Thermally Induced Parallel Magnetic Coupling in Coupled Thin Films

Abstract

Previous investigators reported parallel magnetic coupling between ferromagnetic thin films separated by Pd up to 300 Å thick. They also reported that parallel coupling, while temperature dependent, displayed no thermal hysteresis between − 180° and 300°C. Reported in this paper are: (1) the introduction of large irreversible increases in the parallel coupling field by heat treating the samples between 260° and 345°C, and (2) the irreversible development of parallel coupling through Pd films up to 1500 Å thick, where antiparallel coupling existed prior to heat treatment. The structures on which these effects are observed most readily are similar to those used by the earlier investigators except for their greater Pd thicknesses and deposition at room temperature. However, irreversible increases in the parallel coupling field were also observed for samples with preparation essentially identical to previous work. For a typical sample (two 1000- Å ferromagnetic layers, Hc = 1.5 Oe and 16 Oe, respectively, and a Pd layer 940 Å thick) the coupling field, before heat treating, was 0.3 Oe antiparallel. After treating at 310°C for 4 h, the coupling field was 1.1 Oe parallel. The temperature and time dependence of the development of the parallel component of the coupling field cannot be described by a classical diffusion process characterized by a single activation energy, which suggests a mixture of grain boundary and volume diffusion. The ability to achieve a desired value of parallel coupling by heat treating provides a convenient method for establishing the required coupling for device applications.