The properties of doped and natural impurities in lithium niobate single crystals are studied using the whispering gallery mode method at low temperatures as a function of magnetic field. The study reveals considerable coupling of microwave photon modes to the ${\mathrm{Fe}}^{3+}$ spin ensemble in iron-doped and nondoped crystals. The $S=5/2$ structure of the ${\mathrm{Fe}}^{3+}$ impurities demonstrates zero field splittings of $11.21$ and $20.96\mathrm{GHz}$, a significant asymmetry of the Zeeman lines, and additional lines with anomalous $g$ factors of $1.37$ and $3.95$. Also, interactions between different transitions of the ${\mathrm{Fe}}^{3+}$ ion are observed. An additional ion impurity ensemble with a splitting of about $1.7\mathrm{GHz}$ is shown to couple to the dominating ${\mathrm{Fe}}^{3+}$ spins, and the effect on the $Q$ factors of the microwave photon modes due to the ${\mathrm{Fe}}^{3+}$ ion ensemble is also demonstrated. Measurements down to less than one photon level are made, with a loss tangent of order ${10}^{-5}$ determined.

• Received 28 April 2015
• Revised 2 July 2015

DOI:https://doi.org/10.1103/PhysRevB.92.060406