Spectral spin diffusion in dilute ruby

The broadening of the + 1/2↔- 1/2 or low-field EPR absorption line of Cr³⁺ 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 H₀ parallel to the crystal axis, while it tends to be homogeneously broadened when observed with H₀ perpendicular to the crystal axis. A qualitative model is discussed in which the effect is attributed to the local-field modulation at the Cr³⁺ sites by ²⁷Al 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 ²⁷Al nuclei create spectral spin diffusion in the electron Cr³⁺ spin system which is seen directly as an increase of the width of the central spin packet of the Cr³⁺ absorption line.