Annealing of Co x Cu 1-x / Cu Multilayers
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Annealing of CoxCu1-x / Cu Multilayers Jörg Ebert, Mohammad Ghafari, Branko Stahl, and Horst Hahn Darmstadt University of Technology, Institute of Material Science, Thin Films Division Petersenstraße 23, 64287 Darmstadt, Germany ABSTRACT In the multilayer system cobalt / copper at the second antiferromagnetic coupling maximum (2. AFM) with a copper thickness of dCu = 2,2 nm it is possible to reduce magnetoresistive hysteresis by the use of either very thin Co-layers or by alloyed magnetic layers Co1-xCux. It was possible to achieve values for the giant magnetoresistance effect of GMR ≈ 20 % for as prepared samples. A heat treatment was applied to study the degeneration of the system. For annealing at moderate temperatures (Tanneal ≤ 250°C) an increase up to GMR ≈ 25 % was observed. Annealing at slightly higher temperatures lead to an rapid decrease in the GMR effect. To study the structural changes the method of x-ray reflectivity was utilized showing changes in interface roughness as well as in bilayer thickness. INTRODUCTION Since the discovery of the Giant Magnetoresistance effect (GMR)1 there has been great effort to utilize it for industrial applications. The first branch that used GMR based devices was the computer industry. The assemblies are called Spin-Valves and are used in read heads of hard disc drives. In principle these devices consist of two ferromagnetic films with different coercivities separated by a nonmagnetic material. The thickness of the nonmagnetic layer must be high enough so that there is no magnetic interaction between the ferromagnetic layers. Another basic principle is the use antiferromagnetically coupled multilayers based on Co and Cu. To reduce undesirable hysteresis Co is often alloyed with Fe2. Some alternative approaches are discussed in literature like reducing the thickness of the magnetic layer3 or alloying the magnetic layer with the material of the spacer layer4. Besides the reduction of the hysteresis the main duty is to guaranty the capability of the functional layers to withstand the high temperatures of complete production process of a magnetic sensor (Tstandard ≈ 300°C)2. The equilibrium phase diagram5 of Co and Cu shows that the two elements are not miscible promising a relative stable layer configuration with no intermixing. Nevertheless the preparation in UHV by sputtering with high energies may lead to implantation of the atoms of one species into the layer of the other element. As the dimensions of the single layers are in the nanoscale and the GMR is known to be an interfacial effect the control of the interface structure and the knowledge of its changes due to thermal annealing is of great importance for the successful application of GMR devices. There are different approaches to obtain the desired flat interfaces. One is the use of so called buffer layers made of Fe, Ni80Fe20, or Ta6. The other is the utilization of surfactants like Pb or Ag7, which float on the surface during the preparation process and inhibit island like growth leading to ferromagnetic
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