Magnetic Dynamics of Iron-Oxide Nanoparticles in Frozen Ferrofluids and Ferronematics

Abstract

We present a detailed study of the magnetic behavior of iron-oxide (γ-Fe 2 O 3 and Fe 3 O 4 ) nanoparticles constituents of ferrofluids (FF's) with average particle sizes <d> = 2.5 and 10 nm. The particles were dispersed in the frozen liquid carrier (pure FF) and in a frozen lyotropic liquid crystalline matrix in the nematic phase or ferronematic (FN) (ferrolyomesophase). Both FF and FN phases displayed superparamagnetic (SPM) behaviour at room temperature, with blocking temperatures T B ~ 10 and 100 K for <d> = 2.5 and 10 nm, respectively. Dynamic ac susceptibility measurements showed a thermally activated Néel-Brown dependence of the blocking temperature with applied frequency. Our results show that dipolar interactions are small, but non-negligible, as compared to the single-particle energy barriers from magnetic anisotropy. From the fit of ac susceptibility we calculated the effective magnetic anisotropy constant K eff for 2.5 nm maghemite particles. Although interparticle interactions present in highly diluted samples do not appreciably modify the dynamic magnetic behavior of isolated particles, the calculated magnetic anisotropy were abut one order of magnitude larger that the bulk materials, suggesting the existence of large surface anisotropy. Using the thermally activated model to fit the dynamic data yielded effective energy barriers E a = 3.5x10 -21 J. From these data, we obtained K eff = 422 kJ/m 3 for the single-particle effective magnetic anisotropy.

1H-NMR study of the spin dynamics of fine superparamagnetic nanoparticles

“…26 An estimate of the Zeeman energy E Z at 174426-3 3.4 and 14.4 kOe yields values of the order of 10 2 -10 3 erg while the particle magnetic anisotropy energy E A as calculated from the bulk magnetocrystalline anisotropy constant K bulk = 4.7 × 10 4 erg/cm 3 is only of the order of 10-10 2 erg. However, it is quite common to witness a difference of two orders of magnitude 16,27,28 between the bulk anisotropy energy density and the effective anisotropy energy density in a nanoparticle with size d 5 nm, because additional sources of anisotropy come into play (i.e., shape, surface, magnetostriction contributions, and the dipolar interaction contribution). Thus, it is not unusual to have E Z < E A and a double-minima (or multiminima) energy landscape even at the high fields commonly found in solid-state NMR experiments.…”

Blocking phenomena studies on ferronematics

“…Thus, optical measurements were performed and the optical orientation behavior is very similar to the magnetic one for the ferronematics doped with smaller sized magnetic particles [3,6] as well as the ones doped with the larger magnetic particles [3]. The time scales are different, of course, because the optical orientation involves the rotation of the directors, as well as the magnetic measurements.…”

“…Concurrently with dipolar interactions, other sources of magnetic anisotropy (e.g. surface effects) can have a major impact on the blocking process [6]. In this work, we discard the hypothesis of agglomeration phenomena in these ferronematic samples.…”

Ferrofluids: Synthetic Strategies, Stabilization, Physicochemical Features, Characterization, and Applications

“…[207] In the case of dipolar interactions, the sources of magnetic anisotropy such as surface effects may significantly influence the blocking phenomenon. [208] The cryo-TEM images of Fe 3 O 4 dispersions (Figure 12 d,e) clearly demonstrate the increasing effect of dipolar attraction between the MNPs as their size increases. This could be explained in terms of the value of the interaction parameter (l) for which the expected value for 16 nm Fe 3 O 4 particles with dominant isotropic interactions is approximately 2, whereas for 21 nm Fe 3 O 4 particles exhibiting linear chain structures, the l value is approximately 7.…”