Magnetic dipole transitions in crystals

Following Wybourne's concept of the so-called intensity borrowing mechanism contributions that are due to the magnetic dipole transitions are described. The approach is based on double perturbation theory, within which the perturbative effect of the crystal field and spin-orbit interaction are taken into account. The contributions to the transition amplitude up to the fourth order are derived, and new selection rules for the magnetic dipole transitions are presented. Finally, some estimates of the relative importance of the fourth-order contributions are presented.