KEYWORDS: Solar concentrators, Magnetic tunnel junctions, Magnetic sensors, Analytic models, Sensors, Magnetism, Space operations, Shape analysis, Ruthenium, Annealing
To develop sensors for sub-nT magnetic field detection, we fabricate micron-sized magnetic tunnel junctions with symmetric response. Their sensitivity is improved by carefully tuning the exchange field of the soft-pinned free-layer. Furthermore, the use of a flux concentrator significantly enhances the sensor sensitivity at low field. The flux concentrator design is optimized to reach high gain values thanks to a particularly narrow air-gap of 10 µm. We demonstrate an amplification gain of a factor 350 with a flux concentrator realized by electro-chemical deposition. This result opens the way to the development of integrated magnetic sensors for ultra-low field detection.
Magnetic tunnel junctions with perpendicular magnetic anisotropy for Magnetic Random Access Memory need to combine high speed and low critical switching current. Higher spin transfer torque (STT) write efficiency is required. This can be achieved introducing a switchable assistance layer, which can be designed to maximize the STT efficiency independently of the switching direction. At the same time, the assistance layer also increases the retention in standby. The reversal process was confirmed with time-resolved measurements. The outlook for scaling to the sub-20 nm diameter range will also be reviewed looking at STT driven switching in perpendicular shape anisotropy cells.
We present a detailed investigation of a novel platform for integration of spintronic memory elements and a photonic network, for future ultrafast and energy-efficient memory. We designed and fabricated magnetic tunnel junction (MTJ) structures based on (Tb/Co)x5 multilayer stack with optically switchable magnetization. Optical single-pulse measurements allowed us to estimate the value of the stray field present in the parallel configuration, which prevents the structure from all-optical switching. We performed numerical calculations based on the Finite Difference Time Domain method and ellipsometry measurements of (Tb/Co)x5 to compute the absorption by the MTJ structure. Simulation results are in good agreement with the experimental measurements, where we implemented a thermal model to estimate effective absorption in the pillar. These estimations showed up to 14% absorption of the incident optical power in 300-nm-wide MTJ. Moreover, we designed and realized an integrated optical network with focusing structures to efficiently guide and couple the light into the MTJs. We show a chain of necessary steps to obtain the threshold value of the switching energy, and our results presenting a path forward for full system integration of optically switchable MRAM technology.
In the context of miniaturization, energy conservation, smart devices and IOT, magnetic field sensors based on magnetic tunnel junctions (MTJ) constitute an attractive choice, with small size, very high intrinsic sensitivity and low power consumption. The specific sensor response curve is determined by the magnetization configuration and hysteresis loop behavior of the soft (sensing) layer of the MTJ. A possible implementation is the vortex-based sensor, in which the junction sensing layer magnetization is in a vortex configuration at zero field, consisting of a small central core with out-of-plane magnetization and an in-plane magnetization rotating around the core with a clockwise or counterclockwise direction. Depending on the sensor element geometry, the vortex can be the natural stable spin configuration of the sensor, with the lowest energy at remanent state. This is the case for circular dots of soft ferromagnetic materials with sufficient thickness. Important characteristics of sensors include the sensitivity, the linearity, the measurement range and the hysteresis. Circular vortex-state sensors typically exhibit much lower sensitivity compared to uniform-magnetization sensors but are naturally highly linear and exhibit low hysteresis in a limited field range.
Here we use micro-magnetic simulations to study the effect of sensor shapes, volume defects, and perimeter defects on the magnetic behavior of a NiFe sensing layer, including hysteresis and vortex stability. In the case of circular dots, we show that the application of a large field results in a hysteresis near zero field, which originates from the combined role of volume defects inside the free layer (modeled by vacancies in the free layer) and perimeter defects (modeled by deviations from circular shape). We explore multiple geometries, sizes and thicknesses, and correlate sensor shape asymmetries, sensitivity axis direction, and hysteresis. The detailed vortex behavior following cycles of vortex expulsion and nucleation is also examined for selected asymmetric dot shapes. The role of different kinds of volume and perimeter defects is explored in determining overall sensor properties. A transverse field bias (transverse to sensing direction) is also shown to result in an increase of hysteresis. Finally, we study the effect of the dot aspect ratio on the sensor sensitivity and linearity and show the impact of the direction of the applied field on the extracted parameters.
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