Magnetic ordering in double perovskites R$_2$CoMnO$_6$ (R = Y, Tb) investigated by high resolution neutron spectroscopy.

We have investigated low energy nuclear spin excitations in double perovskite compounds R(2)CoMnO(6) (R=Y, Tb) by inelastic neutron scattering with a high resolution back-scattering spectrometer. We observed inelastic signals at about 2.1 μeV for Y(2)CoMnO(6) and also for Tb(2)CoMnO(6) at T = 2 K in both energy-loss and energy-gain sides. We interpret these inelastic peaks to be due to the transitions between the hyperfine split nuclear levels of the (59)Co nucleus. The inelastic peaks move towards the central elastic peak and finally merge with it at the magnetic ordering temperature T(C). The energy of the low energy excitations decreases continuously and becomes zero at T(C) ≈ 75 K for Y(2)CoMnO(6) and T(C) ≈ 100 K for Tb(2)CoMnO(6). For Tb(2)CoMnO(6), which contains magnetic rare earth ions, additional quasielastic scattering due presumably to the fluctuations of large Tb magnetic moments was observed. The present study reveals the magnetic ordering of the Co sublattice. The results of this investigation along with that obtained by us for other compounds indicate the presence of unquenched orbital moments in some of the Co compounds