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.

Non-Aqueous Sol-Gel Synthesis of FePt Nanoparticles in the Absence of In Situ Stabilizers

“…Iron platinum (FePt) nanoparticles gained high scientific interest due to their large uniaxial magnetocrystalline anisotropy [ 1 , 2 , 3 ], high saturation magnetization and coercivity [ 4 , 5 , 6 , 7 ], as well as good chemical stability [ 1 , 8 ]. Those characteristics are promising with regard to potential applications, such as magnetic hyperthermia [ 4 , 9 , 10 ], biomedical imaging [ 11 , 12 ], ultrahigh density magnetic recording [ 13 , 14 , 15 , 16 , 17 ], or advanced permanent magnets [ 18 , 19 , 20 , 21 ]. Wet-chemical synthesis of nanosized FePt enables the preparation of monomodal nanoparticles with well-defined morphologies and particle sizes [ 1 , 16 ].…”

Effect of B content on the magnetic properties, phase evolution, and aftereffect of nanocrystalline FeCoPtB ribbons

“…On the other hand, it is noted that high reduced remanence ratio ͑ r / 12 kOe ͒ of 0.78-0.86 is obtained for this series ribbons, and it is higher than 0.68 for the bulk, 12 and 0.5 for an assembly of randomly oriented uniaxial magnets. 13 Such high remanent magnetization and reduced remanence ratio suggest that multiple magnetic phases with strong exchange-coupling effect might exist in the present quaternary FeCoPtB ribbons. In this study, a B r of 10.1 kG, i H c of 5.4 kOe, and ͑BH͒ max of 15.7 MG Oe have been obtained in ͓͑Fe 0.7 Co 0.3 ͒ 0.725 Pt 0.275 ͔ 85 B 15 ribbons.…”

Effect of boron on the magnetic properties and exchange-coupling effect of FePtB-type nanocomposite ribbons

“…It is noted that high reduced remanence ratio ͑ r / 12 kOe ͒ of 0.75-0.86 is obtained, which is much higher than ϳ0.68 for the bulk, 5 and 0.5 for an assembly of randomly oriented uniaxial magnets. 6 Such high remanent magnetization and reduced remanence ratio suggest that multiple magnetic phases with strong exchange-coupling effect might exist in the present ternary FePtB ribbons. In this study, a B r of 8.9 kG, i H c of 8.2 kOe, and ͑BH͒ max of 13.2 MGOe and a B r of 9.4 kG, i H c of 7.5 kOe, and ͑BH͒ max of 14.0 MGOe have been obtained in ͑Fe 0.675 Pt 0.325 ͒ 82 B 18 and ͑Fe 0.675 Pt 0.325 ͒ 84 B 16 ribbons, respectively.…”