Perovskite properties put scientists in a spin

A team at the University of Utah has discovered that a class of ‘miracle materials’ called organic-inorganic hybrid perovskites could be a game changer for future spintronic devices.

Spintronics uses the direction of the electron spin – either up or down – to carry digital information. A spintronic device can process exponentially more data than traditional electronics that use the ebb and flow of electrical current to generate digital instructions, but physicists have struggled to make spintronic devices a reality.

The new study, reported in a paper in Nature Physics, is the first to show that organic-inorganic hybrid perovskites are a promising material class for spintronics. The researchers discovered that perovskites possess two contradictory properties necessary to make spintronic devices work – the electrons' spin can be easily controlled, and the spin direction can be maintained for long enough to transport information, a property known as spin lifetime.

"It's a device that people always wanted to make, but there are big challenges in finding a material that can be manipulated and, at the same time, have a long spin lifetime," says Sarah Li, assistant professor in the Department of Physics & Astronomy at the University of Utah and lead author of the study. "But for this material, it's the property of the material itself that satisfies both."

Organic-inorganic hybrid perovskites are already famous in scientific circles for being amazingly efficient at converting sunlight into electricity. "It's unbelievable. A miracle material," says Valy Vardeny, distinguished professor in the Department of Physics & Astronomy and co-author of the study, whose lab studies perovskite solar cells. "In just a few years, solar cells based on this material are at 22% efficiency. And now it has this spin lifetime property. It's fantastic."

The material's chemical composition makes it an unlikely candidate for spintronics, however. The hybrid perovskite inorganic frame is made of heavy elements, which is actually good for spintronics, because the heavier the atom, the easier it is to manipulate the electron spin. But other forces also influence the spin. When the atoms are heavy, the spin lifetime is usually short, explains Li.

"Most people in the field would not think that this material has a long spin lifetime. It's surprising to us, too," says Li. "We haven't found out the exact reason yet. But it's likely some intrinsic, magical property of the material itself."

Cellphones, computers and other electronic devices have silicon transistors that control the flow of electrical currents like tiny dams. As these devices get more compact, transistors must handle the electrical current in smaller and smaller areas.

"The silicon technology, based only on the electron charge, is reaching its size-limit," says Li, "The size of the wire is already small. If it gets any smaller, it's not going to work in a classical way that you think of."

"People were thinking, 'How do we increase the amount of information in such a small area?'" adds Vardeny. "What do we do to overcome this limit?"

Spintronics uses the spin of the electron itself to carry information. Electrons are basically tiny magnets orbiting the nucleus of an element. Just as the Earth has its own orientation relative to the sun, electrons have their own spin orientation relative to the nucleus, and this can be aligned in two directions: ‘up’, which represents a one; and ‘down’, which represents a zero. Physicists relate the electron's ‘magnetic moment’ to its spin.

Adding spin to traditional electronics would allow it to process exponentially more information. "With spintronics, not only have you enormously more information, but you're not limited by the size of the transistor. The limit in size will be the size of the magnetic moment that you can detect, which is much smaller than the size of the transistor nowadays," says Vardeny.

Tuning an electron spin is like tuning a guitar, but with a laser and a lot of mirrors. First, the researchers formed a thin film of the hybrid perovskite methyl-ammonium lead iodine (CH₃NH₃PbI₃) and placed it in front of an ultrafast laser that shoots very short light pulses 80 million times a second. The researchers are the first to use light to set the electron's spin orientation and to observe the spin precession in this material.

They split the laser into two beams. The first beam hits the film to set the electron spin in the desired direction. The second beam bends through a series of mirrors like a pinball machine before hitting the perovskite film at increasing time intervals to measure how long the electron holds the spin in the prepared direction.

They found that the perovskite has a surprisingly long spin lifetime – up to nanosecond. The spin flips many times during one nanosecond, which means a lot information can be easily stored and manipulated during that time.

Once they determined the long spin lifetime, the researchers tested how well they could manipulate the spin with a magnetic field.

"The spin is like the compass. The compass spins in this magnetic field perpendicular to that compass, and eventually it will stop spinning," says Li. "Say you set the spin to 'up’, and you call that 'one'. When you expose it to the magnetic field, the spin changes direction. If it rotated 180°, it changes from one to zero. If it rotated 360°, it goes from one to one." They found that they could rotate the spin more than 10 turns by exposing the electron to different strengths of magnetic field.

According to Vardney, the potential for this material is enormous: it could process data faster and increase random-access memory. "I'm telling you, it's a miracle material," he says.

This story is adapted from material from the University of Utah, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.