Suppression of the Néel temperature in hydrothermally synthesized α-Fe2O3 nanoparticles

Abstract

Magnetic measurements up to 1000 K have been performed on hydrothermally synthesized α-Fe2O3 nanoparticles (60 nm) using a Quantum Design vibrating sample magnetometer. A high vacuum environment (1×10 −5 torr) during the magnetic measurement up to 1000 K leads to a complete reduction of α-Fe2O3 to Fe3O4. This precludes the determination of the Néel temperature for the α-Fe2O3 nanoparticles. In contrast, coating α-Fe2O3 nanoparticles with SiO2 stabilizes the α-Fe2O3 phase up to 930 K, which allows us to determine the Néel temperature of the α-Fe2O3 nanoparticles for the first time. The Néel temperature of the 60-nm α-Fe2O3 nanoparticles is found to be 945 K, about 15 K below the bulk value. The small reduction of the Néel temperature of the α-Fe2O3 nanoparticles is consistent with a finite-size scaling theory. Our current results also show that coating nanoparticles with SiO2 can effectively protect nanoparticles from oxidation or reduction, which is important to technological applications.

Finite-size scaling law of the Néel temperature in hematite nanostructures

“…Both the microscopic length scale and the exponent for a-Fe 2 O 3 are very close to those for the antiferromagnetic CoO, suggesting a common origin. It is interesting that a data point reported earlier for a nanoparticle sample 22 also falls close to the solid line although the heat stick used for the previous sample is different from the one used for the current samples. This consistency suggests that the absolute temperature uncertainties in our heat sticks are rather small.…”

“…To do so, it is necessary to suppress both particle growth and phase transformation at high temperatures, which was partially fulfilled by coating them with silica. 22 It might be better to use thermally more stable hematite nanostructures to determine their N eel temperatures more accurately.…”

“…To be consistent, we choose the mean t and d values, which were determined by XRD spectra. 22,25 The solid line in Fig. 5 is the curve that goes through the data points for the nanoring, nanotube, and bulk samples.…”

Magnetic properties and Mössbauer spectroscopy of iron nanowire arrays prepared by electroless deposition in a magnetic field

“…Previously, our research team has synthesised and characterised several monodisperse magnetic nanoparticle systems [13][14][15][16][17]. Here we demonstrate that iron (Fe) nanowires can be formed in AAO templates immersed in ferrous chloride (FeCl 2 ) solution by the electroless deposition method.…”

Synthesis and magnetic properties of nickel three-dimensional superlattice structure

“…In our previous work, we have synthesised highly monodisperse magnetic nanoparticles with different size distribution [18][19][20][21]. Herein, we report a facile chemical method for controllable synthesis of nickel superlattices.…”