| Switching behavior of synthetic antiferromagnet (SAF) structure to be used as the memory elements of the "Toggle-Mode Magnetoresistive Random Access Memory (T-MRAM)" recently invented as a breakthrough for solving the critical problem on operating field margin and scalability encountered in developing MRAMs was investigated.; By using analytical methods with an aid of numerical calculations, the critical field curves for switching an SAF, which is composed of an astroid-like spin switching curves and an oval-like spin saturation curve, were obtained for the first time. This enabled us to optimize the parameters fulfilling the requirements for practical application, such as operating fields, bit densities, and thermal stability for memory retention, and propose a new scheme of T-MRAM element in which the shape and intrinsic uniaxial anisotropies are aligned orthogonally to compensate each other, which has been found to give the maximum margin with the minimum operating field. Further, it has been clarified that the DC-bias field can not only reduce the necessary operating field strength but also increase the field margin. The attainable margin vs. operating field strength curves have been calculated imposing the same requirements for the memory retention time and half-select robustness, for the new scheme of T-MRAMs with/without biasing and for conventional Stoner-Wohlfarth (SW) type MRAM. It has been found that the margin increases monotonically with increasing the operating field for the T-MRAMs, while it reaches a constant value at a critical operating field for the ordinary SW type and that the biased-T-MRAM can have the largest margin in the whole range of the operating field, with T-MRAM without biasing coming next, and SW-MRAM giving similar margin as the latter in a lower operating field range.; Some dynamic simulations of T-MRAM switching using Landau-Lifshitz-Gilbert equations were performed to study the magnetization evolutions and switching for the toggle-operation. The results were found consistent with the above analysis for relatively long field pulse width and rise/fall time, while for the field pulses with width and rise/fall time in nanosecond order some discrepancy was found, indicating the challenge for T-MRAM switching in this short pulse regime. |
