STT-MRAM memory could enable super efficient computers and sensors

Most parts of present computer systems are made of volatile devices, and the power to supply them to avoid information loss causes huge energy losses. We can eliminate this meaningless energy loss by utilizing the non-volatile function of advanced spin-transfer torque magnetoresistive random-access memory (STT-MRAM) technology and create a new type of computer, i.e., normally off computers . Critical tasks to achieve normally off computers are implementations of STT-MRAM technologies in the main memory and low-level cache memories. STT-MRAM technology for applications to the main memory has been successfully developed by using perpendicular STT-MRAMs, and faster STT-MRAM technologies for applications to the cache memory are now being developed. The present status of STT-MRAMs and challenges that remain for normally off computers are discussed.

Normally off computers could use less than 1% of the power

The room temperature (RT) tunnel magneto-resistance (TMR) effect found in Al-O based magnetic tunnel junctions (MTJs) has enabled a new type of non-volatile memory, i.e., the magneto-resistive random access memory (MRAM). The concept of “ instant-on computers” has attracted attention around 2000 as an application of MRAMs. MRAMs were expected to reduce the start-up time of computers and to reduce user frustration. MRAMs play an important role only when computers start up in instant-on computers. However, we believe that the potential of MRAMs is not limited to start up and they have hidden potential to change the computer architecture. The researchers proposed the concept of ” normally off computers” in 2001 from this point of view.

Suppose that you are typing on a keyboard. During the approximately 100 ms to move your finger from one key to the next, the computer needlessly wastes energy waiting for your input. This is because most parts of present computers are made of volatile devices, i.e., transistors and dynamic RAMs (DRAMs), which lose information when powered off. The present computers are designed on the premise that power will always be supplied, i.e., they will be normally on. If computers are redesigned so that power consumption is zero during any short intervals when users are absent from the job without them even being aware of it, very energy efficient computers such as mobile personal computers running on solar batteries or hand-cranked dynamos can turn into a reality.

We need high performance non-volatile devices that do not require a power supply to retain information to create normally off computers and simultaneously guarantee sufficiently high speed operation to manipulate the information. The main memory, for example, requires performance as fast as 10 to 30 ns (image below) and density as high as 1 Gbit per chip.

Many problems still remain to be solved to achieve normally off computers. The problems are not only limited to materials and MTJ devices but circuits, memory architectures, operating systems, and peripherals, which should also be redesigned. This paper reports efforts to attain normally off computers and discusses the challenges that remain.

Layered structure of computer systems. Typical access times for smartphone, personal computer, and supercomputer systems are shown.
Citation: J. Appl. Phys. 115, 172607 (2014); http://dx.doi.org/10.1063/1.4869828

Journal of Applied Physics

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