Large Scale Shell Model Studies of M1 Strength in Argon and Calcium Isotopes

We have calculated the M1 strength distributions in the $^{36-40}$Ar and $^{40}$Ca isotopes within large-scale shell model studies which consider valence nucleons in the sd and pf shells. While the M1 strength in $^{36}$Ar is well reproduced within the sd shell, the experimentally observed fragmentation of the M1 strength in $^{38}$Ar and $^{40}$Ca requires n-particle n-hole excitations with $n\ge4$ from the $sd$ to the $pf$ shell. The mechanism of M1 strength fragmentation and the role of different n-particle n-hole cross-shell excitations are discussed.