Effect of cobalt substitution on the magnetic properties of nickel chromite

Abstract 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 AB 2 O 4 is currently drawing much attention, where A refers to tetrahedral and B to octahedral sites. In the particular case of NiCr 2 O 4 the material undergoes several structural phase transitions associated with the magnetic ordering. It is therefore necessary to study the magnetic behaviour of NiCr 2 O 4 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 NiCr 2 O 4 by doping with Co 2+ at A sites. In order to achieve the afore mentioned, (Ni 1− x Co x )Cr 2 O 4 (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 Cr 2 O 3 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 NiCr 2 O 4 and CoCr 2 O 4 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 0.5 Co 0.5 )Cr 2 O 4 and (Ni 0.25 Co 0.75 )Cr 2 O 4 , respectively. The (Ni 0.5 Co 0.5 )Cr 2 O 4 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 0.5 Co 0.5 )Cr 2 O 4 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 NiCr 2 O 4 can easily and dramatically be modified by doping Co 2+ at A sites.