Contents lists available at ScienceDirect Materials Characterization journal homepage: www.elsevier.com/locate/matchar Canted surface spins driven exchange anisotropy in erbium substituted nickel ferrite nanoparticles Mritunjoy Prasad Ghosh, Samrat Mukherjee ⁎ Department of Physics, National Institute of Technology Patna, Patna 800005, Bihar, India ARTICLE INFO Keywords: Ferrite nanoparticles Raman spectra Tauc plots Exchange bias Magnetization Superparamagnetism ABSTRACT Single-phase nickel ferrite nanoparticles doped with rare earth Er 3+ ions have been prepared using conventional chemical co-precipitation technique. Existence of pure spinel cubic structure of all samples was confirmed by the X-ray diffraction profiles and Raman modes. Average crystallite sizes ranged from 4 nm to 15 nm were estimated using Williamson-Hall plot. The crystallite size was found to decrease with increasing Er dopants in nanosized nickel ferrites. Compressive microstrain was detected for all the synthesized nanoferrites. Lattice constant in- creased with the increase of Er concentration due to its large ionic diameter. The crystal symmetry of octahedral sublattices was lowered for higher Er content nanoferrites as detected by the Raman spectra. Indirect optical band gaps increased with Er doping as observed in Tauc plots. Existence of exchange anisotropy in Er content nickel nanoferrites was seen by field cooled (7 T) M(H) plots recorded at 5 K. Both the exchange bias and coercivity were observed to increase at 5 K with the increment of Er percentage. Saturation magnetization also decreased with the increase of Er content. The highest Er content nanoferrites exhibited superparamagnetic nature at room temperature as seen from M(T) plots. The irreversibility temperature as well as blocking tem- perature was reduced because of mean size reduction of nanoparticles with increasing Er percentage. 1. Introduction Nanosized magnetic particles have drawn significant attention of researchers in past two decades due to their excellent adjustable structural, electronic, optical and magnetic properties along with var- ious technological and biomedical applications. Several physical prop- erties of a system containing nanoparticles can be tuned by varying size, shape, preparation method, dopant ions and their concentration [1–5]. Due to large number of atoms present on the surface of nanoparticles together with reduction of their co-ordination number with respect to bulk, magnetic nanoparticles exhibit various superior and completely new phenomena [6–8]. In magnetic nanoparticles, ferrimagnetic spinel cubic ferrite nanocrystals have been investigated broadly in recent years. Ferrite nanoparticles also have wide applications in nanoelec- tronics, hyperthermia, targeted drug delivery and electromagnetism [9,10]. Among spinel ferrites, nanosized nickel ferrite is widely used for its semiconducting nature, soft magnetic behavior, better chemical stabilities, moderate saturation magnetization (M S ), coercive field (H C ) and Néel temperature (T N ). It also displays almost complete inverse spinel cubic structure with an inversion factor i = 0.99 in bulk form [3,10,11]. Rare earth Er 3+ ions display paramagnetic behavior above 84 K (T N ) and also favour to occupy octahedral voids in spinel structure due to its large ionic radii (0.89 Å) with respect to transition metal ions [8,12]. The substitution of large size Er dopants in nanosized nickel ferrites within the percolation restriction alters the structural and magnetic properties significantly. Due to the large diameter of Er do- pants, the doping of Er 3+ ions on nickel nanoferrites distort the lattice symmetry and lead to change in structural and electronic properties [8,11]. Rare earth ions also display strong spin-orbit coupling along with strong magnetic moment at low temperature due to quenching of orbital momentum, the magnetic moment of transition metal ions come from spin contributions only [12,13]. Rare earth ions also prevent the crystal growth during chemical synthesis because of their large size and limited solubility in spinel ferrites which result in reduction of mean crystallite size [8]. Magnetic ferrite nanocrystals can be synthesized using several bottom up and top down approaches. Spinel ferrite nanoparticles of average diameter less than 10 nm exhibit considerable exchange anisotropy effect at low temperature [7,8,14]. It is due the coupling between ordered core spins and canted surface spins at the interface of the nanoparticles. The exchange ani- sotropy has been first observed in 1956 by Meiklejohn and Bean in Co/ CoO nanoparticulates [15,16]. Exchange bias phenomenon has been utilized in storage devices, spintronics devices and permanent magnetic applications [17,18]. The induced anisotropy due to strong pairing of https://doi.org/10.1016/j.matchar.2020.110203 Received 9 January 2020; Received in revised form 13 February 2020; Accepted 13 February 2020 ⁎ Corresponding author. E-mail address: samrat.udc@gmail.com (S. Mukherjee). Materials Characterization 162 (2020) 110203 Available online 14 February 2020 1044-5803/ © 2020 Elsevier Inc. All rights reserved. T