Unusual Behavior in The Magnetic Anisotropy of Ultra-Thin Co Sandwiches: The Role of Au Underlayers.
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ABSTRACT We have used in situ polar Kerr effect measurements to study the magnetic anisotropy of X/Co/Y sandwich structures grown by MBE on Cu(l 11) buffers, where X and Y are variable thicknesses of Au. For fixed values of Y and in the case of an underlayer wedge, e.g. variable X value, we have found a sharp minimum in both coercive field and perpendicular anisotropy at -1 atomic layer of the Au underlayer. This anisotropy behavior is opposite to that of an Au overlayer deposited on a Co film, i.e. variable Y and fixed X.
INTRODUCTION AND BACKGROUND In the past few years a great amount of research has been devoted to the study of the magnetic surface and interface anisotropies in ferromagnetic ultra-thin films. However the underlying fundamental mechanism remains a puzzling problem in modem magnetism. Besides the magnetocrystalline interface anisotropy [1], possible explanations include the strain-induced magnetoelastic anisotropy [2-3] and altered electronic structure at the surfaces and interfaces [4]. The recent advent of sensitive in situ magnetic measurement techniques gives the researcher ways of investigating the evolving behavior of anisotropy during the growth of thin films. Thus, it has been discovered that the perpendicular anisotropy of cobalt thin films on different buffer layers such as Pd(l 11) [51, Au(1 11) [6], Ag(1 11) [7] or Cu(1 11) [8] displays a drastic increase during the deposition of a non-magnetic metallic overlayer. A pronounced peak for both coercivity and anisotropy was observed for an overlayer of about one monoatomic layer (ML). Similar non-monotonic behavior has also been observed for Cu(001)/Co/Cu films with an in-plane anisotropy [9]. As this effect was observed with a large variety of metals used for the buffer layer as well as for overlayer, this phenomenon can be more likely explained by a change in the electronic structure of the overlayer due to its restricted dimensionality, or to hybridization with cobalt, than by a strain-induced magneto-elastic anisotropy. Indeed, there have been recent reports of quantum-well-type confinement effects with Cu on Co(0001) [10] , Co on Cu(lll) [11] or Au on W(Ill) [12]. Thus, by changing the layering sequences of different metallic layers over or under a cobalt thin film, we aim to probe the effect of confinement on the magnetic anisotropy. We report here a series of experiments using in situ polar Magneto-Optical Kerr effect measurements (pMOKE) with cobalt films embedded between gold layers on a Cu(1 11) buffer layer, showing a pronounced decrease in the anisotropy for a Au underlayer of about I ML.
EXPERIMENTAL Film Growth The thin films were deposited at room temperature in the growth chamber of our Molecular Beam Epitaxy (MBE) machine on single-crystalline Cu(1 11) buffer layers epitaxially grown on Si 221 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
(111) substrates. The background pressure during deposition was < 5 x 10° torr. Optical-feedbackcontrolled e-beam evaporators were used to deposit the Au (= O.I
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