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

Kelly L. Pisane; Emily C. Despeaux; Mohindar S. Seehra

doi:10.1016/j.jmmm.2015.02.038

paper

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

arXiv:1501.06842 · doi:10.1016/j.jmmm.2015.02.038

Abstract

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-Fe $_{2}$ O $_{3}$ ) NPs. Transmission electron microscopy (TEM) of the sol-gel synthesized $γ$ - $F e$ $_{2}$ $O$ $_{3}$ 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 K to 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 x 10 $^{10}$ Hz and anisotropy constant $K$ $_{a}$ = 5.48 x 10 $^{5}$ ergs/cm $^{3}$ 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 $σ$ $\approx$ $<$ $μ$ $>$ $/2$ . 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.

Submitted to the Journal of Magnetism and Magnetic Materials

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Magnetic Nanoparticles for Neural Engineering

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

Abstract

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