Terahertz Laser Pulses Transform Ferromagnetism

Researchers have used terahertz gentle pulses to induce ferromagnetism in a crystal at temperatures far above its regular transition temperature, paving the best way for optically managed reminiscence and computing gadgets with increased velocity and effectivity.

Scientists in Germany and the USA have proven for the primary time that terahertz (THz) gentle pulses can stabilize ferromagnetism in a crystal at temperatures greater than thrice its standard transition temperature. As the workforce stories within the journal Nature, utilizing pulses simply tons of of femtoseconds lengthy (a millionth of a billionth of a second), a ferromagnetic state was induced at excessive temperature within the rare-earth titanate YTiO₃ which continued for a lot of nanoseconds after the sunshine publicity. Below the equilibrium transition temperature, the laser pulses nonetheless strengthened the present magnetic state, growing the magnetization as much as its theoretical restrict.

Using gentle to regulate magnetism in solids is a promising platform for future applied sciences. Today’s computer systems primarily depend on the circulation {of electrical} cost to course of info. Moreover, digital reminiscence storage gadgets make use of magnetic bits that have to be switched exterior magnetic fields. Both of those points restrict the velocity and vitality effectivity of present computing methods. Using gentle as an alternative to optically swap reminiscence and computing gadgets may revolutionize processing speeds and effectivity.

YTiO₃ is a transition steel oxide that solely turns into ferromagnetic, with properties resembling these of a fridge magnet, under 27 Ok or –246° Celsius. At these low temperatures, the spins of the electrons on the titanium atoms align in a particular direction. It is this collective ordering of the spins which gives the material as a whole a macroscopic magnetization and turns it ferromagnetic. In contrast, at temperatures above 27 K, the individual spins fluctuate randomly so that no ferromagnetism develops.

Using a powerful THz light source developed at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, Germany, the team managed to achieve ferromagnetism in YTiO₃ up to nearly 100 K (­–193° C), far above its normal transition temperature. The light-induced state also persisted for many nanoseconds. The intense light pulse is designed to ‘shake’ the material’s atoms in a coordinated way, allowing the electrons to align their spins.

“The frequencies of the pulses are tuned to drive specific vibrations of the YTiO₃ crystal lattice, called phonons,” explains lead author Ankit Disa. “We found that when we excite one particular phonon at its natural frequency of 9 THz, the collective order of the spins and the orbitals of the electrons are modified, leading to a stronger tendency towards a ferromagnetic state. When driving other phonons, we observed totally different results: Excitation of a phonon at 4 THz actually worsens the ferromagnetism and of one at 17 THz enhances it – but not as strongly as the 9 THz phonon does.”

Below the usual transition temperature of 27 K, the 9 THz phonon excitation significantly increased the magnetization, raising it by around 20 percent to its theoretical maximum – a level that had not been achieved to date.

The THz source used in these experiments delivers intense pulses and is capable of exciting a very narrow frequency region in the material, making it an extremely precise tool. It has already been deployed in several other MPSD-led studies on light-enhanced superconductivity and magnetism. However, this work revealed for the first time that qualitatively different effects can be produced by exciting a range of lattice vibrations.

Apart from deepening scientists’ understanding of intense and ultrafast light-matter interactions, these results are important stepping stones toward the optical control of magnetic components.

“This work does not only demonstrate the switching of magnetism on and off on demand, it also gives us a foretaste of what could be done to store and process information at ultra-high speeds,“ explains Andrea Cavalleri, Director of the MPSD’s Condensed Matter Dynamics Department.

“Beyond this demonstration, our work underscores the ability to create order in disordered, fluctuating phases of matter, something akin to freezing water with light. Controlling these processes has been a long-standing goal of our group. Over the years we have reported a number of other realizations that flank this work, including photo-induced high-temperature superconductivity^[1,2] and photo-induced ferroelectricity.^[3]”

Reference: “Photo-induced high-temperature ferromagnetism in YTiO₃” by A. S. Disa, J. Curtis, M. Fechner, A. Liu, A. von Hoegen, M. Först, T. F. Nova, P. Narang, A. Maljuk, A. V. Boris, B. Keimer and A. Cavalleri, 3 May 2023, Nature.
DOI: 10.1038/s41586-023-05853-8

At Harvard University and the University of California – Los Angeles, postdoc Jon Curtis and his advisor Prineha Narang supplied essential theoretical contributions. In Germany, the collaboration included the teams of Bernhard Keimer on the Max Planck Institute for Solid State Research (Stuttgart) and Andrey Maljuk on the Leibniz Institute for Solid State and Materials Research (Dresden).

Notes

1. “Light-Induced Superconductivity in a Stripe-Ordered Cuprate” by D. Fausti, R. I. Tobey, N. Dean, S. Kaiser, A. Dienst, M. C. Hoffmann, S. Pyon, T. Takayama, H. Takagi and A. Cavalleri, 14 January 2011, Science.
   DOI: 10.1126/science.1197294
2. “Possible light-induced superconductivity in K₃C₆₀ at high temperature” by M. Mitrano, A. Cantaluppi, D. Nicoletti, S. Kaiser, A. Perucchi, S. Lupi, P. Di Pietro, D. Pontiroli, M. Riccò, S. R. Clark, D. Jaksch and A. Cavalleri, 8 February 2016, Nature.
   DOI: 10.1038/nature16522
3. “Metastable ferroelectricity in optically strained SrTiO₃” by T. F. Nova, A. S. Disa, M. Fechner and A. Cavalleri, 14 June 2019, Science Advances.
   DOI: 10.1126/science.aaw4911