The attainable operating field margin, giving the same requirements for the memory retention time and half-select robustness, was compared for the zero-total-anisotropy toggle (T)-MRAM with/without biasing and ordinary Stoner-Wohlfarth (SW) type MRAM by analytic/numeric method. 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. The biased-T-MRAM can have the largest margin in the whole range of the operating field, T-MRAM without biasing comes next, with SW-MRAM giving similar margin as the latter in a lower operating field