Uncovering the magnetic properties of the AgxNiy (x+y=55) nanoalloys in the whole composition range

Abstract

Nickel and silver are metals with interesting properties of technological relevance: nickel is a well known ferromagnet and silver has antibacterial properties. Both exist in the face centered cubic phase but are immiscible. In the context of alloys at the nanoscale, one can play with the size to fine tune a desired property, or to achieve new properties and functionalities that do not exist at the macroscopic regime. In this work, we explore how the subtle interaction between Ni and Ag triggers the chemical order, the electronic structure, and the magnetic properties of a AgNi nanoalloy of 55 atoms, a size that can accommodate core/shell configurations with sizable parts. Calculations are conducted within the density functional theory at the generalized gradient approximation for exchange and correlation. We determine, in the whole composition range, the chemical order, absolute and relative stabilities by means of binding energy, excess energy and second energy difference, as well as total and part-projected spin-polarized electronic densities of states and local charge and spin magnetic moments distribution. Ni-core/Ag-shell structures are particularly stable, but contrary to what one would expect by simply extrapolating the properties of the pure Ag and Ni clusters or of pure fcc bulks, we find unexpected behaviors along the composition range, such as quenched magnetic moments in Ni, total magnetic moments essentially contributed in some cases by Ag, or electronic charge transfer that changes its sign depending on the stoichiometry. These behaviors lead to magnetic transitions as a function of the composition, and differ, in some cases, from those of the smaller 13-atoms AgNi nanoalloys of the same symmetry with which we compare, a further demonstration of the complex nature of nanostructures. The above trends are robust against ionization and electron capture.