Magnetic nano-grains from a non-magnetic material: a possible explanation

Abstract

Abstract. Based on positive semidefinite operator properties, an exact ground state solution is deduced for a 2D Hubbard model with periodic boundary conditions on small samples. The obtained ferromagnetic behavior is used as a possible explanation of the ferromagnetism occurring in nano-samples made of non-magnetic but metallic materials. IntroductionIt is known that if a sample is constructed from non-magnetic metal (for example Au), and the size of the object is decreased to nanoscale values, the material can become ferromagnetic [1,2]. Below we provide a possible explanation for this effect, which requires only Coulomb repulsion between itinerant carriers, closed surface and quantum mechanical effects. For this reason we analyze a two dimensional L L × square lattice at arbitrary but finite L size with periodic boundary conditions in both directions, containing itinerant electrons. The Coulomb repulsion in this many-body system is screened by the itinerant system, consequently is of short-range type, and hence is taken into account for simplicity by the on-site Coulomb repulsion alone. Below this Hubbard system is solved exactly for the ground state in a restricted region of the parameter space, obtaining a ferromagnetic solution for small L values.The solution procedure (see [3][4][5][6][7] for details) is based on positive semidefinite operator properties. The technique first transforms in exact terms the Hamiltonian Ĥ of the system in a positive semidefinite form ˆĤ P C = + where P is a positive semidefinite operator while C is

Nanograin ferromagnets from non-magnetic bulk materials: the case of gold nanoclusters

“…Our aim in this paper is to show that the main conclusions of 24 concerning ferromagnetism in gold nanograins remain true et exact level also in the case of two-band treatment, and presence of ĤSO in the many-body 2D Hamiltonian containing the on-site Coulomb repulsion in the correlated band.…”

“…The model presented in 24 shows a potential possibility, but in fact is a toy model that should be pushed more closely to the real analysed system. First, the published studies show that the multiband character of Au is important to be taken into consideration.…”

“…We underline here the many-body correlations since the current literature strongly stresses that the correlation effects in Au are important [21][22][23] . Properly described correlation effects in describing gold nanoparticles have been considered in 24 . Here a two-dimensional lattice is considered with itinerant spin-1/2 carriers and periodic boundary conditions in both directions as describing a closed surface.…”

Exact results relating spin–orbit interactions in two-dimensional strongly correlated systems

“…Indeed, magnetism can appear at the interface of two, othervise non-magnetic perovskites 18 ; at the interface between Cr 2 O 3 and overlayers of Pd or Pt 19 ; Cu or Mn in thin films interfaced with organic molecules 20 ; nanoparticle surfaces with extremely high surface/volume ratio fabricated from otherwise macroscopically non-magnetic materials (e.g. Au, or Pd) 21,22 ; or interfaces in multilayers 23 . In several of these cases also anomalous magneto-transport measurements have been reported 8,17,24 .…”