Abstract
The spinel ferrimagnetic compound CoCr2O4 demonstrates a spin spiral (TS) ordering at 25 K, as well as an anomaly at 15 K termed as lock-in transition (TL). From crystallographic perspective CoCr2O4 retains the cubic phase down to 11 K. On the other hand, the normal spinel CuCr2O4 crystallizes into the cubic phase above 850 K, below which Jahn-Teller (J-T) activity of the Cu reduces the crystal symmetry by transforming it to a tetragonal phase. Contraction of CuO4 tetrahedra towards the formation of a square planar structure accounts for the tetragonal to orthorhombic structural transition at ∼ 130 K associated with the ferrimagnetic Curie temperature (TC). Considering the differences in crystal structure and magnetism of these two compounds, the current work investigates the modification in crystal structure and magnetic behaviour by mixing Co site with Cu in CoCr2O4. To achieve this, (Co1–xCux)Cr2O4 (x = 0.5, 0.25 and 0.75) nanoparticles were prepared by chemical routes. X-ray diffraction (XRD) revealed the retention of cubic structure for the samples calcined at a temperature of 600 °C for x = 0.25 and 0.5. On the other hand, J-T distortion becomes prominent for x = 0.75. Hence, only the compositions with x = 0.25 and 0.5 were studied in detail as unusual cubic phase retention is observed in these compounds. The temperature dependent magnetization studies revealed that the TC values of both the samples, 103 K for (Co0.5Cu0.5)Cr2O4 and 99 K for (Co0.75Cu0.25)Cr2O4, compare well with the value reported for CoCr2O4. However, the feature related to TS is quite prominent for x = 0.25, whereas it is suppressed for x = 0.5. The electronic properties of the cations associated with these compounds, probed using X-ray photoelectron spectroscopy (XPS), indicate that Cu and Co mostly has a 2+ oxidation state whereas that of Cr is 3+.
Highlights
The most fascinating features of this compound relates to the magnetic transitions that occur in the temperature range below the TC
Further increasing x or the calcination temperature to 800 ○C leads to a tetragonal distortion in crystal structure due to J-T effect
Ferrimagnetic TC of (Co0.5Cu0.5)Cr2O4 samples was observed to be higher in comparison to the (Co0.75Cu0.25)Cr2O4 powders
Summary
Multiferroic CoCr2O4 forms a normal cubic spinel structure of the form AB2O4 (A = tetrahedral site occupied by Co2+ ions, B = octahedral site occupied by Cr3+ ions).. The most fascinating features of this compound relates to the magnetic transitions that occur in the temperature range below the TC. These transitions observed at the temperatures associated with the spiral ordering and lock-in transitions, that occur at TS ≈ 25 K and TL ≈ 15 K.1. In order to examine the low temperature magnetic structure, temperature dependent neutron diffraction measurements of the polycrystalline samples were carried out.. The observed low temperature magnetic transitions were substantiated through heat capacity (Cp) and dielectric measurements. These transitions observed at the temperatures associated with the spiral ordering and lock-in transitions, that occur at TS ≈ 25 K and TL ≈ 15 K.1 Experimental findings evidenced that the origin of the multiferroicity stems from the conical spin modulation below TS. In order to examine the low temperature magnetic structure, temperature dependent neutron diffraction measurements of the polycrystalline samples were carried out. Lawes et al. confirmed the stability of the cubic phase up to 11 K and indicated the signature of complex magnetic ordering in this compound. The observed low temperature magnetic transitions were substantiated through heat capacity (Cp) and dielectric measurements.
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