Tuning crystal structure and magnetic property of dispersible FePt intermetallic nanoparticles

Dispersible FePt intermetallic nanoparticles (NPs) with tunable composition were synthesized by thermal annealing of MgO coated Al-FePt (or Al-FePt-Fe3O4) NPs followed by an acid treatment to remove MgO. High-temperature annealing facilitates the conversion of FePt from disordered alloy to ordered intermetallics. Under the protection of MgO, the diffusion of Fe and Pt atoms was limited, making it possible for the atom reconstruction in the lattice to give discrete FePt intermetallic NPs after a facile acid etching process. FePt intermetallic NPs formed face-centered cubic and face-centered tetragonal structures with their magnetic properties tuned by composition. The saturation magnetization was adjusted from 8 to 52 emu g−1 by increasing the Fe concentration, while the coercivity reached a maximum of 33 kOe when Fe concentration was 44%. After surface modifications by hydrophilic or hydrophobic molecules containing thiol groups, FePt intermetallic NPs could be dissolved into water or hydrocarbon solvents. The hydrophilic L10-FePt intermetallic NPs were applied as contrast agents for magnetic resonance imaging, showing a high transverse relaxivity of 328.6 mmol−1 L s−1, which indicated the great potential of FePt intermetallic NPs as molecular probes for cancer diagnosis.