Vol.2 No.3 2009

Research paper : Creating non-volatile electronics with spintronics technology (S. Yuasa et al.)−196−Synthesiology - English edition Vol.2 No.3 (2009) 1.3 Room temperature TMR effect and its applicationsA magnetic tunnel junction (MTJ) device consists of an insulation layer that is no thicker than a few nanometers (tunnel barrier) sandwiched between two ferromagnetic metal layers (ferro-magnetic electrodes) (Fig. 4). Insulators do not normally carry current, but when the insulation is less than a few nanometers thick, minute currents flow due to quantum effects, a phenomenon called the “tunneling effect.” The current and electrical resistance generated by this effect are referred to as the tunnel current and the tunnel resistance. If the electrode layers are ferro-magnetic, the tunnel resistance is small in the parallel magnetization state (P state: Fig. 4(a)) and a larger current flows. In the anti-parallel magnetization state (AP state: Fig. 4(b)), on the other hand, the tunnel resistance is large and the tunneling current is small. That phenomenon is called the tunnel magnetoresistance (TMR) effect. MTJ devices can be switched between the P state and the AP state by application of a magnetic field (Fig. 4(d)), creating magnetoresistance. Also, because ferromagnets possess a magnetic hysteresis characteristic, they are bistable at zero magnetic field, having a P state or an AP state. An MTJ device can thus act as a non-volatile memory to store one bit of information.The low-temperature TMR effect has been known since the 1970s, but room-temperature magnetoresistance had not been obtained and there was little interest in the phenomenon in the following ten years. With the discovery of GMR in 1988, however, came much R & D on magnetic sensors (HDD magnetic heads, etc.) and TMR also began to attract interest again. In 1995, Miyazaki et al.[2] and J. Moodera et al.[3] used amorphous (random arrangement of atoms) aluminum oxide (Al-O) for the tunnel barrier and polycrystalline transition metals such as Fe or Co for the ferromagnetic electrodes to fabricate MTJ devices that had MR ratios of close to 20 % at room temperature and low magnetic field (Fig. 3). That was the highest room temperature MR ratio at the time, and the achievement thrust the TMR effect into the limelight. Subsequently, there was vigorous work on optimizing the method for making the Al-O tunnel barrier and the electrode material, resulting in achievement of room temperature MR ratios of over 70 % for the TMR effect.The room temperature TMR effect was put to practical use in an HDD magnetic read head (TMR head) in 2004, about ten years after it had been implemented (Fig. 5). Combining that TMR head with a perpendicular magnetic recording medium achieved a high recording density of 100 Gbit/inch2. Furthermore, a relatively low capacity (4 Mbit to 16 Mbit) non-volatile MRAM (Fig. 6) product based on the MTJ device was commercialized in 2006. That product drew attention as a unique non-volatile memory that featured high reliability (unlimited write endurance). The reason for the unlimited write endurance of the MRAM is that reversal of the spin direction (the rewrite operation) causes no degradation of the material at all. In addition, MRAM operation is faster than DRAM and nearly as fast as the high-speed SRAM used in the CPU. However, there remains little margin for improvement in the performance (room temperature MR ratio) of MTJ devices based on an amorphous Al-O tunnel barrier, and that was a serious problem for achieving higher Fig. 3 History of the magnetoresistance effect and its industrial applications.Fig. 2 Spintronics and magnetoresistance effect. - e ChargeElectronicsSpintronicsElectron spinMagnetics・Diode・Transistor・LaserNew field that uses both electron charge and spin・Magnetic recording・Permanent magnets ・TransformerElectronLSIHard disk driveNSMagnetoresistanceeffect1985 1990 1995 2000 2005 2010 MR headGMR headHDD magneticheadInductive headMRAM Spin RAMNon-volatilememoryTMR headMgO-TMR head1857 A. Fert, P. Grünberg Lord KelvinAMR effectMR = 1~~2% GMR effectMR = 5~~15% MR = 200~600% MR = 20~~70% (Nobel Prize in Physics,2007) TMR effect Miyazaki, Moodera Yuasa, S. ParkinGiant TMR effectYearMR ratio (room temperatureand low magnetic field)Industrial applications of themagnetoresistance effectNew devicesAchievement of themagnetoresistance effectCommercialization inonly three years! Microwave oscillator, random number generator, etc.In research and developmentAlready commercialized


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