Vol.2 No.3 2009

Research paper : Creating non-volatile electronics with spintronics technology (S. Yuasa et al.)−195−Synthesiology - English edition Vol.2 No.3 (2009) consumption is a problem with the HDD. Reduction of the size (diameter) of the recording medium disk called “platter” is generally an important factor in reduction of HDD power consumption, and the key to that is higher recording density. At this time, HDDs that have platters that are 3.5-inches in diameter are mainstream, but those drives consume a great deal of power (typically 5 W per unit). The smaller 2.5-inch drives, on the other hand, consume only about one-fifth the power required by a 3.5-inch drive. If the current 3.5-inch HDDs can be replaced by 2.5 inch drives that have a higher recording density, the overall power consumption of HDDs could be greatly reduced. AIST is also doing R & D aimed at increasing the recording density of HDDs.1.2 Spintronics technology for non-volatile electronicsSpintronics is a new field in which new functions are created by using both the electrical (charge) and magnetic (spin) properties of electrons (Fig. 2). Silicon-based electronic devices that use only electron charge are the foundation of information technology, but they are inadequate for implementing non-volatile memory. Magnetics, which uses only electron spin on the other hand, is good for non-volatile memory, but does not perform well in terms of logic operations or power consumption. For future technology, spintronics opens up the possibility of realizing non-volatility together with features such as high reliability, low power consumption and logic operations at the same time. Non-volatile memory that offers large capacity, high-speed and high reliability together will be the core technology for non-volatile electronics for the ultimate normally-off computers.Spintronics uses quantum mechanical phenomena to correlate electron charge and spin. Of those phenomena, the most important is magnetoresistance. Magnetoresistance (MR) is the change in the electrical resistance of a solid or a solid-state device when a magnetic field is applied to it. The relative rate of change in electrical resistance is referred to as the magnetoresistance ratio (MR ratio). The magnetoresistance effect can be used to convert a magnetic field signal into an electrical signal, so it can be applied to sensing magnetic fields in the design of magnetic read heads for hard disk drives (HDDs). Furthermore, magnetic hysteresis characteristics of ferromagnets can be used to implement the same kind of non-volatile memory as is possible with magnetic recording.For devices that use the magnetoresistance effect, the MR ratio at room temperature and low magnetic fields (below a few milli-tesla) serves as an index of performance, because the magnetic fields that can be generated in ordinary electronic circuits are small, several milli-tesla at most. Larger MR ratios at room temperature and low magnetic field mean that devices of higher performance can be developed. This important metric of practical application had values of only from 1 to 2 %, which was never considered as having potential for practical use. Then, A. Fert et al. and P. Grünberg et al. discovered the giant magnetoresistance effect (GMR effect) of metallic magnetic multi-layers in 1988, achieving an MR ratio at room temperature and low magnetic field of about 10 %, an order of magnitude higher than any previous value. That discovery earned Fert and Grünberg the 2007 Nobel Prize in Physics. About ten years after its discovery, GMR was applied to the magnetic read head of hard disk drives (GMR head), after which the capacity of HDDs increased rapidly (Fig. 3). Furthermore, the discovery of the GMR effect stimulated vigorous research and development on magnetoresistance around the world. It was also linked to achievement of the TMR effect (explained below) at room temperature (Fig. 3). We do not cover GMR here; for more information on that subject, see the formal paper for the 2007 Nobel Prize in Physics[1]. The TMR effect is described in more detail in the next section.Fig. 1 Current electronics and future non-volatile electronics. Structure of computer for futureimplementationGreat reduction in HDD power consumption by achieving ultra-high recording densitySpin RAMLow powerconsumptionHDD and SSDNon-volatiledisplayNon-volatileexternalmemoryLarge capacity,high speed andhighly reliablenon-volatile memoryNon-volatileCPUConfiguration of currentcomputerHigh output MTJ device exhibits giantmagnetoresistance at room temperatureand low magnetic fieldHigh power consumptionVolatileHDDDisplayNon-volatileexternalmemorySRAMDRAMCPU・ Slow start-up Current electronics are “volatile” Spintronics technology realizes “non-volatile electronics“ Quick-on Normally off・ Power is consumed even in standby mode(instantaneous start-up technology )(ultimate green IT technology )


page 22