Critical behavior of Zn0.6−xNixCu0.4Fe2O4 ferrite nanoparticles.

We have investigated the critical behavior of Zn 0.6−x Ni x Cu 0.4 Fe 2 O 4 (x = 0, 0.2 and 0.4) ferrite nanoparticles near the ferromagnetic-paramagnetic (FM-PM) phase-transition temperature ( T C ). Experimental results reveal that all samples undergo a second-order phase transition. Through various techniques such as modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis, the estimated critical exponents are close to those expected for three-dimensional Heisenberg class for x = 0 ( β = 0.386 ± 0.002, γ = 1.271 ± 0.012 and δ = 4.387 at T C = 305 K). Whereas for a high amount of Ni, these exponents belong to a different universality class ( β = 0.716 ± 0.063, γ = 0.807 ± 0.008 and δ = 2.010 at T C = 565 K for x = 0.2 sample) and ( β = 0.785 ± 0.004, γ = 0.797 ± 0.002 and δ = 2.061 at T C = 705 K for x = 0.4 sample). This is due to the fact that the substitution of Zn 2+ (non-magnetic ions) by Ni 2+ (magnetic ions) increases the A – B interaction sites of AB 2 O 4 spinel ferrite which in turn increases the magnetic disorder when increasing Ni content. Using the magnetic entropy change equation: | Δ S M m a x | = a ( H ) n , we have studied the relationship between the exponent n and the critical exponents of our samples. The obtained n value are 0.641, 0.843 and 0.873 for x = 0, 0.2 and 0.4, respectively. These values are in good agreement with those deducted from the critical exponents using the KF method. [ABSTRACT FROM AUTHOR]