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Ferroic materials such as ferromagnets and ferroelectrics underpin modern data storage, yet face limits: They switch slowly, or suffer from unstable polarization due to depolarizing fields respectively. A new class, ferroaxials, avoids these issues by hosting vortices of dipoles with clockwise or anticlockwise textures, but are hard to control.

Researchers at the Max-Planck-Institute for the Structure and Dynamics of Matter (MPSD) and the University of Oxford now show that bi-stable ferroaxial states can be switched with single flashes of polarized terahertz light. This enables ultrafast, light-controlled and stable switching, a platform for next-generation non-volatile data storage. The work is in the journal Science.

Modern society relies on digital technologies, where all information is fundamentally encoded in a binary system of 0s and 1s. Consequently, any physical system capable of reliably switching between two stable states can, in principle, serve as a medium for digital data storage.

Ferroic materials are solids that can be switched between two such stable states. The most familiar examples are ferromagnets, which can be magnetized in opposite directions, and ferroelectrics, which can hold opposite electric polarizations. Because these states are readily switchable by magnetic or electric fields, these ferroic materials are widely used in today's data storage and electronic technologies.

However, these systems also come with drawbacks: They are vulnerable to external influences—such as strong magnetic fields near a hard drive—and tend to degrade over time. This makes the search for alternative data storage technologies highly attractive.

Ferroaxial materials are a recent addition to the ferroic family. Instead of magnetic or electric states, these solids host vortices of electric dipoles that can be oriented in two opposite directions without creating a net magnetization nor electric polarization. These are very stable and are unaffected by external fields, but for the same reason very difficult to control, which has limited their exploration until now.

The research team, led by Andrea Cavalleri, used circularly polarized terahertz light pulses to switch between clockwise and anti-clockwise ferroaxial domains in a material termed rubidium iron dimolybdate (RbFe(MoOâ‚„)â‚‚).

"We take advantage of a synthetic effective field that arises when a terahertz pulse drives ions in the crystal lattice in circles," says Zhiyang Zeng, lead author of this work. "This effective field is able to couple to the ferroaxial state, just like a magnetic field would switch a ferromagnet or an electric field would reverse a ferroelectric state," he adds.

"By adjusting the helicity, or twist, of the circularly polarized light pulses, we can selectively stabilize a clockwise or anti-clockwise rotational arrangement of the electric dipoles," states fellow author Michael Först. "In this way enabling information storage in the two ferroic states. Because ferroaxials are free from depolarizing electric or stray magnetic fields, they are extremely promising candidates for stable, non-volatile data storage."

"This is an exciting discovery that opens up new possibilities for the development of a robust platform for ultrafast information storage," says Andrea Cavalleri. "It also shows how circular phonon fields, first achieved in our group in 2017, are emerging as a new resource for the control of exotic materials phases."