Phase transformation and magnetic properties of Fe-Pt-based bulk alloys

The ordering transformation and magnetic properties of Fe_59.75Pt_39.5Nb_0.75 bulk alloys have been investigated in detail by using different homogenization temperatures and different low-temperature annealing times to obtain samples with different microstructure and different atomically ordered states. The experimental results indicate that the disorder–order transformation temperature for the presently studied alloys is around 950 °C. When using high homogenization temperatures (in the range 1100–1350 °C) and medium quenching rates, one can reach high nucleation rates of the atomically ordered phase, so that a fine microstructure of ordered regions with dimensions of 10–20 nm is obtained. A uniform nanoscale microstructure with a wide distribution in the degree of order can be obtained by properly controlling the quenching rate and the annealing time. In this way, a high remanence ratio (m_r=0.74) can be obtained by profiting from the nanocomposite exchange coupling between nearest-neighbor ordered regions with different magnetic anisotropy. We found that, in the present off-stoichiometric magnetic alloy system, the fct phase is magnetically not very hard, like the Nd₂Fe₁₄B phase, and that the fcc phase is not truly magnetically soft due to an increase in the degree of atomic order. Therefore, the samples do not possess the features of an exchange spring magnet as the iron-rich Nd–Fe–B magnets, although the exchange coupling still results in a strong remanence enhancement.