Stability of Spin Torque Oscillators for MAMR: Perspectives of Materials and Design

We live in a world with exponentially increased data volumes created by various things, which demands innovations on new technologies for data storage to meet the high data volume increasing. Currently, the main data storage technology is still magnetic recording, which needs to maintain a high density growth rate for exponentially increased data volume. The conventional ways to keep a high density growth rate is realized by scaling down both the grain size and the number of grains per bit. However this is limited by the superparamagnetic effect [1]. To solve this issue, the recording media should have increasingly higher anisotropy energy that in turn demands stronger head fields in order to write desired information into recording media. While the strongest writing field is limited by the available materials, being about 2.4 Tesla. New configurations of magnetic recording have to be implemented to enhance the write-ability. Heat-Assisted Magnetic Recording (HAMR) and Microwave Assisted Magnetic Recording (MAMR) are the two main technologies that were put forward to enhance the write-ability [2], [3]. At this moment, HAMR is mainly pursued by Seagate Technology, while Western Digital works on MAMR. It is believe that the MAMR has similar recording potential as HAMR. Furthermore, MAMR consumes less energy (no or negligible heating), and is easier to integrate with the current recording system with slightly modifications on the head [4], [5]. The centre part of MAMR is the use of spin torque oscillator (STO) for the generation of localized ac magnetic field in the microwave frequency regime of 20-40 GHz for effectively assisted writing on high anisotropy recording media [6]. Therefore the development of STO is one of the key enabling technologies that take MAMR drives to the market. However, the close proximity of STO to the writing head, together with ultralow flying height of 2-3 nm, puts STO in a very complex magnetic environment, which inevitably disturbs the STO behavior, leading to its instability. This is one of the key concerns for MAMR to take off. In this paper, we will present the materials selection, layer stack and geometry optimization for STO to enhance its stability against stray field for microwave assisted magnetic recording. Our recent experimental work on STO materials will be presented as well.