Forbidden Hyperfine Spectra in Electron Spin Resonance due to Hyperfine Anisotropy and Nuclear Zeeman Interaction

Electron spin resonance of ${\mathrm{Co}}^{2+}$ in Zn ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2$ ${\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·6${\mathrm{H}}_2$O has been studied at 35 and 70 GHz at liquid-helium temperatures. In addition to the usual eight-line hyperfine spectrum from ${\mathrm{Co}}^{59}$ ($I=\frac{7}{2}$), a seven-line "forbidden" hyperfine spectrum was also observed with the applied magnetic field nearly perpendicular to the symmetry axis of the crystalline field. The "forbidden" transitions do not result from nuclear quadrupole coupling, but arise because of the very large anisotropy of the hyperfine interaction, more specifically because ${\mathrm{Ag}}_{\parallel }\gg B{g}_{\perp }$. As a consequence of this anisotropy, the axis of spin quantization of the nucleus deviates appreciably from the direction of the applied field, and the nuclear Zeeman interaction contains terms in $I_x$ that at high fields and certain angles can become comparable to the terms in $I_z$. Since the intensity of the forbidden spectra is a direct measure of the field at the nucleus, it is found that there exists at the cobalt nucleus a pseudo-magnetic-field the same order of magnitude as the applied magnetic field. The measured pseudofield, which results from the induced magnetization of the electron charge distribution by the external field, is in order-of-magnitude agreement with calculations.