in a large array with the same current, the applied field needs to be larger than the mean "selected" switching field by greater than 6σ. In addition,the applied field must be kept below a maximum value. Thus, this "conventional" MRAM must keep these two distributions well-separated. As a result, there is a narrow operating window for programming fields; and only inside this window, can all the bits be programmed without errors or disturbs. In 2005, a "Savtchenko switching" relying on the unique behavior of a synthetic antiferromagnet (SAF) free layer is applied to solve this problem. The SAF layer is formed from two ferromagnetic layers separated by a nonmagnetic coupling spacer layer. For a synthetic antiferromagnet having some net anisotropy Hk in each layer, there exists a critical spin flop field Hsw at which the two antiparallel layer magnetizations will rotate (flop) to be orthogonal to the applied field H with each layer scissoring slightly in the direction of H. Therefore, if only a single line current is applied (half-selected bits), the 45° field angle cannot switch the state. Below the toggling transition, there are no disturbs all the way up to the highest fields. This approach still requires a fairly substantial current to generate the field, however, which makes it less interesting for low-power uses, one of MRAM's primary disadvantages. Additionally, as the device is scaled down in size, there comes a time when the induced field overlaps adjacent cells over a small area, leading to potential false writes. This problem, the half-select (or write disturb) problem, appears to set a fairly large minimal size for this type of cell. One experimental solution to this problem was to use circular domains written and read using the giant magnetoresistive effect, but it appears that this line of research is no longer active. A newer technique, spin-transfer torque (STT) or spin-transfer switching, uses spin-aligned ("polarized") electrons to directly torque the domains. Specifically, if the electrons flowing into a layer have to change their spin, this will develop a torque that will be transferred to the nearby layer. This lowers the amount of current needed to write the cells, making it about the same as the read process. There are concerns that the "classic" type of MRAM cell will have difficulty at high densities because of the amount of current needed during writes, a problem that STT avoids. For this reason, the