Measurement of the giant magnetoresistance effect in cobalt–silver magnetic nanostructures: nanoparticles

Abstract

Cobalt-silver (Co-Ag) core-shell nanoparticles with different silver thicknesses were prepared by the microemulsion method in a two-step reduction process. Transmission electron microscopy (TEM) characterization revealed the almost monodispersity and nanometric size (in the range 3-5 nm depending on the shell thickness) of the synthesized nanoparticles. However, it was the use of high-resolution TEM that revealed the correct core-shell formation of the nanometric material. The selected area electron diffraction pattern indicated the fcc (face-centered cubic) and hcp (hexagonal close packed) nature for silver and cobalt, respectively. Cyclic voltammetry also allowed the correct core-shell formation to be assured. The magnetic properties revealed the presence of both superparamagnetic and ferromagnetic contributions. Because of the lack of methodology, it was necessary to develop a method to measure the magnetotransport properties of the prepared nanoparticles. The strategy which followed was successful as it was possible to measure these properties: giant magnetoresistance values of 0.1% at room temperature were obtained. The numerical analysis of magnetic and magnetoresistance data indicated the presence of superparamagnetic particles showing interaction among the magnetic moments.

Effect of chemical composition and Co-core size on the magnetic and magneto-transport properties of CoyAg100−y core–shell nanocrystallites

“…It seems a result of forming and anchoring a sufficiently rigid shell (nonmagnetic) on a ferromagnetic core of cobalt so that it behaves as an ideal single magnetic domain. A kind of a nonmagnetic shell serves as a strong pinning barrier which supports devising an effectively large H c in the ideal single magnetic domains of small fcc-Co crystallites bounded in an Ag-shell [12,16]. These results are in good agreement with the earlier reported work of Co@Au core-shell NPs, wherein with an increase in shell thickness the H c values increased due to enhanced interfacial pinning effect [9].…”

“…4 , when compared to Co 36.6 Ag 63.4 crystallites. Consistent with the results reported in the literature by Garcia-Torres et al on similar Co-Ag alloys [12], another key factor which controls the MR properties in such materials is the thickness of the shell, and as the shell thickness increases, the MR properties deteriorate significantly. It supports a lower MR value observed in the sample Co 20.6 Ag 79.4 over an increased number of magnetic centres in the sample Co 36.6 Ag 63.4 which has a larger Co-content.…”

“…However, true core-shell structures are envisaged as better alternative over thin films or composites as the particle size could be effectively controlled to near ideal situation of two ferromagnetic single domains separated by a non-ferromagnetic layer. A plethora of chemical routes such as co-precipitation, thermal decomposition, reverse micelle route and transmetallation have been reported thus far to synthesise such CoAg CSNCs [11][12][13][14][15][16]. In comparison to the physical methods, the chemical methods provide an additional advantage of precisely controlling the chemical composition, size and shape (also surface topology) in CSNCs.…”

Strong circular dichroism of core-shell magnetoplasmonic nanoparticles

“…Such nanoparticles can be synthesized in the laboratory and are being extensively studied experimentally [6,[15][16][17][18] and theoretically [19][20][21][22][23][24][25][26][27][28], since they exhibit interesting electronic, magnetic, optical, chemical, and biological properties. In our calculations, we use the actual optical and magneto-optical constants of the constituent materials, ϵ z , ϵ r , ϵ κ , ϵ s , in the spectral region that interests us here, deduced from the experiment [29][30][31], while μ g 1 and μ s 1.…”

“…(18), (19), and (22) that, in general, the cross sections depend on the polarization and the direction of propagation of the incident plane wave. In the particular case of a spherically symmetric particle, e.g., if in our case the core is also made of a nongyrotropic (isotropic) material, the T matrix becomes diagonal: T Plm;P 0 l 0 m 0 T Pl δ PP 0 δ ll 0 δ mm 0 , and the cross sections solely depend on the T matrix, since P m jA 0 Plm ·êj 2 2π2l 1.…”