Magnetic properties of amorphous iron

Ultrasonic irradiation of iron pentacarbonyl was shown recently to result in the formation of fine ({}30 nm) amorphous, elemental-iron particles. We have measured the magnetic properties of an aggregate of these particles and find that they are soft ferromagnets with a Curie temperature in excess of 580 K. The exchange stiffness, as manifested through the coefficient of the T3/2 temperature dependence in Bloch's law, is 30% of that of crystalline Fe. The low-temperature magnetization curves approach saturation as the inverse square root of applied field, which is characteristic of systems with random anisotropy fields. We argue that the results are consistent with the properties of an aggregate of amorphous Fe particles of nonspherical shape, giving rise to local shape anisotropy.