Fig. 1: Writing probability measured for both writing direction, and both voltage polarities. Writing is better when the voltage is decreased gradually (b) than abruptly (a).
During writing of Thermally Assisted Switching Magnetic Random Access Memory (TAS-MRAM), the torque due to a voltage pulse may be used to help writing. As part of collaboration between Spintec and Crocus Technology, we brought out that most of the influence of the spin transfer torque is exerted during the last part of the voltage pulse, and can be enhanced by bringing back the voltage to 0 gradually.
Writing of TAS-MRAM may be assisted by a spin transfer torque exerted during the writing voltage pulse. We found that by decreasing the voltage gradually over 70ns at the end of the voltage pulse, the write error rate may be decreased by one order of magnitude.
TAS-MRAMs benefit from exchange bias between a ferromagnetic layer and an antiferromagnetic one to ensure a good retention of information in the antiferromagnetic layer, while the ferromagnetic layer magnetization is easy to write. For writing, the exchange bias is suppressed by heating by Joule effect thanks to the voltage pulse. The end of writing requires cooling while keeping the magnetization carrying information in the desired direction, which may be done thanks to spin transfer torque.
This result was obtained by examining the writing properties of magnetic tunnel junctions processed in PTA (Plateforme Thechnologique Amont), from a magnetic stack developed jointly by Spintec and Crocus Technology. Writing tests at the 0.5% accuracy were performed at Spintec with 50 ns long pulses, which falling edge duration was varied from 3ns to 150ns.
This result gives a new point of view concerning the thermal dynamics of these nanometric devices, and suggests new writing method to reach really low writing error rate in MRAMs.
This work was funded by ANR under project EXCALYB ANR-13-NANO-0010 and realized in collaboration with Crocus Technology in the framework of CIFRE PhD study of Antoine Chavent.
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Last update : 10/29 2015 (1126)