Orbital magnetism in ensembles of gold nanoparticles

Abstract

The last two decades have witnessed various experiments reporting very unusual magnetic properties of ensembles of gold nanoparticles surrounded by organic ligands, including ferromagnetic, paramagnetic, or (large) diamagnetic responses. These behaviors are at odds with the small diamagnetic response of macroscopic gold samples. Here we theoretically investigate the possibility that the observed unusual magnetism in capped gold nanoparticles is of orbital origin. Employing semiclassical techniques, we calculate the orbital component to the zero-field susceptibility of individual as well as ensembles of metallic nanoparticles. While the result for the orbital response of individual nanoparticles can exceed by orders of magnitude the bulk Landau susceptibility in absolute value, and can be either diamagnetic or paramagnetic depending on nanoparticle size, we show that the magnetic susceptibility of a noninteracting ensemble of nanoparticles with a smooth size distribution is always paramagnetic at low magnetic fields. In particular, we predict that the zero-field susceptibility follows a Curietype law for small nanoparticle sizes and/or low temperatures. The calculated field-dependent magnetization of an ensemble of diluted nanoparticles is shown to be in good agreement with existing experiments that yielded a large paramagnetic response. The width of the size distribution of the nanoparticles is identified as a key element for the quantitative determination of the orbital response. arXiv:1808.00815v2 [cond-mat.mes-hall]

Nanoparticles Surface as a Spheroidal Defect in the Parent Material

“…But, for example, for the occurrence of magnetism when removing a cube from a "nonmagnetic" matrix, the most important is the homolytic breaking of bonds inside the crystal, as a result of which orthogonal surfaces appear on the surface of the removed cube -orbitals with unpaired electrons (spins). As a result of exchange interactions between them, a magnetic domain appears, covering the NP surface, and it becomes magnetic [17,18]. At the band approach, it is assumed that an electron from the valence or conduction band gets to the surface level appearing due to the broken bond (ie, a defect).…”

“…In addition, the concept of uncompensated magnetic sublattices, various types of spin subsystems, the emergence of a new crystal structure that promotes magnetism, etc. [78][79][80][81][82] were attracted. These approaches were suitable for describing the properties of a narrow range of materials and were difficult to use when considering systems other than those based on such approaches.…”

NMR study of Au/Al nanosytems in solution

“…We associate the origin of this signal with the paramagnetic state of the reduced 5d 10 6s 1 gold in the nanoparticle with an odd number of atoms. The general model of the orbital origin of the paramagnetic properties of the ensemble of gold nanoparticles was considered in [28]. The fine structure is attributed to quantum confinement of the nanoparticle electron structure [15,29], which is predicted theoretically [30].…”

Spontaneous orbital magnetization of mesoscopic dipole dimers

“…Numerous puzzling experiments have reported over the last two decades that assemblies of gold nanoparticles encapsulated with organic ligands can present either a paramagnetic behavior [2][3][4][5][6][7][8][9], a diamagnetic response larger than the one of the bulk [4,7,[10][11][12], or, even more surprisingly, a ferromagnetic instability [6,7,[9][10][11][13][14][15][16][17][18][19]]. 1 Recently, the paramagnetic behavior of relatively dilute samples of noninteracting gold nanoparticles has been theoretically elucidated in terms of orbital magnetism of the confined conduction electrons [21]. Such an effect is a purely quantum-mechanical phenomenon, which in the bulk gives rise to the Landau diamagnetic susceptibility [22] χ L = − e 2 k F 12π 2 m * c 2 .…”