A sizing composition and method for manufacturing such, which is suitable for use in manufacturing fiber-reinforced composites which have desirable properties such as high strength, high resistance to water and chemical degradation, and improved coloring. The system and method uses an oscillating magnetic field to move nanospheres comprised of single-domain nanoparticles. The armature includes an armature coil opposed to the rotor from one side in a rotation axis direction parallel to the rotation axis. After conduction for a predetermined time, the gain of the proportional-plus-integral controller is set to zero to maintain an integrated value constant and accordingly fix a voltage command value. In the process, a polymer composition of crosslinked component A and soluble components B and C is formed from mono-olefinic and multi-olefinic monomers in the presence of catalyst and initiator.