Spin Memory Devices

  

 


  SOT-MRAM


  Spin-orbit torque magnetoresistive random-access memory (SOT-MRAM) is a promising novel memory device due to sub-ns operation speed and non-volatility. It also has higher reliability compared to spin-transfer torque magnetoresistive random-access memory (STT-MRAM) because of the decoupled read and write path. Writing operation involves the current flowing through the heavy metal located adjacent to the magnetic tunnel junction (MTJ). This current induces in-plane torque driven by spin Hall effect (SHE) and/or Rashba effect. Unlike STT, of which the direction is perpendicular, SOT has the in-plane directed torque therefore it only lays down the initially perpendicular magnetization (z-direction) towards the spin-polarized direction in the x-y plane. Numerous methods are proposed for the deterministic switching, such as applying in-plane magnetic field or inducing STT current. SOT-MRAM is expected to be used to fill the speed gap between DRAM and SRAM or used in the embedded memory application which requires low-power operations. In this laboratory, we conduct micromagnetic simulations which are based on Landau-Lifshitz-Gilbert (LLG) equation to simulate SOT-MRAM.

Available tools in our lab: mumax3, oommf, HSPICE






  STT-MRAM


  Spin-transfer torque based magnetoresistive random access memory (STT-MRAM) has been considered as one of the promising candidates for future memory device due to its innate advantages of nonvolatility and short switching time. The magnetic tunnel junctions (MTJs) based on thin MgO/CoFeB layers have exhibited fairly low switching current density and outstanding tunneling magnetoresistance (TMR) above 100%. In recent years, there have been lots of efforts to overcome several shortcomings of conventional STT-MRAM. Novel materials called Heusler alloys have shown potentials to be used as the recording layer of future 10 Gbit MRAM. They have stimulated considerable interest due to ultra-low Gilbert damping constant. In this laboratory, we perform self-consistent DFT-NEGF simulations using SIESTA-SMEAGOL package which enables non-collinear spin calculations, especially for CoFe and various Heusler alloy-based MTJ structures. Furthermore, we investigate strain effect on MTJs such as I-V characteristics, TMR and so on.