Ion-irradiation-induced amorphization of cobalt nanoparticles

• Published in: Physical review. B, Condensed matter and materials physics Condensed matter and materials physics, 2010-04, Vol.81 (15), Article 155414
• Main Authors: Sprouster, D. J., Giulian, R., Araujo, L. L., Kluth, P., Johannessen, B., Kirby, N., Nordlund, K., Ridgway, M. C.
• Format: Article
• Language: English
• Subjects: ABSORPTION SPECTROSCOPY; AMORPHOUS STATE; ASYMMETRY; ATOMS; CALCULATION METHODS; CHALCOGENIDES; CHARGED PARTICLES; COBALT; COHERENT SCATTERING; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; DEBYE-WALLER FACTOR; DIFFRACTION; DISTANCE; ELEMENTS; FORECASTING; INTERATOMIC DISTANCES; IONS; IRRADIATION; METALS; MOLECULAR DYNAMICS METHOD; NANOSTRUCTURES; OXIDES; OXYGEN COMPOUNDS; PARTICLES; PHYSICAL RADIATION EFFECTS; RADIATION EFFECTS; SCATTERING; SILICON COMPOUNDS; SILICON OXIDES; SIMULATION; SMALL ANGLE SCATTERING; SPECTROSCOPY; SURFACES; TEMPERATURE DEPENDENCE; TRANSFORMATIONS; TRANSITION ELEMENTS; X-RAY DIFFRACTION; X-RAY SPECTROSCOPY
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.81.155414

The amorphization of Co nanoparticles embedded in SiO{sub 2} has been investigated by measuring their structure and size, before and after ion irradiation, by x-ray absorption spectroscopy and small-angle x-ray scattering, respectively. Compared to bulk material, unirradiated crystalline nanoparticles exhibited increased structural disorder and a decreased average coordination number as a result of finite-size effects. Upon irradiation, there was no variation in nanoparticle size yet significant structural change. The coordination number decreased further while the mean value (bondlength), variance (Debye-Waller factor), and asymmetry (third cumulant) of the interatomic distance distribution all increased, as consistent with theoretical predictions for an amorphous elemental metal. Furthermore, the interatomic distance distribution for irradiated Co nanoparticles was in excellent agreement with our molecular dynamics simulations for bulk amorphous Co, and we have thus attributed the observed structural changes to the formation of an amorphous phase. Though such a crystalline-to-amorphous phase transformation is not readily achievable in bulk material by ion irradiation, we suggest that the perturbed structural state prior to irradiation and the amorphous surrounding matrix both contribute to nucleating and stabilizing the amorphous phase in irradiated Co nanoparticles. In addition to the structural properties, the vibrational properties of the amorphous phase were also probed, using temperature-dependent x-ray absorption spectroscopy measurements. The Einstein temperature of the unirradiated crystalline nanoparticles was lower than that of bulk material due to loosely bonded surface/interfacial atoms. In contrast, that of the irradiated amorphous nanoparticles was substantially higher than the bulk value. We attribute this apparent bond stiffening to the influence of the rigid surrounding matrix.