Optical coherence of $^{166}$Er:$^{7}$LiYF$_4$ crystal below 1 Kelvin

We explore spin dynamics of isotopically purified $^{166}$Er:$^{7}$LiYF$_4$ crystal below 1 Kelvin and at weak magnetic fields $ 1.5$ K and time scales of $\mu$s, spin coherence of Er:LYF crystal appears only at magnetic fields above 1 Tesla. In the present work, we demonstrate spin coherence of Er:LYF crystals at the field range compatible with ZEFOZ transitions of $^{167}$Er isotope and with working conditions of superconducting quantum circuits.