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We studied the magnetic and magnetoresistance characteristics of pseudospin-valve magnetic tunnel junctions (MTJs) based on CoFe/Pd multilayer electrodes with perpendicular magnetic anisotropy and an MgO barrier. The MTJs at annealing temperature (T a) of 473 K showed a tunnel-magnetoresistance (TMR) ratio of 1.5%. An fcc (111)-oriented texture of the bottom and top Co90Fe10/Pd multilayer electrodes, together with an imperfectly crystallized MgO, were revealed by cross-sectional TEM images. The TMR properties of perpendicular MTJs with a Co20Fe60B20 or Co50Fe50 layer inserted between the CoFe/Pd multilayer electrodes and the MgO barrier were also studied. The TMR ratio with Co20Fe60B20 insertion was 1.7% at T a= 473 K and monotonically decreased at T a over 523 K. The TMR ratio with Co50Fe50 insertion increased up to 3% at T a= 573 K and then decreased to 0.4% at T a= 598 K. The influence of the Pd layer on CoFeB was studied by using the simplified structures of Pd/CoFeB/MgO/CoFeB/Pd and Ta/CoFeB/MgO/CoFeB/Ta with inplane anisotropy. A former structure with Pd resulted in reduced TMR ratio which decreases with increasing T a, whereas MTJs with a Ta-based structure showed a monotonic increase of a TMR ratio. The low TMR ratio observed in Pd-containing structures appears to result from crystallization of CoFeB in an unfavorable crystal orientation.

A 1.8 V 2-Mb SPRAM (SPin-transfer torque RAM) chip using 0.2-μm logic process with MgO tunneling barrier cell is reviewed, which demonstrates the circuit technologies for potential low power non-volatile RAM, or universal memory. This chip features: an array scheme with bit-by-bit bi-directional current write to achieve proper spin-transfer torque writing of 100-ns, and parallelizing-direction current reading with low voltage bit-line that leads to 40-ns access time.

We studied several proposals to address the problems encountered when single-pole-type giant magnetoresistive (SPT-GMR) heads are applied to an actual recording system. The first area of problems was regarding side erasure when there is a head skew. We fabricated a trapezoid-shaped main pole using a focus-ion-beam trimming or using a new wafer-processing technique and examined their write performances and their effects on reducing side erasure. The second area of problems concerned the stray field robustness of SPT-GMR heads when combined with a double-layered perpendicular recording medium. We tried to optimize the robustness and write performance by changing the width of a yoke in the main pole or by positioning a shield above the main pole. The stray field robustness and write performance were adequate in both cases, but a tradeoff relationship exists between both performances.

Recording and signal decay characteristics of perpendicular CoCr-alloy thin film media are investigated in comparison with those of longitudinal media. A perpendicular medium shows recording resolution 25-32% higher than that of a longitudinal medium with similar low density output. The medium S/N of a 25 nm thick perpendicular medium is comparable to that of a 16 nm thick CoCrPt longitudinal medium at high linear densities. The signal decay rate of the high Mr/Ms perpendicular medium is below 0.2% per decade of time for all linear densities. These results and thermal stability considerations based on the temperature dependence of KuV/kT indicate that CoCr-alloy perpendicular recording media are promising to extend the linear density well beyond 400 kFCI.

Single-layered perpendicular recording media with high resolution and low noise characteristics, which can be used for 10 Gb/in/sup 2/ level magnetic recording, have been developed. Noise characteristics of highly oriented CoCr/sub 19/Pt/sub 10/ media are improved, by employing CoCr/sub 35//TiCr/sub 10/ dual-underlayer, by reducing magnetic crystal grain diameter, and by adding Ta to the magnetic layer. A linear recording density resolution (D/sub 50/) value of 260 kFCI and a S/sub 0//N ratio of 36 dB are confirmed for the low noise 50-nm-thick CoCr/sub 19/Pt/sub 10/Ta/sub 2/ dual-underlayer medium using a flying MR head at magnetic spacing of 40 nm.

The detailed micromagnetic calculation results on the dynamics of spin transfer torque magnetization reversal is reported. Nano-pillars with CoFe/Ru/CoFe/Cu/CoFe film stacks are utilized with CoFe/Ru/CoFe trilayer as the strongly anti-ferromagnetic-coupled pinned layer. Magnetization reversal plots from anti-parallel (AP) to parallel (P) configuration with the nano-pillar shape ellipsoid and rectangular are determined. Results show that the threshold current for rectangular-shaped nano-pillars dramatically increases which is due to the pinning effect at the corners or the said pillars. In the case of magnetization reversal from P to AP configuration, the tendency is completely same. On the other hand, the simulation results indicate that the threshold current is not dependent on the aspect ratio of nano-pillars. Comparing the threshold currents for P to AP case between temperatures T=77 K and T=300 K, it is noted that the threshold current for the short pulse at T=77 K is smaller than that for T=300 K. This is also the same result for the AP to P case. These results suggest that a decrease in the coherent time of the precession movement of magnetization caused by the thermal scattering increases the time of spin transfer torque magnetization reversal.

We first report a multi-level-cell (MLC) spin-transfer torque memory (SPRAM) with series-connected magnetotunnel junctions (MTJs). The series MTJs (with different areas) show multi-level resistances by a combination of their magnetization directions. A four-level operation by spin-transfer-torque writing was experimentally demonstrated. A scheme for the write/read operation of the MLC SPRAM was also presented.

A magnetic-tunnel-junction (MTJ) circuit model, which considers spin dynamics under finite temperature, electrical bias, a stochastic process, and spin-transfer torque, was developed. Switching behaviors simulated by this model were verified by experimental measurements. Moreover, a disturbance-free read scheme for Gbit-scale spin-transfer torque RAM (SPRAM) was also developed. The feasibility of this scheme was confirmed by circuit simulation using the model and on-chip measurement of switching probability.

The effect of Co20Fe60B20 insertion on tunnel magnetoresistance (TMR) properties of MgO barrier magnetic tunnel junctions (MTJs) was studied with Co90Fe10/Pd multilayer electrodes showing perpendicular anisotropy. The TMR ratio, which depended on the sputtering power density of the CoFeB layer, reached 43% at 1.77W/cm in MTJs with a 1.8-nm-thick CoFeB layer inserted on both sides of the MgO barrier. With increasing CoFeB layer thickness, the optimal annealing temperature (Ta) realizing the maximum TMR ratio increased along with the TMR ratio. The MTJs with 3.0-nm-thick CoFeB deposited at 1.77W/cm showed a TMR ratio of 91% at Ta 1⁄4 250 C, where the perpendicular magnetic anisotropy is maintained. # 2010 The Japan Society of Applied Physics

An analytical magnetic force microscopy has been applied to investigate the relationship between the media S/N and the microscopic magnetization irregularities of CoCr-alloy longitudinal and single-layer perpendicular media recorded at linear densities up to 450 kFCL. When the S/N is compared between longitudinal and perpendicular media, a 16-nm-thick CoCrPt longitudinal medium shows a little higher S/N than that of a heat-treated 25-nm-thick CoCrTa perpendicular medium. The medium S/N is found to deteriorate with increasing the size and the number of magnetization transition irregularity.

We propose a tunnel magneto resistance (TMR) design methodology for SPRAM that takes into account the disturbances during read operations and the data retention periods. We have clarified that the thermal stability factor (E/kBT) of TMR must be higher than 64 to ensure a 10-year data retention and a continual non-destructive read-operation. Moreover, the thick synthetic ferromagnetic free layer with Fe-rich CoFeB can achieve a E/kBT of 64 while maintaining a low write cell current of less than 400 muA/cell with a 100 times 160 nm2 TMR.

A 1.8 V 2-Mb SPRAM (SPin-transfer torque RAM) chip using 0.2-mum logic process with MgO tunneling barrier cell demonstrates the circuit technologies for potential low power non-volatile RAM, or universal memory. This chip features: an array scheme with bit-by-bit bi-directional current write to achieve proper spin-transfer torque writing of 100-ns, and parallelizing-direction current reading with low voltage bit-line that leads to 40-ns access time.

A 1.8V 2Mb spin-transfer torque RAM chip using a 0.2mum logic process with an MgO tunneling barrier cell demonstrates the circuit technologies for potential low-power non-volatile RAM, or universal memory. This chip features an array scheme with bit-by-bit bidirectional current write to achieve proper spin-transfer torque writing in 100ns, and parallelizing-direction current reading with a low-voltage bit-line that leads to 40ns access time.

A new method to evaluate microscopic magnetization intensities has been developed based on a magnetic force microscopy (MFM) observation. The MFM tip-to-sample distance is varied to get quantitative information at the magnetic layer surface. The method is applied to investigate the origin of medium noise in single-layer perpendicular media. The irregular magnetization in the DC erased state and the average size of magnetization irregularities are quantitatively evaluated for CoCr-alloy perpendicular media with different noise characteristics.