The 3d transition metals and their alloys with large coercivity and high Curie temperatures are expected to overcome the disadvantages of rare-earth magnets for tomorrow’s green energy needs. Magnetic nanoparticles of 3d transition metals and their alloys not only serve as ultra-high density magnetic recording media, but also pre-constructed units of permanent block in bottom-up way to realize miniaturization of magnetic components. First-principles calculations indicate that the spin-orbit coupling leads to prominent magnetocrystalline anisotropy in monoclinic Fe₃Se₄. Ferromagnetic exchange couplings between magnetic ions in Fe_3-xCr_xSe₄ single crystals were enhanced by suitable Cr substitution in Fe₃Se₄. The lower dimensionality may provide additional tuning capabilities for their magnetism. Monoclinic Fe_3-xCr_xSe₄ nanocacti (0≤x≤2.5) were synthesized using a high-temperature solution chemical method. The average size of the nanocacti was about 1μm with the spikes in a diameter between 60 to 80 nm and a length range from 200 to 500 nm. With increasing the Cr doping, the peak positions in the X-ray diffraction patterns of Fe_3-xCr_xSe₄ nanostructures slightly shifted to lower 2θ values due to the changes in lattice parameters. Expansions in the unit cell volumes of Fe_3-xCr_xSe₄ nanostructures (x>0.3) may have been responsible for enhancing the ferromagnetic (FM) interaction between magnetic ions, which resulted in a significant increase in the Curie temperature from 331 K for Fe₃Se₄ to 429 K for FeCr₂Se₄, distinctly differing from the magnetic properties of the corresponding bulk materials. A room-temperature coercivity analysis showed an obvious increase from 3.2 kOe for Fe₃Se₄ to 12 kOe for Fe_2.3Cr_0.7Se₄ nanostructure, but gradually decreased upon further increasing the Cr content.

Fig. 1.TEM images of (a) a Fe_2.3Cr_0.7Se₄ nanocactus, (b) the spikefeature growing perpendicularly to the surface of the Fe_2.3Cr_0.7Se₄nanocactus in (a). The inset in (b) shows a HRTEM image of the selectedarea in a rod with the lattice fringe spacing of 0.28 nm representing the(2？？02) planes of Fe_2.3Cr_0.7Se₄.

Fig. 1. Cr content dependence of the unit cell volumes, Curie temperature and coercivity of the as-prepared Fe_3-xCr_xSe₄ nanocacti. The inset shows a linear fitting between the TC and the unit cell volume.