Zero-Field Spin Absorption in Paramagnetic Salts

The spin absorption spectrum in the absence of a static magnetic field has been observed in several paramagnetic salts at liquid-helium temperatures. The absorptive component of the complex susceptibility ${\chi }^{\prime \prime }$ was measured as a continuous function of frequency over the range 130-4000 Mc/sec by observing the influence of powdered samples on the transmission of a tunable coaxial resonant cavity. Exchange narrowing in qualitative agreement with the theory of Wright was observed in cupric salts. Moments of the shape functions obtained by expressing the results in terms of an empirical, fitted function yield, on the basis of the theories of Wright and Caspers, values of the exchange constant $A$ of 3.3 and 3.7 for the cupric potassium and cupric ammonium Tutton salts, respectively. The cupric salt experimental curves are lower and broader than the curves proposed by Locher and Gorter. The absorption band in chromic potassium alum has a "flat-topped" appearance, and a width approximately three times the dipolar relaxation frequency ${\nu }_0$. No temperature dependence of the shape functions was observed.