AbstractThe observation by magnetic force microscopy (MFM) of Sm2Fe17N3 (SFN) particles 2 to 4 µm in diameter (D), about 15 particles in all, revealed that the particles show a single‐domain state and/or more than three domain states. The magnetic and domain wall energies of the single‐domain particles (SDP) and those of two‐ and three‐domain particles (2DP, 3DP) were calculated from the measured saturated polarization (Js) and the domain wall energy (γ). The calculation results are as follows: (1) the energy difference between 2DP and 3DP is less than 6% for total energy, in the range of ESDP > E3DP > E3DP; (2) 3DP becomes stable, E2DP > E3DP, in particles with D > 3.0 µm, assuming that the critical diameter is 2 µm. A local demagnetizing field and the presence of macroscopic defects in a particle strongly influence the nucleation of domain wall formation, and it is obvious that 2DP has a higher static magnetic energy than 3DP. Therefore, it can be considered that the morphological complexity of real particles results in greater stability of 3DP than 2DP in the sample particles. On the other hand, the discrepancy between the calculated critical diameter of SDP (dc ≈ 0.18 µm) and the measured value (dc ≈ 2 µm) can be explained by the temperature dependence of the relative ratio of the crystal magnetic anisotropy and the saturation polarization in the SFN particles, that is, K = K(T)/KR.T.) versus . It is revealed that the decrease in the anisotropy is more pronounced in the higher temperature region near the Curie temperature than the saturation polarization . This is probably the reason that the measured SDP size is greater than the critical radius theoretically calculated using K1 and Js at room temperature. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(1): 1–8, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20212