Conventional electronics only consider the charge of electron and ignore the electron spin. Spintronics, or spin electronics, study the role played by electron spin in solid state physics. New spin-based phenomena, including spin-transport, spin transfer toque or spin Hall effect have brought new horizons to electronics and magnetism, which have realized lots of potential applications.




(1) Spin-transfer torque

Spin transfer predicted by Slonczewski and Berger has attracted a great deal of attention in recent years. Nanomagnetic device with perpendicular magnetic anisotropy is very important for further magnetic random access memory (MRAM) applications, because it can not only satisfy the thermal stability requirement but also have no limit of cell aspect ratio, and have an advantage of scaling for high packing density. Our group has demonstrated the very first spin momentum transfer in a nanomagnetic device with perpendicular magnetic anisotropy for both free and fixed magnetic layers.

spintronics Spintronics

Figure 1.(Left) Hysteresis loop of the sheet film. The field was applied out-of-plane. (Right) Resistance-current scanning loop. It shows the clear spin transfer torque switching. The current density is about 1.0 × 108 A/cm2.


Related publications:

H. Meng and J. P. Wang, “Spin transfer in nanomagnetic devices with perpendicular anisotripy” Appl. Phys. Lett. 88, 172506 (2006).


(2) Ultrafast spin transfer torque switching

Ultrafast spin transfer torque (STT) switching in the sub-ns regime is one of the key issues for spin transfer torque random access memory (STT-RAM) development. One of the crucial limitations for ultrafast switching is the incubation delay induced by pre-switching oscillation. Several approaches have been proposed to minimize pre-switching oscillations in order to improve the switching speed in spin valves (SVs), such as developing all perpendicular structures, applying hard axis field to set the free layer equilibrium away from the easy axis, and adding an extra perpendicular polarizer. As of now, limited work has been done on sub-nanosecond STT switching in magnetic tunnel junctions (MTJs). We have reported ultrafast switching (165 ps–10 ns) in CoFeB–MgO MTJs with good tunnelling magnetoresistance (TMR) ratio around 100% and large coercivity (100 Oe) under zero bias field. With a basic conventional stack structure, the sample exhibits ultrafast switching in the sub-200 ps regime while maintaining all the requirements for STT-RAM application.


Figure 2. (a) MTJ resistance versus magnetic field loop at room temperature. (b) Switching probability dependence on pulse width with various pulse amplitudes on P-AP state. (c) Pulse voltage as a function of pulse width at 50% switching probability for AP-P and P-AP switching.


Related publications:

1. H. Zhao, et al., “Sub-200 ps spin transfer torque switching in in-plane magnetic tunnel junctions with interface perpendicular anisotropy” Journal of Physics D: Applied Physics. 45, 025001 (2011).

2. H. Zhao, et al., “Low writing energy and sub nanosecond spin torque transfer switching of in-plane magnetic tunnel junction for spin torque transfer random access memory” Journal of Applied Physics 109, 07C720 (2011).


(3) Spin-logic devices

Programmable spintronics logic devices show many potential advantages compared to tranditional semiconductor logic devices, such as nonvolatility, rapid, unlimited reconfigurable variations and low-power consumption. Here, a progrannable spintronics logic device have been designed and fabricated based on a single pinned magnetic tunnel junction (MTJ) element. A current input line C passing through the MTJ element itself was introduced. Two separated input current lines (A and B) could switch the magnetization of the pinned layer under the heat assistance from line C. Full logic functions (AND, OR, NAND, NOR, XOR, and XNOR) can be realized based on a normal pinned and a synthetic pinned MTJ element. A Wheatstone bridge was engineered to read this single MTJ element logic device.


Figure 3. (a) Schematic of a programmable spin-logic device based on a single MTJ element with two independent input lines A and B, a third input line C, and an output line. (b) MTJ with a normal bottom pinned structure for logic gates (AND, OR, NAND, NOR, and XOR). (c) MTJ with a synthetic bottom pinned structure for logic gates and (AND, OR, NAND, NOR, and XNOR).



Figure 4. Working principle of the circult with three MTJs connected in parallel to form the input and connected in series with the output to form the voltage divider circuit. It could realize direct communication between MTJs.


Related publications:

1. J. Wang, H. Meng and J. P. Wang. “Programmable spintronics logic device based on a magnetic tunnel junction element” Journal of Applied Physics 97, 10D509 (2005).

2. H. Meng, J. Wang and J. P. Wang, “A spintronics full adder for magnetic CPU” IEEE Electron Device Letters, 26, 360 (2005).

3. A. Lyle, et al., “Direct communication between magnetic tunnel junctions for nonvolatile logic fan-out architecture” Applied Physics Letters 97, 152405 (2010).


(4) Spin-orbit Torque Characterization from Topological Insulators

Three-dimensional (3D) topological insulators are known for their strong spin–orbit coupling (SOC) and the existence of spin-textured surface states that might be potentially exploited for “topological spintronics.” Here, we use spin pumping and the inverse spin Hall effect to demonstrate successful spin injection at room temperature from a metallic ferromagnet (CoFeB) into the prototypical 3D topological insulator Bi2Se3. The spin pumping process, driven by the magnetization dynamics of the metallic ferromagnet, introduces a spin current into the topological insulator layer, resulting in a broadening of the ferromagnetic resonance (FMR) line width.


Figure 5. Spin Hall effect generated by an topological insulator Bi2Se3.

Related publications:

M. Jamali, et al. “Giant Spin Pumping and Inverse Spin Hall Effect in the Presence of Surface and Bulk Spin− Orbit Coupling of Topological Insulator Bi2Se3” Nano Letters 15, 7126 (2015).