Steven C. Sanders

Surface degradation of superconducting YBa2Cu3O7−δ thin films

The surface degradation of c‐axis oriented YBa2Cu3O7−δ thin films due to air, CO2, N2, O2, and vacuum exposure has been studied with reflection high‐energy electron diffraction (RHEED), scanning tunneling microscopy, and contact resistivity measurements. The formation of an amorphous surface reaction layer upon exposure to air and CO2 is monitored with RHEED and correlated with an increase in contact resistivity. The contact resistivity of samples exposed to air increases with time t as ρc = (1.0 × 10−7 Ω cm2)e√t/640 min. Surfaces exposed to CO2 show a similar degradation while surfaces exposed to N2 showed a slightly different degradation mechanism. Vacuum exposed surfaces how little increase in contact resistivity, indicating no long‐term surface oxygen loss.

Magnetostriction and giant magnetoresistance in annealed NiFe/Ag multilayers

Magnetostriction data are reported for NiFe/Ag multilayer thin films displaying giant magnetoresistance. Magnetostriction and magnetoresistance were measured as functions of annealing temperature for NiFe/Ag samples having different numbers of NiFe/Ag bilayers and Ag spacer thicknesses. They varied systematically with annealing temperature in a manner consistent with residual stress reduction and microstructural changes such as grain‐boundary diffusion and grain growth. Zero magnetostriction concurrent with high magnetoresistance ratio (5%) and field sensitivity [7.5%/(kA/m) (0.6%/Oe)] was observed for an optimal multilayer configuration and annealing temperature. This combination of zero magnetostriction and high magnetoresistive response makes the NiFe/Ag multilayer system an attractive candidate for high‐performance magnetic recording read‐head sensors.

Size and self-field effects in giant magnetoresistive thin-film devices

Giant magnetoresistance (GMR) was measured as a function of device size for patterned NiCoFe/Cu and NiFe/Ag films. For the quasi-granular NiCoFe/Cu films, the normalized maximum change in resistivity /spl Delta//spl rho///spl rho/ was 8% for most of the samples. For the NiFe/Ag films, antiparallel alignment was achieved through magnetostatic coupling, not exchange fields, with a /spl Delta//spl rho///spl rho/ of 4.5%. The films were patterned into stripes with Au current leads for size-effect measurements. The height of the stripes varied from 0.5 to 16 /spl mu/m and the track width varied from 1 to 16 /spl mu/m. Discrete switching events and anomalous low-field dips in the response were observed for both materials for small device sizes. Self-field and heating effects due to the applied current were investigated for the NiFeCo/Cu films. The effect of the self-field produced by the applied current was separated from the thermal contribution and was found to reduce the response by over 32% for a current density of 10/sup 7/ A/cm/sup 2/. >

Telegraph noise in silver-Permalloy giant magnetoresistance test structures

We report noise data for discontinuous Ni/sub 82/Fe/sub 18//Ag multilayer test structures. Examination of the noise data for this material indicates that random telegraph fluctuator (RTF) noise of the resistance is the predominant noise source. Analysis of the RTF noise in these structures presents an opportunity to estimate magnetic domain or magnetic cluster strengths and the domain-domain interactions.

Size effects in submicron NiFe/Ag GMR devices

We have measured the magnetoresistive response of submicron NiFe/Ag giant magnetostrictive (GMR) devices as a function of current density and field angle. In addition to magnetostatic broadening, we observe large lumps in the magnetoresistive response (Barkhausen jumps) due to domain switching. These effects lead to irregular device-specific magnetoresistive response curves, The large Barkhausen jumps are more pronounced at low current density while at high current densities the response is smoother due to self field stabilization. The detailed structure of the Barkhausen jumps is very sensitive to the angle of the applied magnetic field. These effects are general properties of a wide class of GMR materials that rely on incoherent reversal of many small magnetic domains. We compare the experimental data with a micromagnetic simulation which incorporates a phenomenological GMR transport model. The model qualitatively describes the experimental data and provides insight into the detailed micromagnetic behavior of these films.

Insulating boundary layer and magnetic scattering in YBa2Cu3O7−δ /Ag interfaces over a contact resistivity range of 10−8–10−3 Ω cm2

We have measured interface transport in thin‐film YBa2Cu3O7−δ /Ag interfaces having resistivities ranging from 10−8 to 10−3 Ω cm2. Analysis of the interface I‐V data indicates that tunneling is the predominant transport mechanism even for the in situ interfaces having contact resistivities of 1–7×10−8 Ω cm2. Zero‐bias conductance peaks are also observed for the entire range of interface resistivity. The similarity of the zero‐bias conductance peaks among these widely varying interfaces suggests that the low‐temperature interface transport is governed by the same mechanism in each case. These conductance peaks are analyzed in the framework of the Appelbaum–Anderson model for tunneling assisted by magnetic scattering from isolated magnetic spins in the interface.

Insulating nanoparticles on YBa2Cu3O7−δ thin films revealed by comparison of atomic force and scanning tunneling microscopy

The surface topography of YBa2Cu3O7−δ thin films has been studied with both atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The AFM images reveal a high density of small distinct nanoparticles, 10–50 nm across and 5–20 nm high, which do not appear in STM images of the same samples. In addition, we have shown that scanning the STM tip across the surface breaks off these particles and moves them to the edge of the scanned area, where they can later be imaged with the AFM.

Oxygen annealing of ex-situ YBCO/Ag thin-film interfaces

The resistivity of YBCO/Ag interfaces has been measured for different oxygen annealing temperatures for a series of ex-situ fabricated thin-film contacts having sizes from 16 /spl mu/m/spl times/10 /spl mu/m down to 4 /spl mu/m/spl times/4 /spl mu/m. The interface resistivity began to decrease after annealing at 10 minutes in one atmosphere oxygen. After annealing at 400/spl deg/C, the contact resistivity decreased by several orders of magnitude to the 10/sup -7/ range. The 500-nm thick Ag layer showed surface diffusion and agglomeration for annealing temperatures above 400/spl deg/C; this temperature thus represents a practical limit for oxygen annealing the YBCO/Ag interface system for more than 10 minutes. Rapid cooling of the chip after annealing led to a severe loss of critical current density in the YBCO layer, which could be restored by reannealing and cooling at a slower rate of 50/spl deg/C/min. The relative shape of the conductance-vs.-voltage characteristics of the YBCO/Ag interface were essentially unaltered by oxygen annealing; the overall parabolic shape, superconducting gap features, and magnetic-scattering zero bias anomaly remained constant, even though the contact conductance increased by several orders of magnitude. These data suggest main reduction in interface resistivity enhancement of the effective contact area, not a change in interface conduction mechanism.<<ETX>>

Size effects and giant magnetoresistance in unannealed NiFe/Ag multilayer stripes

We have observed giant magnetoresistance (GMR) in unannealed NiFe/Ag multilayer thin‐film stripes. Rectangular stripes having constant thickness and a constant 11:1 length‐to‐width aspect ratio, but varying widths down to 0.5 μm, were measured. Two types of multilayer configurations were tested, a system of five NiFe/Ag bilayers with 5.5‐nm‐thick Ag spacer layers, and a system of nine bilayers with 4.4‐nm‐thick Ag layers. In contrast to the characteristic of annealed NiFe/Ag multilayer stripes, the unnannealed stripes produced increasing GMR ratios for decreasing stripe sizes, with the 0.5‐μm‐wide stripe of the five‐bilayer system exhibiting a ΔR/R of 2.5%. Barkhausen noise and response broadening also increased with decreasing stripe size, however. The results are discussed in terms of magnetostatic coupling of the NiFe layers within the stripes.

Evidence for tunneling and magnetic scattering at in situ YBCO/noble-metal interfaces

We report low-temperature conductance data for in situ YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO)/Ag, YBCO/Au, and YBCO/Pt planar c-axis interfaces. Analysis of the conductance data for these interfaces, which have resistivities as low as 1/spl times/10/sup -8/ /spl Omega//spl middot/cm/sup 2/, indicates that tunneling is the predominant transport mechanism. Zero-bias conductance peaks are present for all of the in situ interfaces. These peaks are analyzed in the framework of the Appelbaum model and are attributed to the presence of isolated magnetic spins at the interface. The presence and similarity of the peaks for each noble-metal overlayer supports the hypothesis that the magnetic spins are inherent to the YBCO surface.<<ETX>>