Surface segregation in chromium-doped NiCr alloy nanoparticles and its effect on their magnetic behavior

Murtaza Bohra (1) , Panagiotis Grammatikopoulos (1) , Rosa E. Diaz (1) , Vidyadhar Singh (1) , Junlei Zhao (2) , Jean-François Bobo (3) , Antti Kuronen (2) , Flyura Djurabekova (2) , Kai Nordlund (2) , Mukhles Sowwan (4)

1 OIST - Okinawa Institute of Science and Technology Graduate University
2 Department of Physics [Helsinki]
3 CEMES - Centre d'élaboration de matériaux et d'études structurales
4 Al-Quds University

Surface segregation designates the phenomenon of variation in chemical composition between the surface and the bulk of an alloy, which can have a beneficial or detrimental effect on its physical and chemical properties. This is even more pronounced in nanoalloys, i.e., alloy systems comprised of nanoparticles, with significant surface-to-volume ratios. In this case study we demonstrate the element-specific Cr segregation in Ni-rich NiCr alloy nanoparticles and nanogranular films grown by gas-phase synthesis methods. In situ annealing measurements (300-800 K), performed under vacuum using aberration-corrected environmental transmission electron microscopy (E-TEM), and vibrating sample magnetometry (VSM) revealed progressive Cr segregation with annealing temperature and subsequent complete transformation into core-satellite structures at 700 K. Simultaneously, atomistic computer simulations (molecular dynamics (MD) and Metropolis Monte Carlo (MMC)) elucidated the resultant structures, explaining the driving force behind segregation energetically. Most importantly, we emphasize the significant effects of Cr segregation on magnetic properties, namely, (i) the highly nonsaturated M-H loops (below the Néel temperature of antiferromagnetic Cr) with reduced coercivities and (ii) the uncompensated high Curie temperatures, TC, compared to the NiCr bulk, which approach bulk Ni values upon annealing. Both are clear evidence that the distribution of Cr in the nearest-neighbor shells of Ni atoms differs from that of the bulk NiCr alloy, reconfirming our structural findings.

Mots clés: Alloys; Annealing; Molecular dynamics; Monte Carlo methods; Nanomagnetics; Nanoparticles; Nickel; Synthesis (chemical); Transmission electron microscopy; Annealing temperatures; Atomistic computer simulation; Chemical compositions; Environmental transmission electron microscopy; High Curie temperature; Metropolis Monte Carlo; Physical and chemical properties; Vibrating sample magnetometry; Surface segregation

Domaines: Physique [physics]

Murtaza Bohra, Panagiotis Grammatikopoulos, Rosa E. Diaz, Vidyadhar Singh, Junlei Zhao, et al.. Surface segregation in chromium-doped NiCr alloy nanoparticles and its effect on their magnetic behavior. Chemistry of Materials, 2015, 27 (9), pp.3216-3225. ⟨10.1021/acs.chemmater.5b00837⟩. ⟨hal-01718888⟩