Guo-meng Zhao

Temperature dependence of magnetic anisotropy constant in iron chalcogenide Fe3Se4: Excellent agreement with theories

Magnetic hysteresis loops were measured for ferrimagnetic iron chalcogenide [Formula: see text] nanoparticles in the whole temperature range below the Curie temperature [Formula: see text] (315 K). The coercivity of the material is huge, reaching about 40 kOe at 10 K. The magnetic anisotropy constant K was determined from the magnetic hysteresis loop using the law of approach to saturation. The deduced anisotropy constant at 10 K is [Formula: see text], which is over one order of magnitude larger than that of [Formula: see text]. We also demonstrated that the experimental magnetic hysteresis loop is in good agreement with the theoretical curve calculated by Stoner and Wohlfarth for a noninteracting randomly oriented uniaxial single-domain particle system. Moreover, we show that K is proportional to the cube of the saturation magnetization [Formula: see text], which confirms earlier theoretical models for uniaxial magnets.

Unusual temperature dependence of the magnetic anisotropy constant in barium ferrite BaFe12O19

We report magnetic hysteresis loops in a wide temperature range (4-700 K) for silica-coated barium ferrite BaFe12O19 nanoparticles. The saturation magnetization Ms and the first magnetic anisotropy constant K are determined simultaneously from the magnetic hysteresis loop using the law of approach to saturation. It is remarkable that K is linearly proportional to Ms and varies precisely with temperature as K(T) = K(0)[1 − (T/TC)1.58] in the whole temperature range below the Curie temperature TC (740 K). The unusual temperature dependence of the anisotropy constant and its linear relation with the saturation magnetization in BaFe12O19 are not predicted from the existing theoretical models.

Finite-size scaling relation of the Curie temperature in barium hexaferrite platelets

High-temperature magnetic measurements were carried out on barium ferrite BaFe12 O19 nanoparticles coated with amorphous silica. We find that the Curie temperature of this material decreases with decreasing particle size, in agreement with the finite-size scaling theory. In contrast to what one expects, the observed particle-size dependence of the Curie temperature does not follow a finite-size scaling relation for a zero-dimensional magnetic system. Instead, the data follow a finite-size scaling relation for a two-dimensional magnetic system with the scaling exponent ν = 0.78±0.06. The validity of the two-dimensional scaling relation in this material is due to the fact that the nanoparticles have a platelet-like shape.

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

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 Neel 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 Neel temperature of the α-Fe2O3 nanoparticles for the first time. The Neel 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 Neel temperature of the α-Fe2O3 nanoparticles is consistent with a finite-size scaling theory.

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

We report high-temperature magnetic properties of single-crystalline hematite (α-Fe2O3) nanostructures with different shapes. Magnetic measurements under a high vacuum (<9.5 × 10−6 Torr) up to 920 K were used to characterize thermal stability of the nanostructures. The onset temperature of the α-Fe2O3 to Fe3O4 phase transformation and the transformed fraction were found to depend strongly on the shape of the nanostructure. The data demonstrate that the phase transformation mainly occurs at the (001) surfaces. The high thermal stability of the nanoring and nanotube samples allows us to accurately measure their Neel temperatures. The Neel temperatures of the nanoring and nanotube samples were found to decrease with decreasing the mean wall-thickness of the nanoring/nanotube assembly. The data confirm the two-dimensional finite-size scaling law for the Neel temperature.

MAGNETIC PROPERTIES AND MÖSSBAUER SPECTRA OF Fe7S8 NANORODS

Fe7S8 nanorods have been successfully synthesized using a chemical evaporation method. X-ray diffraction pattern showed that the as-prepared products were Fe7S8 with no impurity phase. The results of scanning electron microscopy indicated that the samples synthesized at 750°C and 900°C were rod and sheet-like, respectively. The magnetic properties of the iron sulfide nanorods were measured over a wide temperature range (4 K–750 K) using a quantum design vibrating sample magnetometer. It was found that the nanorods were ferromagnetic with the Curie temperature of about 581 K. The Mossbauer spectra showed that the iron sulfide nanorods consisted of hexagonal pyrrhotites, whose spectra were asymmetrical according to correlation between the isomer shift and the hyperfine field.