Magnetic behavior ofCoions in the exotic spin-chain compoundCa3Co2O6fromCo59NMR studies

We have performed field-swept ${}^{59}\mathrm{Co}$ NMR measurements on the “exotic” spin-chain material, ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$, crystallizing in ${\mathrm{K}}_4{\mathrm{CdCl}}_6$-type rhombohedral structure, believed to exhibit two magnetic transition temperatures (around 24 and $12\mathrm{K}$). This is the first NMR investigation of this family of compounds. We are able to detect the NMR signal below about $15\mathrm{K}$, which is conclusive proof for the existence of nonmagnetic $\mathrm{Co}$ ions (${\mathrm{Co}}^{3+}$-low-spin $3d^6$, in addition to magnetic $\mathrm{Co}$ ions) at least in a narrow temperature range around $10\mathrm{K}$, thereby resolving a controversy on this issue in the literature. In addition, we find that the spin-lattice and spin-spin relaxation times undergo a dramatic increase below about $12\mathrm{K}$, with a profound influence on the intensity of the NMR signal as a function of temperature, providing microscopic evidence for the existence of the $12\mathrm{K}$-transition. The spectral features present quite an interesting situation.