Structural, Magnetic, and Electronic Properties of Mixed Spinel NiFe_2–<i>x</i>Cr_<i>x</i>O₄ Nanoparticles Synthesized by Chemical Combustion

Abstract

A series of nickel-chromium-ferrite NiFe_2-xCr_xO₄ (with x = 1.25) nanoparticles (NPs) with a cubic spinel structure and with size d ranging from 1.6 to 47.7 nm was synthesized by the solution combustion method. A dual structure of all phonon modes revealed in Raman spectra is associated with metal cations of different types present in the spinel lattice sites. Mössbauer spectra of small NPs exhibit superparamagnetic behavior. However, the transition into the paramagnetic state occurs at a temperature that is unusually high for small particles (T_N is about 240 K in the d = 4.5 nm NPs). The larger NPs with d > 20 nm do not exhibit superparamagnetic properties up to the Neel temperature. From the magnetic and Mössbauer data, the cation occupation of the tetrahedral (A) and octahedral [B] sites was determined (Fe_0.75Ni_0.25)[Ni_0.75Cr_1.25]O₄. The saturation magnetization M_S in the largest NPs is about (0.98-0.95) μ_B, which is more than twice higher the value in bulk ferrite (Fe)[CrNi]O₄. At low temperatures the total magnetic moment of the ferrite coincides with the direction of the B-sublattice moment. In the NPs with d > 20 nm, the compensation of the magnetic moments of A- and B-sublattices was revealed at about T_com = 360-365 K. This value significantly exceeds the point T_com in bulk ferrites NiFe_xCr_2-xO₄ (about 315 K) with the similar Cr concentration. However, in the smaller NPs NiFe_0.75Cr_1.25O₄ with d ≤ 11.7 nm, the compensation effect does not occur. The magnetic anomalies are explained in terms of highly frustrated magnetic ordering in the B sublattice, which appears due to the competition of AFM and FM exchange interactions and results in a canted magnetic structure.