Contents lists available at ScienceDirect Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb Cr doped ZnO: Investigation of magnetic behaviour through SQUID and ESR studies P.E. Amami a , J. Das a,b,* , D.K. Mishra c , V.V. Srinivasu a , D.R. Sahu d , B.K. Roul e a Department of Physics, College of Science, Engineering and Technology, University of South Africa, Johannesburg, 1710, South Africa b Department of Physics, Silicon Institute of Technology, Bhubaneswar, 751024, Odisha, India c Department of Physics, Faculty of Engineering and Technology (ITER), Siksha ‘O’ Anusandhan Deemed to be University, Khandagiri Square, Bhubaneswar, 751030, Odisha, India d Department of Physics, Natural and Applied Sciences, Namibia University of Science and Technology, Windhoek, Namibia e Institute of Materials Science, Planetarium Building, Acharya Vihar, Bhubaneswar, Odisha, India ARTICLE INFO Keywords: Semiconductors Sintering Photoluminescence spectroscopy Raman spectroscopy Electron spin resonance ABSTRACT Polycrystalline Zn 1-x Cr x O (0.01 ≤ x ≤ 0.09) samples synthesised by solid state reaction technique were sintered at different temperatures following slow step sintering schedule, investigated for optical and magnetic properties using suitable characterisation techniques. Cr 2 O 3 and CrO 2 phases have been detected in the Raman spectra of Zn 1-x Cr x O samples with x ≥ 0.05. Photoluminescence study indicated improved optical property of the samples compared to undoped ZnO. While low percentage Cr doped samples show diamagnetic behaviour, different types of magnetic orderings are observed in the samples with higher percentage of dopants (x ≥ 0.05) for different sintering temperatures. The spin system and magnetic properties were analysed through Electron Spin Resonance study; g-value of 1.97 indicates Cr in 3+ valence state in ZnO. Presence of both Cr 3+ and Cr 4+ in ZnO understood to facilitate super exchange interactions to promote room temperature ferromagnetism. ESR study ensures improved magnetic homogeneity through slow step sintering process. 1. Introduction It has been a well accepted fact that considerable spin polarised carriers could be created by incorporating magnetic ions onto the host lattice sites of an oxide based semiconductor to behave as a diluted magnetic semiconductor (DMS) [1]. In the past few decades, oxide semiconductors such as ZnO, TiO 2 , and SnO 2 ; with exclusive combined properties of ferro-electricity, high permittivity, superconductivity, magnetism and photo-electricity, have been extensively used for many practical applications. In oxide semiconductors, all these excellent properties may be combined in single configuration to design hyper- intelligent devices. However, for feasible optical spintronics devices like quantum computers and MRAM etc., these candidates should ex- hibit high Curie temperature (T C ) magnetism together with good optical (luminescence) properties. In this regard, transition metal (TM) doped ZnO has claimed wide popularity for reported room temperature fer- romagnetism [2,3] along with its excellent optical properties. In ZnO crystal structure (hcp), tetrahedral sites are occupied by Zn atoms, whereas the octahedral sites are plentily available to accommodate the intrinsic defects and/or extrinsic dopants. As per local density approximation theory [4], in ZnO, 3d states of TM- atoms (Fe, Co, Ni, Cr etc) are capable of showing ferromagnetic ordering, without any extra carrier doping treatment. In the optical absorption spectra of these systems, multiple structures are seen due to the Coulomb interaction between the 3d electrons. Similarly, magnetic interaction between the localized 3d spins and the carriers in the host valence band arises due to hybridization between 3d states of TM atoms and the valence band of the host [5]. By virtue of its direct band gap structure and formation of the stable exciton state [6], ZnO itself can radiate near-ultraviolet light. Doping with transition metal ions also reduces the effect of re- combination radiation in its UV region. However, the vital problem in selecting the material for application is: the dopant materials in ZnO segregate forming precipitates or clusters which can be responsible for the ferromagnetic properties. In addition, there have been various controversies [2,7–9] related to the origin of ferromagnetism in the system which have to be addressed carefully. Among the transition metal elements, Cr could be an important dopant candidate for ZnO, which has close ionic radius to that of Zn and can form a solid solution by substituting Zn in the crystal structure [10–15]. Unlike many other metals, Cr is reported to exhibit https://doi.org/10.1016/j.physb.2019.07.056 Received 21 May 2019; Received in revised form 30 July 2019; Accepted 31 July 2019 * Corresponding author. Department of Physics, Silicon Institute of Technology, Bhubaneswar, 751024, Odisha, India. E-mail addresses: jayashree304@gmail.com, jayashree@silicon.ac.in (J. Das). Physica B: Condensed Matter 572 (2019) 60–65 Available online 01 August 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved. T