Relativistic many-body calculations of magnetic dipole transitions in Be-like ions

Reduced matrix elements and transition rates are calculated for all magneti c dipole (M1) transitions within 2l2l' configurations and for some 2l3l' - 2l2l' transitions in Be-like ions with nuclear charges ranging from Z = 4 t o 100. Many-body perturbation theory (MBPT), including the Breit interactio n, is used to evaluate retarded M1 matrix elements. The calculations start with a (1s)(2) Dirac-Fock potential and include all possible n = 2 configur ations, leading to 4 odd-parity and 6 even-parity states, and some n = 3 co nfigurations. First-order perturbation theory is used to obtain intermediat e coupling coefficients. Second-order MBPT is used to determine the matrix elements, which are evaluated for all 11 M1 transitions within 2l2l' config urations and for 35 M1 transitions between 2l3l' and 2l2l' states. The tran sition energies used in the calculation of oscillator strengths and transit ion rates are obtained from second-order MBPT. The importance of negative-e nergy contributions to M1 transition amplitudes is discussed.