Effect of cobalt substitution on the magnetic properties of nickel chromite

Highlights: • Structural and magnetic properties of (Ni_1−xCo_x)Cr₂O₄ (size from TEM: 50 – 100 nm) were presented using chemical co-precipitation techniques. • The values of T_C determined for both NiCr₂O₄ and CoCr₂O₄ was higher than the reported values with noticeable increase upon substitution of Ni by Co. • For the case of (Ni_0.5Co_0.5)Cr₂O₄, high coercivity of 3.6 T and shifting of the M-H loop under field cooling condition was observed. • X-ray photoelectron spectroscopy (XPS) of (Ni_0.5Co_0.5)Cr₂O₄ suggests Ni and Co are in 2+ oxidation state and Cr in 3+ oxidation state. • EXAFS measurements confirmed that no redistribution of A and B site cations occur at low temperatures by doping Co²⁺ at A sites. It is of interest to study the magneto-structural coupling in geometrically frustrated antiferromagnets, where structural distortion elevates the ground state degeneracy, leading to a long-range magnetic order. In this regard a cubic spinel compound of the form AB₂O₄ is currently drawing much attention, where A refers to tetrahedral and B to octahedral sites. In the particular case of NiCr₂O₄ the material undergoes several structural phase transitions associated with the magnetic ordering. It is therefore necessary to study the magnetic behaviour of NiCr₂O₄ by substituting either A or B sites, or both systematically with suitable cations. The current work therefore aims at the modification of magnetic properties of NiCr₂O₄ by doping with Co²⁺ at A sites. In order to achieve the afore mentioned, (Ni_1−xCo_x)Cr₂O₄ (0 ≤ x ≤ 1) were prepared using chemical co-precipitation techniques. X–ray diffraction (XRD) results indicate that the samples are in the expected phase without any trace of Cr₂O₃ impurities after calcination. Transmission electron microcopy (TEM) analyses of these samples show that the particles are mostly bi-pyramidal in shape, with sizes ranging from 50 nm to 100 nm. In the present study the ferrimagnetic transition temperatures (T_C) of the various samples were determined utilizing magnetization as function of temperature measurements. T_C for NiCr₂O₄ and CoCr₂O₄ was determined to be 82.4 ± 0.8 K and 99.5 ± 0.5 K, respectively. These values are higher than those previously reported in the literature for both these compounds. Substitution of Ni by Co, results in an increase in the T_C, giving values of 89.2 ± 0.7 K and 90.6 ± 0.9 K for (Ni_0.5Co_0.5)Cr₂O₄ and (Ni_0.25Co_0.75)Cr₂O₄, respectively. The (Ni_0.5Co_0.5)Cr₂O₄ sample demonstrated a high coercivity of 3.6 ± 0.1 T and a shift in the hysteresis loop observed under field cooled measurement, not previously reported in literature. X–ray photoelectron spectroscopy (XPS) of (Ni_0.5Co_0.5)Cr₂O₄ suggests that the oxidation states of Ni and Co are 2+, while that of Cr is 3+. In order to investigate the local structure around the cations, low temperature extended x–ray absorption fine structure (EXAFS) measurements were performed. From EXAFS it is confirmed that no redistribution of A and B site cations occur at low temperatures. Present findings demonstrate that the magnetic properties of NiCr₂O₄ can easily and dramatically be modified by doping Co²⁺ at A sites.