Spectral spin diffusion in dilute ruby

The broadening of the $+ \frac{1}{2}\leftrightarrow - \frac{1}{2}$ or low-field EPR absorption line of ${\mathrm{Cr}}^{3+}$ in dilute ruby has been studied at 2.7 GHz at liquid-helium temperature. Experimental evidence is given that the line is inhomogeneously broadened when observed with the external magnetic field $H_0$ parallel to the crystal axis, while it tends to be homogeneously broadened when observed with $H_0$ perpendicular to the crystal axis. A qualitative model is discussed in which the effect is attributed to the local-field modulation at the ${\mathrm{Cr}}^{3+}$ sites by ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ nuclei. A new technique of double resonance at microwave and radio-frequency energy has been used in which the phase of the wave at harmonic frequency is detected as a function of the external magnetic field and rf frequency. It is shown that spin transitions induced in the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ nuclei create spectral spin diffusion in the electron ${\mathrm{Cr}}^{3+}$ spin system which is seen directly as an increase of the width of the central spin packet of the ${\mathrm{Cr}}^{3+}$ absorption line.