Study of Magnetic Interlayer Coupling in Synthetic Antiferromagnets for use in MRAM Devices

Synopsis

In recent years, there has been an ever increasing level of effort focused on creating novel technologies based on spintronics. One of the most exciting of these technologies is spin transfer torque random access memory (STT-MRAM), which is a solid state non-volatile memory technology that is orders of magnitude better than flash memory in terms of speed and write endurance. One critical component of an STT-MRAM memory cell is the so-called fixed magnetic layer. The direction of magnetization of this layer must remain fixed so that it can act as a reference for the reading and writing of information to the cell. This layer is ideally composed of a synthetic antiferromagnet (SAF) with zero net magnetization because it can offer thermal stability, have its magnetization be less effected by external magnetic fields, and have reduced stray fields. One challenge with integrating a SAF into an STT-MRAM memory cell is that they are typically not thermally robust. Creating an STT-MRAM device generally requires at least one annealing step at temperatures between 200 and 300C. During this annealing process, the antiferromagnetic coupling (AFC) within the SAF changes dramatically and usually becomes ferromagnetic, thereby eliminating the SAF and all of its advantages. The work in this thesis centers around understanding exactly how and why this magnetic coupling changes during the annealing process, and how to prevent it so that a SAF fixed magnetic layer can be used in STT-MRAM devices. We start by depositing thin films containing two FeCoB layers coupled across several different non-magnetic spacer layers of varying thicknesses. We determine the magnitude and direction of the magnetic coupling between the two FeCoB layers both before and after annealing my analyzing ferromagnetic resonance (FMR) and magnetostatic measurements. Next, we study the role that boron has on the magnetic coupling by co-depositing it into the Ru spacer layer of samples with the structure NiFe/Ru/FeCo. From this, we conclude that the presence of boron within the FeCoB layer leads to increased diffusion of magnetic atoms into the non-magnetic spacer layer during the annealing process, which is responsible for the change in coupling seen in SAF structures. In order to prevent this, we insert diffusion barriers next to the FeCoB layers within a SAF. We find that with the diffusion barriers, we are able to create a thermally robust SAF structure that maintains AFC coupling even after annealing at temperatures of up to 350C.