The German researchers used terahertz laser pulses to twist the lattice to favor a particular ground state

Yttrium titanate crystals are relatively weak magnets. At 0K, tiny quantum fluctuations weaken the microscopic arrangement of electrons and spins on which the material's magnetism depends. At 27K, magnetism disappears completely. Andrea Cavalleri of the Max Planck Institute for the Structure and Dynamics of Matter in Germany and his collaborators used strong terahertz laser pulses to not only boost the magnetism of YTiO3, but also preserve it at temperatures higher than 80K. The method allows the researchers to create light-activated switches that can create or destroy magnetism. That ability could eventually lead to storage devices being able to encode a 0 or a 1 "faster than ever before," Cavalleri said.

The low temperature fluctuations that inhibit the magnetism of YTiO3 occur because the crystal alternates randomly between three different ground states. In turn, these states occur because titanium atoms have a valence electron that can choose from three different orbitals. Any alignment between the valence electron spins is lost as the crystal alternates between these degenerate states. "The name of the game is to eliminate those degradations and make it one of the three tracks," Cavalleri said. Nature doesn't give you things like that."

The current work stems from a 2007 study in which Cavalleri and his team reported using terahertz laser pulses to twist a lattice in favor of a specific ground state. The pulse excites a specific quantized vibration that alters the electronic state of the crystal, causing the resistance transient to drop by five orders of magnitude.

For their new experiment, the researchers chose three laser frequencies that were each coupled to one of several possible lattice distortions in YTiO3. Using a magneto-optical pump probe device, they examined how each excitation affected the crystal's structure and magnetism. Specifically, they looked to see if the polarization of the light reflected off the crystal changed when viewed from the opposite direction. A clockwise/counterclockwise shift in the polarization of the reflected light would be a clear indication of the invariance of the time inversion, which occurs only in the presence of a magnetic order.

They found that an ultrafast laser pulse tuned to the 9THz phonon frequency would cause the YTiO3 crystal to become fully magnetized at just above zero K. This order doesn't disappear at 27K, but stays stable at at least 80K, the highest temperature they've ever measured. What's more, the magnetism lasted for many nanoseconds, six orders of magnitude longer than femtosecond-long laser pulses. The team attributes this persistent state to the stability of structural deformations caused by laser deposition energy.

Alexey Kimel, who studies ultrafast magnetism at Radboud University in the Netherlands, says the temperature increase that would allow the spin order to persist is huge. Other researchers who have used light-driven methods to achieve ordering in magnetic semiconductors have reported temperature increases of about 1 percent. Cavalleri's team, by contrast, saw 300 percent growth. David Hsieh of the California Institute of Technology, who studies ultraffast control of quantum materials, points out that most previous efforts have focused on suppressing, or converting, pre-existing magnetic sequences. "This new work suggests that using light to suppress electron fluctuations may be a general strategy for enhancing the tendency of electrons to order in materials."

Source: NetEase