Narrowing of Resonance Line by Spin-Lattice Relaxation in the System K3(Cr-Fe-Co) (CN)6

The effect of Fe 3+ ions on the line width of Cr 3+ spin resonance in K 3 (Cr 0.01 Fe x Co 0.99- x (CN) 6 is studied with varying the concentration of Fe (or Co) ions. At room temperature the resonance lines of Cr 3+ ions have small widths in spite of coexistence of paramagnetic ions Fe 3+ . However, with decreasing temperature, line widths of Cr 3+ ions are broadened and the resonance fields are shifted to lower fields. The widths at lowest temperatures exceed the calculated dipolar width, suggesting the existence of certain amounts of exchange coupling between Cr 3+ ions and Fe 3+ ions. The temperature variations of the line widths and the shifts are reasonably interpreted in terms of the random modulation of the interactions between Cr 3+ ions and Fe 3+ ions by the rapid spin-lattice relaxation of Fe 3+ ions following the generalized theory of magnetic resonace by R. Kubo and K. Tomita (J.Phys. Soc. Japan 9 (1954) 888).