Effect of nitrogen substitution on the structural and magnetic ordering transitions of NiCr2O4

Abstract

The nitrogen (N) doped spinel NiCr₂O₄ has been synthesized at 773 K (N500) and 873 K (N600) by ammonolysis of NiCr₂O₄ powders to study the effect of anion doping on its structural and magnetic properties. The N contents are determined by thermogravimetric oxidation, yielding a composition that can be described as NiCr₂O_3.68N_0.21 (N500) and NiCr₂O_3.55N_0.30 (N600). X-ray photoelectron spectroscopic studies suggest that N³⁻ species partly substitute the oxygen in the lattice and oxygen vacancies exist in the N doped samples. There is evidence that in the nitrided sample, the Cr ion is most likely in a mixed oxidation state. As the N content increases, the structure at room temperature changes from tetragonal to the cubic phase; N500 is only partially tetragonal; N600 is completely cubic. Such structural change is the consequence of the depression of the cooperative Jahn–Teller effect of Ni²⁺ in the tetrahedral A site caused by the presence of N³⁻. Combined heat capacity and temperature dependent magnetic susceptibility measurements give clear evidence of the magnetic and structural transitions in the N doped NiCr₂O₄. The Jahn–Teller transition temperatures decrease with increasing N content; this is likely due to increased covalency and hence enhanced contribution of the angular momentum and the spin–orbit coupling to local chemical bonding around Ni²⁺. Antiferromagnetic transitions are observed at T_S = 23 K and 22 K for N500 and N600, respectively. Hence there is indeed a lowering of transition when compared to pure NiCr₂O₄ (28 K). The magnetic loops at different temperatures confirm that the material behaves as a paramagnet over a wide range of temperatures T ∼ 80–350 K. The material also exhibits a canted ferrimagnetic structural transition between 30 and 70 K. We also report evidence for increased frustration and lowered correlation length in N doped compounds compared to the parent NiCr₂O₄. The present study on N⁻ doping effects on the structure and magnetic properties of this NiCr₂O₄ is expected to be useful for tailoring the ferric phase transitions through anion substitutions.