  We investigate magnetic properties of an ultracold Fermi gas with population
imbalance. In the presence of population imbalance, the strong-coupling theory
developed by Nozieres and Schmitt-Rink (which is frequently referred to as the
NSR theory, or Gaussian fluctuation theory) is known to give unphysical results
in the BCS-BEC crossover region. We point out that this problem comes from how
to treat pseudogap effects originating from pairing fluctuations and many-body
corrections to the spin susceptibility. We also clarify how to overcome this
problem by including higher order fluctuations beyond the ordinary T-matrix
theory. Calculated spin susceptibility based on our extended T-matrix theory
agrees well with the recent experiment on a 6Li Fermi gas.
