Role of Ni substitution on structural, magnetic and electronic properties of epitaxial CoCr₂O₄ spinel thin films

Abstract

Cubic spinel CoCr 2 O 4 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 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 confirmed through X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A 1g and F 2g modes of the CoCr 2 O 4 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 (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 transition temperatures significantly get enhanced from that of CoCr 2 O 4 . 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. CoCr 2 O 4 is a ferrimagnetic spinel compound with complex magnetic interactions at low temperatures [1]. The magnetic cations Co 2+ and Cr 3+ ions occupy the tetrahedral A and octahedral B sites, respectively, of the spinel structure [1]. Below Curie temperature (T C = 93 K), a collinear ferrimagnetic ordering occurs, with further lowering the temperature to 26 K (T S ) the compound develops a short-range-ordered (SRO) spiral component [1]. Theoretical calculations has also evidenced the T S feature due to the nesting of the Fermi surface having high degree of magnetic instability in the system associated with the nesting vector q along the [110] direction [2]. Recent experiments have shown the size dependence of these magnetic transitions [3]. Below a critical particle size of d c , spiral = 4.4 (1) nm, the spiral ordering vanishes [3] and for size less than d c , col = 3.3 (1) nm, the collinear magnetic order disappears [3]. A lock-in transition at a temperature T L = 15 K appears because of the incommensurate-to-commensurate magnetic phase transition [1]. At T L the period of the spin spiral eventually "locks in" to the lattice parameter [1]. This spiral component is responsible for inducing the multiferroicity in CoCr 2 O 4 [1]. Structurally, CoCr 2 O 4 is stable and remains cubic with space group Fd ̅ m at all temperatures as the sample is cooled down to 10 K [4].On the other hand for NiCr 2 O 4 the crystal ...

Ce0.8Sr0.2CoxFe1‐xO3‐δ (x=0.2, 0.5, 0.8) – A Perovskite‐type Nanocomposite for Application in the Oxygen Evolution Reaction in Alkaline Media

“…Its normalised Co 2p core level was therefore subtracted from that of the other two samples to give the different spectras “CSCF3‐CSCF1” and “CSCF2‐CSCF1” displayed in the bottom part of Figure 4b as blue and red open circles, respectively. Their line shape was fitted with two spin‐orbit doublets, ascribed to Co 2+ and Co 3+ oxidation states, for the main peaks and one for the high BE satellite [70, 71]. The overall fit is found to superimpose well with the experimental data and confirms the difference in the line shape between the two spectra.…”

Plasmon photothermal-promoted solar photocatalytic hydrogen production over a CoCr2O4/g-C3N4heterojunction

“…32 The O 1s spectrum can be divided into three groups of peaks centred at 529.72 eV, 531.44 eV, and 532.82 eV, which are ascribed to the characteristic peaks of lattice oxygen (O latt ), adsorbed oxygen (O ads ), and free H 2 O, respectively. 33 Particularly, it has been widely reported that the peak at 531.5 eV indicates the presence of a large number of oxygen vacancies in the CoCr 2 O 4 samples, 34,35 which are greatly favourable for enhancing the surface adsorption capacity of the catalysts as well as improving their light absorption properties. After the compositing of CoCr 2 O 4 and g-C 3 N 4 , the binding energies of C and N elements are shifted slightly to the lower energy region, while the binding energies of Co and Cr elements are shifted to the higher energy region.…”

Oxygen-Defect-Mediated ZnCr2O4/ZnIn2S4 Z-Scheme Heterojunction as Photocatalyst for Hydrogen Production and Wastewater Remediation

“…It has been generally recognized that the characteristic peak at 531.4 eV signifies the possible existence of oxygen vacancy defects in the structure of ZnCr 2 O 4 . The presence of abundant oxygen vacancies will form a defective energy level that is able to greatly improve the photocatalytic performance of ZnCr 2 O 4 . , It is worth noting that the relative intensity values of the O ads peak to the O latt peak in the ZCO-ZIS-0.6 composite sample are substantially higher than those in the ZnCr 2 O 4 sample, suggesting that the relative concentration of oxygen vacancies in the composite increases sharply, which should be related to the employment of the strongly reducing glycerol in the compositing experiment. To further verify the existence of oxygen vacancy defects in the ZnCr 2 O 4 sample as well as in the ZCO-ZIS-0.6 composite, electron paramagnetic resonance (EPR) tests are performed, and the results are supplied in Figure f.…”