ABSTRACT
In nanoscale magnetic systems, the possible coexistence of structural disorder and competing magnetic interactions may determine the appearance of a glassy magnetic behavior, implying the onset of a low-temperature disordered collective state of frozen magnetic moments. This phenomenology is the object of an intense research activity, stimulated by a fundamental scientific interest and by the need to clarify how disordered magnetism effects may affect the performance of magnetic devices (e.g., sensors and data storage media). We report the results of a magnetic study that aims to broaden the basic knowledge of glassy magnetic systems and concerns the comparison between two samples, prepared by a polyol method. The first can be described as a nanogranular spinel Fe-oxide phase composed of ultrafine nanocrystallites (size of the order of 1 nm); in the second, the Fe-oxide phase incorporated non-magnetic Au nanoparticles (10-20 nm in size). In both samples, the Fe-oxide phase exhibits a glassy magnetic behavior and the nanocrystallite moments undergo a very similar freezing process. However, in the frozen regime, the Au/Fe-oxide composite sample is magnetically softer. This effect is explained by considering that the Au nanoparticles constitute physical constraints that limit the length of magnetic correlation between the frozen Fe-oxide moments.
ABSTRACT
We study the mechanism of heat generation, induced by an alternating magnetic field, in magnetite nanoparticles doped with manganese, produced by thermal decomposition from organometallic precursors. We investigate a set of four samples obtained by varying the duration of the reflux treatment carried out at a temperature of 300 °C during the synthetic procedure. On increasing this parameter from 60 to 180 minutes, the mean size of the nanoparticles increases, though remaining below 10 nm, as well as the saturation magnetization, which in all the samples, thanks to the Mn doping, is higher than that in magnetite nanoparticles taken as a reference. The combination of these two events has two main consequences. First, it determines the intensity of dipolar interactions between the nanoparticles, thus influencing their magnetic relaxing behavior, which, in turn, is closely related to the heating efficiency. Secondly, in a heating test, it is possible to operate in the regime of non-linear magnetic response of the nanoparticles at values of amplitude and frequency of the alternating field usually employed for biomedical applications. We show that, in this regime, the Specific Absorption Rate (SAR) in each sample depends linearly on the fraction of nanoparticles that are not superparamagnetic. This opens the possibility of modulating the heating capacity of the produced nanoparticles, so as to match specific needs, changing only a single synthesis parameter and opportunely exploiting the strict connection between structural features, magnetic properties and measurement conditions.
