k − ( π/a, π/a ). The wavenumbers of the true spin and of the true magnetic field that appear there remain unchanged, however. The end result is that the true magnetic susceptibility χ$_{⊥}$ (+ , k , ω ) in the hidden N´ eel state is given by that in the hidden ferromagnetic phase with the exchange J ‖ 1 ↔ J ⊥ 1 alone, whereas the hidden magnetic susceptibility χ$_{⊥}$ ( − , k , ω ) there is given by that in the hidden ferromagnetic phase with the previous exchange along with the replacement k → k + ( π/a, π/a ) in the wave numbers. [Cf. Eq. (4).] Equation (9) and the discussion above yield true static magnetic susceptibilities χ$_{⊥}$ (0) = ( J$_{0}$ + 4 J ⊥ 1 + 4 J ⊥ 2 ) − 1 for the hidden ferromagnetic state and χ$_{⊥}$ (0) = ( J$_{0}$ + 4 J ‖ 1 + 4 J ⊥ 2 ) − 1 for the hidden N´ eel state. It also yields static susceptibilities for hidden magnetic order given by χ$_{⊥}$ ( − , k , 0) = 2 s/ Ω$_{−}$, which correctly diverge as k approaches the wavenumber that corresponds to either long-range ferromagnetic or N´ eel order. Finally, the poles of the dynamical susceptibility for hidden magnetic order (9), χ$_{⊥}$ ( − , k , ω ), represent two spin- wave excitations of momentum ¯ h k that correspond to the two possible orientations of the transverse component of the hidden moment, S$_{⊥}$ ( − , k , ω ). They both lie at an energy ¯ h (Ω$_{+}$Ω$_{−}$) 1 / 2 above the groundstate energy. These spin-wave excitations are also reflected by the poles in the dynamical susceptibility for true magnetic order (9), χ$_{⊥}$ (+ , k , ω ). As mentioned above, however, the wave number k shifts by the characteristic amount ( π/a, π/a ) in the case of hidden N´ eel order. Last, both spinwaves have spectral weights in the true (+) and the hidden ( − ) channels that are read off from the imaginary part of the dynamical susceptibility: The peaks in the dynamical structure function above signal spin-wave excitations that are observable by inelastic neutron scattering only in the true magnetic (+) channel. Recalling the discussion following Eq. (9), the true dynamical susceptibility χ$_{⊥}$ (+ , k , ω ) is common to a hidden ferromagnet and to a hidden N´ eel state that are related by the transformation (2). Inelastic neutron scattering is therefore unable to discriminate between these two possible types of hidden magnetic order! Figure 2 depicts the linear spin-wave spectrum for a cSDW with intervening hidden ferromagnetic (N´ eel) order at two extreme values for the Hund’s rule coupling: absent and 9 Im χ$_{⊥}$ ( ± , k , ω ) = A ( ± , k )( δ [ ω − (Ω$_{+}$Ω$_{−}$) 1 / $^{2}$] − δ [ ω + (Ω$_{+}$Ω$_{−}$) 1 / $^{2}$]) . (12) A ( ± , k ) = πs (Ω$_{∓}$ / Ω$_{±}$) 1 / 2 (11)