Petr Holec

Surface spin effects in La-doped CoFe2O4 nanoparticles prepared by microemulsion route

A comparative study of pure CoFe2O4 nanoparticles and La-doped CoFe2O4 nanoparticles, prepared by microemulsion route has been performed. The samples were characterized using x-ray diffraction and transmission electron microscopy in order to obtain average particle size. The doping of small amount of La3+ ions (up to 3 molar %) causes significant reduction of the particle size using the identical preparation route. The samples were investigated by magnetization measurements, which revealed the coercivity values strongly dependent on particle size, but not significantly on level of La3+ doping. Detailed in-field Mossbauer spectroscopy studies were performed in order to determine spin canting angles and cation distribution within the spinel network. The non-negligible canting angles up to 40° in the La-doped samples were observed. The presence of the spin surface effects was also supported by magnetic measurement as the magnetization did not saturate even in considerably high magnetic fields (7 T). Moreover...

Magnetocaloric phenomena in Mg-ferrite nanoparticles

A comparative study of magnetocaloric effect (MCE) in superparamagnetic (SPM) regime is reported in two different types of magnesium ferrite nanostructures. The samples were prepared either by microemulsion method as MgFe2O4 nanoparticles encapsulated in amorphous SiO2, or as matrix-less nanoparticles using hydrothermal synthesis in supercritical water conditions. The particle diameter in all prepared samples was obtained from XRD measurements and TEM analysis. All samples show a SPM behavior above the blocking temperature, TB. The entropy change, ΔS was finally derived from the measurements of magnetization, M(H,T) curves at defined temperature intervals. We observed, that all samples show a broad peak of ΔS in the temperature range that is fairly above the TB. The values of the ΔS also depend on the particle size, and they are of about two orders lower than those reported in the famous giant magnetocaloric materials.

ACr2O4 /SiO2 (A = Zn, Cu, Cd) nanocomposites, their preparation and physical properties

This article presents preparation and characterization of zinc, copper and cadmium chromites nanocrystals embedded in a silica matrix. The ZnCr2O4/SiO2, CuCr2O4/SiO2 and CdCr2O4/SiO2 samples were prepared by a conventional sol-gel method using HNO3 as an acid catalyst, formamide as a modifier, methanol as a solvent and TEOS. Final heat treatment of the nanocomposites was carried out at temperatures in the range of 900 – 1100°C. The resulting samples were characterized by X-ray diffraction, High Resolution Transmission Electron Microscopy, and magnetic measurements.

Observation of surface effects in La-doped CoFe2O4/SiO2 nanocomposites

In magnetism of nanoparticles, surface spin effects play a key role. The so-called spin canting at the surface leads either to lowering of saturation magnetization, or even to lack of saturation in high magnetic fields. The value of the saturation magnetization depends on cation distribution within the spinel network and on the spin canting angles. We have investigated structure and magnetic behavior of CoFe2O4 nanoparticles doped with La (Co1−xLaxFe2O4, x = 0.05 − 0.5 and CoLaxFe2−xO4, x = 0.05 − 0.2) embedded in amorphous silica matrix prepared by a sol-gel method. The samples were characterized using X-ray diffraction and TEM. The zero-field cooled and field cooled measurements of magnetization showed that the blocking temperature is well above the room temperature. The high coercivity was observed (up to 2 T at 10 K) and no saturation of the magnetization even in applied field of 7 T was obtained, suggesting considerable spin canting effects. Finally, the Mossbauer spectroscopy under magnetic field and down to low temperature was performed to determine the cation distribution and the spin canting angles.