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Condition of resonance and angular dependence of H_res

In polar coordinate, H. Suhl [H. Suhl, Phys. Rev. 97, 555 (1954)] derived the following expression for the intrinsic contribution of the linewidth:

$\Delta H=\frac{\alpha}{\left|d \omega / dH \right|} \ \frac{\gamma}{M} \left(\frac{\partial^2 E}{\partial \theta^2} + \frac{1}{\mathrm{sin}^2 \theta} \frac{\partial^2 E}{\partial \phi^2}\right)$