Scientists use terahertz pulses of light to induce ferromagnetic states, which could improve photo-controlled storage and high-speed computing

A team of scientists from Germany and the US have shown for the first time that terahertz (THz) pulses of light can stabilise ferromagnetism in crystals at more than three times the usual transition temperature. By using terahertz pulses of light to induce ferromagnetism in crystals at this particular transition temperature, this technique paves the way for light-controlled memory, computing devices with greater speed and efficiency.

As the team reports in Nature, pulses only a few hundred femtoseconds long (millionths of a second in billionths of a second) were used to induce a ferromagnetic state at high temperatures in the rare earth titanate YTiO3, which persisted for many nanoseconds after light struck it. Below the equilibrium transition temperature, the laser pulse still strengthens the existing magnetic state, increasing the magnetization to its theoretical limit.

Using light to control magnetism in solids would be one potential future application: today computers rely mainly on the flow of electric charges to process information. In addition, digital memory storage devices make use of magnetic potential that must switch external magnetic fields. Both of these aspects limit the speed and energy efficiency of current computing systems, while using light to optically switch memory and computing devices can revolutionize processing speed and efficiency.

YTiO3 is a transition metal oxide that becomes ferromagnetic only at temperatures below 27 K or -- 246°C, with properties similar to refrigerator magnets. At these low temperatures, the electron spins on titanium atoms align in certain directions. It is this collective ordering of spins that makes the material macromagnetized and makes it ferromagnetic when taken as a whole. In contrast, at temperatures above 27 K, individual spins fluctuate randomly, so no ferromagnetism is produced.

Using a powerful terahertz light source developed at the Max Planck Institute for Structure and Dynamics of Matter (MPSD) in Hamburg, Germany, the team managed to achieve ferromagnetism up to nearly 100 K (-- 193°C) in the YTiO3 material, well above its normal transition temperature. In addition, the photoinduced state lasted for many nanoseconds. The bright light pulses are designed to "shake" the atoms of the material in a coordinated manner, allowing electrons to align their spins.

The pulse frequency is adjusted to drive specific vibrations of the YTiO3 lattice, called phonons. The researchers found that when they excited a particular phonon at a natural frequency of 9 THz, the collective order of the spins and the orbits of the electrons were modified, leading to a stronger ferromagnetic trend. When driving other phonons, we observed completely different results: excitation of 4 THz phonons actually worsened ferromagnetism, while excitation of 17 THz phonons enhanced ferromagnetism - but not as strongly as 9 THz phonons.

Below the usual 27 K transition temperature, 9 THz phonon excitation significantly increases the magnetization, raising it by about 20% and reaching the theoretical maximum -- the highest level yet to be reached.

The terahertz source used in these experiments sends out strong pulses that are able to excite a very narrow frequency region in the material, making it an extremely precise tool. It has been deployed in several other MPSD-led studies on light-enhanced superconductivity and magnetism. However, this work reveals for the first time that effects of different qualities can be produced by exciting a series of lattice vibrations.

In addition to deepening scientists' understanding of strong and ultra-fast light interactions with matter, these results will help achieve better optical control of magnetic components.

Source: OFweek