Vol.:(0123456789) 1 3 Journal of Materials Science: Materials in Electronics (2020) 31:435–443 https://doi.org/10.1007/s10854-019-02546-9 Efect of doping diferent rare earth ions on microstructural, optical, and magnetic properties of nickel–cobalt ferrite nanoparticles Kamar Tanbir 1  · Mritunjoy Prasad Ghosh 2  · Rakesh Kumar Singh 1  · Manoranjan Kar 3  · Samrat Mukherjee 2 Received: 10 August 2019 / Accepted: 7 November 2019 / Published online: 19 November 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract The infuence of doping diferent rare earth ions (Gd 3+ , Sm 3+ , and Eu 3+ ) on the structural, optical, and magnetic properties of Ni–Co spinel ferrite nanoparticles synthesized via chemical co-precipitation method was investigated. The formation of pure cubic spinel phase along with complete solubility of large-sized rare earth ions inside the crystal structure was verifed by the X-ray difractograms. A slight reduction in crystallite size due to hindered crystal growth by the large-sized rare earth ions was also observed. The mean particle size estimated from HRTEM micrograph also followed the X-ray difraction results closely. The paramagnetic nature of rare earth ions at room temperature also weakens the superexchange interactions between sub-lattices. The coercivity, saturation magnetization, and Curie temperature were observed to reduce in comparison to the pristine sample values due to rare earth ion substitution. The enhancement in indirect allowed optical band gap for all the nanoferrites from the bulk value is attributed to the nanosize efect of the prepared particles. 1 Introduction The tremendous progress in ferrite nanoparticles research in last two decades is due to their promising application in storage device, ferrofuids technology, magnetic imaging, sensors, hyperthermia, and drag delivery [1–4]. Recently, mixed cubic spinel nanoferrites have become the center of attraction for researchers because of their tunable properties together with signifcant applications in modern technology. The tuning of magnetic and dielectric properties of ferrites created a wide application in electronics and electromagne- tism. Among mixed ferrites group, the nickel–cobalt spinel ferrites have moderate coercivity and signifcant saturation magnetization [1, 5]. Tuning of structural, magnetic, opti- cal, and dielectric properties of spinel ferrites using various kinds of dopant ions has been performed widely. In cubic spinel ferrite group, the mixed nickel–cobalt nanoferrites exhibit inverse spinel structure where Fe 3+ ions are almost equally distributed among octahedral and tetrahedral voids. Nickel ferrite is well known for its soft magnetic and semi- conducting nature. Nickel ferrite exhibits smaller coercive feld (around 500 Oe) and it is easier to magnetize or demag- netize using weak magnetic feld as well as its magnetic energy losses are also smaller with respect to hard cobalt ferrite. It also exhibits indirect optical band gap near 1.65 eV and can be considered as a semiconductor among other spi- nel ferrites. The substitution of minute amount of Co 2+ ions (20%) in place of Ni 2+ ions in nickel ferrites also results in signifcant changes in magnetic and microstructural behavior due to its high efective anisotropy constant [6–8]. In nano- size, as the overall behavior of any system is determined by the surface atoms, thereby the system displays interesting and superior phenomenon in comparison to its bulk form. Several surface efects like as spin pinning, spin canting, and magnetically dead surface layer also infuence the magnetic properties of nanosize ferrite particles [9]. The substitution of diferent rare earth ions (Gd 3+ , Sm 3+ , and Eu 3+ ) in Ni–Co nanoferrites also afects various struc- tural and physical properties signifcantly. This is due to their large ionic radii in comparison to Fe 3+ , Co 2+ , and Ni 2+ ions along with magnetic ordering well below room temperature. The rare earth Gd 3+ ions exhibit magnetic moment of 8.9 μ B below 289 K along with paramagnetic behavior at room temperature [2, 10–14]. The other two rare earth Sm 3+ and Eu 3+ ions display signifcant diference in * Samrat Mukherjee samrat.udc@gmail.com 1 Aryabhatt Centre for Nanoscience and Nanotechnology, Aryabhatt Knowledge University, Patna, Bihar 800001, India 2 Department of Physics, National Institute of Technology Patna, Patna, Bihar 800005, India 3 Department of Physics, Indian Institute of Technology Patna, Patna, Bihar 800013, India