Critical Current Distribution in Spin Transfer Switched Magnetic Tunnel Junctions

The spin transfer switching current distribution within a cell was studied in magnetic tunnel junction based structures having alumina barriers with resistance-area product (RA) of 10 to 30 Ohm-um2 and tunneling magneto-resistance (TMR) of ~20%. These were patterned into current perpendicular to plane configured nano-pillars having elliptical cross-sections of area ~0.02 um2. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse width, was found to be 7.5% and 3.5% for cells with thermal factor (KuV/kBT) of 40 and 65 respectively. The distribution width did not change significantly for pulse widths between 1 s and 4 ms. An analytical expression for probability density function, p(I/Ico) was derived considering the thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within cell critical current distribution width.Comment: 12 pages, 4 figure

Spin-Polarized Current Induced Torque in Magnetic Tunnel Junctions

We present tight-binding calculations of the spin torque in non-collinear magnetic tunnel junctions based on the non-equilibrium Green functions approach. We have calculated the spin torque via the effective local magnetic moment approach and the divergence of the spin current. We show that both methods are equivalent, i.e. the absorption of the spin current at the interface is equivalent to the exchange interaction between the electron spins and the local magnetization. The transverse components of the spin torque parallel and perpendicular to the interface oscillate with different phase and decay in the ferromagnetic layer (FM) as a function of the distance from the interface. The period of oscillations is inversely proportional to the difference between the Fermi-momentum of the majority and minority electrons. The phase difference between the two transverse components of the spin torque is due to the precession of the electron spins around the exchange field in the FM layer. In absence of applied bias and for a relatively thin barrier the perpendicular component of the spin torque to the interface is non-zero due to the exchange coupling between the FM layers across the barrier.Comment: 6 pages, 3 figure

Spin Transfer Switching and Spin Polarization in Magnetic Tunnel Junctions with Mgo and Alox Barriers

We present spin transfer switching results for MgO based magnetic tunneling junctions (MTJs)with large tunneling magnetoresistance (TMR) ratio of up to 150% and low intrinsic switching current density of 2-3 x 10 MA/cm2. The switching data are compared to those obtained on similar MTJ nanostructures with AlOx barrier. It is observed that the switching current density for MgO based MTJs is 3-4 times smaller than that for AlOx based MTJs, and that can be attributed to higher tunneling spin polarization (TSP) in MgO based MTJs. In addition, we report a qualitative study of TSP for a set of samples, ranging from 0.22 for AlOx to 0.46 for MgO based MTJs, and that shows the TSP (at finite bias) responsible for the current-driven magnetization switching is suppressed as compared to zero-bias tunneling spin polarization determined from TMR.Comment: To appear in Appl. Phys. Lett. soo

The characteristic and size–frequency distribution of rocks at the Zhurong landing site, Mars

The rock characteristic and size–frequency distribution (SFD) on Mars are important for understanding the geologic and geomorphic history of the surface, for evaluating the trafficability of roving, and for planning the potential infrastructure construction. Tianwen-1, China’s first autonomous Mars exploration mission, formed an excavated depression during touchdown, which has been the deepest depression on the Martian surface so far compared with others. According to the images captured using the Navigation and Terrain Cameras (NaTeCams) onboard the rover, Zhurong, the SFD of rocks is calculated and compared inside the excavated depression, within and out of the blast zone. For the first time, the rock size distribution inside the excavated depression is obtained, exposing the geological features of the shallow subsurface on Mars at a depth of tens of centimeters, which will surely be important for future drilling missions. It is found that the rock abundance in the depression is smaller than the original abundance on the surface, and the distribution of rocks in the blast zone on the surface is greatly influenced by the touchdown. In addition, based on the fractal dimension of rock sizes, the rocks (>10 mm) at the shallow subsurface of the Zhurong landing site may experience two different geological processes

Phenomenology of plume–surface interactions and preliminary results from the Tianwen-1 landing crater on Mars

The plume–surface interaction (PSI) is a common phenomenon that describes the environment surrounding the landers resulting from the impingement of hot rocket exhaust on the regolith of planetary bodies. The PSI will cause obscuration, erosion of the planetary surface, and high-speed spreading of dust or high-energy ejecta streams, which will induce risks to a safe landing and cause damage to payloads on the landers or to nearby assets. Safe landings and the subsequent scientific goals of deep-space exploration in China call for a comprehensive understanding of the PSI process, including the plume flow mechanics, erosion mechanism, and ejecta dynamics. In addition, the landing crater caused by the plume provides a unique and insightful perspective on the understanding of PSI. In particular, the PSI can be used directly to constrain the composition, structure, and mechanical properties of the surface and subsurface soil. In this study, we conducted a systematic review of the phenomenology and terrestrial tests of PSI: we analyzed the critical factors in the PSI process and compared the differences in PSI phenomena between lunar and Martian conditions; we also reviewed the main erosion mechanisms and the evolution and development of terrestrial tests on PSI. We discuss the problems with PSI, challenges of terrestrial tests, and prospects of PSI, and we show the preliminary results obtained from the landing crater caused by the PSI of Tianwen-1. From analysis of the camera images and digital elevation model reconstructions, we concluded that the landing of Tianwen-1 caused the deepest crater (depth > 40 cm) on a planetary surface reported to date and revealed stratigraphic layers in the subsurface of Martian soil. We further constrained the lower bounds of the mechanical properties of Martian soil by a slope stability analysis of the Tianwen-1 landing crater. The PSI may offer promising opportunities to obtain greater insights into planetary science, including the subsurface structure, mineral composition, and properties of soil

DataSheet1_The characteristic and size–frequency distribution of rocks at the Zhurong landing site, Mars.pdf

The rock characteristic and size–frequency distribution (SFD) on Mars are important for understanding the geologic and geomorphic history of the surface, for evaluating the trafficability of roving, and for planning the potential infrastructure construction. Tianwen-1, China’s first autonomous Mars exploration mission, formed an excavated depression during touchdown, which has been the deepest depression on the Martian surface so far compared with others. According to the images captured using the Navigation and Terrain Cameras (NaTeCams) onboard the rover, Zhurong, the SFD of rocks is calculated and compared inside the excavated depression, within and out of the blast zone. For the first time, the rock size distribution inside the excavated depression is obtained, exposing the geological features of the shallow subsurface on Mars at a depth of tens of centimeters, which will surely be important for future drilling missions. It is found that the rock abundance in the depression is smaller than the original abundance on the surface, and the distribution of rocks in the blast zone on the surface is greatly influenced by the touchdown. In addition, based on the fractal dimension of rock sizes, the rocks (>10 mm) at the shallow subsurface of the Zhurong landing site may experience two different geological processes.</p

High-spin Co3+ in cobalt oxyhydroxide for efficient water oxidation

Abstract Cobalt oxyhydroxide (CoOOH) is a promising catalytic material for oxygen evolution reaction (OER). In the traditional CoOOH structure, Co3+ exhibits a low-spin state configuration ( $${t}_{2{{{{{\rm{g}}}}}}}^{6}{e}_{{{{{{\rm{g}}}}}}}^{0}$$ t 2 g 6 e g 0 ), with electron transfer occurring in face-to-face $${t}_{2{{{{{\rm{g}}}}}}}^{*}$$ t 2 g * orbitals. In this work, we report the successful synthesis of high-spin state Co3+ CoOOH structure, by introducing coordinatively unsaturated Co atoms. As compared to the low-spin state CoOOH, electron transfer in the high-spin state CoOOH occurs in apex-to-apex $${e}_{{{{{{\rm{g}}}}}}}^{*}$$ e g * orbitals, which exhibits faster electron transfer ability. As a result, the high-spin state CoOOH performs superior OER activity with an overpotential of 226 mV at 10 mA cm−2, which is 148 mV lower than that of the low-spin state CoOOH. This work emphasizes the effect of the spin state of Co3+ on OER activity of CoOOH based electrocatalysts for water splitting, and thus provides a new strategy for designing highly efficient electrocatalysts