AC Susceptibility Studies under DC Fields in Superspinglass Nanomaghemite-Multiwall Carbon Nanotube Hybrid

Abstract

Magnetic properties of maghemite (γ-Fe2O3) nanoparticles grown on activated multiwall carbon nanotubes have been studied by alternating current (AC) magnetic susceptibility experiments performed under different temperatures, frequencies, and applied magnetic fields. Transmission electron images have suggested that the γ-Fe2O3 nanoparticles are not isolated and have an average size of 9 nm, but with a relatively broad size distribution. The activation energies of these 9 nm γ-Fe2O3 nanoparticles, determined from the generalized Vogel–Fulcher relation, are reduced upon increasing the direct current (DC) field magnitude. The large activation energy values have indicated the formation of a superspinglass state in the γ-Fe2O3 nanoparticle ensemble, which were not observed for pure γ-Fe2O3 nanoparticles, concluding that the multiwall carbon nanotubes favored the appearance of highly concentrated magnetic regions and hence the formation of superspinglass state. Magnetic relaxation studies, using Argand diagrams recorded for DC probe fields (<20 kOe) below the magnetic blocking temperature at 100 and 10 K, have revealed the presence of more than one relaxation process. The behavior of the ensemble of γ-Fe2O3 nanoparticles can be related to the superspinglass state and is also supported by Almeida–Thouless plots.

The Effect of a DC Magnetic Field on the AC Magnetic Properties of Oleic Acid-Coated Fe3O4 Nanoparticles

“…Using the value of the peaks’ temperatures in Figure 2 b for both frequencies, it is possible to evaluate the Mydosh parameter [ 31 ] following the definition: Φ = ΔT p /(T Lf Δlog(f)) where f is the frequency of the AC magnetic field, T P is the peak’s temperature and T Lf is the peak’s temperature at low frequency. Typically, the value obtained for Φ is in the range of 0.05–0.18 for a super-spin-glass system and in the range of 0.3–0.5 for a system of non-interacting nanoparticles [ 28 ].…”

“…On the other hand, in the literature, the existence of a super-spin-glass behaviour for MNPs is also reported [ 28 , 37 , 38 ]. In that case, the field dependence of the peak temperature (T SSG ) of χ″(T) follows the relation: T SSG = T f [1 − (H/H 0 ) 2/3 ] where T f is the freezing temperature of the system without a magnetic field, and H 0 is the theoretical transition field for a spin-glass state at zero temperature.…”

“…where f is the frequency of the AC magnetic field, T P is the peak's temperature and T Lf is the peak's temperature at low frequency. Typically, the value obtained for Φ is in the range of 0.05-0.18 for a super-spin-glass system and in the range of 0.3-0.5 for a system of non-interacting nanoparticles [28].…”

“…The idea is that depending on the type of transition that characterises the nanoparticles, the DC field produces a different influence on the blocking temperature [ 24 , 25 , 26 , 27 ]. In particular, for a super-spin-glass system, the presence of a DC field produces a variation of the freezing temperature (related to the super-spin-glass transition) [ 28 ] that is different from the one usually expected for the blocking temperature in a sample of non-interacting MNPs [ 29 , 30 ]. The results obtained with this approach can be compared with the evaluation of the Mydosh parameter [ 31 ].…”

“…In a similar way to the effect of a superimposed DC field, the effect of the AC field frequency on the blocking temperature is different for non-interacting nanoparticles with respect to a super-spin-glass system. In particular, by evaluating the variation of the blocking temperature in a decade of frequencies, it is possible to empirically separate three ranges for the value of the Mydosh parameter in the case of non-interacting, clusterised and super-spin-glass nanoparticle systems [ 17 , 28 ]. In this work, a MNPs sample has been studied showing the presence of a double peak in the imaginary part of AC susceptibility.…”