Use of Magneto-Optic Kerr Effect Measurements to Study Strain and Misfit Accommodation in Thin Films of Ni/Cu (100)
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USE OF MAGNETO-OPTIC KERR EFFECT MEASUREMENTS TO STUDY STRAIN AND MISFIT ACCOMMODATION IN THIN FILMS OF Ni/Cu (100) H. E. INGLEFIELD, C. A. BALLENTINE, G. BOCHI, S. S. BOGOMOLOV, R. C. O'HANDLEY, and C. V. THOMPSON Massachusetts Institute of Technology, Cambridge, MA ABSTRACT We have detected magnetic transitions in Ni/Cu (100) films as a function of Ni thickness through in situ measurements of the magneto-optic Kerr effect (MOKE). Crystalline quality was monitored using in situ RHEED and Auger electron spectroscopy. Films were deposited by molecular beam epitaxy on silicon wafers and cleaved sodium chloride with varying epitaxial Ni layer thicknesses between 10 and 200 A. High-resolution TEM images of these films indicate decreasing misfit dislocation spacing and decreasing strain as measured by moir6 fringe analysis with increasing Ni thickness. These observations have been correlated with changes in magnetic anisotropy as measured by MOKE. MOKE, therefore, may provide a tool for in situ monitoring of the kinetics of misfit accommodation in magnetic thin films. INTRODUCTION Misfit dislocations tend to form at the interface of a heteroepitaxial system as a mechanism of relieving the strain caused by the difference in lattice parameters. As the thickness of the epilayer increases, the spacing between the dislocations will tend to decrease to relieve the strain. These dislocations are an important subject for study since the properties of films can have strong effects on performance in devices. The equilibrium dislocation spacing in such films is commonly determined by balancing the energy associated with a dislocation free strained layer with the energy associated with the presence of dislocations. This approach gives the following equation for the thickness dependence of dislocation spacing [11: S=1
(-1vcos2l [bcos
-8h cos 2 X
1+v
In4t
[
JV b
(1)
where f is the misfit between the two lattices (2.6% in the case of Ni/Cu), b is the Burger's vector of a misfit dislocation, h is the film thickness, v is Poisson's ratio, I5and X are the angle between b and the dislocation line and between b and the normal to the dislocation in the plane of the interface, respectively. In the Ni/Cu system, the dislocations formed are predominantly 600 type, so that P and k are both 60*. A similar equation can be derived for the thickness dependence of the average strain in the film [7]:
Mat. Res. Soc. Symp. Proc. Vol. 308. ©1993 Materials Research Society
766
-=G2
b(1 -v)
20G4h(1 +v)cos
+j, ( -VCOS2)ln -.1 4nr
(1 -v)
b]
(2)
where Gi and Go are the shear modulii of the interface and overlayer, respectively. The Ni/Cu (001) system has been extensively studied both for its magnetic properties [2],[3] and its microstructure [4],[5],[6]. However, rarely have these two properties been studied together as part of the same project. The presence of strain in a thin film can have a dramatic effect on magnetic anisotropy due to magneto-elastic coupling. It is likely therefore that the process of misfit accommodation, which is ch
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