Impurity spin resonance ofMn2+in antiferromagnetic FeBr2

Electron spin resonance of substitutional ${\mathrm{Mn}}^{2+}$ ions in antiferromagnetic Fe${\mathrm{Br}}_2$ host has been investigated in the liquid-helium temperature range using a conventional homodyne spectrometer. Experimental results show that the exchange constants between the impurity and the host are weaker than the host ones. Therefore, the impurity spin behaves nearly as a "paramagnetic" spin in an effective molecular field. From the experimental results, we have deduced an effective-spin Hamiltonian of the impurity. The leading term is the impurity-host effective field, its value being 2900 G. Moreover, the deviations $\Delta {g^{\prime }}_{\parallel }=-0.06\mathrm{}$ and $\Delta {g^{\prime }}_{\perp }=-0.49\mathrm{}$ of the spectroscopic factor, and the anisotropy term $D^{\prime }=-770\mathrm{}$ G are unusually large. A theoretical analysis including the effect of the exchange interactions between the impurity and the host has allowed us to get the physical origin of the various terms of the spin Hamiltonian. The values of diagonal exchange impurity-host parameters have been found (approximately 1 ${\mathrm{cm}}^{-1\mathrm{}}$). We have shown that $\Delta {g^{\prime }}_{\parallel }$ and $\Delta {g^{\prime }}_{\perp }$ were, respectively, proportional to the susceptibilities ${\chi }_{\parallel }$ and ${\chi }_{\perp }$ of the host, and that the large negative value of the anisotropy term resulted from large nondiagonal exchange impurity-host coupling.