Magnetic properties and coupled spin‒phonon behavior of M‒type Sr1-xSmxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles

Abstract

We thoroughly investigated the effects of Sm³⁺ doping on the magnetic, structural, hyperfine, and vibrational properties of Sr1-xSmxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles, which were synthesized using the proteic sol-gel process. The Rietveld refined XRD patterns confirmed the presence of M-type SrFe12O19, alongside a minor fraction of α-Fe2O3. As Sm³⁺ doping increased, the unit cell volume fluctuated, decreasing from 691.73 ų at x = 0 to 690.67 ų at x = 0.10 and slightly rising thereafter. The incorporation of Sm³⁺ induced a conversion of Fe3+ to Fe2+, affecting lattice parameters and crystallinity. At x = 0.10, maximum crystallite size (56.48 nm) and X-ray density (4.83 g/cm³) were attained, alongside decreased microstrain, indicating improved crystallinity and reduced defects. However, at x = 0.15, an increase in microstrain suggested lattice instability. FTIR confirmed that bands in the range 400–600 cm⁻1 might be due to the stretching vibration of oxygen and metal (Fe–O) ions, confirming the formation of hexaferrite. Temperature-dependent Raman spectroscopy studies confirmed the presence of spin-phonon coupling in all samples in the ferrimagnetic transition at ∼725-735 K. Mössbauer spectroscopy elucidated that the substitution of Sr2+ by Sm3+ induced the most pronounced effect on the 12k, 4f2, and 2a sites, corroborating the evolution of the lattice constants and the fact that these three sites have Sr sites in their close vicinity. The room-temperature hysteresis loops exhibited narrow features, indicative of soft magnetic behavior. Saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) varied with Sm3+ doping content (x). Ms initially decreased from 62.62 emu/g and 22.03 emu/g at x = 0.00 to 54.96 emu/g and 19.63 emu/g at x = 0.10, respectively, and then increase to 59.77 emu/g and 21.86 emu/g at x = 0.15. Conversely, Mr and Hc showed a non-monotonic trend with x, suggesting complex interactions between dopant concentration and magnetic properties.