Katsuya Nishiyama

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 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.

-FePt Electrodes

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

In the CoFeB∕MgO∕CoFeB magnetoresistive tunneling junctions, the bias-dependent magnetic switching curves are investigated and the spin transfer torque effect and the thermal activation effect on the magnetic switching are estimated. The different switching behaviors at the positive and negative biases were observed, which are the asymmetric changes of the coercive force and the shift field. In order to investigate the asymmetric reduction of the coercive force according to the bias direction, the magnetic excitation effect due to the hot electron was introduced. Consequently, from the analysis by using the current versus magnetic field phase diagram, the asymmetric reduction of the coercive force at the positive and negative biases was explained quantitatively by the relationship between the thermal activation effect due to the Joule heating and the magnetic excitation effect due to the hot electron.

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

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...

Estimation of spin transfer torque effect and thermal activation effect on magnetization reversal in CoFeB∕MgO∕CoFeB magnetoresistive tunneling junctions

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

Conceptual material design for magnetic tunneling junction cap layer for high magnetoresistance ratio

Spin-RAM technologies for operation speed faster than 30 ns, memory capacity larger than 1 Gbits and practically infinite read/write endurance have been developed by using magnetic tunnel junctions with perpendicular magnetization layers. Combination of Spin-RAM and power-gating technology will enable ultra low power computer called Normally-Off Computer.

1.8 V Power Supply 16 Mb-MRAMs With 42.3% Array Efficiency

Dual-spin-valve-type double magnetic tunnel junctions (double MTJs) of sputtered Ir–Mn/CoFe/AlOx/CoFeNi/AlOx/CoFe/Ir–Mn were fabricated using photolithography and ion-beam milling. The double MTJs were subjected to long-time annealing at various temperatures (185–400°C) in order to investigate the thermal stability due to the interdiffusion. Magnetoresistance (MR) ratio and resistance were measured at room temperature before and after annealing. The thermal changes of MR ratio are well-explained by considering three phenomena with effective activation energies of 2.6 eV, 0.26 eV, and 1.9 eV. These values are in good agreement with the activation energies of the interdiffusion based on the vacancy mechanism. The three phenomena with the effective activation energies are well-explained by considering the interdiffusion and redistribution of O and Mn at the AlOx/Co–Fe/Ir–Mn interfaces. Based on the effective activation energies, it is evaluated that there would be no significant changes in the MR ratio in MTJs with CoFe(3 nm)/IrMn pinned layers for a period of more than 10 years at 160°C.