Magnetic Fe@FeO_x, Fe@C and α-Fe₂ O₃ single-crystal nanoblends synthesized by femtosecond laser ablation of Fe in acetone

There are few reports on zero-field-cooled (ZFC) magnetization measurements for Fe@FeO_x or FeO_x particles synthesized by laser ablation in liquids (LAL) of Fe, and the minimum blocking temperature (T_B) of 120 K reported so far is still much higher than those of their counterparts synthesized by chemical methods. In this work, the minimum blocking temperature was lowered to 52 K for 4–5 nm α-Fe₂ O₃ particles synthesized by femtosecond laser ablation of Fe in acetone. The effective magnetic anisotropy energy density (K_eff) is calculated to be 2.7–5.4 × 10⁵ J/m³, further extending the K_eff values for smaller hematite particles synthesized by different methods. Large amorphous-Fe@α-Fe₂ O₃ and amorphous-Fe@C particles of 10–100 nm in diameter display a soft magnetic behavior with saturation magnetization (M_s) and coercivities (H_c) values of 72.5 emu/g and 160 Oe at 5 K and 61.9 emu/g and 70 Oe at 300 K, respectively, which mainly stem from the magnetism of amorphous Fe cores. Generally, the nanoparticles obtained by LAL are either amorphous or polycrystalline, seldom in a single-crystalline state. This work also demonstrates the possibility of synthesizing single-crystalline α-Fe₂ O₃ hematite crystals of several nanometers with (104), (113), (116) or (214) crystallographic orientations, which were produced simultaneously with other products including carbon encapsulated amorphous Fe (a-Fe@C) and Fe@FeO_x core-shell particles by LAL in one step. Finally, the formation mechanisms for these nanomaterials are proposed and the key factors in series events of LAL are discussed.