For many decades charge-based memory (e.g. dynamic random access memory (DRAM), flash memory, etc.) technologies have been successfully scaled down to achieve higher speed and increased density of memory chips at lower bit cost. However, memories based on charge storage are gradually approaching the physical limits of scalability. Unlike DRAM and flash memories a future universal memory should not require electric charge storing and can be based on alternative principles of information storage. For the successful application a new universal memory must also exhibit low operating voltage, low power consumption, high operation speed, long retention time, high endurance, and a simple structure. Alternative principles of information storage include the resistive switching phenomenon in insulators, the effect of changing the magnetoresistance, the domain wall motion along magnetic racetracks, the ferroelectric effect, and others. From technologies which utilize new storage principles the most promising candidates for future universal memory are spin transfer torque MRAM (STT-MRAM) and resistive/redox RAM (RRAM).
Non-uniformity of device characteristics appears a major challenge for large-scale manufacturing of RRAM. First and foremost, one needs a better understanding of the resistive switching phenomena to solve this problem. Development of accurate and flexible models of switching is paramount for future progress in RRAM technology. In the thesis a new stochastic model of resistive switching is presented. Simulation results obtained with the stochastic model are in good agreement with experimental results.
For STT-MRAM the main challenge is to reduce the switching current density without compromising the thermal stability factor. Micromagnetic simulations significantly contribute to solving this problem through the optimization of STT-MRAM memory cells. In the thesis, a new concept of a STT-MRAM structure with a composite free layer is proposed, simulated, and optimized. In addition, reliability issues of STT-MRAM are studied. A new mechanism for switching failure in a MTJ-based STT-MRAM through transverse domain wall formation in a free layer is discovered. A method of utilizing this parasitic switching effect for constructing an efficient spin-torque oscillator is shown. By performing extensive micromagnetic modeling it is proved that the structure exhibits a wide tunability range of oscillation frequencies from a few GHz to several tens of GHz.