SPIN-DEPENDENT AND DEFORMATION-DEPENDENT ORBITAL M1 STRENGTH IN RARE-EARTH NUCLEI

The M1 excitations in three isotope chains, Nd-142, Nd-146, Nd-148, Nd -150, Sm-144, Sm-148, Sm-150, Sm-152, Sm-154, and Gd-156, Gd-158, are studied within the quasi-particle random-phase approximation using a m ean field given by a deformed Woods-Saxon potential and including quad rupole-quadrupole and spin-spin residual inter-actions. A residual rot ation-vibration coupling ensures the exclusion of the spurious state. The behaviour of the M1 strength distribution is studied systematicall y in the whole range of experimentally explored excitation energies an d compared with the large variety of experimental information, which i ncludes both low-lying orbital and higher-lying spin-flip 1+ excitatio ns. We have found a quadratic dependence of the Ml strength summed up to 4 MeV on the nuclear deformation reproducing at the same time the m ain features of the low-lying Ml spectra. The double-peaked structure of the spin M1 strength distribution found experimentally in several n uclei is qualitatively reproduced and interpreted as isoscalar and iso vector peaks.