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

• Published in: Journal of magnetism and magnetic materials Vol. 384; pp. 148 - 154
• Main Authors: Pisane, K.L., Despeaux, E.C., Seehra, M.S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15-06-2015
• Subjects: Coercive force; Constants; Interparticle interaction; Maghemite nanoparticles; Magnetic moment; Magnetic moment distribution; Magnetic properties; Magnetic relaxation; Magnetization; Nanoparticles; Particle size distribution; Standard deviation; Maghemite nanoparticles; Particle size distribution; Interparticle interaction; Magnetic relaxation; Magnetic properties; Magnetic moment distribution
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2015.02.038

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〉=7.04nm and standard deviation σ=0.78nm. Magnetization, M, vs. temperature (2–350K) of the NPs was measured in an applied magnetic field H up to 90kOe along with the temperature dependence of the ac susceptibilities, χ′ and χ″, at various frequencies, fm, from 10Hz to 10kHz. From the shift of the blocking temperature from TB=35K at 10Hz to TB=48K at 10kHz, the absence of any significant interparticle interaction is inferred and the relaxation frequency fo=2.6×1010Hz and anisotropy constant Ka=5.48×105erg/cm3 are determined. For T<TB, the coercivity HC is practically negligible. For T>TB, the data of M vs. H up to 90kOe at several temperatures are analyzed two different ways: (i) in terms of the modified Langevin function yielding an average magnetic moment per particle μp=7300(500) μB; and (ii) in terms of log-normal distribution of moments yielding 〈μ〉=6670µB at 150K decreasing to 〈μ〉=6100µB at 300K 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.
•Magnetic properties of γ-Fe2O3 nanoparticles, size=7.04(0.78)nm, are reported.•Attempt frequency fo=2.6×1010 Hz and no interparticle interactions are inferred.•M vs. H above TB analyzed using modified Langevin yields µp≈7300µB per particle.•M vs. H above TB fit to distribution of moments yields results consistent with µp.•Modified Langevin fit is meaningful if width of moment distribution is <0.83.