Magnetic random access memory with selective toggle memory cells
View Patent ↗A toggle MTJ is disclosed that has a SAF free layer with two or more magnetic sub-layers having equal magnetic moments but different anisotropies which is achieved by selecting Ni ˜0.8 Fe ˜0.2 for one sub-layer and CoFeB or the like with a uni-axial anisotropy of 10 to 30 Oe for the higher anisotropy sub-layer. When a field is applied at <10° angle from the easy axis, magnetic vectors for the two sub-layers rotate to form different angles from the easy axis. A method is also described for selectively writing to bits along a word line that is orthogonal to bit line segments and avoids the need to “read first”. A bipolar word line pulse with two opposite pulses separated by a no pulse interval is applied in the absence of a bit line pulse to write a “0”. A bit line pulse opposite the second word line pulse writes a “1”.
1. A method of selectively writing to a plurality of toggle MTJ cells formed along a word line in a MRAM array wherein said toggle MTJ cells are formed between an array of bit line segments and said word line, and are comprised of a SAF free layer having a plurality of “n” magnetic sub-layers with equal magnetic moments but different anisotropies wherein “n” is an even integer, an anti-parallel coupling layer between adjacent magnetic sub-layers, and an easy axis parallel to a bit line segment and normal to said word line, comprising:
(a) applying a first current pulse in said word line during a first time interval, said first current pulse generates a magnetic field larger than a spin-flop field for the SAF free layer with sufficient margin to write a “1” memory state in the SAF free layer in each of the plurality of toggle MTJ cells;
(b) reducing the first current pulse to zero in a second time interval that immediately follows the first time interval;
(c) applying a second pulse in the word line current that is equal and opposite the first current pulse during a third time interval that immediately follows the second time interval, said second pulse generates a magnetic field larger than said spin-flop field with sufficient margin to write a “0” memory state in the SAF free layer in said plurality of toggle MTJ cells; and
(d) reducing the second pulse to zero in a fourth time interval that immediately follows the third time interval.
2. The method of claim 1 wherein there is no pulse applied to a bit line current and a “0” memory state is written to the plurality of toggle MTJ cells along said word line.
3. The method of claim 1 further comprised of applying a bit line current pulse in each of said bit line segments during said third time interval and reducing said bit line current pulse to zero during the fourth time interval, said bit line current pulse is opposite that of the second word line pulse and is overlaid on the second word line pulse during the third time interval to write a “1” memory state in said plurality of toggle MTJ cells.
4. The method of claim 3 wherein the overlay of the bit line current pulse with the second word line pulse yields a large field angle away from the easy axis of the SAF free layer to greatly increase the spin-flop field relative to the applied current field and thereby avoids writing a “0”.
5. The method of claim 2 wherein the spin-flop field is minimized by applying a magnetic field along said easy axis or at an angle less than about 10° away from the easy axis.