Quantification of Dipolar Interactions in Fe_3–xO₄ Nanoparticles

Abstract

A general method for the quantification of dipolar interactions in assemblies of nanoparticles has been developed from a model sample constituted by magnetite nanoparticles of 5 nm in diameter, in powder form with oleic acid as a surfactant so that the particles were solely separated from each other through an organic layer of about 1 nm in thickness. This quantification is based on the comparison of the distribution of energy barriers for magnetization reversal obtained from time-dependent relaxation measurements starting from either (i) an almost random orientation of the particles' magnetizations or (ii) a collinear arrangement of them prepared by previously field cooling the sample. Experimental results and numerical simulations show that the mean dipolar field acting on each single particle is significantly reduced when particles' magnetizations are collinearly aligned. Besides, the intrinsic distribution of the energy barriers of anisotropy for the non-interacting case was evaluated from a reference sample where the same magnetic particles were individually coated with a thick silica shell in order to make dipolar interactions negligible. Interestingly, the results of the numerical simulations account for the relative energy shift of the experimental energy barrier distributions corresponding to the interacting and non-interacting cases, thus supporting the validity of the proposed method for the quantification of dipolar interactions.

Structural and magnetic properties of core-shell Au/Fe3O4 nanoparticles

“…The M s value is in agreement with the measured value whereas the K eff value is in good agreement with the reported value for bulk magnetite29. Differences between the experimental and calculated FC curves could be assigned to the occurrence of particle-particle interaction, once the effect of those interactions is the flattening of the FC curves, in agreement with the literature30. In the low temperature region the blocking temperature distribution has been assigned mainly to the contribution of non-interacting magnetite NPs.…”

Beyond the qualitative description of complex magnetic nanoparticle arrays using FORC measurement

“…Physically, these extra features cannot fully be due to the interaction fields, but they have been previously shown to be potentially from thermal fluctuations [51]. Other magnetic measurements, such as ferromagnetic resonance, have shown that interaction fields cause broadening and/or shifts of the P Hc peak [52][53][54][55], but not necessary inducing extra local minima/maxima. Similar problems arise with P Hu distributions in figure 5.…”

Magnetic hyperthermia performance of magnetite nanoparticle assemblies under different driving fields

“…Herein, exchange coupling is neglected and long-range dipolar interaction is taken into consideration. 21,22 For an ensemble of randomly distributed NPs, this energy barrier is thus revised by the mean-field approximation: 23,24 …”