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Here, we report the effect of reduction in particle size on the temperature dependent magnetization of chemically synthesized BiFeO 3 nanocrystals with average grain size of 55 nm. The X-ray photoelectron spectroscopy results show a significant broadening of binding energy peaks associated to Fe 3+ 2p 3/2 core levels due to the reduced size. Additionally, due to the nanosize effect, the M-H loops show a significant coercivity starting from 390 K with an anomaly located in the vicinity of 150 K in our H c vs T as well as M r /M s(50 kOe) vs T curves. At this temperature, both H c and M r /M s(50 kOe) undergo minima. Additionally, our results for the first time show the evidence of existence of a low temperature anomaly due to spin-glass transition in the range from 40-44 K in the field cooled magnetization curves. In bulk single crystals, this transition is reported to be situated at around 50 K, however, this transition remained so far undiscovered in the recent studies on BiFeO 3 nanoparticles due to the insufficient temperature resolution. The significant shift in this transition toward lower temperature can be attributed to size dependent effects. Our results clearly present new information on the size dependent properties of BiFeO 3 nanoparticles.

The rare earth orthochromites are extremely interesting due to the richness of their optical, dielectric, and magnetic properties as well as due to their multiferroic properties which make them suitable materials to study in the nanoregime. However, the wet-chemical synthesis of these materials in nanosize is nontrivial. Here, we report for the first time, the detailed Raman spectra as well as magnetic and dielectric properties of chemically synthesized GdCrO3 nanoparticles of size ranging from 40 to 60 nm. The magnetic properties are dictated by competing Cr3+–Cr3+, Gd3+–Cr3+, and Gd3+–Gd3+ superexchange interactions in different temperature regions, resulting into an antiferromagnetic ordering at 167 K due to the Cr3+–Cr3+ followed by weak ferromagnetic ordering due to the onset of Cr3+–Gd3+ interactions. At lower temperature, it shows weak antiferromagnetic ordering due to Gd3+–Gd3+ interaction. Below 95 K, GdCrO3 nanoparticles showed the presence of negative magnetization due to Gd3+ and Cr3+ interactions resulting into weak ferromagnetic coupling. The Raman spectroscopy shows the characteristic Raman shifts indicating that below 450 cm−1, Gd3+ ions play a dominant role in determining the phonon frequencies of GdCrO3, and above 450 cm−1, the Cr+3 ions dominate. We also present for the first time the low temperature dielectric constant and loss tangent data for GdCrO3 in a broad temperature and frequency range. The dielectric constant shows a decrease in comparison to the bulk values due to the size dependent effects. It also shows a peak centered at around 320 K above which it shows a sharp decrease. The dielectric loss value in GdCrO3 nanoparticles is quite small and shows an interesting frequency dependent anomaly at lower temperature which might be due to the coupling between magnetic and dielectric order parameters.

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Colloidal gold particles synthesized in water have been capped with octadecanethiol (ODT) molecules rendered water-soluble by threading with α-cyclodextrin (α-CD) molecules. Thereafter, the gold nanoparticles could be transferred into an organic solvent such as chloroform by vigorous shaking of a biphasic mixture of the α-CD-threaded ODT-stabilized gold hydrosol and chloroform. The phase transfer of the gold nanoparticles could be observed as a rapid transfer of color from the aqueous phase to the organic phase. During shaking of the biphasic mixture, it is believed that α-CD molecules are dislodged from the octadecanethiol molecules, rendering the gold particles hydrophobic and amenable to phase transfer. Transmission electron microscopy studies of films of the gold nanoparticles in chloroform formed by solvent evaporation indicate a uniform size distribution of the nanoparticles and hexagonal close-packing of the particles in the film.