Record silicon spin alignment, and at room temperature

Researchers at the University of Utah have reported room temperature spintronics in silicon. They describe using a spintronic transistor to align the magnetic spins of electrons in silicon chips. The spin alignment was maintained for 276 trillionths of a second. It might not sound like much, but the researchers claim it as a record.

So how have the done it? The trick, as they call it, is not to cool silicon but to inject electrons with their spins already aligned into the semiconductor.

The researchers explain that they used magnesium oxide as a tunnel barrier to get the aligned electron spins to travel from one nickel-iron electrode through the silicon semiconductor to another nickel-iron electrode. The magnesium oxide stabilises the spin-alignment. If it were not there, the electrons would quickly revert to a random but 50/50 distribution of up and down spin polarisations.

First they deposited an ultra-thin layer of magnesium oxide on a one-inch by 0.3 inch silicon wafer. On to that they deposited 12 transistors which would be used to inject and later detect the spin-aligned electrons.

From the University announcement: The nickel-iron transistor had three contacts or electrodes: one through which electrons with aligned spins were injected into the silicon and detected, a negative electrode and a positive electrode used to measure voltage.

During the experiment, the researchers send direct current through the spin-injector electrode and negative electrode of each transistor. The current is kept steady, and the researchers measure variations in voltage while applying a magnetic field to the apparatus.

"By looking at the change in the voltage when we apply a magnetic field, we can find how much spin has been injected and the spin lifetime," said Ashutosh Tiwari, an associate professor of materials science and engineering at the university.

Tiwari says work like this is needed for spintronics to really take off. "Spintronics will become useful only if we use silicon," says Tiwari.

"Optical methods cannot do that [orient electron spin] with silicon, which is the workhorse of the semiconductor and electronics industry, and the industry doesn't want to retool for another material," Tiwari says.

The paper, published in Applied Physics Letters is here.