Physical and Magnetic Properties of [Co/Pd] based Spin-Valves with Perpendicular Anisotropy for Spintronic Device Application

Abstract

In recent years there has been an increased interest in magnetoresistive devices with perpendicular anisotropy driven by the technical promise of high thermal and magnetic stability. In particular, for the implementation of spin-transfer switched (STS) magnetic random access memory applications (MRAM), scalability, low critical currents and high stability against thermal fluctuations have been predicted. In this thesis, [Co/Pd] based giant magnetoresistance (GMR) pseudo spin-valves (PSV) with perpendicular anisotropy are explored as a potential candidate for spin-transfer switched spintronic devices. Firstly the structure of the Co/Pd multilayers and PSVs were optimized with respect to the perpendicular anisotropy and GMR ratio by considering the thicknesses of the Co and Pd layers, number of bi-layers and seed layer materials. The use of a Ta seed layer allowed for initial smooth interface which promoted the crystalline structure of the Co and Pd layers, leading to enhancement of perpendicular anisotropy, due to the stress induced anisotropy from the interface between the meta-stable hcp a-Co (100) and fcc Pd (111), and Co crystalline anisotropy. Subsequently, in order to reduce the critical current density, an approach of reducing the soft layer coercivity by the insertion of NiFe and Co between the soft [Pd/Co]2 layer and the Cu spacer was considered. An insertion of NiFe (0.4nm)/Co (0.2nm) at the interface between soft layer and Cu spacer was found to achieve an optimum condition where the soft layer coercivity is reduced while maintaining higher GMR ratio in the [Co/Pd] based PSVs. Secondly, it was theoretically and experimentally verified that the interlayer coupling in the spin-valves with perpendicular anisotropy dominantly followed a Ruderman-Kittel-Kasuya-Yosida (RKKY) oscillation coupling rather than a topologically induced coupling. In addition a model that the GMR in the PSV with perpendicular anisotropy is proportional to the sine of the angle formed between the soft and hard layer magnetizations along the perpendicular direction during the magnetic reversal of the soft layer by the applied magnetic field was proposed.

Thirdly, magnetic force microscopy and GMR measurements demonstrated that the nano-patterned [Co/Pd] based PPSV exhibited a single as well as a coherent domain switching behaviour and a stable GMR performance even at lower dimensions below 90 ? 90 nm2 device size. Fourthly, the nature of anomalous peaks in extraordinary Hall effect (EHE) measurement of exchange biased GMR spin-valves with perpendicular anisotropy (PA-SVs), that were accidently observed during the course of this thesis work, was explored. It was experimentally and theoretically confirmed that the physical nature of anomalous EHE peaks originated from the abrupt change in magnetostatic energy caused by the free or pinned layer reversal as well as the dependence of the EHE coefficient RS on the applied magnetic field in PA-SVs. Finally, the GMR and STS performance of the [Co/Pd] based spin-valves were studied. Current perpendicular-to-plane (CPP) GMR spin-valve devices based on the optimized structure were successfully fabricated down to 100nm diameter dimensions. CPP GMR of the 150nm and 100nm diameter devices was measured to be ~ 0.89% and 1.2% respectively. Finally STS measurements of the CPP devices were found to exhibit a critical switching current density of to be JAP-P = -2.6?107 A/cm2 to -3.2?107 A/cm2 and JP-AP = 3.8?107 A/cm2 to 5.5?107 A/cm2? which is lower than or comparable to the switching current densities reported for other spin-valves with perpendicular anisotropy.