Abstract:  We analyze the consequences of neutrino "secret interactions" with gauge bosons beyond the Standard Model at the epoch after neutrino decoupling. 
Our findings show neutrinos of the background could undergo a ferromagnetic phase transition before the condition of free streaming if the coupling of the new 
light gauge boson is between $2.2\times 10^{-13} \left(\frac{m_\nu}{10^{-4}\,\rm{eV}}\right)^2<g<2.3\times 10^{-7}$