![]() The orderly oriented carboxylate groups of PAA play an important role in dense packing of nanoparticles with an ordered arrangement inside the clusters. Post-functionalization of these nanoparticles with surfactant cetyltrimethyl ammonium bromide (CTAB) and subsequently with polyelectrolyte polyacrylic acid (PAA) result in irregular aggregation of nanoparticles and nanoparticle clusters respectively. Oleic acid capped iron oxide nanoparticles with sizes of 5 ± 2 nm are used as the primitive assembly of subunits. In this study, a reassembling strategy of iron oxide nanoparticles prior to the formation of nearly identical nanoclusters of size 80 ± 20 nm as the secondary nanostructures is reported, where ordered arrangement of nanoparticle subunits with a narrow distribution of interparticle spacing is observed. Nanoparticle clusters have become attractive secondary nanostructures due to their collective physical properties, which can be modulated as a function of their internal structure. These results have been tentatively explained considering size distribution effects and the presence of superparamagnetic and spin glass-like contributions arising from the frustration of the antiferromagnetic order owing to surface effects and an insufficient filling of the akaganéite channels with Cl- anions. The study of the respective dependences of the dc magnetization and the ac susceptibility on temperature and exciting magnetic field revealed complex magnetic relaxation processes, high coercivity values at low temperature, and exchange bias effect. In addition, a small amount of rodlike akaganéite particles with 23 ± 5 nm in length and 5 ± 1 nm in width was also detected. Structural investigations using X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) revealed that the sample was mainly constituted by rather-equiaxial akaganéite nanocrystals with mean diameter of 3.3 ± 0.5 nm. The resulting product was characterized by several analytical techniques. In the present work, ultrafine akaganéite nanoparticles were prepared by the hydrolysis of FeCl solutions at room temperature induced by the presence of NaOH. The magnetic properties of low dimensional materials of several iron oxyhydroxide phases, such as akaganéite (β-FeOOH) or lepidocrocite (γFeO(OH)), remain poorly explored, probably due to their specific preparation as single crystalline phase requires special conditions owing to their structural instability. An observed nonthermal activation type relaxation mechanism at 12 K is attributed to possible quantum tunneling effect in Mn-Zn ferrite nanoparticles. The peak is also observed in a plot of χ''/χ' versus temperature, which may mean the existence of magnetic aftereffect, and furthermore, it has an Arrhenius as well as Vogel-Fulcher law type dependence. The temperature at which the maximum in the ac-susceptibility curve is observed is well accounted by the Vogel-Fulcher law for both χ' and χ''. ![]() The complex magnetic susceptibility was measured as a function of temperature for frequencies ranging from 67 to 1800 Hz. The observed higher value of standard deviation is due to the interparticle interaction. The relevant parameters give the values of particle diameter (D) 80 A˚, standard deviation 0.3 in ln(D), and the anisotropy constant K to be 5.8×105 erg/cm3. The zero-field-cooled magnetization data are simulated by assuming noninteracting magnetic particles with uniaxial anisotropy and lognormal particle size distribution. The zero-field-cooled peak temperature decreases with increasing magnetic field and obeys the well known de Almeida-Thouless line. Static and dynamic magnetic properties of oleic acid/oleyamine coated Mn-Zn ferrite nanoparticles of diameter 82 A˚ are reported. ![]()
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