Effect of praseodymium valence change on the structure, magnetic, and microwave absorbing properties of M-type strontium ferrite: the mechanism of influence of citric acid dosage and calcination temperature

Abstract

SrFe12-xPrxO19(x = 0–0.5) powders were synthesized and the effects of praseodymium doping amount, citric acid dosage, and calcination temperature on its structure, magnetic properties, and microwave absorbing properties were studied. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that praseodymium doping leads to the lattice expansion and makes strontium ferrite easier to form spherical polyhedron in microscopic morphology. The Fe2O3 impurity phase disappears and FePr1-ySryO3-δ impurity phase appears with the increase of the calcination temperature. It was inferred from Raman spectra and X-ray photoelectron spectroscopy that some Pr3+ ions form Pr4+ ions and the latter may occupy iron or strontium sites, and the transition from the Fe3+ to Fe2+ may occur at 4f2 site. The vibrating sample magnetometer results show that the saturation magnetization decreases first and then increases; the coercivity increases and the anisotropy field decreases after doping praseodymium. Praseodymium-doped strontium ferrite has strong absorption in low frequency band (C-band) or medium frequency band (X-band), while the matching thickness corresponding to high frequency band (Ku-band) is small. The maximum reflection loss reaches −50.95 dB and the effective absorption bandwidth (RL < −10 dB) is 4.76 GHz. Increasing the calcination temperature or doping amount will reduce the matching thickness, while increasing the amount of citric acid will disperse the grains and reduce the dielectric loss. It can be concluded that praseodymium valence change leads to the increase of Fe2+ ions and oxygen defects and there is also interface polarization between M-phase and heterogeneous phase, which together increases the dielectric loss of the material. Meanwhile, praseodymium doping can significantly increase the magnetic loss caused by natural resonance. These show that praseodymium doping improves the impedance matching of the material, increases the attenuation constant, and significantly improves the wave absorption performance.