Universality of a Critical Magnetic Field in a Holographic Superconductor

We study aspects of holographic superconductors analytically in the presence of a constant external magnetic field.It is shown that the critical temperature and the critical magnetic Geld can be calculated at nonzero temperature.We detect the Meissner effect in such superconductors.A universal relation between black hole mass M and critical magnetic field Hc is proposed to be（Hc）/(M2/3)（?）≤0.687365.We aiso discuss some aspects of phase transition in terms of black hole entropy and Bekenstein's entropy to the energy upper bound.