Fine Structure of the Giant \textit{M}1 Resonance in $^{90}$Zr

The High-Intensity $\vec{\gamma}$-ray Source of the Triangle Universities Nuclear Laboratory, in connection with high-resolution Ge detectors, opens up the possibility to deduce precisely the strengths and locations of individual $M1$ transitions, comprising the giant $M1$ resonance, and to distinguish them unambiguously from $E1$ and $E2$ deexcitations. We will present the results of an investigation of the fine structure of the giant $M1$ resonance in $^{90}$Zr performed with polarized and mono-energetic photon beams from 7 to 11 MeV. The strength of numerous $M1$ transitions will be compared to calculations using the quasiparticle-random-phase approximation in a Wood-Saxon potential and with the Instantaneous Shape Sampling model in order to study the observed fragmentation.