Paramagnetic resonance has been observed from 2 kMc/sec to 110 kMc/sec in ${\mathrm{Fe}}^{3+}$-doped single crystals of rutile at 78°K, 4.2°K, and 1.4°K. The rather large zero field splittings measured between the three doublets are 43.3±0.1 kMc/sec and 81.3±0.1 kMc/sec. The derived constants for the spin Hamiltonian $\{\mathcal{H}=g\beta \mathrm{H}·\mathrm{S}\times D({S_z}^2-\frac{35}{12})+E({S_x}^2-{S_y}^2)+(\frac{a}{6}\left)\right({S_x}^4+{S_y}^4+{S_z}^4-,\frac{707}{16)}\left\}\right\{+\left(\frac{7}{36}\right)F[{S_z}^4-(\frac{95}{14}){S_z}^2+\frac{81}{16}].\mathrm{}\}$ are $D=20.35\mathrm{}\pm 0.1$ kMc/sec, $E=2.21\mathrm{}\pm 0.07$ kMc/sec, $a=1.1\mathrm{}\pm 0.2$ kMc/sec, $F=-0.5\mathrm{}\pm 0.3$ kMc/sec, $D=20.35\mathrm{}\pm 0.1$ kMc/sec, $E=2.21\mathrm{}\pm 0.07$ kMc/sec, $a=1.1\mathrm{}\pm 0.2$ kMc/sec, $F=-0.5\mathrm{}\pm 0.3$ kMc/sec, $g=2.000\mathrm{}\pm 0.005$. The average line width is 35 Mc/sec for 2×${10}^{19}$ spins per ${\mathrm{cm}}^3$, and the average spin-lattice relaxation time $T_1$ at $T=1.4\mathrm{}°$K is 4 msec. $T_1$ still has a very slow inverse temperature dependence even at $T=78\mathrm{}°$K. An increase in sensitivity over metal walled cavity spectrometers was achieved by using the pieces of Fe-doped rutile as the microwave resonators.

• Received 20 January 1960

DOI:https://doi.org/10.1103/PhysRev.118.1485