Traditional electronics utilize semiconductors to transmit data through bursts of charged carriers, such as electrons or holes, to convey binary messages in "1s" and "0s." Spintronic devices can process significantly more information by using the orientation of electrons' magnetic poles-known as spin-as the binary code: an "up" spin represents a 1, and a "down" spin represents a 0.

A major challenge in commercializing spintronics has been setting and maintaining electron spin orientation. Most devices rely on ferromagnets and magnetic fields for this purpose, which is cumbersome and unreliable. Extensive research has shown that carriers lose their spin orientation when moving from materials with high conductivity to those with low conductivity, such as from metallic ferromagnets to undoped silicon and conjugated polymer materials used in most modern semiconductors.

For the first time, scientists have converted existing optoelectronic devices into ones that can control electron spin at room temperature without the need for ferromagnets or magnetic fields.

Typically, optoelectronic devices like LEDs control only charge and light, not the spin of electrons. In a new study led by physicists at the University of Utah and researchers at the National Renewable Energy Laboratory (NREL), scientists replaced the electrodes of commercial LEDs with a patented spin filter made from hybrid organic-inorganic halide perovskite material. These modified LEDs produced circularly polarized light, indicating that the filter had injected spin-aligned electrons into the LED's existing semiconductor structure, marking a major advancement for spintronics technology.

"It's a miracle. For decades, we've been unable to efficiently inject spin-aligned electrons into semiconductors because of the mismatch of metallic ferromagnets and non-magnetic semiconductors," said Valy Vardeny, Distinguished Professor in the Department of Physics and Astronomy at the U and co-author of the paper. "All kinds of devices that use spin and optoelectronics, like spin-LEDs or magnetic memory, will be thrilled by this discovery."

The study was published in the journal Nature on June 19, 2024.

Spin Filters

In 2021, the same team developed a technology that acts as an active spin filter made of two successive layers of material, known as chiral hybrid organic-inorganic halide perovskites. Chirality refers to a molecule's symmetry, where its mirror image cannot be superimposed on itself. Human hands are a classic example; they are mirrors of each other but cannot be perfectly aligned without flipping one.

Certain molecules, such as DNA, sugar, and layers of chiral hybrid organic-halide perovskites, exhibit chiral symmetry. The spin filter works by using a "left-handed" chiral layer to allow electrons with "up" spins to pass while blocking those with "down" spins, and vice versa. Previously, the scientists suggested this discovery could transform conventional optoelectronics into spintronic devices by incorporating the chiral spin filter. The new study successfully demonstrated this concept.

"We took an LED from the shelf. We removed one electrode and put the spin filter material and another regular electrode. And voila! The light was highly circularly polarized," said Vardeny.

Chemists from NERL fabricated the spin LEDs by stacking several layers, each with specific physical properties. The first layer is a common transparent metallic electrode; the second layer blocks electrons with the wrong spin direction, functioning as a chirality-induced spin filter. The spin-aligned electrons then recombine in the third layer, a standard semiconductor used in regular LEDs. The injected spin-aligned electrons cause this layer to emit photons that move in unison along a spiral path, rather than in a conventional wave pattern, producing the LED's characteristic circular polarized electroluminescence.

"This work demonstrates the unique and powerful ability for these emerging 'hybrid' semiconductors to combine and take advantage of the interplay of the distinct properties of organic and inorganic systems," said Matthew Beard, coauthor of the study of NREL. "Here the chirality is borrowed from the organic molecules and provides control over spin while the inorganic component both orients the organic component and provides conductivity or control over charge."

After installing the filter in a standard LED, Xin Pan, a research assistant in the Department of Physics and Astronomy at the University of Utah, confirmed the device operated as intended, specifically through spin-aligned electrons. However, further research is needed to elucidate the precise mechanisms that create polarized spins.

"That's the $64,000 question for a theorist to answer," said Vardeny. "It's really a miracle. And the miracle is without knowing the exact underlying mechanism. That's the beauty of being experimentalist. You just try it."

The researchers believe that other scientists can apply this technique using various chiral materials, such as DNA, in numerous applications.

Research Report:Room-temperature spin injection across a chiral perovskite/III-V interface