Field Dependence of Blocking Temperature in Magnetite Nanoparticles

Abstract

Spherical magnetite nanoparticles having average particle size <d> = 5 nm have been synthesized by coprecipitation of Fe(II) and Fe(III) salts in KOH with Polyvinylalcohol (PVA). The resulting dry powder displayed superparamgnetic (SPM) behaviour at room temperature, with a transition to a blocked state at T B ~ 45 K for applied field H app = 500 Oe. The effect of dipolar interactions was investigated by measuring the dependence of T B on the applied field H ap and driven ac field in susceptibility data. A thermally activated model has been used to fit the dynamic data to obtain the single-particle energy barriers E a = K eff V, allowing us to estimate the contributions of dipolar interactions to the single-particle effective magnetic anisotropy K eff . We have measured the dependence of T B with H ap in order to draw the transition contours of a H-T diagram. Two different regimes are found for the (T B -T 0 ) ~H λ dependence at low and high fields, that can be understood within a pure SPM relaxation-time (Néel-Brown) landscape. The T B (H) data shows a crossover from λ = 2/3 to λ ~2 for applied magnetic fields of ≈ 550 Oe.

Fabrication and magnetic properties of Fe3O4 nanowire arrays in different diameters

“…This phenomenon was also observed by other researchers [33]. At a high field, the relationship between the blocking temperature and the applied field can be described by power dependence of H 2/3 p(1ÀT B /T 0 ), where T 0 is the blocking temperature at zero field [30]. Our experimental results above 200 Oe can be well fitted by this relation (see Fig.…”

“…4b). The filed dependence of blocking temperature has also been observed by other groups [30,31]. The reason is that high field can lower the energy barriers between the two easy axis orientations, and therefore lower the blocking temperature.…”

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

“…The position of the maximum decreases with increasing field, as it would be expected. 16 The corresponding magnetization loop at 5 K is shown in the inset of Fig. 2: the magnetization does not reach a saturation value either at room temperature or at 5 K and it stands well below the value 80 emu/g, the saturation magnetization in bulk maghemite.…”

“…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.…”

Sol–gel synthesis and anomalous magnetic behaviour of NiO nanoparticles

“…Lai et al [50] calculated the effective crystalline anisotropy constant of K eff epitaxial (1 1 1) grown NiO film from micromagnetic modelling as 2.4 Â 10 3 J/m 3 . Large values of K eff are usually attributed to dipolar interactions and/or surface effects [51] ( Table 2). …”