Giant Magnetic Susceptibility of Gold Nanorods Detected by Magnetic Alignment

Abstract

We have determined the magnetic properties of single-crystalline Au nanorods in solution using an optically detected magnetic alignment technique. The rods exhibit a large anisotropy in the magnetic volume susceptibility (Á V ). Á V increases with decreasing rod size and increasing aspect ratio and corresponds to an average volume susceptibility ( V ), which is drastically enhanced relative to bulk Au. This high value of V is confirmed by SQUID magnetometry and is temperature independent (between 5 and 300 K). Given this peculiar size, shape, and temperature dependence, we speculate that the enhanced V is the result of orbital magnetism due to mesoscopic electron trajectories within the nanorods. DOI: 10.1103/PhysRevLett.111.127202 PACS numbers: 75.75.Àc, 73.22.Àf, 75.20.En, 78.67.Qa Bulk Au is a diamagnetic material, i.e., one with a negative volume magnetic susceptibility Au . Recently, it was reported that Au nanoparticles (NPs), with functionalized surfaces, show a broad range of magnetic behavior, ranging from (enhanced) diamagnetic [1,2] to (super)paramagnetic [3][4][5] and even ferromagnetic up to room temperature [6,7]. The NP size and the type of capping molecules, strongly binding to or weakly interacting with Au, appear to influence the magnetic response. Several explanations were suggested, such as competing magnetic contributions of the NP core and surface [3], the formation of a magnetic moment due to the exchange of charges at the Au-ligand interface [5,6,8], the creation of large orbital moments due to electron motion within surface clusters [9], and the occurrence of persistent currents in the Au core [2]. However, so far, the origin of this unexpected magnetism and why it differs strongly between different types of NPs is not yet understood [2,10,11].We employ a novel magnetic alignment technique to measure the magnetic properties of rod-shaped Au NPs in solution. We focus on relatively large NPs (all dimensions >7 nm) that are single crystalline. The degree of alignment is measured optically, through the magnetic field-induced linear dichroism and birefringence, across the Au surface plasmon resonance (SPR) that arises due to collective oscillation modes of the conduction electrons [12,13]. We find an enhanced (dia)magnetic behavior, which does not depend on temperature (in the range 5-300 K). We speculate that this enhanced magnetism is an orbital effect, resulting from mesoscopic electron trajectories within the NPs [2,14].The optically detected magnetic alignment technique relies on the anisotropy of both the optical and magnetic properties of the Au nanorods. Because of their shape, the rods exhibit an anisotropic optical response, determined by their longitudinal ( k ) and transverse ( ? ) polarizabilities [15]. Polarized light, therefore, provides a sensitive tool to determine the alignment of rods [16][17][18][19]. In this Letter, rod alignment is induced by a magnetic field (B) because of the difference in the magnetic susceptibility parallel ( k ) and perpendicular ( ? ) to the lo...

Superconducting fluctuations and large diamagnetism of low-Tcnanoparticles

“…Thus, they are just an example and provide motivation. Although the strength of superconducting fluctuations at such high temperatures is a truly general and remarkable phenomenon, and it otherwise explains all other features of the data, including the very weak temperature dependence up to ∼ 10 5 T c , something (as far as explaining the results of Ref [1]) is still missing! The inherent difficulty of this problem stems from the fact that the bulk volume susceptibility of Au, χ b ∼ several ∼ 10 −5 , results from the dense core electrons, which should not change very much with the arrangement and binding of the atoms (for example, in the metal or the nanoparticle).…”

“…According to Refs [1,3], the effect of Ref. [1] is intrinsic to the metal, and not due to chemical interactions with a capping layer.…”

“…[3] proposed an explanation of this effect in terms of the persistent currents flowing on the surfaces of these nanoparticles in response to the magnetic flux, using a model of ballistic, noninteracting electrons. According to Refs [1,3], the effect of Ref.[1] is intrinsic to the metal, and not due to chemical interactions with a capping layer.The experience [2,4,5] in explaining such mesoscopic currents shows however, that just finite-size effects due to noninteracting electrons fall short in explaining them both in sign and in magnitude. The reason being the alternating sign of the response as function of the azimuthal quantum number.…”

“…Ra,74.25.Bt Very recently [1], a novel nanoscale effect-a large average diamagnetic susceptibility of rod-shaped, down to ten-nanometer scale, gold nanoparticles-has been discovered experimentally. Such a susceptibility should be due to persistent currents [2] flowing in these nanoparticles in response to a magnetic field.…”

“…[3] proposed an explanation of this effect in terms of the persistent currents flowing on the surfaces of these nanoparticles in response to the magnetic flux, using a model of ballistic, noninteracting electrons. According to Refs [1,3], the effect of Ref.…”