Second Order Magneto-Optic Effects in Epitaxial FE(110)/MO(110) Bilayers
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ABSTRACT The signal measured during a Magneto-Optic Kerr Effect (MOKE) experiment is usually assumed to be linear in the magnetization (or the magnitude of the magneto-optic coupling vector Q that is proportional to the magnetization) so that a plot of the magnetization versus applied field can be obtained. We have observed an appreciable contribution from the Q 2 term in the magneto-optic response of epitaxial Fe(110)/Mo(110) bilayers. The Q 2 term in the magneto-optic response is much larger than that predicted by existing theory. We re-derive and modify the existing theory to fit the Q2 term. INTRODUCTION MOKE has proven its usefulness in a number of magnetic thin film systems where it has been used to investigate the thickness dependence of the Curie temperature, search for magnetic ordering, and determine the anisotropy'. The latter measurement requires a magnetization curve in order to determine the anisotropy or saturation field. To obtain such a curve, the MOKE signal must be proportional to the magnetization Mi (the absolute value of which is proportional to Q, the absolute value of the magneto-optic coupling vector2 ). Most workers in magneto-optics have assumed that the higher order terms in the magnetooptic response are negligible due to the smallness of Q and because for purely in-plane magnetization the second order term is proportional to the product of the components of M lying parallel and perpendicular to the plane of incidence" 3 . In an isotropic thin film, one of these two components is usually zero because the external field Hl is applied parallel or perpendicular to the plane of incidence so that the Q2 terms vanish. In films with in-plane magnetic anisotropy, however, there can be a non-zero component of MC4 perpendicular to H; the anisotropy forces M to reverse by coherent rotation when H is close to parallel to the hard axis. We shall show that in a magnetically anisotropic Fe(110)/Mo(110) bilayer, the large size of the component of 4 perpendicular to HI creates a measurable Q 2 term in the magneto-optic response. EXPERIMENTAL A (110)-oriented Fe/Mo bilayer was grown following a well-known procedure that guaranteed epitaxy 4 . A seed layer of Mo was deposited onto a A12 0 3 substrate heated to above 6500 C which was subsequently allowed to cool to below 1000 C before deposition of the Fe layer. Torque magnetometry determined the uniaxial and biaxial anisotropy constants to be respectively 9.0 x 104 ergs/cm 3 and 1.6x 104 ergs/cm3 , with an easy axis along the in-plane [001] direction. The magnetization curve (measured with vibrating sample magnetometry (VSM)) along the hard axis could be simulated assuming pure rotation (no domain wall motion). The demagnetizing field confined Mi to the plane of the sample. A He-Ne laser beam (633 nm) polarized parallel to the plane of incidence ('p'-polarized) was incident on the sample at an angle from the film normal between 5' and 290. Both 491
Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
rotation and ellipticity (Ok and Ck,
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