Magnetic and Transport Behavior of Af-Coupled Layers with a Limited Number of Repetitions
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MAGNETIC AND TRANSPORT BEHAVIOR OF AF-COUPLED LAYERS WITH A LIMITED NUMBER OF REPETITIONS M.M.H. WILLEKENS*, H.J.M. SWAGTEN*, A.M. DUIF*, P.J.H. BLOEMEN**, R.J.T. VAN KEMPEN*, S.K.J. LENCZOWSKI** AND W.J.M. DE JONGE* *Department of Physics; Eindhoven University of Technology; P.O.Box 513, 5600 MB Eindhoven, The Netherlands. "**Alsoat Philips Research Laboratories, Eindhoven, The Netherlands.
ABSTRACT We investigated the magnetization behavior of [(Co/Pd)4-Ru]x samples with perpendicular anisotropy and a limited number of repetitions (x = 1,..,22). In these systems the Co/Pd multilayers behave as single magnetic entities. A detailed analysis and comparison of the magnetization curves observed by MOKE and VSM permits us to observe the magnetization reversal and hysteresis of the individual layers and to determine the antiferromagnetic coupling J between each pair of layers. A gradual increase in J is observed in all samples when going from the bottom layer to the top layer. Magnetoresistance curves show the same sharp transitions as the magnetization curves. A clear distinction can be made between an outer layer and an inner layer. These results will be compared with model calculations.
INTRODUCTION The last few years magnetic multilayers have been the subject of intensive investigation. Especially a large (perpendicular) anisotropy, an oscillating exchange coupling and a giant magnetoresistance have attracted much attention [1,2,3]. Values for anisotropy (K) and exchange coupling (J) are often determined from transition and/or saturation fields of magnetization curves. However this determination is not a trivial calculation. Calculations of the magnetization [4,5] and magnetoresistance [5] for a sandwich containing two AF-coupled magnetic layers with uniaxial anisotropy have been performed in two limiting cases. In one case the system is always in a state of absolute minimum energy. In the other case the system finds itself in a local energy minimum due to anisotropy induced energy barriers. Measurements on these systems however show a wide variety of magnetization curves lying between the two limiting curves [6,7]. It is believed that the exact shape of the magnetization curve is determined by microscopic phenomena such as local anisotropy fluctuations and pinholes. Therefore, the derived equations for the limiting cases are not applicable. In the case of a multilayer also the number of repetitions can play a crucial role. To gain more insight in the magnetic behavior of these systems we have grown a series of multilayers with perpendicular anisotropy and AF-coupling. The number of layers was varied between 1 and 22. Magnetization loops have been measured with.the Magneto Optical Kerr Effect (MOKE) as well as Vibrating Sample Magnetometer (VSM). Magnetoresistance measurements have been performed at room temperature and at low temperature. A detailed investigation of the magnetization and magnetoresistance effect leads to an analysis in which it is possible to distinguish between the magnetization reversal and hys
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