Mössbauer Observation of the Quantum Levels of ${\mathrm{Fe}}^{3+}$ Ions Doped in 1D Ising Ferromagnet C${\mathrm{a}}_3$C${\mathrm{o}}_2$${\mathrm{O}}_6$

We have measured the Mössbauer effect of ${}^{57}{\mathrm{Fe}}^{3+}(S=5/2\mathrm{})$ ions doped in a 1D Ising ferromagnet C${\mathrm{a}}_3$C${\mathrm{o}}_2$ ${\mathrm{O}}_6$. The spectra with hyperfine fields corresponding to the exchange polarized Fe electron spins with $m_z=\pm 5/2,\pm 3/2,\pm 1/2$ are first observed in the magnetically dense system for $T\le 20\mathrm{K}$, suggesting slow magnetic relaxation at the ${\mathrm{Fe}}^{3+}$ sites. The key lies in the large difference between magnetic characters of the host (Ising) and impurity (Heisenberg) spins, making the impurity spins extremely isolated in the host 1D Ising spin system.