**2. Experimental**

Nanosized NiCrFeO4 , CoCrFeO4 , and Zn CrFeO<sup>4</sup> fuel lean, stoichiometric and fuel rich samples, and fuel rich CoCrx Fe2−xO4 (with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by solution self-combustion technique using analytical grade corresponding to high purity metal nitrates (Merck-Germany) and citric acid (C6 H8 O7 H2 O) as oxidizing agents and fuel, respectively. The ratio of citric acid to metal nitrate (F/N ratio) was taken as 0.65 for fuel lean sample, 1 for stoichiometric sample, and 1.35 for fuel rich samples. The solutions of three nitrates (Fe, Cr, Co/Ni/Zn) and citric acid were prepared separately, mixed, and stirred completely to form a homogenous solution using magnetic stirrer. The homogenous solution was evaporated on a magnetic hot plate. On evaporation, solution gets thickened to a thick gel. The temperature was then increased to 200°C to start the auto-ignition process, which resulted in the formation of a fluffy residue [12–16]. It was powdered and used for further studies.

The X-ray diffraction spectrum of the samples was taken by an X-ray diffractometer (Rigaku MiniFlex) with Cu-Kα radiation. The X-ray diffraction (XRD) patterns of all the samples were fitted with MAUD Rietveld refinement program. DC magnetization measurements were obtained using vibrating sample magnetometer (VSM- Lake Shore 7304) with a maximum applied field of 10 kOe [16].
