Light-Induced Reorientation Transition in an Antiferromagnetic Semiconductor

Antiferromagnets, unlike ferromagnets, exhibit ordered atomic spins without generating a macroscopic magnetic moment, making them highly promising for spintronic technologies that require minimal interference between neighboring devices. However, their lack of a net magnetic moment also makes them difficult to control, necessitating new approaches for manipulating their magnetic states. In this study, we demonstrate a novel method to manipulate antiferromagnets using ultrafast optical pulses. We investigate the antiferromagnetic semiconductor CaMn2⁢Bi2, driving it out of equilibrium using an ultrafast laser pulse. This excitation promotes electrons above the material’s energy band gap, disrupting its long-range spin order. Recent studies in nonmagnetic materials suggest that such nonequilibrium conditions can lead to metastable states distinct from the material’s equilibrium phase. Using time-resolved second harmonic generation, an advanced optical technique for tracking spin dynamics, we observe that CaMn2⁢Bi2 enters a metastable antiferromagnetic state just 10 ps after excitation, and this state persists for at least 150 ps. Remarkably, while the material remains an antiferromagnet, its spins rotate by a finite angle—something that is thermodynamically forbidden in equilibrium and achievable only through optical excitation. This discovery reveals a new pathway for manipulating antiferromagnetic order using light, presenting exciting possibilities for ultrafast magnetic-device technologies. By leveraging light-induced transitions to metastable states, future research could explore novel spintronic applications that harness ultrafast phase transitions, further expanding the potential of antiferromagnetic materials in next-generation computing and information storage.

Figure: (a) Magnetic unit cell of CaMn2Bi2. The angle γ is such that the Mn spin does not lie precisely along any particular crystallographic axis. (b) Schematic of the rotational-anisotropy second harmonic generation SHG (RASHG) setup. (c) RASHG intensity at 8 K in the Pin–Pout polarization channel as a function of time for the AFM order parameter. The insets beneath the RASHG plots depict schematically the value of the Néel vector 2 ps before and 12 ps after photoexcitation.