To make a functional spintronic device, it’s important to measure spin direction accurately. Right now, electrons with aligned spins can be injected into a material, but determining whether those electrons maintain their spins is important if that spin is to be controlled in a device.
In their experiment, Boehme and his colleagues read the spin in a polymer OLED by measuring the current coming out of it. They attached electrodes to the device and bombarded it with a microwave pulse every 500 microseconds.
Boehme explains that spin can be thought of as a tiny bar magnet, pointing in a certain direction. In an LED, when the voltage is applied in a certain direction, negatively charged electrons and positively charged holes form pairs. Each pair subsequently decays, or loses some energy, emitting a photon. Because the electron and hole each have a specific spin, the electron-hole pair can take one of four spin states: up-up, up-down, down-up, and down-down. “Only one of these four can decay and produce light,” Boehme says. This means that OLEDs made from the polymer will probably not reach efficiencies higher than 25 percent, he adds.
At the same time, a particle’s spin direction can change. So among all the electron-hole pairs formed in the LED material, Boehme says, “one of those that cannot [emit light] can all of a sudden flip and turn into one of the four states that can produce light.” More light-emitting states increase the material’s light output, but since the electrons and holes get annihilated, the current decreases.
The microwave pulse changes the spins in the polymer OLED in a manner determined by the length and frequency of the pulse. The result is to alternately create more or fewer light-emitting states, decreasing and increasing the current. The greater the frequency of the microwave pulse, the faster the current increases and decreases.
“We have shown that when you coherently manipulate spins, when you turn them around from up to down and everything in between, you can see the imprint of spin motion on the current that you measure,” Boehme says.
The researchers believe that their work could also help improve OLEDs. Introducing impurities into the polymer materials could change the rate at which electrons in the material flip their spins, Boehme says. That could create more and more light-emitting states, increasing the efficiency of OLED over 25 percent and lead to brighter devices.