Schematic of the spin-injection experiment.
Electronics has revolutionized the modern world, owing to continuous improvements in microprocessor technology since the 1960s. However, this process of refinement is projected to stall in the near future, due to constraints imposed by the laws of physics. Some of these bottlenecks have already taken effect: for instance, the clock rate (the rate at which transistors perform digital operations) has been unable to exceed a few gigahertz, or several operations per nanosecond, for the past twenty years, a limitation stemming from the electrical resistance of silicon. This has led to an increasingly urgent global search for superior alternatives to semiconductor electronics.
One of the leading candidates, called spintronics, is based on the idea of carrying information via the ‘spin’ of electrons, rather than the direct motion of the electrons. Using spin currents to transport information is an exciting prospect because they are expected to operate with lower energy consumption. There are, however, numerous practical difficulties that remain to be overcome. One of the most serious is the ‘spin injection problem’: transferring a spin current from one material to another (e.g., from a magnetic metal to a semiconductor) tends to scramble the spins, destroying the information carried by them.
Now, a team of scientists from Nanyang Technological University (NTU), the National University of Singapore (NUS), and the Agency for Science, Technology and Research (A*STAR) has achieved a breakthrough in the speed and efficiency of spintronics. They have shown that an ultrashort pulse of spin current, lasting less than a picosecond (one trillionth of a second), can be injected from a metal to a semiconductor with amazing efficiency, breaking the previous spin injection record by over 10000 times. These findings were described in a pair of papers recently published in the leading scientific journals Nature Physics and Advanced Materials.
In these experiments, ultrashort spin current pulses are produced by shining a laser pulse on cobalt, a magnetic metal. This generates a swarm of excited electrons with ‘spin-polarisation’, meaning that the spins mostly point in the same direction. The spin-carrying electrons then travel outward, diffusing into other adjacent materials.
Photograph of four of the team members. Left-to-right: Prof. M. Battiato, Prof. J. C. W. Song, Dr. L. Cheng, and Prof. E. Chia. Photo credit: M. Fadly
“We wanted to show that these ultrashort spin current pulses can be used for efficient spin injection,” says Marco Battiato, a Nanyang Assistant Professor at NTU and a member of the research team, who had advanced the first theoretical prediction of this phenomenon in 2016. He notes that the outward diffusion of spin current pulses takes place over several hundred femtoseconds (one femtosecond is one thousandth of a picosecond), up to a thousand times faster than conventional electronic devices operate, making it potentially useful for future high-speed spintronic devices.
The extreme speed of the spin diffusion, though exciting, also makes the phenomenon difficult to study in experiments utilizing present-day electronic technologies. “We had to devise a careful strategy to measure the spin currents flowing into the semiconducting part of the device,” says Associate Professor Elbert Chia, who supervised the experimental part of the project at NTU. “To accomplish this, we used a semiconductor containing heavy elements, which converts spin currents into ultrashort electrical currents. The entire sample then becomes an electromagnetic antenna, emitting radiation at terahertz frequencies (intermediate between microwaves and infrared light). We are able to measure this radiation, then work backwards to figure out the original spin current.”
By carefully selecting the materials in their spintronic device, the team was able to conclusively show that a spin-polarised current was being injected into the semiconductor part. Astonishingly, the strength of this spin current turned out to be over ten thousand times larger than the previous record. “In real devices, such strong spin currents will not be required, so one can get away with considerably weaker excitations,” notes NTU’s Associate Professor Chia. In follow-up experiments, the authors have been able to accurately determine how long it took for the spin current to form and decay away.
“Possibly the most striking aspect is that all this was demonstrated using a very relatively simple metal-semiconductor interface, without the complicated and costly structural engineering one sees in other spintronic experiments,” says Nanyang Assistant Professor Justin Song, a theoretical physicist and National Research Foundation Fellow (NRFF) who was also part of the project.
“These results represent a fundamental step in the development of ultrafast spintronics based on spin current superdiffusion,” says Nanyang Assistant Professor Battiato. In the future, the team envisions this efficient spin injection process becoming one of the key technologies behind high-speed spintronic computers.
This work has been featured in Asian Scientist magazine and Nature Physics.