Magnetization and electron paramagnetic resonance of Co clusters embedded in Ag nanoparticles

Abstract

We report magnetization and electron paramagnetic resonance (EPR) measurements on Co clusters embedded in Ag obtained by an inverse micellar technique. The cluster size (∼10 atoms), the saturation magnetization (M s), and the anisotropy constant were obtained from magnetization measurements at low temperature. In the as-prepared sample we found that M s /M bulk s = 0.44±0.05. Reduction of the sample under a H 2 flux at 373 K leads to an enhancement of the ratio M s /M bulk s to 1.16 ± 0.19-consistent with Co-cluster-beam experiments-and to an anisotropy constant of K = (8.5 ± 6.5) × 10 7 erg cm −3 , much larger than the corresponding bulk value. The EPR clearly shows the effect of dynamic narrowing due to superparamagnetic relaxation. Modelling the EPR spectra, we obtained independently the anisotropy energy value and the volume dispersion.

On the behavior of Ni Magnetic Nanowires as studied by FMR and the effect of “blocking”.

“…We concluded this is what happens in some nanoparticles at low temperature [DeBiasi et al, 2004; that evidence an intensity drop as the effective anisotropy field becomes greater than /. In previous works [Antoniak et al, 2005;Berger et al, 2001;Sánchez et al, 1999] this intensity loss was associated with "blocking" in the sense that the magnetic system was not able to follow the microwave excitation frequency   10 10 GHz (X band) below this "blocking" temperature. If that would be the case then, at the same temperature, the system would not be able to show any sign of resonance at a higher frequency, such as Q-band (35GHz), which is not the case (see also, [Ennas et al, 1998]).…”

“…Yet, a crucial difference between  ac and FMR is that the first technique is usually carried out at H = 0 while FMR is not. The implications of this decisive difference have been overlooked and lead some authors to the conclusion that "blocking" occurs in FMR [Antoniak et al, 2005;Berger et al, 2001;Lubitz et al, 2004;Sánchez et al, 1999;Trunova et al, 2009]. All these articles, dealing with magnetic nanoparticles, show that the ferromagnetic line broadens and its intensity decreases at low T, similarly to what is observed in  ac for T < T B .…”

Study of structural and ferromagnetic properties of pure and Cd doped copper ferrite

“…They reported that the shape anisotropy has the same functional dependence as a uniaxial anisotropy term, which is different from what was originally predicted [20]. Only recently [21][22][23] these theories have been applied to systems to obtain the physical parameters like anisotropy and magnetic moment, from the line shape of the FMR curves. In one such attempt, while probing Co/Ag clusters, Sánchez et al [22] showed that FMR and DC magnetization yield consistent results regarding the particle volume and anisotropy.…”

“…It is a standard tool for probing spin waves and spin dynamics, so it is a powerful characterization technique for magnetic nanoparticle systems [18][19][20][21][22]. One of the advantages of FMR over 3 conventional magnetization measurements is that it yields information on the dynamics of the system.…”

“…Only recently [21][22][23] these theories have been applied to systems to obtain the physical parameters like anisotropy and magnetic moment, from the line shape of the FMR curves. In one such attempt, while probing Co/Ag clusters, Sánchez et al [22] showed that FMR and DC magnetization yield consistent results regarding the particle volume and anisotropy. These theories rely on the assumption that the effective field acting on each particle can be considered as the addition of the applied field and the anisotropy field modified by thermal excitations.…”