Magnetic Exchange Coupling in Asymmetric Trilayers of Co/Cr/Fe
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EXPERIMENTAL To study the coupling behavior of a system similar to Fe/Cr(001), but with a different
second FM/NM interface, we have grown a (3.0 nm Co)/Cr/(5.6 nm Fe) trilayer with a wedge-shaped (0.5 - 3.0 nm thick) Cr spacer by MBE method; the growth temperature was 300'C. To grow this system epitaxially we employed MgO(001) substrates with Cr(001) buffers. The sample surface was covered with a Cr cap layer (for protection) subsequent to growth. The Co layer grows in the hcp phase in the (1120) orientation with the c-axis laying in the film plane. Since the uniaxial (1120) Co structure is grown on bcc Cr(001) with a four-fold crystallographic symmetry, there are two equivalent orientations for the Co to grow: with the c-axis parallel to either the [110] and [J10] axes of the Cr, respectively. This results in a twinned crystallographic domain structure [6]. 165 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
S.015
We have also grown superlattices with 10 periods of the se-
3.Onm Co/2.Onm Cr/5.6nm Fe 0-
quence [Co/Cr/Fe/Cr] and sam-
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ples with individual Co or Fe layers, respectively. In the latter case the magnetic films are embedded in Cr layers. The layer thicknesses for all samples are chosen to be thick enough such that the magnetization is in the film plane. To study the magnetic properties of the Co/Cr/Fe trilayer sample we measured hysteresis loops with the magneto-optical Kerr effect (MOKE). We measured in the longitudinal configuration as a function of the Cr interlayer thickness; the 100 A-diameter laser spot was moved along the wedge shaped sample. For each Cr thickness we performed a complete inplane sample rotation to determine the easy and hard axes. By plotting the Kerr angle in remanence as a function of the sample orientation, indicated by the angle '1 'H of the external field with respect to the Cr[001] in-plane direction, a four-fold anisotropy was obtained. In Fig. la, we present such a plot for a Cr thickness of 2.0 nm. The hysteresis loops, however, show a nearly uncoupled behavior (insets of Fig. la). We show also plots of an individual 8.9 nm thick Fe layer (Fig. lb) and an individual 4.7 nm thick Co layer (Fig. 1c). Notice the much higher coercitivity of the Co-layer. The maxima of the Kerr angles in remanence indicate the inplane easy axis. For Fe films they correspond to the [100] and [010] axes; this reflects the well-known result of a positive first order cubic anisotropy parameter KUb [7]
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