4 Reports on Experimental Results 4.18 FMR and MOKE characterization of Co40Fe40B20 thin Þlms

Magnetic tunneling junctions are being widely used for read heads for hard discs, sensors in robotics or machine controllers. The number of potential Þelds of application is growing rapidly following the increase in the strength of the tunneling magnetoresistance (TMR) effect. The ad-vantages of using single crystalline barriers for achieving large magnetoresistance ratios in tun-neling junctions was Þrst predicted for Fe/ZnSe/Fe and Fe/MgO/Fe systems [14]. However, it was only technically possible to accomplish large tunneling magnetoresistance values with a MgO barrier [5, 6]. Later theoretical works predicted even larger TMR ratios for junctions with Co and CoFe electrodes and a MgO barrier [7]. This was experimentally conÞrmed by TMR values above 220 % at room temperature [8]. A major breakthrough was provided by the introduction of the alloy CoFeB. First attempts reached TMR values above 200 % at room temperature [9, 10]. With further control of the exact stack composition and of the annealing conditions values as large as 472 % [11] and 604 % [12] have been reported. These results have triggered a large interest in the properties of CoFeB alloys with different stoichiometry. The stuctural properties have been widely studied in the above cited works and others related to tunneling junctions fabrication. Considering dynamic properties, only few works for certain compositions have been published [13, 14]