Figure 1 (a) shows the TEM image of as-synthesized FePt NPs, the NPs are typical concave-cube shape with uniform size. TEM images of the samples after annealing under 0, 6 and 12 T HMF are showed in Fig. 1 (b)~(d), respectively. From the TEM images, it can be found that the shape-anisotropic concave-cube NPs transform to sphere after annealing, the size uniformity is poor and some NPs had grown abnormally in the annealed samples. Although the NaCl medium is employed to avoid aggregation of the NPs during annealing, the random abnormal growth of the NPs is still unavoidable [11, 26]. As shown in Fig. 1 (e1) and (e2), the lattice fringes of as-synthesized and 0 T-annealed FePt nanoparticles display interplanar spacing of 0.20 and 0.27 nm in the particle, which match well respectively with those of the (111) and the (110) planes of the A1 and L12 phases. The SAED patterns of as-synthesized and 12 T-annealed NPs are showed in Fig. 1 (f1) and (f2), respectively. After annealed at 12 T, the diffraction rings of (001), (110) and (002) faces are collected, which indicates that the L10-FePt NPs are generated at 12 T.
The grain sizes of the NPs are collected from TEM images, the size distributions are showed in Fig. 2(a). The distributions of the NPs fit well with Gauss Function, average size of the as-synthesized NPs is 11.55 nm, and most of sizes are between 9 and 14 nm. After annealed, the size distributions are moved to left-side, most sizes are between 5 and 11 nm, and the grain size of the FePt NPs decreases to about 8.51 nm. The average sizes of the NPs annealed under 0, 6 and 12 T HMF are consistent within the error range, which means the HMF strength shows no effect on sizes of the annealed NPs.
The EDS patterns of the NPs are showed in Fig. 2 (b). It should be noticed that the relative intensity of Pt Lα peak decreases after annealing, and the Fe content in the NPs is increased from 51.6% to about 72.1%, which means the Pt atoms in FePt NPs are decreased after annealing. Generally, the composition of the NPs is consistent before and after annealing. In this research, the as-synthesized concave-cube FePt NPs are shape and element anisotropic, the NPs will become spherical and the Pt-rich corners will fall off from the FePt concave-cubic NPs during annealing. Due to the higher Pt content and smaller size of the corners, it is hard to be collected through centrifugation, and lead to the decreasing of Pt content in the NPs.
Fig. 3 shows the XRD patterns of as-synthesized and post-annealed FePt NPs. The as-synthesized FePt NPs is A1-FePt phase with a disorder fcc structure [9]. After annealing without of the HMF, the diffraction peaks of (001), (110) and (201) faces are detected, considering the composition of the NPs closes to Fe70Pt30, the peaks can be indexed as L12-Fe3Pt phase. To diffraction patterns of samples applied the 6 and 12 T HMF during annealing, a remarkable change can be found that the (002) peak of A1-FePt divides into two peaks, which indicates that the L10-FePt phase forms at the 6 and 12 T HMF. The separation of (200) and (002) peaks is the most important characteristic of L10-FePt, the distance between those two peaks relates to the ordering degree of L10-FePt, which suggest that the ordering degree of L10-FePt is obviously enhanced at the 12 T HMF [26, 27].
The ordering degree s of the NPs calculates by s = 0.85×[I(001)/I(002)]0.5, where I(001) and I(002) are the intensity of (001) and (002) peaks, respectively. The s increases from 0.46 to 0.65 when increasing the HMF from 6 to 12 T, also indicating that the ordering degree can be enhanced by increasing HMF strength. Both the diffraction peaks of L12-Fe3Pt and L10-FePt can be found in the HMF-annealed samples, the content of L10-FePt phase in the samples annealed at the 6 and 12 T has been roughly calculated and it increases from 16.8–25.1%, which means the increasing of the HMF strength also increases the content of the L10-FePt NPs. The crystal structure of the concave-cube A1-FePt NPs transforms to L12-Fe3Pt after annealed, application of the HMF will induce formation of L10-FePt, the ordering degree and the content of L10-FePt increases with the enhancing of HMF strength.
The room-temperature hysteresis loops of the as-synthesized and the post-annealed FePt NPs are showed in Fig. 4. The nonzero coercivity is detected in the post-annealed FePt NPs, it means the as-synthesized superparamagnetic FePt NPs transform to ferromagnetic after annealing. While application of the 0, 6 and 12 T HMF during annealing, the coercivity of the FePt NPs increases from 266 to 363 and 489 Oe, respectively. The coercivity of the annealed FePt NPs relates to the grain size and effective anisotropy [23]. Although the HMF shows no effects on grain sizes of NPs, the HMF induces formation and ordering degree enhancement of L10-FePt NPs increase the effective anisotropy and coercivity of FePt NPs. The coercivity of the sample annealed at 12 T is not so high, because the larger component deviation (far from 1:1) will lead to the less content and lower ordering degree of L10-phase in the annealed FePt NPs [23, 26, 27].
The effects of HMF annealing on concave-cube FePt NPs are summarized in Fig. 5. The as-synthesized concave-cube FePt are both shape and element anisotropic, the Pt atoms rich in the corner-site. After annealing, the shape anisotropic FePt NPs become spherical, the Pt-rich corner is fell off from the concave-cube FePt NPs. As the mass of the Pt-rich corner is too small to be centrifugal collected, the Pt content and grain size of the annealed FePt NPs is decreased. The Fe/Pt atoms ratio of FePt NPs closes to 3:1 after annealing, which leads to the formation of orderly L12-Fe3Pt phase. This result also indicates that the temperature of spheroidization of concave-cube FePt NPs is lower than that of disorder-order transformation temperature, the Pt-rich corner are fell off from the NPs firstly, and then the residual Fe and Pt atoms are diffused to form the ordered phase.
The strength of the HMF cannot affect the shape, size and composition evolutions of FePt NPs during annealing, but induces the formation of L10-FePt NPs. In the equilibrium phase diagram of FePt alloys, the maximum Fe content in L10-phase is about 70% [27, 28]. The Fe content in annealed NPs is about 72.1%, closes to the critical value, which suggests the possibility of formation L10-FePt NPs. As the temperature of spheroidization is lower, the Pt-rich corner are fell off still. However, the HMF prefers the formation of L10-phase, because the magnetocrystalline anisotropy energy of L10-phase is higher than that of L12-phase. The nucleation of L10-phase and ordering diffusion of Fe/Pt atoms will be enhanced by the HMF induced magnetization energy [19–23]. Therefore, the content and ordering degree of L10-FePt in the annealed NPs is increased by increasing HMF strength. However, as the Fe/Pt atoms ratio of the annealed FePt NPs is far from 1/1, only a very small amount of L10-FePt NPs with lower orderly degree can be generated.