X. Bian

STUDY OF DENSITY IN PULSED-LASER DEPOSITED AMORPHOUS CARBON FILMS USING X-RAY REFLECTIVITY

We report the accurate determination of the absolute density of the pulsed‐laser deposited amorphous carbon ultrathin films as well as the film density evolution with the substrate temperatures Ts (22–300 °C) using high‐resolution x‐ray reflectivity. The density values ρ for the films deposited with laser power density of 8×108 W/cm2 varied from 3.10 to 2.40 g/cm3 as the substrate temperature increased from 22 to 300 °C. This result, together with the optical band‐gap values obtained by transmittance measurements, clearly indicates a diamond‐like to graphite‐like microstructure change near Ts=200 °C.

Effect of annealing on the giant magnetoresistance of sputtered Co/Cu multilayers

Co/Cu multilayers with the form of glass/Cu 50 A[Co11.5 A/Cu(tCu)]30/Cu50 A, prepared by rf triode sputtering, exhibit oscillatory magnetoresistance (MR), as a function of the Cu layer thickness with a period of ≂12 A and maximum MR values of 36.8% and 22.3% at 77 and 295 K, respectively. In order to study the effect of annealing on the structural and magnetic properties, five samples with Cu thicknesses between 9 and 34 A were heat treated at temperatures up to 300 °C and analyzed by low‐ and high‐angle x‐ray diffraction, MR, and mageto‐optic Kerr effect measurements. Annealing at moderate temperatures for the samples with Cu thicknesses around the second and third MR peaks leads to an initial increase in the MR. In contrast, annealing causes only a monotonic MR decrease for the sample at the first peak with a Cu thickness of 9 A.

Giant magnetoresistance with low saturation field in (NixCo100−x/Cu) multilayers

We have investigated giant magnetoresistance (GMR) in NixCo100−x/Cu multilayers for x in the range 20–100. The GMR at room temperature is greater than 12% for magnetic alloys with x near 80, where the magnetocrystalline anisotropy is small. The smallest saturation fields are found near x=60 in the region where the magnetostriction vanishes. This combination of large GMR and small saturation field results in large MR field sensitivities up to 0.16%/Oe at room temperature. In order to maximize these parameters, we have also studied the dependence of the MR on the magnetic layer thickness and the number of bilayers.

Pulsed laser deposition of giant magnetoresistance Co-Cu films

Abstract The pulsed laser deposition (PLD) technique has been used to produce giant magnetoresistance (GMR) Co x Cu 1− x granular alloy films. An MR as large as 8% has been found for the annealed film with x Co = 0.18 at 4.2 K in a magnetic field of up to 1 T. GMR and its thermal evaluation are shown to depend on the density and size distribution of the Co precipitates. The influence of the characteristics of the PLD technique on the GMR effect is also discussed.

(Ni80Co20/Cu) multilayers: Giant magnetoresistance with low saturation field

We have observed giant magnetoresistance (MR) in Ni80Co20/Cu multilayers. Saturation magnetoresistance exceeding 16% and very well‐defined oscillations in MR as a function of Cu spacer thickness are observed at room temperature with an oscillation period and phase quite similar to that found in other Cu‐based multilayered systems. Of particular interest are the small saturation fields (HS<120 Oe) for Cu thickness near the second peak (∼20 A) which result in large MR field sensitivities up to 0.12%/Oe at room temperature.

Structural and magnetoresistance studies in granular (Ni81Fe19,Ni80Co20)/Ag synthesized from annealed multilayers

The structural and magnetotransport properties have been studied in (Ni81Fe19,Ni80Co20)/Ag granular alloy films synthesized by the annealing of sputtered multilayers containing ultrathin magnetic layers. The magnetic concentration, particle size, and morphology of the precipitates can be controlled by adjusting the magnetic layer thickness during the deposition process. Magnetoresistance as large as 30% was found at 4.2 K with a rather small saturation field (<2 kOe), leading to relatively high field sensitivity. Magnetoresistance and saturation field in these alloy films essentially depend on the magnetic precipitate size, concentration, and annealing temperature. The temperature dependence of magnetization and magnetoresistance was also studied, and the correlations among the microstructure, magnetotransport, and magnetic properties are discussed.

Cumulative interface roughness and magnetization in antiferromagnetically coupled NiCo/Cu multilayers

Cumulative interface roughness and its influence on the magnetization process in antiferromagnetically coupled (Ni80Co20/Cu)×N multilayers is studied. In these multilayers, Cu and Ni80Co20 thicknesses are fixed at 20 and 15 A, respectively, in order to obtain the antiferromagnetic coupling at the second oscillation peak of giant magnetoresistance (GMR) versus Cu thickness. Low‐angle x‐ray reflectivity measurements show that cumulative interface roughness increases with increasing bilayer number N. In‐plane magnetization hysteresis measured with both SQUID and surface magneto‐optic Kerr effect (SMOKE) magnetometers are compared. When the cumulative interface roughness is significant, SMOKE hysteresis loops, which are sensitive to the top 5 or 6 magnetic layers, display a nonlinear plateau region at small fields. Comparison of low‐angle x‐ray, and SMOKE results show that interfaces of relatively high quality in top layers only exist for sputtered multilayer with N<10.

Spin polarized neutron scattering study of NiCo/Cu multilayers

Spin polarized neutron reflectometry measurements were performed on Ni80Co20/Cu multilayers with a Cu spacer thickness of 20 A, corresponding to the second oscillation peak in the magnetoresistance of the NiCo/Cu multilayer system. Measurements in a 15 Oe field indicate a nearly perfect antiferromagnetic stacking of the magnetic moments in successive Ni80Co20 layers. The existence of a small magnetic anisotropy in these magnetically soft multilayers leads to the canting of the magnetic moments at an angle of ∼70° with respect to the neutron spin polarization. This interlayer antiferromagnetic coupling can be suppressed by an applied field of ∼200 Oe.

Evolution of structure and magnetoresistance in granular Ni(Fe,Co)/Ag multilayers: Dependence on magnetic layer thickness

Structural and magnetoresistance results on annealed sputtered (Ni81Fe19, Ni66Fe16Co18)/Ag granular multilayers are presented. Structural evolution has shown that highly (111) textured, discontinuous layered structures can persist on annealing up to 400 °C. The average magnetic particle size is controlled by the annealing temperature and the initial magnetic layer thickness. No giant magnetoresistance was observed in the as‐deposited films, while significant MR was found after annealing between 300 °C and 400 °C. Magnetoresistance over 30%, together with a small saturation field, was found at 4.2 K for a starting magnetic thickness of 4 A. Increasing the magnetic layer thickness to 20 A greatly improves the magnetic thermal stability, and leads to high magnetoresistive sensitivities of up to 0.35%/Oe in a field of 10 Oe at room temperature. The magnetization hysteresis, anisotropy, and magnetic interaction in such a granular multilayer are also discussed.

Structural and magnetotransport properties of Co/Re superlattices

Structural properties of sputtered Si/Re 50 A/(Co 20 A/Re x A)40 superlattices have been investigated in detail by low‐ and high‐angle x‐ray‐diffraction measurements. The low‐angle x‐ray reflectivity data have been analyzed using an optical model and the high‐angle data have been treated on the basis of a trapezoidal model. The fitted results indicate that the films with small xRe=3, 5, and 7 A are essentially composed of pure Co layers and Re rich alloy (∼80%) interfaces about 3–5 monolayers thick. The magnetoresistance (MR) data of the antiferromagnetically coupled films have been analyzed using a semiclassical MR model of interfacial spin‐dependent scattering.