Low-density cobalt−phthalocyanine (Co−Pc)/Fe nanocomposite particles with both adjustable electric and magnetic properties were synthesized using one-step thermolysis. The nanocomposite particles were fully characterized by Fourier transform infrared spectroscopy, X-ray diffraction analysis, scanning electron microscopy, high-resolution electron microscopy, and thermogravimetric−differential thermal analysis. The magnetic hysteresis loop and microwave electromagnetic parameters of the nanocomposite particles were measured. The magnetorheological (MR) properties of the MR suspensions based on the nanocomposite particles dispersed in methyl silicone oil were investigated as functions of weight percent of particles, magnetic field strength (H), temperature (T), and shear rate (γ). The results indicated that the Co−Pc/Fe nanocomposite consisted of micrometer-sized regular spheroids with hundreds of thousands of Co−Pc-coated α-Fe nanoparticles on the inside and Co−Pc layers on the surface of the spheroids. It ...