Micromagnetic Modeling of Magnetic Storage Devices

Abstract

Hard disk drives (HDDs) are the dominant mass storage devices for personal and cloud storage due to their low cost and high capacity. Heat-assisted magnetic recording (HAMR) is considered to be next-generation recording technology for HDDs. While HAMR shows the potential for areal density to go beyond one terabit per square inch, this new recording mechanism requires further understanding and optimization before commercialization. First, I examine the relationship between media noise power and linear density in HAMR. I observe that there is a noise plateau at intermediate recording density and show that the plateau can be shifted to different recording density regions depending on the temperature profile. This effect is argued to be a consequence of the competition between transition noise and remanence noise in HAMR. To extend the recording density limit, heat-assisted shingled magnetic recording is studied. The transitions are no longer symmetric about the track center after shingled writing, especially when the transitions are highly curved as a result of the temperature profile generated by the near-field transducer. I propose a new reading scheme by rotating the read head to match the curved transitions. For a single rotated head, more than 10% improvement in user density over that of a single non-rotated head is achieved. I found that the optimal rotation angle generally follows the transition shape. With an array of two rotated heads, a track pitch of 15 nm, and a minimum bit length of 6.0 nm, the user areal density reaches 6.2 terabits per square inch, more than 30% above previous projections for recording on granular media. Magnetoresistive random-access memory (MRAM) is another type of magnetic storage device that is mainly used as computer memory. As semiconductor-based memory begins to hit physical limits, spin-transfer torque (STT) MRAM and spin-orbit torque (SOT) MRAM appear to be strong candidates for future memory applications. I start first by studying SOT switching in magnetic insulators. Magnetic insulators (MIs), in particular rare-earth iron garnets, have low damping compared to metallic ferromagnetic materials due to lack of conduction electrons. Analogous to STT devices, their low-damping nature is presumed to be an advantage for SOT applications. I report that perpendicular magnetic anisotropy (PMA) material with low damping does not favor reliable SOT switching, but increased damping, interfacial Dzyaloshinskii–Moriya interactions, or field-like torques may help SOT switching in some cases. Notches in a nanometer-scale element, which is a more realistic size for practical applications, can also improve switching stability. To fully utilize low damping MIs with SOT, an in-plane exchange-coupled composite free layer SOT-MRAM is proposed. The free layer consists a low-damping soft MI and a high anisotropy material. The adoption of high anisotropy materials, such as L10 alloy, not only facilitates the achievement of ultra-high-density memory but also allows for the reduction of heavy metal layer volume and thus a reduction in write energy not seen in previous CoFeB-based SOT-MRAM. A write energy of 18 attojoules per bit for 1 ns switching is achieved which is only 72 times more than the theoretical limit of 60kBT. It also represents a factor of more than five hundred times improvement relative to state-of-the-art dynamic RAM.