Raffaella Torchio | European Synchrotron Radiation facility (original) (raw)
Structure magnetism and chemical order in the 3d metals and their alloys under extreme pressure conditions
Supervisors: Sakura Pascarelli and Settimio Mobilio
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The structural stability of fcc Ni over a very large pressure range offers a unique opportunity t... more The structural stability of fcc Ni over a very large pressure range offers a unique opportunity to experimentally investigate how magnetism is modified by simple compression. K-edge x-ray magnetic circular dichroism (XMCD) shows that fcc Ni is ferromagnetic up to 200 GPa, contradicting recent predictions of an abrupt transition to a paramagnetic state at 160 GPa. Density functional theory calculations point out that the pressure evolution of the K-edge XMCD closely follows that of the p projected orbital moment rather than that of the total spin moment. The disappearance of magnetism in Ni is predicted to occur above 400 GPa.
The magnetism and structure of compressed Co has been studied up to 120 GPa using K-edge x-ray ma... more The magnetism and structure of compressed Co has been studied up to 120 GPa using K-edge x-ray magnetic circular dichroism (XMCD) and x-ray absorption near edge spectroscopy (XANES). The XMCD signal decreases linearly with pressure and is totally suppressed around 120 GPa at ambient temperature. We detect local fcc-like atomic configurations starting from 80 GPa and at 120 GPa these represent at least 40% of the total. Supporting theoretical calculations indicate that the mixed hcp/fcc phase above 120 GPa is nonmagnetic. Comparison between the pressure evolution of magnetism and structure highlights major differences between the Fe and Co cases.
The magnetic response of nanometer sized Co nanoparticles (NP) prepared using reverse micelle sol... more The magnetic response of nanometer sized Co nanoparticles (NP) prepared using reverse micelle solutions are presented. The use of complementary structural and morphological probes (like transmission electron microscopy, high resolution electron microscopy, X-ray absorption spectroscopy) allowed to relate the magnetic properties to the size, morphology, composition and atomic structure of the nanoparticles. All data agree on the presence of a core–shell structure of NPs made of a metallic Co core surrounded by a thin Co-oxide layer. The core–shell microstructure of NPs affects its magnetic response mainly raising the anisotropy constant.
Co nanoclusters were synthesized by an inverse-micelle chemical route. The magnetic and microstru... more Co nanoclusters were synthesized by an inverse-micelle chemical route. The magnetic and microstructural properties of the nanoparticles have been analyzed as a function of the surfactant (AOT and DEHP) and the drying method. Microstructural analysis has been performed by TEM and XANES; magnetic properties have been studied by hysteresis loops and zero-field cooling – field cooling (ZFC-FC) curves. TEM images show 2 to 4 nm sized particles spherical in shape. XANES measurements point out a significant presence of Co3O4with metallic Co and some Co2+ bound to the surfactant. The presence of antiferromagnetic Co3O4 explains the magnetic transition observed at low T in both ZFC-FC measurements and hysteresis loops. Finally, the presence of magnetic interactions explains the bigger effective cluster size obtained from hysteresis loops fits (6-10 nm) compared to the sizes observed by TEM (2-4 nm).
The structural stability of fcc Ni over a very large pressure range offers a unique opportunity t... more The structural stability of fcc Ni over a very large pressure range offers a unique opportunity to experimentally investigate how magnetism is modified by simple compression. K-edge x-ray magnetic circular dichroism (XMCD) shows that fcc Ni is ferromagnetic up to 200 GPa, contradicting recent predictions of an abrupt transition to a paramagnetic state at 160 GPa. Density functional theory calculations point out that the pressure evolution of the K-edge XMCD closely follows that of the p projected orbital moment rather than that of the total spin moment. The disappearance of magnetism in Ni is predicted to occur above 400 GPa.
The magnetism and structure of compressed Co has been studied up to 120 GPa using K-edge x-ray ma... more The magnetism and structure of compressed Co has been studied up to 120 GPa using K-edge x-ray magnetic circular dichroism (XMCD) and x-ray absorption near edge spectroscopy (XANES). The XMCD signal decreases linearly with pressure and is totally suppressed around 120 GPa at ambient temperature. We detect local fcc-like atomic configurations starting from 80 GPa and at 120 GPa these represent at least 40% of the total. Supporting theoretical calculations indicate that the mixed hcp/fcc phase above 120 GPa is nonmagnetic. Comparison between the pressure evolution of magnetism and structure highlights major differences between the Fe and Co cases.
The magnetic response of nanometer sized Co nanoparticles (NP) prepared using reverse micelle sol... more The magnetic response of nanometer sized Co nanoparticles (NP) prepared using reverse micelle solutions are presented. The use of complementary structural and morphological probes (like transmission electron microscopy, high resolution electron microscopy, X-ray absorption spectroscopy) allowed to relate the magnetic properties to the size, morphology, composition and atomic structure of the nanoparticles. All data agree on the presence of a core–shell structure of NPs made of a metallic Co core surrounded by a thin Co-oxide layer. The core–shell microstructure of NPs affects its magnetic response mainly raising the anisotropy constant.
Co nanoclusters were synthesized by an inverse-micelle chemical route. The magnetic and microstru... more Co nanoclusters were synthesized by an inverse-micelle chemical route. The magnetic and microstructural properties of the nanoparticles have been analyzed as a function of the surfactant (AOT and DEHP) and the drying method. Microstructural analysis has been performed by TEM and XANES; magnetic properties have been studied by hysteresis loops and zero-field cooling – field cooling (ZFC-FC) curves. TEM images show 2 to 4 nm sized particles spherical in shape. XANES measurements point out a significant presence of Co3O4with metallic Co and some Co2+ bound to the surfactant. The presence of antiferromagnetic Co3O4 explains the magnetic transition observed at low T in both ZFC-FC measurements and hysteresis loops. Finally, the presence of magnetic interactions explains the bigger effective cluster size obtained from hysteresis loops fits (6-10 nm) compared to the sizes observed by TEM (2-4 nm).