The M 1 transition and ground state spectra of the even-even 160-164 Dy and 156-160 Gd isotopes are calculated within the framework of the microscopic pseudo-SU͑3͒ shell model. A reasonable description of the M 1 strength function, including its fragmentation, is obtained with an interaction that includes collective as well as single-particle and pairing terms with the parameters of the theory fit to the energy spectrum and E2 transition strengths. ͓S0556-2813͑98͒04403-3͔

The pseudo SU͑3͒ model, in an extended version in which proton and neutron degrees of freedom are incorporated explicitly, is used to study the nature of low-lying magnetic dipole excitations in even-even rare-earth nuclei. Through a linear mapping between invariants of the rotor group and SU͑3͒, a microscopic interpretation of the ''scissors'' mode of the two-rotor model is realized. The model allows for, first, a classification of SU͑3͒ shell-model configurations in terms of collective degrees of freedom and, second, an adjustment of the relative energy of distinct 1 ϩ states. Triaxiality of the parent proton and neutron distributions is shown to add a ''twist'' degree of freedom to the usual ''scissors'' mode picture. This new mode should be observed as: ͑1͒ a low-lying Kϭ0 excitation, and ͑2͒ a second, doubly degenerate, Kϭ1 transition only slightly higher in energy than the usual scissors mode. Fragmentation of the M 1 strength distribution can be reproduced within framework of the model by including representation mixing terms in the Hamiltonian. ͓S0556-2813͑98͒01304-1͔

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