Role of Ni substitution on structural, magnetic and electronic properties of epitaxial CoCr <sub>2</sub> O <sub>4</sub> spinel thin films
P. Mohanty, Sourav Chowdhury, R. J. Choudhary, Anil Gome, V. Raghavendra Reddy, G.R. Umapathy, Sunil Ojha, Emanuela Carleschi, Bryan P. Doyle, A. R. E. Prinsloo, C. J. Sheppard
Abstract
Abstract Cubic spinel CoCr 2 O 4 has recently attained attention due to its multiferroic properties. However, the Co site substitution effect on the structural and magnetic properties has rarely been studied in thin film form. In this work, the structural and magnetic properties of Co 1– x Ni x Cr 2 O 4 ( x = 0, 0.5) epitaxial thin films deposited on MgAl 2 O 4 (100) and MgO (100) substrates to manipulate the nature of strain in the films using pulsed laser deposition (PLD) technique are presented. The epitaxial nature of the films was manifested through x-ray diffraction (XRD), reciprocal space mapping (RSM) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A 1 g and F 2 g modes of the CoCr 2 O 4 with increase in the Ni content. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) studies show a modification of the surface morphology upon Ni substitution. Magnetic measurements disclose that the ferrimagnetic Curie temperature ( T C ) of the CoCr 2 O 4 in thin film grown on MgAl 2 O 4 (100) and MgO (100) substrates were found to be 100.6 ± 0.5 K and 93.8 ± 0.2 K, respectively. With Ni substitution the T C values were found to be enhanced to 104.5 ± 0.4 K for MgAl 2 O 4 (100) and 108.5 ± 0.6 K for MgO (100) substrates. X-ray photoelectron spectroscopy (XPS) suggests Cr 3+ oxidation states in the films, while Co ions are present in a mixed Co 2+ /Co 3+ oxidation state. The substitution of Ni at Co site significantly modifies the line shape of the core level as well as the valence band. Ni ions are also found to be in a mixed 2+/3+ oxidation state. O 1s core level display asymmetry related to possible defects like oxygen vacancies in the films.