Makoto Nagamine

Lower-current and fast switching of a perpendicular TMR for high speed and high density spin-transfer-torque MRAM

We investigate extremely low programming current and fast switching time of a perpendicular tunnel-magnetoresistance (P-TMR) for spin-transfer torque using a P-TMR cell of 50 nm-diameter. A L10-crystalline ordered alloy is used as a free layer that has excellent thermal stability and a damping constant of about 0.03. The programming current of 49 uA and the switching time of 4 nsec are also demonstrated.

Tunnel Magnetoresistance Over 100% in MgO-Based Magnetic Tunnel Junction Films With Perpendicular Magnetic L1

Perpendicular L1₀-FePt/MgO/Fe/L1₀ -FePt magnetic tunnel junction (MTJ) films with the (001) texture were successfully developed to obtain a large tunnel magnetoresistance (TMR) above 100 % at room temperature. The TMR ratio in the L1₀-FePt/MgO/Fe/L1₀-FePt MTJ was strongly dependent on the Fe interfacial layer thickness. The lattice mismatch between the MgO(001) barrier layer and the L1₀ -FePt(001) layer is too large for the MgO barrier layer to grow epitaxially on the L1₀-FePt(001) layer. The insertion of the Fe interfacial layer improves the quality of the MgO(001) barrier layer and achieves an epitaxy in the L1₀-FePt/MgO/Fe/L1₀-FePt stack. As a result, the optimization of the Fe interfacial layer thickness is a key to obtain the large TMR ratio in the MgO-based MTJ with the L1₀-FePt electrodes.

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We have investigated the mechanism of negative bias temperature (NBT) degradation of p/sup +/-gate p-MOSFETs having SiON and SiO/sub 2/ films. As a result, it was found that NBT degradation of SiO/sub 2/ is improved by fluorine incorporation, while no effect is observed in that of SiON, and that the activation energy of NBT degradation in SiON is lower than that in SiO/sub 2/. From these experimental results, it is inferred that nitrogen-originated NBT degradation dominates NBT degradation in SiON. It was also found that non-energetic holes existing in the inversion layer contribute to NBT degradation for both SiON and SiO/sub 2/ films, and that the oxide field is indispensable for NBT degradation.

-FePt Electrodes

Abstract We have reported both the experimental results of the flat interface formation and its mechanism based upon the theoretical analysis of the oxygen radical transport in the grown SiO2. We obtained the logarithmic dependence of the oxide film thickness on the oxidation time on the assumption that the deactivation of the oxygen radicals is proportional to the concentration. The characteristic length “a” of the oxygen radicals plays an important role in forming the flat interface.

NBTI mechanism in ultra-thin gate dielectric - nitrogen-originated mechanism in SiON

We report on the spin-transfer magnetization switching properties of CoFe/Pd-based perpendicularly magnetized giant magnetoresistive cells over a wide current pulse duration time range. Analytic expressions without empirical parameters like attempt frequency are tested experimentally for the thermally assisted and precessional regimes. Good agreement with the experiment data is obtained using a common parameter set in both regimes, which leads to a comprehensive understanding of the switching properties including the origin of the attempt frequency.

A study of atomically-flat SiO 2 /Si interface formation mechanism, based on the radical oxidation kinetics

We studied the spin-transfer switching probability (Psw) in giant magnetoresistance (GMR) device with perpendicular magnetizations using short nanosecond and sub-nanosecond current pulses. A switching time of 510 picoseconds was achieved with the application of 7.5 mA, which is 4.3 times larger than the critical current at 0 K, without the application of an assisting magnetic field. Experiments with longer pulses revealed an exponential decay of the nonswitching probability (1-Psw) as a function of pulse width. Extrapolation of the results predicts an error rate of 10-19 for a pulse width of about 4.8 ns. To understand the observed pulse width dependence of Psw, we developed a formula using a macro spin model for the perpendicular magnetization system which includes the influence of thermal fluctuations in the initial magnetization direction of the free layer. The formula easily reproduces the qualitative nature of the observed Psw distributions in all time ranges.

Unified understanding of both thermally assisted and precessional spin-transfer switching in perpendicularly magnetized giant magnetoresistive nanopillars

Spin torque transfer switching magnetic random access memory (Spin-MRAM) using MgO-magnetic tunnel junction (MTJ) is considered to be the most promising candidate for high-density, high-speed, and non-volatile RAM. Notwithstanding its excellent potential, the breakdown mechanism of MgO-MTJ has not been well understood although a thorough understanding is essential for commercialization of Spin-MRAM. In this paper, we demonstrate for the first time the modeling of dielectric breakdown phenomena of MgO-MTJ by time-dependent dielectric breakdown (TDDB) measurement concerning the effect of self-heating using simulation and conclude that E-model with the effect of self-heating at MgO-MTJ during current stress (power) removed gives the best fitting as a degradation model of MgO-MTJ ultrathin dielectrics.

High-Speed Spin-Transfer Switching in GMR Nano-Pillars With Perpendicular Anisotropy

Magnetoresistive Random Access Memory (MRAM) is a promising device for high-density (over Gbits scale), high-speed (equal to DRAM or better) non-volatile RAM, and much research has been done over several years with a view to overcoming the problems regarding practical use. Spin Torque Transfer switching MRAM (STT-MRAM) is considered to be the most promising candidate and there already are some papers on this new device. MgO is expected to be the best material for magnetic tunnel junction (MTJ) of STT-MRAM, because MgO-MTJ is known to show large Magnetoresistance (MR) and enhance spin polarization by the coherent tunneling effect, resulting in decrease of writing current of MTJ. MgO-MTJ has been shown to be an excellent barrier with little resistance drift compared with MTJ using alumina. Notwithstanding its excellent potential, the degradation mechanism of MgO-MTJ has not been well understood. In this paper, we will demonstrate for the first time the degradation of coherent tunneling and trapping phenomena of MgO-MTJ and discuss its mechanism.

Effect of Self-Heating on Time-Dependent Dielectric Breakdown in Ultrathin MgO Magnetic Tunnel Junctions for Spin Torque Transfer Switching Magnetic Random Access Memory

A conceptual material design for magnetic tunneling junction cap layer realizing a steep NiFe∕AlOx interface is proposed. Tunnel magneto resistance stack of cap∕NiFe∕AlOx∕CoFe∕Ru∕CoFe∕PtMn∕Ta∕∕sub was prepared. Maximum magnetoresistance (MR) ratios of nonmagnetic-NiFeZr, Zr, Ta, Ru, and Rh caps at 0 V were 55%, 28%, 50%, 43%, and 42%, respectively. The decrease of MR ratio and the increase of resistance area product RA with Ru cap compared to Ta cap correlate with the partial oxidation of the NiFe∕AlOx interface occurring in additional postannealing, which was confirmed by focused-ion-beam–transmission-electron-microscope–energy-dispersive-x-ray-fluorescence observation. Since standard electrode potential is Ta<Fe<Ni<Ru, it is supposed that NiFe with Ru cap is positively charged, the NiFe∕AlOx interface is easily oxidized during annealing by negatively charged oxidizing species, and increase of RA and decrease of MR ratio occur. RA with Rh cap was even higher than that with Ru cap, consistent with the hig...

Resistance drift of MgO magnetic tunnel junctions by trapping and degradation of coherent tunneling

Technologies for realizing high density MRAM were developed. First, new circuitry to lower the resistance of programming wires was developed. Second, both MTJ plane shape and cross-sectional structure were optimized to lower the programming current. Based on these two technologies, 16 Mb MRAM was designed, fabricated with 130 nm CMOS process and 240 nm back end MTJ process. As a result, a 1.8 V power supply MRAM with 42.3% array efficiency was successfully demonstrated