Abstract: Altermagnets are a new class of magnets with zero net magnetization and no stray fields, but with strong spin polarization that depends on momentum. That suits spintronic memory devices: the spin polarization gives a good readout. Writing, however, has usually needed a net magnetization, which altermagnets lack in equilibrium. Overcoming this limitation needs a way to write without the stray fields of a magnetic knob, and light can be that knob. In this talk, I start with why altermagnets compare well with ferromagnets and antiferromagnets for memory devices. Then I turn to writing with light. Using Floquet theory, linear response and Lindblad dynamics, I show that periodic light can create a highly tunable magnetization in a two-dimensional altermagnet, which has none in its static state. The spin, orbital and electrical responses (magnetoresistance and Hall) change on femtosecond-to-picosecond timescales. I cover single-color circularly or linearly polarized light, two-color drives, and a cavity setup where an infrared-active phonon of the material strengthens the effect. The results suggest a dynamic way to write altermagnetic memory on ultrafast timescales. 

Based on: 

1. M. Yarmohammadi, U. Zülicke, J. Berakdar, J. Linder, and J. K. Freericks, Phys. Rev. B 111, 224412 (2025)
2. M. Yarmohammadi, M. Berritta, M. Bukov, L. Šmejkal, J. Linder, and P. M. Oppeneer, Phys. Rev. B 113, L060403 (2026)
3. M. Yarmohammadi, L. Šmejkal, and J. K. Freericks, Phys. Rev. Lett. 136, 146904 (2026)
4. M. Yarmohammadi, P. M. Gunnink, J. Sinova, and J. K. Freericks, arXiv:2606.21281
5. M. Yarmohammadi, D. Jo, M. Berritta, L. Šmejkal, J. K. Freericks, and P. M. Oppeneer, arXiv:2606.31867

Short Bio: Mohsen Yarmohammadi is currently a research scientist at Georgetown University. He received his PhD from TU Dortmund in 2022 and was a postdoctoral researcher at the University of Texas at Dallas (2022–2024) and at Georgetown University (2024–2026). He has also been a visiting scholar at the Max Planck Institute for the Physics of Complex Systems in Germany and at Uppsala University in Sweden since 2022. His research in theoretical condensed matter physics focuses on the dynamic control of quantum materials.