Noise and transport studies in spin valve structures

Magnetic multilayers have been the focus of intensive research due to rich physics involved and promising applications in spintronics devices, such as ultra sensitive magnetic sensors and non-volatile Magnetic Random Access Memory (MRAM). In this thesis, resistance noise due to magnetization fluctuations in the magnetic layers of spin valves and magnetic tunnel junctions were studied. The noise power exhibits pronounced field dependence indicating that both magnetically free and pinned layers contribute to the resistance noise. Exchange coupled magnetic multilayers are good candidates for study of individual layers, as the magnetic noise of one of the layers can be suppressed either via strong exchange coupling, or large applied fields. The studies were focused on the pinned layer noise. A strong correlation between resistance susceptibility and noise magnitude was observed for a wide range of fields and samples. Resistance noise can be modeled as thermally activated magnetization fluctuations as in the Fluctuation Dissipation Theorem (FDT). FDT can quantitatively describe the magnitude and field dependence of the observed noise. Temperature studies revealed a non-monotonic behavior indicative of strong temperature dependence of the magnetic losses. Also noteworthy is the effects of the hysteresis on the magnetic noise.;The performance of magnetic sensors in the form of spin valves can be improved by employing Micro-electromechanical Systems (MEMS) flux concentrators (FCs). FCs can improve the sensitivity of a sensor by an enhancement factor depending on the shape and the permalloy used. Oscillating FCs can also be used to mitigate the noise by shifting the operating regime of the SV to higher frequencies. However, first it has to be shown that the FCs (stationary) do not contribute significant excess noise. Our studies indicate that this is indeed the case.;Finally, hybrid structures involving an organic spacer layer (rr-P3HT) sandwiched between ferromagnetic layers (La0.67Sr0.33MnO 3- La0.67Sr0.33MnO3 and La0.67 Sr0.33MnO3-Co) were also studied. Large magnetoresistance were observed ranging from 1% (room temperature) to 20% (low T). Strong field and temperature dependences as well as the bias dependence are indicative of GMR as the mechanism. Initial noise measurements are also reported.