Exchange coupling in FePt permanent magnets

Abstract

FePt (for 40–60 at. % Fe) exhibits an order–disorder transformation. The disordered phase is face centered cubic and magnetically soft while the ordered phase is tetragonal and shows high magnetic anisotropy. Since the changes in volume between the two phases are small, it is easy for the soft and hard phases to coexist in a uniform manner. Thus, we have an ideal system with which to investigate the basic features of exchange coupled magnets. Bulk Fe0.6Pt0.4 exhibits reasonably large permanent magnetic properties with a maximum energy product of ∼15 MG Oe (120 kJ/m3) without the need for special processes to promote grain alignment. The high energy product is partially a result of the high ratio of remanence to the saturation induction which amounts to 0.68 as opposed to the ratio of 0.5 for an assembly of randomly oriented uniaxial magnets. This enhanced remanence is predicted by the exchange-spring magnet model for a mixture of cubic and uniaxial phases. In order to verify that the high remanence of the FePt magnets is indeed caused by this exchange mechanism and not by a fortuitous formation of magnetic texture during sample preparation, we have prepared cubic samples of Fe0.6Pt0.4 and measured the B–H loops along the three principal directions of the cube using a vibrating sample magnetometer. The hysteresis loops are identical in all three directions, indicating the absence of strong texture in this material. After correction of the demagnetizing factor, the ratio of remanence to saturation induction is 0.66–0.68. This is consistent with that in the exchange coupling model. The highest energy product in our study was found for the as-quenched samples. Further annealing, with the intention of promoting the formation of hard magnetic phases, reduced the coercivity and remanence. This suggests that improved results may be achieved by initial suppression of the formation of the hard phases by modification of the quenching process and alloying. An ideal exchange coupled system could be designed based on such a starting material.