Evidence of large exchange bias effect in single-phase spinel ferrite nanoparticles Mritunjoy Prasad Ghosh 1 , Shashank Kinra 2 , Deepak Dagur 2 , Ravi Kant Choubey 2 and Samrat Mukherjee 1 1 Department of Physics, National Institute of Technology Patna, Patna-800005, Bihar, India 2 Department of Applied Physics, Amity Institute of Applied Sciences, Amity University, Noida-201313, Uttar Pradesh, India E-mail: samrat.udc@gmail.com Received 7 July 2020, revised 10 August 2020 Accepted for publication 14 August 2020 Published 26 August 2020 Abstract Sm 3+ doped spinel cobalt ferrite nanoparticles with a generic formula CoSm x Fe 2-x O 4 (x=0.00, 0.06, 0.12 and 0.18) were prepared using wet chemical co-precipitation technique. The structural, optical, magnetic and dielectric characteristics of the samples were investigated carefully. The phase purity and growth of spinel cubic structure was verified by room temperature x-ray diffractograms. Mean crystallite size was observed within the range of 6 nm to 15 nm as calculated from Scherrer’s formula. A blue shift in the indirect optical band gap was noticed with increasing Sm percentage as observed in UV–vis spectra due to the nanosize effect. Superparamagnetic nature at 300 K was detected for all Sm doped ferrite samples. Field cooled (150 kOe) M-H loops obtained at 5 K revealed a large amount of exchange bias field (≈4 kOe) together with high coercivity for the sample having smallest sized particles. Dielectric responses of all samples showed that the hopping of electrons was the fundamental charge conduction mechanism and grain boundaries play a crucial role in determining the dielectric properties. Keywords: ferrite nanoparticles, optical band gap, exchange bias, superparamagnetism, Cole- Cole plot (Some figures may appear in colour only in the online journal) 1. Introduction The exchange bias (EB) phenomena in nanoscale provide a novel approach in improving the anisotropic properties of nanoparticles for prospective applications in spintronics and nanomedicine [1–3]. Recent advancement in the synthesis process of nanoparticles provides a opportunity to explore exchange bias effect in a variety of magnetic nanostructures such as single-phase, core/shell and hybrid composite nanoparticles [4–6]. When two different magnetic phases of a magnetic nanocomposite are in close contact, a new form of unidirectional anisotropy appears due the interactions at the interface. Meiklejohn and Bean were the first to observe this phenomenon in 1956 while working with CoO coated Co particles [7, 8]. A shift in the hysteresis loop was noticed when the system was cooled down through the T N (291 K) of CoO. Although, large number of scientific literature have been reported in past decade on exchange bias system but the development of rigorous modelling still remains incomplete [9–12]. An interface in magnetically coupled system plays a vital role in novel spintronics devices. Importantly, the dynamic behavior of the interface spin structure enables the rise of progressively low dimensional magnetic read heads which are of major importance for non-volatile recording media. Exchange bias and its accompanying properties are fundamental magnetic coupling phenomenon that undergoes at the interfaces of magnetic materials having different anisotropy [13–15]. Nanocrystalline spinel ferrites have been universally scrutinized in last few decades due to their tailorable magn- etic, optical and dielectric properties. The noteworthy growth of spinel ferrites in the diverse field of magnetic storage, magnetic resonance imaging, radio frequency devices, ferro- fluids technology, hyperthermia applications, sensors tech- nology, targeted drug delivery, water purification technology etc is carried out due to their tuneable properties [16]. The Physica Scripta Phys. Scr. 95 (2020) 095812 (11pp) https://doi.org/10.1088/1402-4896/abaf90 0031-8949/20/095812+11$33.00 © 2020 IOP Publishing Ltd Printed in the UK 1