permanent magnetic field
Technical term in physics and engineering for a magnetic field that does not change over time, typically produced by a permanent magnet.
Motors that use a permanent magnetic field are highly efficient.
電磁場 (electromagnetic field) is a broader concept that includes both electric and magnetic fields, and can vary with time, while 永久磁場 specifically refers to a static magnetic field from a permanent magnet.
Compound of 永久 (eternal, permanent) and 磁場 (magnetic field).