Magnetic properties of sintered SrFe12O19–CoFe2O4 nanocomposites with exchange coupling

• Two series of SrFe12O19–CoFe2O4 nanocomposite ferrites were prepared.
• The exchange coupling exists in all the sintered composites.
• The Ms of SrM/CFO is improved together with a small decrease of Hc.
• A dramatic enhancement of Hc in CFO/SrM is obtained.

Two series of nanocomposite ferrites sintered at 700 °C and 900 °C, respectively, were prepared using SrFe12O19 and CoFe2O4 nanopowders obtained via a hydrothermal procedure. For the SrFe12O19/CoFe2O4 ferrites with a mass ratio ranging from 3:1 to 1:3, all the specimens exhibit a typical single-phase magnetic behavior, suggesting the existence of exchange coupling. Compared with single-phase SrFe12O19, after the emergence of exchange coupling, the saturation magnetization of composites increases markedly from 59.2 (700 °C) and 64.7 emu/g (900 °C) to 72.4 and 73.7 emu/g with the increase of CoFe2O4 content, respectively, accompanied with a relatively small decrease of coercivity from 173.1 (700 °C) and 232.8 kA/m (900 °C) to 141.7 and 169.2 kA/m, respectively. It is also found that in sintered CoFe2O4/SrFe12O19 composite ferrites of single-phase magnetic behavior with a mass ratio of SrFe12O19 (Rs) from 2% to 10% (with steps of 2%) of the CoFe2O4 mass, a dramatic enhancement of coercivity up to 93% (Rs = 10%, 900 °C) is obtained if compared with the single-phase CoFe2O4 ferrite, though accompanied with a maximum drop of saturation magnetization up to 9.6% (Rs = 10%, 900 °C). The enhancement of coercivity is ascribed to the pinning from SrFe12O19 and the exchange coupling between CoFe2O4 and SrFe12O19 ferrites. Though it may reduce the coercivity of magnetically hard phase, the exchange coupling could also be used to enhance the coercivity of magnetically soft phase.

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