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Magnetic nanocomposite (hard) SrFe12O19-(soft) La(1-x) SrxMnO3 powders in 4:1 weight ratio was synthesized via a one-pot auto-combustion technique using nitrate salts followed by heat treatment in air at 950°C. Structural and morphological characterizations were performed via x-ray diffraction (XRD) and transmission electron microscopy (TEM). Vibrating sample magnetometer (VSM) was used to study the magnetic properties of the samples. XRD analysis shows presence of pure phase components in the nanocomposite. TEM images show presence of needle shape Sr-Ferrite particles. The room temperature hysteresis loops of the samples showed the presence of exchange-coupling between the hard and soft phases of the composite. The room temperature magnetic measurements revealed the higher Mr/Ms ratio for the nanocomposite than that for the single phase SrFe12O19 which proves the existence of the inter-grain exchange coupling between hard and soft magnetic phases with the exchange spring behavior. The highest Mr/Ms ratio of 0.58 was obtained in SrFe12O19- La0.25Sr0.75MnO3 composite which exhibited 82% increase in the coercivity (Hc∼6.26 kOe) as compared to pure SrFe12O19 (Hc∼3.63kOe). Synthesis of hard-soft exchange couple nanocomposite by controlling the “magnetic softness” of a soft phase instead of more traditional approach of varying concentration of the soft phase could be the promising way to use them in many magnetic applications.

The present study investigates the influence of La 3+ and Pr 3+ doping on the structural, magnetic properties, and hyperfine fields of Sr 0.7 RE 0.3 Fe 12−2x Co x-Al x O 19 , (RE: La 3+ and Pr 3+ , x = 0.0-0.8) hexaferrite compounds prepared via auto-combustion technique. The XRD analysis shows a linear decrease in a and c lattice and unit cell volume contraction with the content x. The room temperature magnetic study shows that for the Pr 3+ doped Sr 0.7 Pr 0.3 Fe 12−2x Co x Al x O 19 (Pr 3+-SrM), the magnetization value monotonically decreases while for La 3+ doped Sr 0.7 La 0.3 Fe 12−2x Co x Al x O 19 (La 3+-SrM) magnetization value shows a noticeable increase in magnetization value with x. The coercivity of the Pr 3+-SrM compound was observed to decrease while that of the La 3+-SrM compound showed a marked 40% increase at x = 0.2 (~5829 Oe) in comparison to undoped SrFe 12 O 19 (~3918 Oe). A difference in Curie temperature was also observed, with Tc ~ 525˚C at x = 0.4 for Pr 3+-SrM and Tc = 505˚C for x = 0.4 for La 3+-SrM compound. The observed differences in magnetic properties have been explained on the basis of the site occupancy of Co 2+ and Al 3+ in the presence of rare-earth ions. The presence of non-magnetic rare-earth ion, La 3+ , improved saturation magnetization, and coercivity and deemed suitable replacement for Sr 2+. The hyperfine parameters namely quadrupole shift showed a decrease with the La 3+ or Pr 3+ doping independent of (Co 2+-Al 3+) ions doping. Overall, the Mossbauer analysis suggests that the (Co 2+-Al 3+) impurities prefer occupancy at 2a site.

The effect of transition metal substitution for Fe and the structural and magnetic properties of Gd 2 Fe 16 Ga 0.5 TM 0.5 (TM = Cr, Mn, Co, Ni, Cu, and Zn) compounds were investigated in this study. Rietveld analysis of X-ray data indicates that all the samples crystallize in the hexagonal Th 2 Ni 17 structure. The lattice parameters a, c, and the unit cell volume show TM ionic radii dependence. Both Ga and TM atoms show preferred site occupancy for 12j and 12k sites. The saturation magnetization at room temperature was observed for Co, Ni, and Cu of 69, 73, and 77 emu/g, respectively, while a minimum value was observed for Zn (62 emu/g) doping in Gd 2 Fe 16 Ga 0.5 TM 0.5 . The highest Curie temperature of 590 K was observed for Cu doping which is 15 and 5% higher than Gd 2 Fe 17 and Gd 2 Fe 16 Ga compounds, respectively. The hyperfine parameters viz. hyperfine field and isomer shift show systematic dependence on the TM atomic number. The observed magnetic and Curie temperature behavior in Gd 2 Fe 16 Ga 0.5 TM 0.5 is explained on the basis of Fe(3d)-TM(3d) hybridization. The superior Curie temperature and magnetization value of Co-, Ni-, and Cu-doped Gd 2 Fe 16 Ga 0.5 TM 0.5 compounds as compared to pure Gd 2 Fe 17 or Gd 2 Fe 16 Ga makes Gd 2 Fe 16 Ga 0.5 TM 0.5 a potential candidate for high-temperature industrial magnet applications.