Magnetic behaviour of nanocrystalline Cu–Fe–Co/Al2O3 composite powders obtained by mechanical alloying

• Cu0.5Fe0.25Co0.25 and Cu0.5Fe0.25Co0.25 + Al2O3 showed similar crystal sizes of 35 nm after MA time.
• Alumina particles delay the diffusion mechanism of Co and Fe into the Cu lattice.
• Visualization of magnetic domains in metallic alloyed and composite powders by MFM.
• Magnetic domain walls are fixed by the alumina, resulting in a decrease of Ms and an increase of Hc.
• With milling time, magnetic behavior tends to be increasingly soft ferromagnetic.

The effects of Al2O3 addition on structural and magnetic property changes during mechanical alloying of Cu, Fe and Co element powders were studied systematically by X-ray diffraction, superconducting quantum interference device (SQUID) and magnetic force microscopy (MFM). Milling the powders for up to 10 h leads to the progressive diffusion of Co and Fe atoms into the Cu. Face-centered cubic Cu(Fe,Co) phases with coherent diffraction domains (cdd) size of ∼35 nm were prepared by mechanically alloying Cu0.5Fe0.25Co0.25 with and without Al2O3 powder (1%, 3% and 10%). The effect of crystal size on magnetic properties was studied. Mechanically alloyed metallic–ceramic composite powder showed lower saturation magnetization than the metallic system but enhanced coercive field. Milling time significantly affects the structure, composition, magnetic properties, and magnetic domains for both metallic and composite systems.