Sunny Yan Hwee Lua

Annealing effects on CoFeB-MgO magnetic tunnel junctions with perpendicular anisotropy

We studied annealing effects on perpendicular anisotropy in CoFeB-MgO magnetic tunnel junctions. The results show that annealing is an effective method to improve the perpendicular anisotropy of a CoFeB-MgO system. It is found that a thicker CoFeB layer requires a higher annealing temperature to buildup its perpendicular anisotropy. However, perpendicular anisotropy could be seriously degraded if the annealing temperature is more than 350 °C. Our study suggests that CoFeB thickness should be optimized so that the required annealing temperature window for perpendicular anisotropy could match the annealing temperature for high magnetoresistance. In this work, the perpendicular anisotropy energy density of 2.5 × 106 erg/cm3 was achieved with tunnel magnetoresistive value exceeding 70%. The use of CoFeB films will enable the development of high density nonvolatile memory with size down to 30 nm.

Reduction of switching current by spin transfer torque effect in perpendicular anisotropy magnetoresistive devices (invited)

Spin transfer torque-based magnetic random access memory with perpendicular magnetic anisotropy (PMA) provides better scalability and lower power consumption compared to those with in-plane anisotropy. Spin transfer torque switching in magnetoresistive spin valves with PMA is investigated. The hard layer is made of (Co/Pd) multilayer, whereas the soft layer is a lamination of (CoFe/Pd) and (Co/Pd). By the insertion of an in-plane spin polarizer adjacent to the perpendicular anisotropy free layer, thus creating a modified-dual spin valve, a significant reduction of about 40% in the current density required for spin torque transfer switching was observed. By using a spin polarized current with different pulse widths down to 10 ns, the barrier energy EB in 100-nm-diameter devices was found to be reduced from 1.1 to 0.43 eV. Besides the reduction of switching current density in a device with PMA, the new structure shows a clear increase in magnetization switching speed as revealed by micromagnetic simulation.

Spin transfer torque switching for multi-bit per cell magnetic memory with perpendicular anisotropy

A novel multi-bit dual pseudo spin valve with perpendicular magnetic anisotropy is investigated for spin transfer torque (STT) switching. The structure consists of two free layers and one reference layer, and all are based on Co/Pd multilayer. STT switching of the multi-bit device shows distinct four resistance levels. The selection of intrinsic properties of each ferromagnetic layer can be controlled for distinct separation of the resistance levels as well as the respective STT switching current. Reversible transitions between different states can be achieved by a pulsed current, in which its critical value is found to be linearly dependent on pulse duration.

Low current density induced spin-transfer torque switching in CoFeB-MgO magnetic tunnel junctions with perpendicular anisotropy

We present the thickness effects of CoFeB free layer on tunnelling magnetoresistive (TMR), perpendicular magnetic anisotropy (PMA) and spin-transfer torque (STT) in CoFeB–MgO based magnetic tunnel junctions (MTJs). It is found that a post-annealing process could significantly improve both TMR and PMA of the MTJ systems. When the free layer thickness is reduced from 1.3 nm to 1 nm, TMR continuously decays from 80% to 20%. On the other hand, PMA is maximized for a 1.28 nm free layer, above which demagnetization becomes stronger and results in lower PMA. If the free layer thickness is very small, dead layer effect could damage interfacial perpendicular anisotropy and PMA is reduced as a consequence. For STT-induced magnetization switching, the lowest intrinsic critical switching current density (Jc0) of 2.1 MA cm−2 is achieved at a free layer thickness of 1.16 nm, accompanied by a TMR of 52% and product of resistance and area (RA) of 16 Ω µm2. Further increasing the free layer thickness will first enhance Jc0 and then reduce it due to the balance between PMA and the total free layer volume. STT studies suggest that the CoFeB free layer thickness should be optimized to make a trade-off among large PMA, high TMR and low switching current density in perpendicular CoFeB–MgO MTJ systems.

Annealing temperature window for tunneling magnetoresistance and spin torque switching in CoFeB/MgO/CoFeB perpendicular magnetic tunnel junctions

Annealing temperature (Ta) and free layer thickness dependencies of magnetic properties and spin-transfer torque switching were investigated in CoFeB-MgO based magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA). Annealing process was found to be critical to buildup PMA. As Ta increases, switching field of free layer and reference layer is enhanced first then drops, corresponding to the improvement and collapse of PMA in both layers. However, it should be noted that PMA of free layer and the tunneling magnetoresistive (TMR) are maximized at different Ta zones. Spin transfer torque study pointed out that switching current density (Jc) depends on the combined effects from PMA, spin polarization, and saturation magnetization, which all depend on Ta values. Thickness dependence study revealed that Jc relies on the competing results of the thickness and PMA. The lowest critical switching current density achieved is 2.1 MA/cm2, accompanied with a TMR around 52% at room temperature.

Magnetization Reversal Process of Tri-Layer Readers for Ultrahigh Density Data Storage

We study the magnetization reversal process of tri-layer readers using micromagnetic simulations. The magnetoresistance response of tri-layer readers has been shown to be distinctive from the conventional spin valve giant magnetoresistance sensors. We look into the challenges for future high storage density when scaling down the read sensor size. The possible sensor reversal process control by engineering the sensor dimensions and material properties has been systematically investigated. We further observe an interesting dependence of magnetization states on the media bit transition length, which could be attributed to the small but finite physical separation between the two coupled ferromagnetic films. The findings in this work are useful for the design of tri-layer sensors for ultrahigh storage density in hard disk drives.

MRAM Device Incorporating Single-Layer Switching via Rashba-Induced Spin Torque

We designed and modeled a nonvolatile memory device that utilizes the Rashba spin-orbit coupling (SOC) to write data onto a free ferromagnetic (FM) layer and uses the tunneling magnetoresistive (TMR) effect for data read-back. The magnetic RAM (MRAM) device consists of a free (switchable) FM multilayer stack, in which a large internal electric field is induced at the interfaces between the oxide and the FM layer. In the FM layer, data writing by magnetization switching occurs via the Rashba-induced spin torque, while the data reading process in the system could be fulfilled via the current-perpendicular-to-plane TMR response. A general equation of motion for the local moments has been obtained by formally deriving the SU(2) spin-orbit gauge field arising due to SOC and the critical current density is estimated to be 1.2 ×108 A/cm2. Micromagnetic simulations were performed to demonstrate the Rashba-induced switching mechanism. By choosing or fabricating alloys with a lower magnetocrystalline anisotropy and enhancing the Rashba coupling strength via surface or interfacial engineering, the critical current may be further reduced to well below 107 A/cm2, a level that may enable the practical realization of a single-layer Rashba-induced magnetization switching memory.

Compact spin transfer torque non-volatile flip flop design for power-gating architecture

This paper proposes a compact spin transfer torque non-volatile flip-flop (STT-NVFF) design. The proposed NVFF adds four transistors and two complementary magnetic tunnel junctions (MTJs) over a standard volatile flip-flop with only 18% area overhead. The NVFF utilizes a low power/ fast switching MTJ that permits the elimination of the write circuitry existing in conventional STT-NVFFs. The proposed NVFF is at least 80% smaller area than conventional STT-NVFFs that uses write circuitry with, at least, the same energy efficiency. It achieves a low backup energy of 111 fJ and restore energy of 6.9 fJ within 3 ns and 0.16 ns respectively. Moreover, it realizes a 72% reduction in break-even point (BEP) and a 10% area reduction compared to an STT-NVFF employing the latch as a writer.

An Optimized Resistance Characterization Technique for the Next Generation Magnetic Random Access Memory

This paper presents an accurate resistance characterization technique for magnetic random access memory (MRAM), such as STT-MRAM. By annulling the mismatch effect of CMOS transistors, this technique produces a resistance distribution profile of MRAM devices in a large array that reflects the actual device statistics. A 1 Kb array of MTJs with an intrinsic 3σ low resistance state distribution modeled with Verilog-A provides the reference device statistics. Monte Carlo simulation results of popular array configurations show the methods generic advantages of tightened distributions of the mean resistance value and standard deviation (SD) of the characterized 1 Kb devices than the reference method. Technology scaling study shows the sustainability of the proposed method with an improvement of the SD of the mean resistance distribution by at least 37.6%. The mean and SD of the standard deviation distribution were improved by at least 25.1% and 67.2% as compared to the reference method, respectively.