Stress in Giant Magnetoresistive Ni 66 Fe 16 Co 18 /Ag Multilayer thin Films
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Stress is an important consideration in thin films for device applications and is strongly dependent on deposition conditions"°. High compressive or tensile stresses can result in reduced device performance and even failure manifested by film buckling or peel-off in extreme cases". Hillock growth can occur during thermal cycling in the presence of compressive stress". The overall stress in a multilayer film is considered the linear combination of the individual layer stresses 12 2. EXPERIMENTAL METHODS Five and ten bilayer films were DC magnetron sputter deposited at 100 W in 2 mTorr of ultra-high purity argon in a Vac-Tec Model 250 side sputtering system with a base pressure of 3 x 10` Torr onto Coming 7059 glass substrates. The geometry of the films is [Ta 100 A/ Ag 25 A/ (NiFeCo 25 A/Ag 50 A)x4 or 9/Ag 25 A/Ta 40 A/ -1 IAm SiOjSior] as suggested in Ref. 1 with the final Ag layer being split in half to surround the NiFeCo layers allowing for even Ag diffusion through each NiFeCo layer. The NiFeCo and Ag thicknesses of 25 and 50 A, respectively, are from Ref. 9. The total number of NiFeCo layers is 5 or 10. A permanent magnet was positioned behind each substrate providing a 90 Oe parallel field resulting in in-plane magnetic uniaxial anisotropy in as-deposited films. This effect is commonly observed in thin ferromagnetic films grown in the presence of a static field' 3 . The sputtering rates were determined from reference film step heights measured on etched samples using a Dek-Tak HA surface profilometer. The stress measurements were performed on a Flexus Thin Film Stress Measurement System Model FLX 2300 with heating capabilities. The mechanism for measurement is to scan a laser beam along the substrate surface before and after film deposition and from the relative position of the reflected beam on a photodetector, the substrate curvature is calculated. Any curvature difference is attributed to film stress resulting in a more concave (tension) or more convex (compression) substrate. The film stress, rf , is calculated using the equation originally developed by Stoney14 and later modified by Hoffman'0 : of = (Ejts 2) / (6 (1-v)tf R), where E, is the elastic modulus of the substrate defined as the slope of the stress vs. strain curve in the elastic regime, t, is the substrate thickness, v is Poisson's ratio for the substrate, tf is the deposited film thickness, and R is the substrate radius of curvature calculated by the equation: R = (RR 2) / (RI-R 2), where R1 is the radius of curvature of the blank substrate and R 2 is the radius of the substrate with the film deposited. A negative stress value indicates a state of compression in the film and a positive stress value indicates tension. Stress vs. temperature cycling and isothermal stress vs. time measurements were performed in an argon atmosphere. Magnetic properties were measured on a Digital Measurement Systems Vibrating Sample Magnetometer (VSM) Model 880. X-ray diffraction was performed on a Rigaku D/Max-2BX XRD System with thin film attachment using Cu Ka
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