A team of researchers from Germany and the United States has demonstrated that terahertz (THz) light pulses can stabilize ferromagnetism in a crystal at temperatures well above its typical transition temperature. Their findings, published in the journal Nature, show that with pulses lasting mere hundreds of femtoseconds (a millionth of a billionth of a second), a ferromagnetic state could be induced in the rare-earth titanate YTiO3 at high temperatures, which persisted for several nanoseconds after exposure to the light. The researchers also found that even below the equilibrium transition temperature, the laser pulses reinforced the existing magnetic state, raising the magnetization to its theoretical maximum.
Optical manipulation of magnetism in solids holds great promise for future technologies. Current computers rely primarily on the movement of electrical charge to process information, while magnetic bits in digital memory storage devices require external magnetic fields to be switched. Both of these factors limit the speed and energy efficiency of modern computing systems. By using light to optically switch memory and computing devices, the speed and efficiency of processing could be revolutionized.
YTiO3 is a transition metal oxide that only exhibits ferromagnetism, where its properties are similar to those of a fridge magnet, below 27 K or -246°C. At these low temperatures, the spins of the electrons on the Ti atoms align in a particular direction, creating a collective ordering of the spins that generates a macroscopic magnetization and turns it ferromagnetic. However, at temperatures above 27 K, the individual spins fluctuate randomly, preventing the development of ferromagnetism.
Using a powerful THz light source developed at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, Germany, the researchers were able to achieve ferromagnetism in YTiO3 at nearly 100 K (≅193°C), far above its typical transition temperature. The light-induced state also persisted for several nanoseconds. The intense light pulse is designed to coordinate the atoms in the material, allowing the electrons to align their spins.
Lead author Ankit Disa explains that the pulses’ frequencies are tuned to drive specific vibrations in the YTiO3 crystal lattice, known as phonons. One particular phonon at its natural frequency of 9 THz modifies the collective order of the spins and electrons’ orbitals, leading to a stronger tendency towards a ferromagnetic state. Other phonons produced entirely different outcomes. Excitation of a phonon at 4 THz worsened the ferromagnetism, while one at 17 THz enhanced it, though not as strongly as the 9 THz phonon.
At temperatures below the usual transition temperature of 27 K, the excitation of the 9 THz phonon notably increased the magnetization, pushing it to around 20% of its theoretical maximum, which had not been accomplished before.
The THz source utilized in these experiments delivers strong pulses and is capable of exciting a narrow frequency range in the material, making it an incredibly precise tool. It has previously been used in several other MPSD-led studies on light-enhanced superconductivity and magnetism. However, this research demonstrates, for the first time, that qualitatively different impacts can be achieved by exciting an array of lattice vibrations.
In addition to deepening scientists’ knowledge of intense and ultrafast light-matter interactions, these findings represent important progress towards the optical control of magnetic components. According to Andrea Cavalleri, Director of the MPSD’s Condensed Matter Dynamics Department, “This work not only demonstrates the ability to switch magnetism on and off on demand, but it also gives us a glimpse of what can be accomplished to store and process information at ultra-high speeds.”
“Beyond this demonstration, our work highlights the ability to create order in disordered, fluctuating phases of matter, something akin to freezing water with light. Our group has been pursuing the goal of controlling these processes for a long time. We have reported a variety of other realizations, including photo-induced high-temperature superconductivity and photo-induced ferroelectricity, alongside this work.”