A new type of light-controlled non-volatile memory

Ferroic materials, like ferromagnets and ferroelectrics, are central building blocks of modern data storage technology. Yet, current platforms face fundamental limits. Ferromagnets suffer from low switching speed, while the ferroelectric polarization is generally unstable because of the depolarizing response of the surrounding material. A newly discovered class of materials, which do not suffer from these limitations, are so-called ferroaxials. They are formed by microscopic vortices of electric dipoles that can either be arranged in a clockwise or anticlockwise texture, yet they are extremely difficult to manipulate. Researchers at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at CFEL and the University of Oxford have now shown that the bi-stable ferroaxial states can be switched on demand using single ultrashort flashes of circularly polarized terahertz light. This discovery establishes a new mechanism that may lead to light-controlled, ultrafast and stable ferroic switching, and a promising platform for next-generation non-volatile data storage technologies. Full article was published in Science.

 

Terahertz light can reversibly switch an unusual form of structural order in solids—called ferroaxiality—between clockwise and counterclockwise rotational patterns. 

© Jörg Harms (MPSD)

 

Ferroic materials are solids that can be switched between two 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, ferroic materials are widely used in today’s data storage and electronic technologies.

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 PI in the Cluster of Excellence ‘CUI: Advanced Imaging of Matter’ and MPSD director, used circularly polarized terahertz light pulses to switch between clockwise and anti-clockwise ferroaxial domains in a material termed rubidium iron dimolybdate (RbFe(MoO₄)₂. “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 added. “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,” continues 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.”