Spin-glass freezing of maghemite nanoparticles prepared by microwave plasma synthesis

Abstract

Magnetic properties of 6 nm maghemite nanoparticles (prepared by microwave plasma synthesis) have been studied by ac and dc magnetic measurements. Structural characterization includes x-ray diffraction and transmission electron microscopy. The temperature scans of zero field cooled/field cooled (ZFC/FC) magnetization measurements show a maximum at 75 K. The ZFC/FC data are fitted to the Brown-Néel relaxation model using uniaxial anisotropy and a log-normal size-distribution function to figure out the effective anisotropy constant K eff . K eff turns out to be larger than the anisotropy constant of bulk maghemite. Fitting of the ac susceptibility to an activated relaxation process according to the Arrhenius law provides unphysical values of the spin-flip time and activation energy. A power-law scaling shows a satisfactory fit to the ac susceptibility data and the dynamic critical exponent (zv % 10) takes value between 4 and 12 which is typical for the spin-glass systems. The temperature dependence of coercivity and exchange bias shows a sharp increase toward low temperatures which is due to enhanced surface anisotropy. The source of this enhanced magnetic anisotropy comes from the disordered surface spins which get frozen at low temperatures. Memory effects and thermoremanent magnetization experiments also support the existence of spin-glass behaviour. All these magnetic measurements signify either magnetic blocking or surface spin-glass freezing at high and low temperatures, respectively. V C 2012 American Institute of Physics. [http://dx.

Microwave Plasma Synthesis of Materials—From Physics and Chemistry to Nanoparticles: A Materials Scientist’s Viewpoint

“…Commercial powders usually contain nanoparticles of sizes ranging from around 5 nm to approximately 200 nm. Similar, broad particle size distributions are observed, when using microwave torches [52,103,109,110], whereas the powders synthesized in low pressure microwave plasma are often characterized by a very narrow particle size distribution [105,107,108,198]. This can be observed for several commercially available nanoscaled powders, like Fe 2 O 3 , ZrO 2 , SnO 2 , or TiO 2 .…”

“…Reference data for particle size dependent saturation magnetization (all symbols in black color) are taken from Han et al [220], Morales et al [222], and de Montferrand et al [227], and finally the bulk reference value is taken from [228], page 993. Literature data of the diverse plasma synthesized F 2 O 3 nanoparticles (colored symbols) are taken from Table 7 (David et al [102,109], Nadeem et al [105], Vollath et al [107,108], Banerjee et al [114], Lei et al [115]). The bars herein are not error bars; they show the spread of indicated particle sizes.…”

The vapor pressure over nano-crystalline ice

“…In addition, the growth model fit does not require a changing aspect ratio to achieve very good agreement with the mea- sured data, excluding an increasing aspect ratio with particle growth. On the basis that metal-oxide nanoparticles produced in similar arrangements have been shown to be compact and spherical (Giesen et al, 2005;Janzen et al, 2002;Nadeem et al, 2012), we estimate the maximum relative uncertainty in p sat, s due to the nonsphericity of the ice particles to 5 %. The main uncertainty in p sat, s is caused by the uncertainty in T s , which is between 0.2 and 0.4 K depending on the applied conditions.…”

Morphology, Microstructure, and Magnetic Properties of Ordered Large-Pore Mesoporous Cadmium Ferrite Thin Film Spin Glasses

“…S8 in ESI). [67,68] Lastly, we note that the magnetic properties of the material employed here differ significantly from those of single crystals. Such single crystals reveal an anomaly in susceptibility at 13 K and have a negative Weiss temperature of -53 K, as described by Kamazawa et al [69] They associated these features with an ordering transition temperature and strong geometrical frustration, respectively.…”