It has been discovered that iron-platinum ferromagnetic particles can be dispersed in a polymer and coated into or onto, or directly linked to or embedded on to, medical devices and magnetized. The magnetized devices are used to attract, capture, and/or retain magnetically labeled cells on the surface of the device in vivo. The magnetic particles have an iron/platinum core. Annealing the Fe/Pt particle is very important for introducing a L10 interior crystalline phase. The Fe:Pt molar ratio for creation of the crystal phase is important and a molar range of 1.2-3.0 Fe to Pt (molar precursors, i.e. starting compounds) is desired for magnetization. The magnetic force as a whole can be measured with a “Super Conducting Quantum Interference Device”, which is a sensitive magnetometer. The overall magnetic force is in the range from 0.1 to 2.0 Tesla.