Measurement of Nucleation Temperature in Saturable Multi-domain Grains in Dy-free Nd2Fe14B Based Sintered Magnets

Magnetic grains in Nd₂Fe₁₄B sintered magnets can be categorized into three types: single-domain grains (SDG), multi-domain grains (MDG) and saturable multi-domain grains (SMDG) that are multi-domain grains in thermally demagnetized state, and can be magnetized into SDG with application of a certain magnetic field. After the magnetization, a portion of the magnetized SMDG return to multi-domain state in the 2nd and/or 3rd quadrants in hysteresis loop, and re-magnetized into single-domain state in inverse direction. Another portion of the SMDG, however, never return to multi-domain state with application of reverse magnetic field, and behave as SDG. A portion of the magnetized SMDG shows the magnetic nucleation of domain structure in the 2nd and/or 3rd quadrants in hysteresis loop. In the measurements of thermal behavior of the magnetized SMDG, the authors found a specific temperature at around 500 K where the magnetization of the samples disappear in the measuring condition of zero applied field (Happl. = 0). The Curie temperature (T_C) of Nd₂Fe₁₄B phase is approximately well-known 583 K. Therefore, this point is clearly different from T_C of the samples. We recognized that this point means the nucleation temperature of domain structure in the SMDG. However, notation of (T_N) (N = nucleation) is used for the well-known Neel temperature, consequently, we use notation of T_K (K means "nucleation" in Japanese = "Kakuseisei") in this report for this characteristic temperature. In the presentation, we would like to show and explain the details of measurements and results, and also the physical meaning of the T_K in the Nd₂Fe₁₄B sintered magnets.