Bichromatic light-induced electronic torques on the Néel vector in Rashba magnetic multipoles from $p$-wave magnets to $i$-wave altermagnets

Abstract

Light can control magnetic order on ultrafast timescales, but for a two-dimensional magnet with an out-of-plane order parameter $\boldsymbol N\parallel\hat z$, the inverse Faraday field is collinear with $\boldsymbol N$ and gives no transverse torque. Here we show that bichromatic $(ω,bω)$ light generates electronic torques perpendicular to $\boldsymbol N$ in Rashba-coupled magnetic multipoles, from $p$-wave ($n=1$) magnets to $i$-wave ($n=6$) altermagnets. Beyond high-frequency effective fields, we evaluate the exact Floquet response and the generalized torque per occupied electron, for prescribed occupations. The momentum-dependent exchange terms dominate this Fermi-sea torque and remove the leading-order zeros (amplitude-ratio blindspots) of the momentum-independent field in the channels tested. For out-of-plane $\boldsymbol N$, rotational symmetry permits a transverse torque only if $\gcd(n,b+1)=1$ for counter-rotating beams or $\gcd(n,b-1)=1$ for co-rotating beams. For in-plane $\boldsymbol N$, the Fermi sea also gives a torque component along $\hat z$ that the momentum-independent field cannot produce. We classify the active channels, with $n=1,\dots,6$ and $b=1,\dots,7$, by frequency scaling and origin (cycle-averaged versus oscillating exchange). As a control, a Rashba ferromagnet ($n=0$) has one symmetry-allowed out-of-plane channel and a weak in-plane response without a selection rule. Finite-momentum occupied-state physics is therefore essential for light-driven torques in unconventional magnets, and the exact Floquet calculation validates and extends the high-frequency description.