Noriaki Kasahara

Low resistance and high thermal stability of spin-dependent tunnel junctions with synthetic antiferromagnetic CoFe/Ru/CoFe pinned layers

In this work, submicron-size (down to 0.273 μm2) spin–dependent tunnel junctions with resistance as low as ∼30 Ω μm2 have been fabricated, where the tunneling barrier of AlOx was formed by in situ natural oxidation. These junctions annealed at 250 °C for 5 h showed tunneling magnetoresistance (TMR) of 14.3% and 25.8% for the pinned layers of CoFe/RuRhMn and CoFe/PtMn, respectively, while the TMR is further increased to 31.6% for a synthetic antiferromagnetic pinned layer of CoFe/Ru/CoFe/PtMn due to less interdiffusion at CoFe/Ru interface. The investigation has indicated that the growth of ultrathin Al layer is very sensitive to the surface roughness of bottom ferromagnetic electrode, and large surface roughness leads to small junction resistance.

Magnetic tunnel junctions on magnetic shield smoothed by gas cluster ion beam

In this work, a technique, gas cluster ion beam (GCIB), was introduced to smooth the bottom NiFe magnetic shield for magnetic tunnel junction (MTJ) read heads. The GCIB treatment can bring the surface roughness of the shield from 15 to 20 A to around 5 A, and the most of scratch marks can be removed. The efficiency of the GCIB process is dependent on the initial surface morphology. The MTJs grown on the magnetic shield smoothed by the GCIB show that the resistance area product RA is increased from 60 to ∼100 Ω μm2 with the GCIB dose up to 1×1016 ions/cm2, arising from a smooth insulating layer, meanwhile, the tunneling magnetoresistance (TMR) is almost constant or slightly decreases. This GCIB process can also improve breakdown voltage (approximately 0.019 V per 1015 ions/cm2) of the MTJs, and slightly increase the ferromagnetic coupling mainly due to the change of the surface morphology. Using this technology, an RA as low as 3.5–6.5 Ω μm2 together with a TMR of 14%–18% can be obtained for MTJs grown on ...

Electrical breakdown of the magnetic tunneling junction with an AlOx barrier formed by radical oxidation

In this work, the dielectric breakdown in magnetic tunnel junctions (MTJs) was studied. The MTJ structure is Ta50/NiFe100/Co20/AlOx/Co30/RuRhMn100/Ta50 with the bottom lead of Ta50/Cu500/Ta50 and the top lead of Cu2000/Ta50 (in A), where the tunneling barrier was formed by 2–20 min radical oxygen oxidation of a 10 A-thick Al layer. The junctions with area from 2×2 to 20×20 μm2 were patterned using the photolithography process, leading to tunneling magnetoresistance up to 17.2% and resistance-area product ranging from 350 Ω μm2 to 200 kΩ μm2. The junctions studied show dc breakdown voltage from 0.7 to 1.3 V, depending on the junction area and the oxidation time. Long oxidation time up to 14 min and a small junction area results in a large dc breakdown voltage. The electrostatic discharge (ESD) of MTJs was tested by using a human body model. The ESD breakdown voltage increases with decreasing junction resistance. These results are discussed in terms of the E-model based on the field-induced distortion of at...

Evaluation of front flux guide-type magnetic tunnel junction heads

Magnetic tunnel junction (MTJ) heads have been successfully fabricated using the free layer as flux guide to prevent electrical short during the definition of the air bearing surface (ABS). For a 6 Gbits/in/sup 2/ design, an output as high as 5740 /spl mu/Vpp was achieved for 3 mA sense current, and the output waveform was stable and noise free. Noise analysis confirmed that shot noise is prevailing in these MTJ heads, suggesting that the junction resistance has to be further reduced to challenge spin-valve heads in terms of signal to noise ratio. Landau-Lifshitz-Gilbert simulations showed that for 20 Gbits/in/sup 2/ application, the flux guide height should not exceed 0.1 /spl mu/m.

A performance study of next generation's TMR heads beyond 200 gb/in/sup 2/

Practical level performance for /spl sim/200 Gb/in/sup 2/ has been verified by AlOx barrier tunneling magnetoresistive (TMR) heads, which resistance area product (RA) is more than 3 ohm/spl middot//spl mu/m/sup 2/, in perpendicular recording mode. In addition, improved AlOx barrier magnetic tunnel junctions (MTJs) formed on plated bottom shield with smoothed surface achieved TMR ratio of 25% and 16% with RA of 1.9 and 1.0 ohm/spl middot//spl mu/m/sup 2/, respectively, indicating over 200 Gb/in/sup 2/ is also possible by the AlOx barrier TMR heads with lower RA. Furthermore, TMR heads with crystalline MgO barrier were fabricated. The MgO barrier MTJs formed on plated bottom shield with smoothed surface achieved TMR ratio of 88% with RA of 2.0 ohm/spl middot//spl mu/m/sup 2/, which is 3.5 times higher than that of AlOx barrier MTJs under similar RA. Dynamic electrical test was also performed for TMR heads with the MgO barrier. As a result, good readback waveform with huge output was obtained. This is the first confirmation of readback waveform generated from TMR heads with crystalline MgO barrier. Our results indicate that the future of TMR heads technology is promising beyond 200 Gb/in/sup 2/ application.

Frequency response of common lead and shield type magnetic tunneling junction head

In this work, the frequency response in magnetic tunnel junction (MTJ) heads was studied. Both stray capacitance and junction resistance, forming a low-pass-filter, have to be reduced to improve the cutoff frequency in MTJ heads. By employing an Ar gas cluster ion beam (GCIB) process, junctions grown on the magnetic shield show a resistance area product as low as 3.6 /spl Omega//spl mu/m/sup 2/ and tunneling magneto-resistance over 14%. The dominant capacitance in common lead and shield MTJ heads was found mainly resulting from the shield-to-shield spacing, whose capacitance can be reduced by using an SiO/sub 2/ gap layer instead of Al/sub 2/O/sub 3/ layer and thus leading to an improved frequency response. Simple analysis indicates that a read amplifier design with low impedance could be helpful to realize a high data transfer rate, and a rate of around 800 Mbps for 100 Gbits/in/sup 2/ recording system can be thus expected.

Electrical performance and reliability of tunnel magnetoresistance heads for 100-Gb/in/sup 2/ application

Tunnel magnetoresistance (TuMR) heads are attractive candidates for future high-density recording. To achieve the high areal density, it is necessary for TuMR heads to get lower resistance and higher delta R/R film. A low resistance and high delta R/R tunneling junction film has been developed and used for this study. The resistance area product and delta R/R are 3 /spl Omega//spl middot//spl mu/m/sup 2/ and 18%, respectively. Reliability that includes lifetime was also studied. We have found TuMR heads can be a promising candidate for 100 Gb/in/sup 2/ application.

Extra current channels in longitudinally biased magnetic tunnel junctions

Experiments on the longitudinal biasing of microsized magnetic tunnel junctions have been conducted using permanent magnets partially overlapping the junction area. The tunneling magnetoresistance ratio showed a strong dependency on the overlap length, since even a 10% overlap of the sensor length resulted in a 25% drop from its initial value without overlap. Analytical and micromagnetic analyses have demonstrated that this decrease comes from extra current channels, located in the regions below the permanent magnets, that shorted a large amount of the sense current from the central active region in the antiparallel magnetization state. The high uniaxial anisotropy field, induced by the permanent magnets in the overlapped regions, created particular magnetic configurations responsible for these low resistance paths. Several alternatives, using antiferromagnetic material in place of the permanent magnets or a modified design of the magnetic tunnel junction structure, are presented and discussed to prevent ...

Canting of exchange coupling direction in spin valve with various pinned layers

One of the problems in spin valve (SV) is the thermal stability of exchange coupling between the pinned magnetic layer and the antiferromagnetic (AFM) layer. During operating in actual hard disk drives, the pinned direction tends to cant toward the longitudinal hard magnet direction and as a result, the output voltage drops. In this study we examine the amount of the cant by the heat and field in SV which used different pinned layer material. The sample we used is Ta(5)/NiFe(9)/Co(1)/Cu(2.7)/pinned layer/AFM/Ta(5) unit nanometers. The pinned layer is three kinds—Co(2), Co(1)/NiFe(1.7), NiFe(3.5), and AFM is two kinds of Ru3Rh15Mn(12 nm), Ru3Rh15Mn(8 nm). The pinned-Co–SV has a larger increase of canting degree than the pinned-Co/NiFe–SV and the pinned-NiFe–SV. The longer the SV is exposed to heat and magnetic field, the more the cant increases. It is likely due to the change of the local pinned direction which in turn may be due to the aftereffect in the minute AFM grains during the heating process.

Low-frequency noise analysis of TMR heads

1/f noise as time traces of the fluctuation and low-frequency noise (pulse noise) were observed in TMR heads. These were then related to the quality of the oxide layer in the TMR heads.