In this work, we investigate effect of the mechanical stress on the performance of magnetic tunnel junctions with perpendicular magnetic anisotropy. We developed a 4-point bending setup that allows us to apply a constant stress over a large substrate area with access to electrical measurements and an external magnetic field. This setup enables us to measure key device performance parameters, such as tunnel magnetoresistance, switching current (I-c(50%)), and thermal stability (Delta), as a function of applied stress. We find that variations in these parameters are negligible: less than 2% over the entire measured range between the zero stress condition and the maximum stress at the point of wafer breakage. Published by AIP Publishing.

STT-MRAM cell design with dual magnetic tunnel junctions (D-MTJ) is a novel design that show a factor of ~2 in switching performance compared to conventional MRAM design. However, the disadvantage is D-MTJ tends to show lower TMR. In this presentation, we demonstrate it's possible to achieve high TMR D-MTJ cell design without compromising its performance gain. We accomplished this by thinning down the secondary MgO barrier. We observe that when thinning down the secondary barrier, the device level TMR reaches a level close to conventional MRAM design of the same free layer while still preserving its high switching performance

We investigate the Gilbert damping and the magnetization switching of perpendicularly magnetized FeCoB-based free layers (FLs) embedded in magnetic tunnel junctions adequate for spin-torque-operated magnetic memories. We first study the influence of the boron content in MgO/FeCoB/Ta systems alloys on their Gilbert damping parameter after crystallization annealing. Increasing the boron content from 20% to 30% increases the crystallization temperature, thereby postponing the onset of elemental diffusion within the FL. This reduction of the interdiffusion of the Ta atoms helps maintaining the Gilbert damping at a low level of 0.009 without any penalty on the anisotropy and the magnetotransport properties up to the 400 °C annealing required in CMOS back-end-of-line processing. In addition, we show that dual MgO FLs of composition MgO/FeCoB/Ta/FeCoB/MgO have a substantially lower damping than their MgO/FeCoB/Ta counterparts, reaching damping parameters as low as 0.0039 for a 3 Å thick tantalum spacer. This confirms that the dominant channel of damping is the presence of Ta impurities within the FeCoB alloy. On optimized tunnel junctions, we then study the duration of the switching events induced by spin-transfer torque. We focus on the sub-threshold thermally activated switching in optimal applied field conditions. From the electrical signatures of the switching, we infer that once the nucleation has occurred, the reversal proceeds by a domain wall (DW) sweeping though the device at a few 10 m/s. The smaller the device, the faster its switching. We present an analytical model to account for our findings. The DW velocity is predicted to scale linearly with the current for devices much larger than the wall width. The wall velocity depends on the Bloch DW width, such that the devices with the lowest exchange stiffness will be the ones that host the DWs with the slowest mobilities.

We have studied experimentally the nanosecond-scale spin-torque-induced switching in perpendicularly magnetized tunnel junctions.

We study whether a direct measurement of the absolute temperature of a Magnetic Tunnel Junction (MTJ) can be performed using the high frequency electrical noise that it delivers under a finite voltage bias. Our method includes quasi-static hysteresis loop measurements of the MTJ, together with the field-dependence of its spin wave noise spectra. We rely on an analytical modeling of the spectra by assuming independent fluctuations of the different sub-systems of the tunnel junction that are described as macrospin fluctuators. We illustrate our method on perpendicularly magnetized MgO-based MTJs patterned in 50*100 nm2 nanopillars. We apply hard axis (in-plane) fields to let the magnetic thermal fluctuations yield finite conductance fluctuations of the MTJ. Instead of the free layer fluctuations that are observed to be affected by both spin-torque and temperature, we use the magnetization fluctuations of the sole reference layers. Their much stronger anisotropy and their much heavier damping render them essentially immune to spin-torque. We illustrate our method by determining current-induced heating of the perpendicularly magnetized tunnel junction at voltages similar to those used in spin-torque memory applications. The absolute temperature can be deduced with a precision of +/- 60 K and we can exclude any substantial heating at the spin-torque switching voltage.

We report the all-optical observation of intrinsic spin dynamics and extraction of magnetic material parameters from arrays of sub-100 nm spin-transfer torque magnetic random access memory (STT-MRAM) devices with a CoFeB/MgO interface. To this end, the interference of surface acoustic waves with time-resolved magneto-optic signals via magneto-elastic coupling was suppressed using a dielectric coating. The efficacy of this method is demonstrated experimentally and via modeling on a nickel nanomagnet array. The magnetization dynamics for both coated nickel and STT-MRAM arrays shows a restored field-dependent Kittel mode from which the effective damping can be extracted. We observe an increased low-field damping due to extrinsic contributions from magnetic inhomogeneities and variations in the nanomagnet shape, while the intrinsic Gilbert damping remains unaffected by patterning. The data are in excellent agreement with a local resonance model and have direct implications for the design of STT-MRAM devices as well as other nanoscale spintronic technologies.

Magnetic tunnel junctions are under active consideration owing to their applications including the potential next generations of spin transfer torque memories. In view of the high symmetry of the magnetic properties and of the high frequency of the system eigenexcitations, out-of-plane magnetized systems are thought to enable a faster and simpler spin-torque-induced switching process than the formerly used in-plane magnetized systems. Unfortunately, the lack of high frequency and low current switching experiments have precluded, so far, the assessment of the speed potential of perpendicular anisotropy systems at low junction dimensions. By time resolving the switching and evidencing its stochastic aspects at the nanosecond-scale, the authors here demonstrate that a complex dynamics happens and persists down to small sized elements with always a very strong asymmetry between the two switching directions. The reversal is explained by the complex interplay between the spatial profile of the stray field emanating from the fixed system of the tunnel junction and the constraint that the switching is preferably initiated from the device edge.

Perpendicular MTJ based Spin-Transfer Torque Magneto-resistive Random Access Memory (STT-MRAM) has been considered as a promising candidate for next generation non-volatile memory due to a combination of fast speed, high endurance, excellent scalability, and ease of integration with the standard CMOS processes. Despite significant progress, there are many remaining hurdles in its productization. They are stemming from a set of requirements imposed on the MTJ performance: simultaneously high magneto-resistance at low RA (Resistance x Area) product, low writing current/voltage and high thermal stability, low write and read error rates and high endurance cycles. We will discuss the inter-dependencies of these parameters, report our progress in meeting these requirements, and our development approaches for future challenges.