angle X-ray scattering. A time-transformation diagram is given for the composition range studied. A new metastable bcc-Co phase with 8 at.% dissolved Zr is found, which has a lattice constant of 2.845 Å, It forms upon quenching from the melt or by polymorphic crystallization of the amorphous alloy. Cooling the bcc-phase leads to a fast transformation. Within about 1 s, a semi-coherent structure is formed where the local concentration and lattice structure is modulated on a length scale of about 30 Å. During this transformation the local composition and crystal structure segregate to fee-Co and to Co 23Zr 6.

NMR study in amorphous CoZr thin film alloys

59Co NMR study has been carried out in a series of magnetic thin film amorphous Co1−xZrx alloys in the concentration range 0.1 <x < 0.4. The analysis shows that every Zr nearest neighbour lowers the NMR frequency on Co in the amorphous CoZr alloys by about 30 MHz and that the alloy structure in Co-rich compositions resembles the polytetrahedrally closed packed crystalline phases.

Induced and local anisotropies in amorphous CoZr–rare earth thin films containing Pr, Nd, and Tb

The induced in-plane and local magnetic anisotropies of rf sputtered (Co93Zr7)100−xREx thin films were investigated as a function of RE content for RE=Pr, Nd, and Tb by transverse biased initial susceptibility measurements at both film/air and glass/film interfaces. The deposition was performed in a magnetic field. The films exhibit a very well defined in-plane anisotropy with negligible long-range fluctuations and a small coercive field Hc, in accordance with the experimental data. The overall variations of the transverse susceptibility are related to the ripple. The variations of the local anisotropy with composition were measured. Both the induced anisotropy Ku and the local anisotropy Kloc increase with the increasing amount of RE in the layers. A clear relationship between Ku and Kloc could be established. The results are discussed in terms of the single-ion anisotropy of the rare earth. While the value of Ku is the same at both interfaces, the ripple constants were found to be slightly different. This last result is believed to be related to some local defects, the origin of which is also suggested. Also the coercive force is discussed and explained in terms of a theoretical model.

NOVEL BINARY SPIN GLASS ALLOYS SYNTHESIZED BY MECHANICAL MILLING

A few novel binary spin glass alloys have been successfully synthesized by mechanical milling of ordered intermetallic compounds in a high energy ball mill. These alloys are amorphous Co 2 Ge , atomically disordered crystalline GdAl 2, and ball-milled crystalline and amorphous CoZr. The characteristic features of spin glasses are observed; these are (a) the sharp cusps at the freezing temperature T f in both ac and dc magnetic susceptibility versus temperature curves and their peculiar sensitivity to the external magnetic field; (b) the irreversibility, i.e. the difference in value and shape between the magnetisation versus temperature curves after zero field cooling (ZFC) and field cooling (FC) at temperatures below T f and (c) the displacement of the FC magnetisation curve relative to ZFC curve and the corresponding remanence in the FC curve at low temperatures. The freezing temperatures are 41, 65, 35, and 11 K, respectively, which are defined by the sharp cusps in the lowest field ac susceptibility versus temperature curves. The freezing temperatures are lowered with increasing external field. The common feature of these materials is that all are binary alloys with a rather high concentration of the magnetic component. The discovery of these novel spin glasses is of significance because they not only represent new classes of spin glass materials but also demonstrate the use of mechanical milling as a novel technique to synthesize various new materials such as spin glasses. The results are reviewed briefly.

Characterization of local anisotropy and anisotropy dispersion of amorphous CoZrRE with rare earth REPr and Nd

The magnetic properties of rf sputtered amorphous (Co 93Zr 7) 100− x (RE) x thin films were investigated where REPr and Nd and for a content of 0 < x < 3.5 by Transverse Biased Initial Susceptibility measurements. Clear relationships could be established between K u and K loc. The data are discussed in terms of single-ion anisotropy of the RE.

Formation of YBa2Cu3O7 films on silicon/silicide/oxide structures formed via amorphous CoZr alloys

Thin films of the superconductor YBa2Cu3O7 have been grown on Si(111)/CoSi2/ZrO2 structures by inverted cylindrical magnetron sputter deposition at 700 degrees C. The CoSi2/ZrO2 bilayer structure was formed simultaneously by annealing an amorphous CoZr alloy layer in O2 at 650 degrees C. The amorphous alloy was formed by solid state reaction of crystalline Co and Zr under long anneals at a temperature below the crystallization temperature of the CoZr alloy. The bilayer formation gave a polycrystalline ZrO2 and an epitaxial CoSi2 layer aligned with the Si(111) substrate, with a channelling yield of 40%. YBa2Cu3O7 films grown on this multilayered structure had a Tc of 81 K and a transition width, Delta T, of 3 K as measured by resistivity readings.

Induced anisotropy in amorphous Co-Zr-M (M=Zr,Nb,Ti) and Co-Zr-Pt thin films: A magnetic and a structural study.

A magnetic and structural investigation was carried out on ${\mathrm{Co}}_{95\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zr}}_{5}$${\mathit{M}}_{\mathit{x}}$ (M=Zr, Nb, or Ti) and ${\mathrm{Co}}_{91\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zr}}_{9}$${\mathrm{Pt}}_{\mathit{x}}$ amorphous thin films in order to determine the physical origin of the induced in-plane uniaxial anisotropy ${\mathit{K}}_{\mathit{u}}$. The films were prepared by rf sputtering and the deposition was performed in a magnetic field. The variations of ${\mathit{K}}_{\mathit{u}}$ were studied as a function of the sputtering parameters, composition, concentration, the thermomagnetic treatment, and structural-relaxation process. As a result of these investigations a model for the origin of ${\mathit{K}}_{\mathit{u}}$ is advanced, which involves the single-ion anisotropy mechanism. ${\mathit{K}}_{\mathit{u}}$ is the sum of two contributions ${\mathit{K}}_{\mathit{u}}^{\mathit{a}}$+${\mathit{K}}_{\mathit{u}}^{\mathit{s}}$ where ${\mathit{K}}_{\mathit{u}}^{\mathit{a}}$ and ${\mathit{K}}_{\mathit{u}}^{\mathit{s}}$ are the induced anisotropies related to the local anisotropy of the octahedral-icosahedral and trigonal-type clusters, respectively. The number of trigonal clusters varies as a function of ${\mathit{P}}_{\mathrm{Ar}}$ and thus explains the variation of ${\mathit{K}}_{\mathit{u}}$ with ${\mathit{P}}_{\mathrm{Ar}}$. In Co-Zr-Pt an extra contribution to ${\mathit{K}}_{\mathit{u}}$ is observed due to the pseudodipolar ordering that results from the magnetic polarization of Pt. Co-Zr-Pt films exhibit a peculiar thermomagnetic relaxation that is explained by the reversibility of the pseudodipolar anisotropy and the thermal instability of trigonal-type clusters resulting from the substitution of Pt.

Decomposition of Nitrogen Monoxide over Amorphous Co&amp;ndash;65Zr Alloys Containing Platinum Group Elements

Surface-activated amorphous Co-Zr alloys containing platinum group elements were used for decomposition of nitrogen monoxide. The alloys were immersed in hydrofluoric acid for activation. The HF-treated amorphous alloys were initially inactive for decomposition of nitrogen monoxide at 773 K, but became active after oxidation by exposure to nitrogen monoxide at 773 K. XRD and XPS analyses revealed that the active catalysts were composed of platinum group elements, Co 3 O 4 and ZrO 2 . Platinum group elements in both metallic and oxidized states were supported on the oxides. The catalytic activity depended on platinum group elements. The palladium-containing alloy showed the highest activity and even higher activity than that of a palladium-black

Influence of in-plane anisotropy and eddy currents on the frequency spectra of the complex permeability of amorphous CoZr thin films

A broadband method for measuring the complex permeability of magnetic thin films, based on the measurement of the S parameters of a stripline and on the numerical solution of Laplace's equation according to the quasi-TEM (transverse electromagnetic) propagation mode, is presented. The method is applied to determine the permeability spectra of CoZr amorphous thin films in the 3 MHz to 3 GHz frequency range. The influence of the exciting magnetic field orientation with respect to the direction of the easy axis is studied. The influence of eddy current on the cutoff frequency observed for permeability spectra is analyzed. >

Magnetic, electronic and electron transport properties of amorphous (Co0.9Zr0.1)100-xXx (X=Al, Si, Cu, Ge and Zr) pseudo-binary alloys

The magnetic phase diagram is constructed for the amorphous (Co0.9Zr0.1)100-xXx (X=Al, Si, Cu, Ge and Zr) psuedo-binary alloys in the concentration range 0<or=x<or=45 through the DC and AC magnetization measurements in the range 4-800 K. The present results are compared with the previous data for the amorphous Co-Zr binary alloys. The ferromagnetism of the amorphous Co90Zr10 master alloy is rapidly weakened and disappears at about x=30, regardless of the third elements Al, Si, Cu and Ge. The concentration dependence of the density of states at the Fermi level is deduced by extracting the band structure contribution from the measured linearly temperature-dependent specific heat coefficient. A significant difference in the electronic structure emerges above x=30, depending on whether the element X added to the Co90Zr10 master alloy is a non-transition metal, like Si and Ge, or the transition metal Zr. A rapid increase in the electrical resistivity with increasing x beyond x=20 for the Si and Ge bearing alloys is attributed to a rapidly decreasing density of states at the Fermi level.

Magnetostriction of amorphous CoZrM (M = Ti, Nb, Pt) thin films

Effect of transition metals (3d, 4d and 5d) on the saturation magnetostriction of the amorphous Co-Zr thin films has been reported. Investigated films were deposited by rf sputtering. Magnetostriction constant λ s was measured at room temperature using the strain modulated ferromagnetic resonance method. The substitution of transition metals as Ti, Nb, Pt for Zr decreases the value of λ s and even change its sign. No correlations have been found between λ s and saturation magnetization for all investigated alloys.

Sign reversal of the Hall coefficient in amorphous Co-Zr thin films

The slope of the Hall curve, above saturation, for all ferromagnetic samples is negative and a change of the sign of R 0 to positive value appears for Co 1- x Zr x at x = 0.50 where films are paramagnetic. This anomalous behaviour is associated with the electronic band structure of these alloys.

Free-Volume-Dependent Atomic Diffusion in Compositionally Modulated Amorphous Co-Zr Films

Atomic interdiffusion in compositionally modulated amorphous Co-Zr films was investigated by means of in situ X-ray diffraction measurements during successive isothermal annealing. The results were analyzed as free-volume-dependent atomic diffusion and activation-type temperature dependence of both the atomic diffusion coefficient, and the annihilation rates of free volume were confirmed with the activation energy QD=8.84(±0.41)×104 J/mol and QA=3.77(±0.96)×104 J/mol, respectively.

Tracer diffusion and thermal stability in amorphous Co-Zr and their relevance for solid-state amorphization.

Measurements have been made of tracer diffusion, thermal relaxation, and crystallization in amorphous ${\mathrm{Co}}_{89}$${\mathrm{Zr}}_{11}$ samples prepared by melt spinning in vacuum. The diffusivity in as-quenched material is enhanced due to the presence of quasivacancies (excess free volume). During an annealing treatment, the quasivacancies are eliminated and the diffusivity approaches a constant value that depends on temperature only and pertains to a metastable equilibrium state. Tracer self-diffusion of $^{57}\mathrm{Co}$ measured over the temperature range 513--693 K in relaxed amorphous material can be described by D${(}^{57}$Co)=8.03\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ exp(- 147 kJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$ / RT ) ${\mathrm{m}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. Tracer diffusion of Au in the temperature range 633--684 K is represented by D${(}^{195}$Au)=7.90\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}1}$ exp(- 274 kJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$ / RT ) ${\mathrm{m}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.Co is a fast diffusor as compared to Au, which very likely diffuses at a similar rate as zirconium. Crystallization studies were made by means of differential scanning calorimetry. The diffusion of Co in amorphous Co-Zr can explain quantitatively the growth rates of amorphous interlayers by solid-state amorphization reactions (SSAR's) in the Co-Zr system reported in the literature. Significant atomic motion at temperatures at which SSAR occurs is restricted to Co diffusion, which explains that diffusion limited growth of an amorphous phase can take place without nucleation of the more stable intermetallic compounds. The activation enthalpy of crystallization (335 kJ mol is much higher than the enthalpy of Co diffusion.