Magnetic properties of nanoparticles of Prussian blue-based molecular magnets M₃[Cr(CN)₆]₂·zH₂O (M=Fe, Co, and Ni)

Abstract

The nanoparticles of Prussian blue-based molecular magnets, M₃[Cr(CN)₆]₂·zH₂O (where M=Fe, Co, and Ni), prepared by a slow addition (drop by drop) of chemicals using the co-precipitation method, are investigated by means of X-ray diffraction, infra red spectroscopy and dc magnetization measurement techniques. The formation of nanoparticles has been confirmed by scanning electron microscopy, whereas the characteristic peak, observed in the range of 1900–2300 cm⁻¹ in the infrared spectra, corresponds to the CN stretching frequency of Cr^+III–CN– M^+II, and confirms the formation of Prussian blue compounds. The results, derived from the Rietveld refinement of X-ray diffraction patterns, reveal that all samples are nanocrystalline in nature with a face-centered cubic crystal structure of space group Fm3m. The particle size and the lattice constants decrease with an increasing atomic number of the transition metals (M=Fe, Co and Ni). The magnetization data show a magnetically ordered state of all nanoparticle samples with a low coercivity (except for the Fe₃[Cr(CN)₆]₂·zH₂O) as well as the remanent magnetization. In addition, by varying M with Fe, Co and Ni, the magnetic ordering temperature increases from ∼12 to ∼28 K, whereas the maximum magnetization and the coercive field decrease from ∼14 to ∼4.5 μ_B/f.u. and ∼554 to ∼22 Oe, respectively. The observed magnetization behavior has been discussed in terms of the structural changes due to the decreasing particle size as well as the varying nature of the metal ions.