Magnetostriction and Microstructure of As-Deposited and Annealed Co Thin Films
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Magnetostriction and Microstructure of As-Deposited and Annealed Co Thin Films Winfried Brückner, Michael Hecker, Jürgen Thomas, Detlev Tietjen, and Claus M. Schneider Institute for Solid State and Materials Research Dresden, D-01171 Dresden, Germany ABSTRACT The magnetostriction of as-sputtered and annealed 400 nm thick Co films has been studied in longitudinal and transverse magnetic fields. The appreciable change of the magnetostriction behavior after annealing above 250 °C is correlated to grain growth and to the related change of the texture (from nearly randomly distributed hcp-Co crystallites to a c-axes texture perpendicular to the film plane). The magnetostriction behavior in the annealed samples cannot be explained by a domain magnetization within the film plane. It is assumed that a rotation of the spontaneous magnetization out of the film plane occurs due to the development of a perpendicular magnetic anisotropy. INTRODUCTION Cobalt thin films are of great interest as soft magnetic single layers as well as in magnetic multilayers, e.g., Cu/Co multilayers [1]. Due to the magnetoelastic coupling, the magnetostriction also affects the magnetic anisotropy. Deposition conditions and post-growth annealing may have a profound influence on the microstructure and therefore also on the magnetostrictive state. The present contribution is focussed on the magnetostriction in sputtered 400 nm thick Co films in the as-deposited state and after annealing up to 450 °C. The results are correlated to findings of microstructural analyses concerning phase formation, texture, and grain morphology. The results are complemented with studies of: (i) the stress development as an indicator for microstructural processes as well as a factor in the magnetoelastic coupling, and (ii) the longitudinal magneto-optical Kerr effect (MOKE) for the characterization of the surface magnetization of the films. EXPERIMENTAL DETAILS The films were deposited by magnetron sputtering onto rotating oxidized 3-inch silicon wafers at room temperature. The deposition conditions were: base pressure 1×10−6 mbar, sputtering pressure 4×10−3 mbar Ar, and sputtering power 150 W for a 4-inch target. The film thickness was measured using a Dektak stylus profiler after wet-chemical etching of an edge into the film, and was determined to be 399 nm. For the annealing procedure, the coated substrate was cut into slabs of 55 mm × 7 mm. The heat treatment was combined with in-situ stress measurements under high vacuum (1×10−5 mbar) using a sensitive laser-optical stress-measurement system described elsewhere [2]. The heating and cooling rate amounted to 4 K/min and the isothermal annealing was done at 150, 250, 350, and 450 °C for 2 h. The annealed slabs were cut into smaller samples for all other measurements. The magnetostriction was determined by a laser-optical measurement of the substrate deflection in an applied magnetic field using a two-beam free-sample set-up described elsewhere [3]. The deflection δ of the substrate at the position of the laser beams (havin
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