| The escalation of the areal density in magnetic recording during the last ten years has been enabled by the development of advanced magnetoresistive materials, which are employed in read transducers. Spin valves (SV's), complex multilayered structures that make use of the giant magnetoresistive effect to achieve a high sensitivity, have been, since their discovery in the early nineties, the front runners in the race to supersede anisotropic magnetoresistive based devices. However, initial SV structures were characterized by an alarming lack of magnetic stability. The magnetic configuration of each individual magnetic layer was difficult to control at the device level, being extremely sensitive to demagnetizing fields and temperature. In order to overcome this problem extensive research was carried out having as a final goal the development of a modified SV system that could operate under the thermal and dimensional conditions demanded in ultra-high density magnetic recording (>10 Gbit/in 2).; This thesis addresses some of the fundamental phenomena that can lead to the loss of the bias state in a SV device once its dimension is scaled down into the submicrometer range, and describes possible solutions for this problem that are based on the concept of a synthetic antiferromagnet. Three different SV systems were studied. Conventional TaNiFeCuNiFeFeMn exchange biased SV's were taken as the reference system. The main magnetic interactions present in this system (exchange bias, interlayer coupling, field-induced anisotropy, shape anisotropy, magnetostatic coupling, and the field induced by the sense current) were studied in depth with the purpose of establishing an unequivocal relationship between each of these interactions and several system parameters such as the growth direction, layer thickness, temperature, device size, and applied current. After identifying the main intrinsic limitations associated with conventional SV sensors, two alternative SV systems were developed: CrCoRuCoCuCoNiFe and RuNiFeCoCuCoRuCoIrMn. Both of these systems make use of a synthetic antiferromagnet (CoRuCo), while the control of the in-plane stability is achieved using two distinct mechanisms: coercivity for the CrCoRuCo based SV's, and exchange bias for the CoRuCoIrMn based SV's. Sensors fabricated using these two alternative systems showed an enhanced magnetic and thermal stability, and the bias point of both the pinned and free layers was shown to be easily achievable, even in submicrometer devices. Due to their improved performance, synthetic antiferromagnet based devices are now considered as one of the key technologies necessary to achieve areal densities up to 100 Gbit/in2. |
