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

TL;DR

This study uses high-resolution neutron spectroscopy to investigate low-energy nuclear spin excitations in double perovskites R$_2$CoMnO$_6$ (R=Y,Tb), revealing magnetic ordering of the Co sublattice and hyperfine interactions at low temperatures.

Contribution

First high-resolution neutron spectroscopy study of hyperfine nuclear spin excitations in R$_2$CoMnO$_6$ compounds, showing magnetic ordering and unquenched orbital moments.

Findings

Inelastic signals at ~2.1 μeV indicating hyperfine transitions.

Energy of excitations decreases and merges with elastic peak at T_C.

Additional quasielastic scattering in Tb compound due to Tb magnetic moments.

Abstract

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 $\mu$eV 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 $^{59}$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 $T_C \approx 75$ K for Y$_2$CoMnO$_6$ and $T_C \approx 100$ 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.