The effect of composition, annealing temperature and thickness on magnetoresistance of ferromagnetic multilayer structures

Abstract

Giant Magnetoresistance (GMR) refers to a phenomenon of a considerable drop in electrical resistance in ultrathin ferromagnetic/non-magnetic layers structure, when a sufficiently high magnetic field is applied to the structure. Since the first time it was observed in late 1980’s, this area of research has attracted a very strong interest due to the deep fundamental physics and its promising technological potential especially in data storage industry. This dissertation discusses the magnetoresistance of ferromagnetic/non-magnetic multilayers fabricated using electron beam evaporation technique. The magnetoresistance value is studied towards the effect of compositional variance, a range of post deposited annealing temperature, and multilayer thickness in terms of number of repetition of the bilayer as well as the non-magnetic layer thicknesses. The results on compositional variance show as deposited Co/Cu/Ni gives the largest magnetoresistance value of 8.75% followed by Ni/Cu/Ni (5.85%), Co/Ag/Co (2.64%) and Ni/Ag/Ni (1.48%). These four ferromagnetic multilayers were given heat treatment with a range of elevating temperature. The temperature ranges between 200°C to 350°C, resulting the magnetoresistance of the above structures increases to 14.25%, 10.68%, 7.45% and 4.38% respectively. However, the thermal stability for each multilayer systems were degraded after further annealing at a temperature called the blocking temperature. From the investigation, it was found that the blocking temperatures (or the optimum annealing temperature) for Co/Cu/Ni, Ni/Cu/Ni, Co/Ag/Co and Ni/Ag/Ni were 280°C, 290°C, 310°C and 280°C respectively. The study of thickness dependence GMR, focus only on Co/Cu/Co structures. It was shown that in [Co/Cu/Co]n structures, the magnetoresistance increase as n increases from 1 to 20. Meanwhile, for Cox/Cuy/Cox structures, the magnetoresistance increases as y decreases from 15 nm to 1 nm with x is fixed at 6 nm. The X-ray diffraction study for the optimum structures shows peaks of Co, Cu, Ni and Ag which describe the multilayers are in the crystalline form. The atomic force microscopy (AFM) image analysis for Co and Cu thin films with elevated annealing temperature reveals that the higher the annealing temperature, the larger the grain size of the multilayer. The EDX spectrum analysis confirmed the presence of Co, Cu, Ni, and Ag in the multilayers.