2026/06/16 by P. A. Usachev, R. V. Pisarev, V. V. Pavlov · 1 voice
Physics and Astronomy · #cond-mat.str-el
Altermagnetism as a new concept in condensed matter physics is currently being thoroughly investigated. Despite the absence of macroscopic magnetization, altermagnets host a hidden spin order whose direct detection remains a challenge. Here we report on the observation of electric and magnetic dipole forbidden optical second-harmonic generation (SHG) in the altermagnet CoF2 with a centrosymmetric lattice and spin order. We demonstrate that below the Néel temperature TN = 38 K the SHG signal is sensitive to the ferrotype magnetic octupole OM which is the order parameter in the antiferromagnetic phase. By combining polarization-resolved SHG experimental data and a phenomenological symmetry analysis, we show that the altermagnetic spin structure of CoF2 enables a ferroic-octupole-induced electric-quadrupolar nonlinear polarization P2ω = \mathrm iε0 c\mathbfχ(3)(OM) :Eω ∇ Eω. The temperature dependence of SHG reveals a phase transition at TN confirming the spin origin of the observed signal. The SHG response is resonantly enhanced by a coherent three-photon process caused by the electronic d-d transitions of the Co2+ ion. Model calculations of SHG polarization rotational anisotropies and temperature dependencies give a reasonable agreement with experimental data, proving the disclosed nonlinear contribution. Our results establish SHG as a novel sensitive tool of ferroic-octupolar spin ordering and highlight the potential of CoF2 and other altermagnets for further nonlinear optical investigations and applications.