Kuei-Hung Shen

Reduction in critical current density for spin torque transfer switching with composite free layer

A composite free layer (CFL) consisting of a soft layer and a hard layer exchange coupled in parallel is proposed. The experimental results showed that the critical current density (Jc) can be reduced from 7.05×106A∕cm2 of strong coupled CoFeB(12.5A)∕Ru(4A)∕NiFe(17.5A) CFL to 2.65×106A∕cm2 of weak coupled CoFeB(12.5A)∕Ru(15A)∕NiFe(17.5A) CFL. The macrospin simulations showed that the soft layer of CFL can assist the hard layer of CFL to switch at a lower Jc. These results suggest that by using CFL, it is possible to reduce the Jc of spin torque transfer switching without compromising the thermal stability.

Interfacial and annealing effects on magnetic properties of CoFeB thin films

The interfacial and annealing effects on magnetic properties of CoFeB thin films with Ta and Ru interfaces were investigated. It is found that the thickness of magnetically dead layer of as-deposited CoFeB film strongly depends on the interfaces. After annealing above 150°C boron atoms seem to diffuse out of CoFeB film and the effective saturation magnetization of CoFeB film increases with annealing temperature and annealing time. In addition, CoFeB film with Ta interfaces remains amorphous while that with Ru interfaces starts to crystallize during the annealing above 250°C. Furthermore, the annealing-induced anisotropy field of CoFeB film can be modified by using different seed and capping materials.

Reduction in critical current density by tuning damping constants of CoFeB for spin-torque-transfer switching

Different capping layers were deposited on 2.5 nm Co72Fe8B20 to investigate their effects on damping constants and critical current density (JC0) for spin-torque-transfer switching. The damping constant of CoFeB is affected by the interfacial conditions and the spin-pumping effects. The Cu capping layer significantly suppresses intermixing and possesses the lowest damping constant of 0.009. The micromagnetic simulations reveal that the low damping constant of CoFeB may result in the nucleation of domains at lower current density, and thus reduces JC0. A reduction of 27% in JC0 can be achieved by replacing the conventional Ta capping layer with a Cu layer.

Wide operation margin of toggle mode switching for magnetic random access memory with preceding negative pulse writing scheme

In this work, a writing scheme with preceding negative pulse wave form for toggle magnetic random access memory (MRAM) is proposed to enhance the switching yield and enable a low current switching. The failure mechanism of toggle switching is studied by micromagnetic analysis. As a result of broadened operation window and reduced switching current, the scalability of MRAM is feasible with the robust toggle operation.

Impact of stray field on the switching properties of perpendicular MTJ for scaled MRAM

For the first time, we show the stray field (Hs) from the pinned layer of perpendicular magnetic tunnel junctions (pMTJs) plays multiple important roles: its out-of-plane component degrades pinned layer stability of scaled MTJ, while its in-plane component assists spin-transfer torque switching. Through stray field engineering, one can retain its advantage, and minimized its detrimental effect. We also show that etching process plays a vital role in the stray-field engineering. Our pMTJ exhibits robust pinned layer performance, symmetrical R-H loop and balanced spin-torque switching current. The switching current of our pMTJ is ~60uA@20us for a 80nm diameter pMTJ.

High density and low power design of MRAM

MRAM structures based on 1T2UMTJ cell and PWWL architecture are proposed to shrink the bit size with a potential down to 6 F/sup 2/ by a so-called ExtVia process and reduce the writing current by a factor of two, combined with the nature of non-volatility and high speed, making the MRAM suitable for universal memory applications.

Improvement of Transport Properties in Magnetic Tunneling Junctions by Capping Materials

The capping effects on the properties of magnetic tunneling junctions (MTJs) have been investigated to improve the magnetoresistive (MR) ratio [Nagamine , Applied Physics, vol. 99, p. 08K703, 2006] and thermal stability [Fukumoto , Japanese Journal of Applied Physics, vol. 45, p. 3829, 2006. In this paper, MgO/Mg and MgO capping layers for MTJs were used to replace our previously used Ta and Ru capping layer [Yen , IEEE Transactions on Magnetics, vol. 42, no. 10, p. 862, 2004]. The MTJs with NiFe and CoFeB free layers were patterned to ellipses of 0.36 mum times 0.72 mum. The MR ratios of the MTJs with an NiFe free layer were enhanced from 26% to about 33% by capping MgO/Mg bilayers. A great improvement was also found on the MTJs with the CoFeB free layer, where the MR ratio was 50.4% with the MgO cap layer enhanced from 44.7% of Ru-capped MTJs. In addition, the influences of those capping layers included Ta, Ru, MgO, and MgO/Mg on thermal stability were also discussed

The switching behaviors of submicron magnetic tunnel junctions with synthetic antiferromagnetic free layers

The switching behaviors in elliptic shaped (aspect ratio=2) submicron magnetic tunnel junctions using CoFeB single free layer and CoFeB∕Ru∕CoFeB synthetic antiferromagnetic (SAF) free layers are studied. It is found that under considerable stray fields originating from pinned layers, junctions with single free layer show complex switching behaviors with larger Hc variations. In contrast, junctions with SAF free layers exhibit kink-free R‐H loops and less Hc variations. The Hc of junctions with SAF free layers is less dependent on the junction size than that with a single free layer. Furthermore, for junctions smaller than a critical size the SAF free layers have a smaller Hc than single free layers.

Scaling Properties of Step-Etch Perpendicular Magnetic Tunnel Junction With Dual-CoFeB/MgO Interfaces

We built and studied the size scaling effect of perpendicular magnetic tunnel junctions (p-MTJs) with stepetch structure and dual-MgO/CoFeB interfaces. The step-etch structure yields symmetrical R-H loop, while dual-MgO/CoFeB interfaces raises cell anisotropy, thus the data retention time. The p-MTJ of 45-nm diameter shows spin-transfer torque switching voltage Vsw with tight temporal sigma (σ(Vsw) <;3.7%). The thermal stability factor is 60. Although the critical switching current (Ic0) reduces with MTJ area, its density (Jc0) increases. One plausible explanation of this observation is that the magnetization reversal of small MTJ follows the single-domain macrospin model, while that of the larger MTJ may be affected by the nucleation of domain during the magnetization reversal, and the wall motion leads to Jc0 lowering; the other may be due to process-induced film damage. Fortunately, the switching efficiency (Eb/|Ic0|) is higher for smaller p-MTJ.

Evidences of Reactive-Ion-Etching-Induced Damages to the Ferromagnet of Perpendicular Magnetic Tunnel Junctions

To minimize the R-H loop shift of a perpendicular magnetic tunnel junction (p-MTJ), a stop-on-tunnel-barrier etch is required to pattern the reference layer such that it is wider than the free layer. Our experiments show that reactive ion etching can induce penetrating damages to the ferromagnetic layers beyond the etched surface. Sufficient sacrificial layer atop the tunnel barrier must be kept to prevent both free and reference layers from damage, which forms a magnetic “step” in the reference layer and results in the R-H loop shift. The optimized p-MTJs revealed an average offset field of 1.4 Oe and also exhibited steady switching behaviors with one-sigma switching voltage distributions less than 6% for both resistance states. A low switching current density of 1.63 × 106 A/cm2 of an 80-nm p-MTJ was measured with a 10-μs pulse at zero external offset field.