To obtain low switching field ($H_{SW}$) we introduced amorphous ferromagnetic $Co_{70.5}Fe_{4,5}Si_{15}B_{10}$ single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures $Si/SiO_2/Ta$ 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/CoFeSiB 7 or CoFeSiB (t)/Ru 1.0/CoFeSiB (7-t)/Ru 60 (in nm) were investigated and compared to MTJs with $Co_{75}Fe_{25}$ and $Ni_{80}Fe_{20}$ free layers. CoFeSiB showed a lower saturation magnetization of $560 emu/cm^3$ and a higher anisotropy constant of $2800;erg/cm^3$ than CoFe and NiFe, respectively. An exchange coupling energy ($J_{ex}$) of $-0.003erg/cm^2$ was observed by inserting a 1.0 nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was frond that the size dependence of the $H_{SW}$ originated from the lower $J_{ex}$ experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower $H_{SW}$ than that of NiFe, CoFe and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial far the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer to submicrometer-sized elements.