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The heat produced by magnetic nanoparticles, when they are submitted to a time-varying magnetic field, has been used in many auspicious biotechnological applications. In the search for better performance in terms of the specific power absorption (SPA) index, researchers have studied the influence of the chemical composition, size and dispersion, shape, and exchange stiffness in morphochemical structures. Monodisperse assemblies of magnetic nanoparticles have been produced using elaborate synthetic procedures, where the product is generally dispersed in organic solvents. However, the colloidal stability of these rough dispersions has not received much attention in these studies, hampering experimental determination of the SPA. To investigate the influence of colloidal stability on the heating response of ferrofluids, we produced bimagnetic core@shell NPs chemically composed of a ZnMn mixed ferrite core covered by a maghemite shell. Aqueous ferrofluids were prepared with these samples using the electric double layer (EDL) as a strategy to maintain colloidal stability. By starting from a proper sample, ultrastable concentrated ferrofluids were achieved by both tuning the ion/counterion ratio and controlling the water content. As the colloidal stability mainly depends on the ion configuration on the surface of the magnetic nanoparticles, different levels of nanoparticle clustering are achieved by changing the ionic force and pH of the medium. Thus, the samples were submitted to two procedures of EDL destabilization, which involved dilution with an alkaline solution and a neutral pH viscous medium. The SPA results of all prepared ferrofluid samples show a reduction of up to half the efficiency of the standard sample when the ferrofluids are in a neutral pH or concentrated regime. Such results are explained in terms of magnetic dipolar interactions. Our results point to the importance of ferrofluid colloidal stability in a more reliable experimental determination of the NP heat generation performance., Authors gratefully acknowledge the financial support of the Brazilian agencies CAPES, CNPq (grants 465259/2014-6, 202340/2015-5, 400849/2016-0, 305796/2016-0, and 422740/2018-7), INCT-FCx (grant 2014/50983-3), and FAP-DF (grants 0193.001569/2017, 0193.001376/2016, and 0193.000.919/2016).

We report on the suitability of core/shell nanoparticles (NPs) for magnetic fluid hyperthermia in a self-regulated and theranostic approach. Aqueous magnetic colloids based on core/shell ZnxMnyFezO4@γ-Fe2O3 and ZnxCoyFezO4@γ-Fe2O3 NPs were produced by a three-step chemical synthesis. Systematic deviations from stoichiometry were observed with increasing Zn substitution for both series of samples. We investigated how the chemical composition affects the saturation magnetization, magnetic anisotropy, and thermomagnetic properties of these core/shell NPs. The heating efficiency through specific power absorption (SPA) was analyzed in the framework of the linear response theory. SPA values obtained for NPs present a different contrast of anisotropy between the core and shell materials, indicating no evidence of the contribution of enhanced exchange coupling to the heating efficiency.

Ultrasmall magnetic particles are notorious for exhibiting a magnetization increase even at quite intense applied fields, behavior which can be interpreted as a non-saturation of the magnetic disordered shell even if the nanoparticle magnetization is reversible. In this work we study two kinds of ultrasmall core@shell nanoparticles (3 nm) with contrasting core anisotropy, composed of MnFe2O4 and CoFe2O4 cores covered by a thin maghemite layer. In order to investigate the saturation criterion associated to the closure of major loops (in contrast to minor loops), we use several procedures to determine, at moderate fields, if the effective anisotropy energy barrier is overcome or not. Firstly, we carefully evaluate the closure field of the hysteresis loop, interpreted as the effective anisotropy field, correspondent to two different contributions. One, arising from the nanoparticle core, is related to the coercivity and the other one is associated to pinned spins of the nanoparticle shell. Secondly, the ZFC-FC magnetization measurements taken at different fields give us information about both the anisotropy energy barrier distribution and the thermal dependence of effective anisotropy field. Thirdly, forced minor loops are performed, measured after either ZFC or FC processes, to counter-check the effective magnetic anisotropy at low temperature of both samples. Finally, we perform major hysteresis loop calculations in order to better understand the magnetization processes involved. To go further, we propose in this study a procedure to extract information about the two contributions which composes the effective magnetic anisotropy field.