In this paper, the mechanisms of inter-phase magnetic coupling in nanostructured materials are discussed. In particular, the magnetic properties of two different systems are reviewed. The first system consists of nanocrystalline Fe samples obtained as bulk by mechanical attrition. Mössbauer spectroscopy allows distinguishing the different physical environment of the atoms in the crystallites and at the grain boundaries, revealing an amorphous-like configuration of the latter. AC and DC susceptibility, remanence and relaxation measurements are consistent with a low-temperature freezing of the crystallite magnetic moments in random directions. The effect is explained in terms of the variation with temperature of the capability of the disordered interface to transmit the exchange interaction to neighboring crystallites. The second investigated system consists of Fe/Fe oxide granular material obtained by cold-compaction of gas-condensed Fe nanoparticles, surrounded by an Fe oxide layer. The low-temperature magnetic behavior is found to be strongly dependent on the exchange coupling between the metallic crystallites and the disordered (both structurally and magnetically) oxide matrix. Such exchange coupling gives rise to a frozen disordered state of the system at low temperatures and determines the temperature variation of the magnetic properties.
Exchange coupling in Iron and Iron/Oxide nanogranular systems
DEL BIANCO, LUCIA;
2005
Abstract
In this paper, the mechanisms of inter-phase magnetic coupling in nanostructured materials are discussed. In particular, the magnetic properties of two different systems are reviewed. The first system consists of nanocrystalline Fe samples obtained as bulk by mechanical attrition. Mössbauer spectroscopy allows distinguishing the different physical environment of the atoms in the crystallites and at the grain boundaries, revealing an amorphous-like configuration of the latter. AC and DC susceptibility, remanence and relaxation measurements are consistent with a low-temperature freezing of the crystallite magnetic moments in random directions. The effect is explained in terms of the variation with temperature of the capability of the disordered interface to transmit the exchange interaction to neighboring crystallites. The second investigated system consists of Fe/Fe oxide granular material obtained by cold-compaction of gas-condensed Fe nanoparticles, surrounded by an Fe oxide layer. The low-temperature magnetic behavior is found to be strongly dependent on the exchange coupling between the metallic crystallites and the disordered (both structurally and magnetically) oxide matrix. Such exchange coupling gives rise to a frozen disordered state of the system at low temperatures and determines the temperature variation of the magnetic properties.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
