Development of world’s First high Reliability Read Method for use in Spin-Torque-Transfer (STT) MRAM

February 11, 2010 · Filed Under Trendy Products 

Fujitsu STT MRAMSpin-Torque-Transfer (STT) MRAM is considered as a potential future form of non-volatile memory which can be used as an alternative to flash memory. Searching for an alternative low power non-volatile memory to the currently used NOR flash memory, which is embedded in microcontrollers in mobile phones as well as other electronic devices, is due to the fact that NOR flash memory is reaching the limits of its feasible miniaturization. The new alternative is expected to continue the necessary miniaturization. The practical implementation of STT MRAM faces several technical challenges, one of such challenges is due to the fact that STT MRAM requires memory storage elements that take advantage of the effect in which a current which is passed through a magnetic material such as a magnetic tunnel junction (MTJ) to revers its direction of magnetization, which has the effect of switching between a low resistance and high resistance, to be used for representing the 1s and 0s of digital information, in order for STT MRAM to be used as a non-volatile memory. However, reading the STT MRAM, in order to discover whether the MTJ offers high resistance to current (“1”) or low resistance (“0”) requires a relatively high amount of voltage to correctly determine whether its resistance is high or low. But current passed at this high voltage leaves very little difference between the read-current and the write-current, resulting in erroneous outcome, since any fluctuation in the electrical characteristics of individual MTJs could cause what was intended as a read-current, to have the effect of a wirte-current, resulting in reversal of the direction of magnetization of the MTJ.
To overcome this technical challenge, Fujitsu Laboratories and the University of Toronto have jointly developed an innovative circuit design that resolves the issue of erroneous writes in STT MRAM during read operations. The newly developed read-method utilizes a negative resistance which is intermediate between the MTJ’s high resistance and low resistance on a parallel circuit, whereby, during the time when MTJ is in a high-resistance state, this new circuit exhibits negative-resistance en the characteristics, and when MTJ is in a low-resistance state, the circuit exhibits a normal-resistance characteristics, allowing the resistance value to be read at lower voltages than before, resulting in suppressing the tendency of the read operation to reverse the direction of magnetization and thus avoiding the problem of erroneous write operations. The practical implementation of STT MRAM still some time away as the R&D effort continues on lowering the write currents and developing process technologies for its further miniaturization.
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