Cheng T. Horng

A Study of Write Margin of Spin Torque Transfer Magnetic Random Access Memory Technology

Key design parameters of 64 Mb STT-MRAM at 90-nm technology node are discussed. A design point was developed with adequate TMR for fast read operation, enough energy barrier for data retention and against read disturbs, a write voltage satisfying the long term reliability against dielectric breakdown and a write bit error rate below 10-9. A direct experimental method was developed to determine the data retention lifetime that avoids the discrepancy in the energy barrier values obtained with spin current- and field-driven switching measurements. Other parameters detrimental to write margins such as backhopping and the existence of a low breakdown population are discussed. At low bit-error regime, new phenomenon emerges, suggestive of a bifurcation of switching modes. The dependence of the bifurcated switching threshold on write pulse width, operating temperature, junction dimensions and external field were studied. These show bifurcated switching to be strongly influenced by thermal fluctuation related to the spatially inhomogeneous free layer magnetization. An external field along easy axis direction assisting switching was shown to be effective for significantly reducing the percentage of MTJs showing bifurcated switching.

A statistical study of magnetic tunnel junctions for high-density spin torque transfer-MRAM (STT-MRAM)

We have demonstrated a robust magnetic tunnel junction (MTJ) with a resistance-area product RA=8 Omega-mum2 that simultaneously satisfies the statistical requirements of high tunneling magnetoresistance TMR > 15sigma(Rp), write threshold spread sigma(Vw)/<Vw> <7.1%, breakdown-to-write voltage margin over 0.5 V, read-induced disturbance rate below 10-9, and sufficient write endurance, and is free of unwanted write-induced magnetic reversal. The statistics suggest that a 64 Mb chip at the 90-nm node is feasible.

Quantitative interpretation of the magnetoresistive response (amplitude and shape) of spin valves with synthetic antiferromagnetic pinned layers

We present an analytical calculation of the shape of the magnetoresistive response of spin-valve structures with synthetic antiferromagnetic pinned layer, i.e., of the form buffer/ferromagnet1/Cu/ferromagnet2/Ru/ferromagnet3/antiferromagnet. The magnetization reversal in the three magnetic layers is assumed to occur via coherent rotation. An analytical expression of the whole hysteresis loop is given as a function of the characteristic parameters of the system (coupling strength through the Ru spacer, ferromagnet3/antiferromagnet pinning energy). We also extended a code based on the Boltzmann equation of transport to calculate the giant magnetoresistance (GMR) amplitude in these structures from the microscopic transport parameters. In order to explain the relatively high GMR amplitude experimentally observed in such spin valves, it is shown that some degree of specular reflection must be introduced at the ferromagnet2/Ru interface. Good agreement with both the shape and amplitude of the experimental magne...

Effect of interfacial specular electron reflection on the anisotropic magnetoresistance of magnetic thin films

We investigated the effect of specular reflection on the anisotropic magnetoresistance (AMR) of magnetic thin films. The sheet conductance is calculated as a function of the angle between magnetization and current from the microscopic transport parameters by using an extension of the Fuchs–Sondheimer theory. The calculation combines specular reflection on the film interfaces with mean-free paths which depend on the angle between the local magnetization and the electron velocity. The theoretical results are compared with experimental ones. Specular reflection can explain the quite large AMR amplitude observed in thin NiFe films used in the last generation of AMR heads.

Study of dielectric breakdown distributions in magnetic tunneling junction with MgO barrier

The breakdown distribution of a magnetic tunnel junction (MTJ) with an ultrathin (∼1.2 nm) MgO barrier was studied, and two distinct distributions were identified. The breakdown distribution with high value demonstrates a wide peak-to-peak separation (∼13.4σ) to the critical spin torque induced switching voltage. However, the peak-to-peak separation is only ∼8.4σ for the devices showing low breakdown value. Both abrupt and gradual breakdown events were observed in two distributions. The dependence of the percentage of low breakdown devices as a function of bias polarity, test and stress conditions, MTJ film properties, and process conditions was investigated. The low breakdown percentage can be significantly reduced by increasing the RA value and MTJ process optimization.

Reduction of AMR effect in giant magnetoresistance spin valve structures

For spin valve heads, the conventional anisotropic magnetoresistance (AMR) effect deteriorates the linearity of transfer curves and degrades its performance. This problem is more severe for thicker free layer spin valve. In this paper, giant magnetoresistance (GMR) and AMR effect of a spin valve structure with laminated free layer (seed/Fa/I/Fb/Cu/AP1/Ru/AP2/AFM/cap, I is a thin high resistive layer) were calculated using semiclassical transport model. It is found that the GMR of spin valve structure with laminated free layer is reduced slightly and DR is comparable with the standard one due to the increase of the sheet resistance. The AMR of the laminated free layer reduced dramatically due to the effective magnetic thickness reduction. Therefore, the AMR/GMR ratio in spin valve was greatly reduced. Film level experimental data confirmed the simulation results. As an example, the AMR of 80 /spl Aring/, 65 /spl Aring/ thickness free layer reduced from 1.6, 1.2% to 0.74, 0.6%, respectively, while GMR ratio reduced slightly due to lamination of free layer. The AMR effect of spin valve structure can be reduced by 50% through the lamination of the free layer. It can be expected that the linearity of the transfer curves can be improved greatly.

5 Gb/in/sup 2/ recording with dual stripe AMR heads and low noise thin film disks

We have demonstrated magnetic recording at an areal density of 5 Gb/in/sup 2/ using dual stripe magnetoresistive heads (DSMR) on low noise and low-glide height quaternary CoCrTa-based alloy thin film disks. The data rate is 120 Mb/sec with a PR4 channel, and the on-track error rate achieved is around 10/sup -9/. The demonstration was accomplished in two cases using two different types of heads. In the first case, it was done with a DSMR head having shield-shield spacing of 0.165 /spl mu/m and 1 /spl mu/m stripe height flying at 25 nm, resulting in a linear density of 321.7 KBPI, and a track density of 15.6 KTPI. The second case was performed with DSMR heads having shield-shield spacing of 0.18 /spl mu/m and 8 /spl mu/m stripe height flying at proximity height of 12 nm, achieving a 358.75 KBPI and 13.8 KTPI. This is the highest linear density published so far for either GMR or AMR heads. It is the direct consequence of high signal/noise of DSMR heads and proximity recording over low noise, low-glide-height media. To explore the large dynamic range of DSMR heads, this study was also done with a variety of low noise disks with Mrt ranging broadly from 0.45 to 1.0 memu/cm/sup 2/ and coercivity of 3000 Oe.