Abstract: Magnetoresistive random access memory (MRAM) based on spin transfer torque (STT) is entering volume production in the semiconductor industry. At the same time, it is becoming increasingly clear that the ultimate energy efficiency, speed, and scalability of STT-MRAM are limited by its current-controlled write mechanism. Therefore, there is a need to explore beyond-STT write mechanisms, materials, and circuit architectures that allow for MRAM to address a wider set of computing applications, particularly at advanced technology nodes.
In this talk, we discuss novel device candidates that may enable approaching the fundamental limits of speed and energy efficiency in spintronic memory. We first review the recent progress and perspectives of voltage-controlled nonvolatile magnetic memory devices, which offer ultralow dynamic energy dissipation as well as reduced standby power due to nonvolatile data retention. We discuss experimental progress in the development of magnetic tunnel junctions using voltage-controlled magnetic anisotropy (VCMA) for switching, which exhibit the lowest power consumption MRAM cells to date (~5 fJ/bit with precessional switching times ~1 ns).
As a strategy to further reduce switching time and improve energy efficiency and scalability of VCMA-based MRAM, we then examine the VCMA effect in new free-layer structures containing antiferromagnetic materials. The large exchange field present in such free layers allows for dramatic reduction of the switching time and write energy. Modeling results show that for sufficiently large applied electric fields that overcome the anisotropy, a high-frequency resonance is excited, which can be used to switch the Néel vector by 180 degrees for voltage pulses shorter than 10 ps. In addition, we discuss the variation of switching behavior and optimum pulse width depending on voltage, external magnetic fields, and exchange interaction. This switching mechanism can enable ultralow-power voltage-controlled or voltage-assisted spintronic devices based on antiferromagnetic materials.
Bio: Pedram Khalili is associate professor of electrical engineering and computer science at Northwestern University. He leads the Physical Electronics Research Lab (PERL), which works on developing nanoscale devices and materials, particularly based on spintronics, which enable computing and sensing systems with unprecedented energy efficiency.