Structural and magnetic properties of hydrothermally synthesized Bi-substituted Ni–Co nanosized spinel ferrites

Abstract In this study, the structural, morphological, and magnetic properties of hydrothermally synthesized Co0.5Ni0.5BixFe2-xO4 (x=0.00–0.10) nanosized spinel ferrites, (CoNiBiFO (x=0.00–0.10) NSFs), were investigated. The formation of CoNiBiFO NSFs phase was confirmed by X-ray diffraction (XRD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the morphology of the NSFs. Magnetization revealed that undoped Co0.5Ni0.5Fe2O4 and Co0.5Ni0.5BixFe2-xO4 NSFs (except with x = 0.04) exhibit ferrimagnetic magnetism at 300 K. Only the hysteresis loops recorded from Co0.5Ni0.5Bi0.04Fe1.96O4 NSFs have superparamagnetic characteristics. Mixed Co0.5Ni0.5Fe2O4 NSFs have maximum remnant magnetization (Mr) of 23.5 emu/g, maximum saturation magnetization (MS) of 63.38 emu/g, and maximum magneton number (nB) 2.66 μB. However, Co0.5Ni0.5Bi0.04Fe1.96O4 NSFs have minimum MS of 28.25 emu/g and minimum nB as 1.22 μB among Bi3+ doped and undoped samples. The coercivity (HC) of undoped Co0.5Ni0.5Fe2O4 NSFs is 656 Oe. However, the Bi3+ ion-doped samples have a wide range of HC values between 101 Oe and 1038 Oe. The squareness ratios are in the range of 0.092–0.418 and multi-domain structure is assigned for all types of samples. The measured positive exchange bias (HE) magnitudes are 268 Oe and 290 Oe, respectively. Hysteresis loops recorded at 10 K proved that the products are ferrimagnetic. These coercivities showed that all the samples are magnetically harder at low temperatures. Positive HE values of 165 Oe and 297 Oe were measured from the nanoparticles (NPs) with x = 0.08 and x = 0.10 at 10 K. The SQR of the Co0.5Ni0.5Bi0.04Fe1.96O4 sample at 10 K is almost equal to the critical value of 0.5, and a single domain structure with uniaxial symmetry can be attributed for this sample. The other samples have SQRs in the range of 0.632–0.782 and multi-domain wall structure is assigned for them at 10 K.