The metal nanostructured films are intensively studied because of their fascinating physical properties and their potentiality in various applications, like magnetic recording industry, catalysis and tribology. For example, smaller particles are required in order to realize advanced magnetic memory units [1], but this request is a major challenge, because of the super-paramagnetic limit for the density of recorded bits, resulting in instability of conventional recording media with three-dimensional particles [2]. The possibility of depositing preformed gas-phase nanoparticles onto surfaces allowed a systematic study of superparamagnetic and ferromagnetic behaviour as a function of particle mass and of film thickness [3,4]. We present the results of an investigation of morphologic and magnetic properties of mass-selected Ni nanoparticles films grown on Si/SiOx and MgO(100) substrates. The films were produced by deposition of preformed Ni nanoparticles, using a gas aggregation nanocluster source and an electric quadrupole mass filter. The linear size of the produced particles ranged between 3 and 10 nm. The morphology of the films, with thickness varying from t = 0.5 to t = 7 nm, was studied with Atomic Force Microscopy and Scanning Electron Microscopy, combining in this way information about height and lateral topography. We observed some flattening of the particles at the early stage of film formation, with the presence of some small aggregations (2 or 3 particles). At increasing coverage, the films grow in agglomerates in a porous structure. Information about the magnetic properties was obtained with Field Cooled-Zero Field Cooled (FC/ZFC) magnetisation curves and Magneto Optical Kerr effect (for thicker films). We observed a reversibility-irreversibility transition at temperatures Ti between 70 and 80 K, and a significant deviation from the superparamagnetic behaviour a T>Ti even for the lowest coverage studied (t = 2 nm, particle size d = 8 nm), revealing inter-particle exchange interaction in competition with single particle random anisotropy, and a global in-plane anisotropy, possibly ascribed to shape anisotropy and dipolar interaction. These hypotheses have been confirmed by Montecarlo simulations of Fc-ZFC curves. [1] S. D. Bader, Rev. Mod. Phys. 78, 1 (2006) [2] D. Weller and A. Moser, IEEE Trans. Magn. 35, 4423 (1999). [3] C. Binns et al., J. Phys. D: Appl. Phys. 38, R357 (2005). [4] A. Kleibert et al, J. Appl. Phys. 101, 114318 (2007)
Morphological and magnetic study of size-selected Ni nanoparticles films
RONCONI, Franco;SPIZZO, Federico
2009
Abstract
The metal nanostructured films are intensively studied because of their fascinating physical properties and their potentiality in various applications, like magnetic recording industry, catalysis and tribology. For example, smaller particles are required in order to realize advanced magnetic memory units [1], but this request is a major challenge, because of the super-paramagnetic limit for the density of recorded bits, resulting in instability of conventional recording media with three-dimensional particles [2]. The possibility of depositing preformed gas-phase nanoparticles onto surfaces allowed a systematic study of superparamagnetic and ferromagnetic behaviour as a function of particle mass and of film thickness [3,4]. We present the results of an investigation of morphologic and magnetic properties of mass-selected Ni nanoparticles films grown on Si/SiOx and MgO(100) substrates. The films were produced by deposition of preformed Ni nanoparticles, using a gas aggregation nanocluster source and an electric quadrupole mass filter. The linear size of the produced particles ranged between 3 and 10 nm. The morphology of the films, with thickness varying from t = 0.5 to t = 7 nm, was studied with Atomic Force Microscopy and Scanning Electron Microscopy, combining in this way information about height and lateral topography. We observed some flattening of the particles at the early stage of film formation, with the presence of some small aggregations (2 or 3 particles). At increasing coverage, the films grow in agglomerates in a porous structure. Information about the magnetic properties was obtained with Field Cooled-Zero Field Cooled (FC/ZFC) magnetisation curves and Magneto Optical Kerr effect (for thicker films). We observed a reversibility-irreversibility transition at temperatures Ti between 70 and 80 K, and a significant deviation from the superparamagnetic behaviour a T>Ti even for the lowest coverage studied (t = 2 nm, particle size d = 8 nm), revealing inter-particle exchange interaction in competition with single particle random anisotropy, and a global in-plane anisotropy, possibly ascribed to shape anisotropy and dipolar interaction. These hypotheses have been confirmed by Montecarlo simulations of Fc-ZFC curves. [1] S. D. Bader, Rev. Mod. Phys. 78, 1 (2006) [2] D. Weller and A. Moser, IEEE Trans. Magn. 35, 4423 (1999). [3] C. Binns et al., J. Phys. D: Appl. Phys. 38, R357 (2005). [4] A. Kleibert et al, J. Appl. Phys. 101, 114318 (2007)I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
