Magnetic relaxation and correlating effective magnetic moment with particle size distribution in maghemite nanoparticles

The role of particle size distribution inherently present in magnetic nanoparticles (NPs) is examined in considerable detail in relation to the measured magnetic properties of oleic acid-coated maghemite (γ-Fe₂O₃) NPs. Transmission electron microscopy (TEM) of the sol-gel synthesized γ-Fe₂O₃ NPs showed a log-normal distribution of sizes with average diameter 〈D〉=7.04 nm and standard deviation σ=0.78 nm. Magnetization, M, vs. temperature (2-350 K) of the NPs was measured in an applied magnetic field H up to 90 kOe along with the temperature dependence of the ac susceptibilities, χ′ and χ″, at various frequencies, f_m, from 10 Hz to 10 kHz. From the shift of the blocking temperature from T_B=35 K at 10 Hz to T_B=48 K at 10 kHz, the absence of any significant interparticle interaction is inferred and the relaxation frequency f_o=2.6×10¹⁰ Hz and anisotropy constant K_a=5.48×10⁵ erg/cm³ are determined. For T<T_B, the coercivity H_C is practically negligible. For T>T_B, the data of M vs. H up to 90 kOe at several temperatures are analyzed two different ways: (i) in terms of the modified Langevin function yielding an average magnetic moment per particle μ_p=7300(500) μ_B; and (ii) in terms of log-normal distribution of moments yielding 〈μ〉=6670 μ_B at 150 K decreasing to 〈μ〉=6100 μ_B at 300 K with standard deviations σ 〈μ〉/2. It is argued that the above two approaches yield consistent and physically meaningful results as long as the width parameter, s, of the log-normal distribution is less than 0.83.