PUB - Publikationen an der Universität Bielefeld

Spintronic devices have attracted a lot of attention in recent years due to possible new applications, e.g., a magnetic random access memory (MRAM), logic and sensors. The spin of the electrons is used as an additional degree of freedom in contrast to common electronic devices. The main constituent of many spintronic devices is the magnetic tunnel junction (MTJ) where two ferromagnets are separated by a thin insulating tunnel barrier. The resistance of such a device depends on the magnetic orientation of the ferromagnets. Usually, R_AP (antiparallel) is higher than R_P (parallel) and a tunnel magnetoresistance (TMR) can be defined as TMR = R_AP-R_P / R_P. For small voltages the resistance is connected to the spin dependent density of states (DOS) at the Fermi level of the ferromagnets. Hence, the TMR value is also given by TMR = (2 P1 P2) / (1-P1 P2) with the spin polarization P1,2. Therefore, materials with a high spin polarization are eligible for applications. A half metallic behavior, i.e., they are 100 percent spin polarized at the Fermi level E_F which has been theoretically predicted for some oxide compounds such as Fe3O4 and CrO2, perovskites (e.g. LaSrMnO3), zinc-blende-type CrAs and Heusler compounds. In particular, Co-based Heusler compounds are promising materials for spintronic applications due to the required high Curie temperatures T_C. Here a Heusler compound is given by the composition X2YZ and a crystallographic L2_1 structure exists. X and Y are transition metal elements and Z is a group III, IV or V element. In 2004, room temperature TMR ratios of more than 100 percent were reported for MgO-based MTJs. Recently Ikeda presented TMR ratios of over 600 percent at room temperature and over 1100 percent at low temperatures for a single MgO tunnel barrier. With the concept of a double barrier system these values can be increased and TMR ratios of more than 1000 percent at room temperature have been reported. High room temperature TMR ratios have also been reported for MTJs containing Heusler compounds as electrodes: 217 percent for Co2MnSi and very recently 386 percent for Co2Fe0.5Al0.5Si. The latter was grown by using molecular beam epitaxy in place of sputtering deposition. However, sputtering is the preferred and established method for industrial applications. From a technological point of view, the aim is also to achieve high TMR ratios by sputtering. The actuality of this topic can by recognized by recent press releases. For example, Toshiba announced the development of a spin transport electronics based metal oxide semiconductor field-effect transistor (MOSFET) cell with a full Heusler compound. However, the predicted half-metallicity for Heusler compounds should lead to much higher TMR ratios. Nevertheless, one has to meet two challenges to achieve half metallicity: 1. crystallization of the Heusler electrode(s) in L2_1 structure 2. coherent interfaces of the Heusler compound and the MgO tunnel barrier In this work we have investigated different Co-based Heusler compounds. We have integrated them into so called half junctions to investigate the crystal growth and magnetic properties of the Heusler electrode and into full MTJs for the transport properties. We describe the optimization of a required seed layer system to induce the preferred (001) texture of the Heusler thin films. Furthermore, we have optimized the Heusler layer in an attempt to achieve a high atomic ordering, represented by a high magnetic moment and a maximum (001) texture. We investigated the transport properties of the full junctions at room temperature and low temperature (13K) respectively, and discuss them in terms of annealing temperature, bias voltage and temperature dependence. Finally, the industrial applicability and integration of Heusler compound electrodes into conventional GMR/TMR systems will be verified. Consequently, Heusler junctions prepared by Singulus NDT GmbH will be compared to our samples. In particular, the growth properties of the Heusler layer will be addressed to determine differences within the sputtering process of the Heusler thin films.