Researchers at Cornell used theoretical techniques to predict that using intense mid-infrared laser light on a titanium perovskite can dynamically induce a magnetic phase transition – taking the material from its ferromagnetic ground state to a hidden anti-ferromagnetic phase. This dramatic shift could have useful applications, particularly in optical information processing.
“It would be a kind of optical switch,” the researchers said. “You have a material where it’s magnetic and ‘non-magnetic.’ It’s going between those two states with light”.
Benedek’s research group uses theory and computational simulations to explore an area called “functional materials,” which do something useful when you apply an external stimulus, such as temperature, pressure, an electric or magnetic field – or, in this case, light.
Previous research into light-induced phase transitions, they said, usually focused on hitting the material with a beam so high in energy that it excites the electrons to a degree that fundamentally changes the material, say from an insulator to a conductor. “That’s interesting, but it’s a pretty blunt hammer.... With our work, we are interested in light pulses that are much lower in energy, and you are exciting specific patterns of atomic displacements in the material. Hopefully, not touching the electrons at all.”
And, they said, depending on how the atoms are displaced, you change the properties of the material in different ways. “It’s much more precise” .
Both states involved in this work are magnetic. In the ferromagnetic state, the electrons’ spins are all aligned the same way and the magnetism is measurable away from the surface. With an anti-ferromagnet, the spins are aligned in such a way (both up and down) that they cancel each other out. These two states are perfectly suited for information processing, the team said.
The other advantage to this type of material manipulation is that it happens very fast. “You’re using light to give you very subtle structural changes,” the researchers explained, “but these changes are leading to a drastic change in the magnetic state.” “They’re happening really, really quickly – on the order of hundreds of femtoseconds [less than a trillionth of a second]. And you can go back and forth [between the two magnetic states].”
Future work will explore light-induced phase transition in other perovskites, including lithium niobate, which is used in mobile phones, piezoelectric (pressure) sensors and optical modulators, which are used in lasers.