Synthesis of PAA/NiFe2O4 composite nanoparticles and the effect of microstructure on magnetism

•Different microstructure of composite nanoparticles were synthesized.•Procedure can influence the microstructure and magnetism of composite nanoparticles.•Unidentate and bridged coordination mode formed according to different procedure.•Magnetism of composite particles are influenced by its microstructure. Nanosized NiFe2O4 and various microstructures composite nanoparticles of polyacrylic acid (PAA)/NiFe2O4 with the mass ratio of PAA to NiFe2O4 being 0.1, 0.2, 0.3 and 0.4, respectively, have been synthesized via two different hydrothermal procedures. The phase structure, morphology and magnetic property of composite nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM) and superconducting quantum interference device (SQUID). The results indicated that two types of coordination mode between PAA chains and NiFe2O4 nanoparticles had formed. One type was unidentate coordination bonds which resulted in core shell structure with a magnetic core of ferrite and a PAA shell. The increasing of PAA content led to thicker coating, lower saturation magnetization and coercivity of composite nanoparticles. Another type was bridged coordination which showed the character of PAA chains twisting to form network and ferrite particles being imbedded into PAA chains. With increased PAA content, their saturation magnetization and coercivity became decreased but with more slowly reduction than those of core shell structure samples. The effect mechanism of microstructure on magnetism was also studied.