Fig. 2: Spectra of GB and NOA with response to P, M, and \({\gamma }_{{{{{{\rm{m}}}}}}}\).
a Magnetic field dependence of the diagonal parts of effective dielectric tensor (\({\varepsilon }_{xx}\) and \({\varepsilon }_{yy}\)) at 4 K in the magnetic field (Hdc|| [110]). Symmetric part of effective dielectric spectra (\({\varepsilon }_{xy}^{{{{{{\rm{S}}}}}}}\), see Eq. (1)) representing the optical rotation of GB under b time reversal and c space inversion. Antisymmetric part of effective dielectric spectra (\({\varepsilon }_{xy}^{{{{{{\rm{A}}}}}}}\)) representing the optical rotation of NOA under d time reversal and e space inversion. The magnetically-induced chirality γm = ±, and equivalently the sign of P, are denoted. f Magnetic field dependence of the peak magnitude of Im[\({\varepsilon }_{xy}^{{{{{{\rm{A}}}}}}}\)] (red) and Im[\({\varepsilon }_{xy}^{{{{{{\rm{S}}}}}}}\)] (blue) and the magnetization M (black line) of CuFe1-xGaxO2 (x = 0.035) (right axis)29. The filled and open circles denote the peak magnitude for (γm = +, +P) and (γm = −, −P) states, respectively. (See Supplementary Note 2 for details of spectra).
