Origin of the high coercivity in FeNi inspired magnets

In samples taken from iron meteorites, the presence of the nanocrystallites of FeNi with a L1₀ crystalline structure embedded in a complex matrix produces a magnetic behavior of the whole sample characterized by large magnetization at saturation and large coercive field. This has created expectations to obtain strong magnets without rare earths based on the L1₀ FeNi phase. In artificially synthetized samples containing several phases, the presence of the L1₀ phase was often deduced from the observation of coercive fields of several hundreds of Oe. Here, the microstructure and the magnetic behavior of crystallized microwires cast from Fe, Ni and P has been thoroughly analyzed. Microwires containing only two phases: plate-shaped, single-domain, nanocrystallites of the soft fcc (A1) FeNi phase embedded in a schreibersite (Fe_xNi_1-x)₃P matrix show large room temperature coercivities of 440 Oe, which decrease substantially by cooling the sample below the Curie temperature of the matrix. It is shown experimentally that such behavior indicates that the origin of the high coercivity is the microstructure that isolates magnetically the crystallites of the soft FeNi A1 phase. The results point out that the presence or absence of L1₀-FeNi ordered phase is irrelevant in first order to achieve coercivity of hundreds of Oe in these types of alloys. In fact, it is the combination of microstructure and shape anisotropy that causes the high coercivity.