Composite ferrite/steel bar magnets

The design of large bar magnets for producing field at a distance is examined. One of the applications now being considered is the temporary degaussing of merchant ships, where conventional coil systems would be too difficult and expensive to install. Among possible magnet materials, ferrite is stabler and far cheaper than conventional alloys such as Alnico 5. However, it has a comparatively low magnetic moment/mass ratio (MMR), which is a disadvantage in mobile applications. To overcome this disadvantage ferrite is combined with mild steel. Simple magnetostatic analysis indicates that the overall MMR may be optimized to reach the Alnico value. An early composite magnet of simple design gave a lower MMR result, but lost no moment in a year of field testing. The present design consists basically of mild steel rods alternated with ferrite disks. Its proportions are optimized theoretically so as to raise the MMR nearly to the Alnico value, which is confirmed by experiment. Prospective manufacturing details for magnets of size 0.5-5 tons are outlined.