Magnetic nanoparticles are applicable in imaging, diagnosis, therapy, and biomaterial separation. The magnetic nanoparticles are represented as (Fe.sub.wGd.sub.x).sub.vZ.sub.y, wherein w is from 99.9% to 97.5%, x is from 0.1% to 2.5%, Z is an element of the group VIa, and v, y are positive numbers.