  Cubic spinel CoCr2O4 has attained recent 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 Co1-xNixCr2O4 (x = 0, 0.5) epitaxial
thin films deposited on MgAl2O4 (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 confirmed through X-ray
diffraction (XRD) and Rutherford backscattering spectrometry (RBS)
measurements. Raman measurements revealed a disappearance of characteristic A1g
and F2g modes of the CoCr2O4 with increase in the Ni content. Atomic force
microscopy (AFM) studies show a modification of the surface morphology upon Ni
substitution. Magnetic measurements disclose that the ferrimagnetic Curie
temperature (Tc) of the CoCr2O4 in thin film grown on MgAl2O4 (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 transition temperatures significantly
get enhanced from that of CoCr2O4. X-ray photoelectron spectroscopy (XPS)
suggests Cr3+ oxidation states in the films, while Co ions are present in a
mixed Co2+/Co3+ 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.
