Proposal of four-valued MRAM based on MTJ/RTD structure

We proposed a novel four-valued magnetic random access memory (MRAM), that uses a double barrier magneto tunnel junction (MTJ) and a resonant tunneling diode (RTD) connected in series. The double barrier MTJ in the form FM1/I/FM2/I/FM1, where FM1 and FM2 represent ferromagnetic materials with different coercive forces and I represents an insulator, respectively, can take four distinct resistance values, depending on the direction of magnetization of each ferromagnetic layer The RTD can increase the tunneling magneto-resistance (TMR) ratio of the MTJ without area penalty due to the nonlinear nature of the NDR characteristics and compactness. The SPICE simulation showed that the RTD with its peak-to-valley current ratio of 12 could increase the effective TMR ratio from 15% to more than 100%. The cell area per bit is 2F/sup 2//bit, which is suitable for an ultra-high density memory.