Tuning of magnetic transition temperatures in nanoparticles of CoCr ₂O₄ multiferroic by B-site mixing

Pure CoCr_{2 - x}Fe_xO₄ nanoparticles synthesized through conventional coprecipitation technique show particle size distribution in the range of 16-20 and 6-10 nm for x = 0.1 and 0.2 respectively. Magnetic and specific heat measurements show an enhancement of paramagnetic to collinear ferrimagnetic transition temperature, T_c from 100 to 110 K and to a short range non-collinear spiral ordering, T_s from 18 to 36 K with increasing x from 0.1 to 0.2 respectively. In addition, a strong disagreement between the paramagnetic moment obtained from the fitting of χ^-1 = T/C + 1/χ₀ - b/(T - θ) and ferrimagnetic moment measured from the M vs. H loop at 10 K corroborates the nonsaturation behavior of magnetization at 50-100 kOe field and an order of magnitude higher coercivity (H_c). The enhancement of T_c and T_s with increasing Fe concentration is attributed to an intrinsic change in non-collinear to collinear spin structure and strong J_A-B interaction.