Field dependence of blocking and irreversibility temperature in Fe3O4 magnetic nanoparticles coated by oleic and citric acid

Abstract

The magnetic nanoparticles can develop superparamagnetic properties in certain conditions1 and this feature attracted the researchers that began to study this class of materials in order to exploit their magnetic properties for applications. The main magnetic peculiarities which make these samples so interesting are the high magnetization and near zero coercive field that allow to use the magnetic nanoparticles for biomedical purposes2,3. Among all the magnetic nanoparticles, the Fe3O4 (magnetite) ones are very attractive due to their biocompatibility and because they show a low toxicity suggesting their possible use in different scopes4,5. Unfortunately, the magnetite nanoparticles are very sensitive to the action of oxygen, thus some of them might undergo oxidation to ferric hydroxide (Fe(OH)3) or to maghemite (γ-Fe2O3) phases. To limit this undesired effect and also the aggregation of primary nanoparticles of low dimension (10-30 nm), specific surface treatments and techniques of encapsulation are currently used to allow their utilization6.In this work, two samples of Fe3O4 magnetic nanoparticles coated with oleic acid (Fe3O4-OA) and citric acid (FeO4-CA) have been studied by means of DC magnetization measurements as a function of temperature (T) in Zero Field Cooling (ZFC) and Field Cooling (FC) conditions at different magnetic fields. The measurements have been performed by means of a Quantum Design PPMS equipped with a VSM option. The M(T) measurements have shown a superparamagnetic behavior in both the samples although with different characteristics which have been highlighted studying the field dependence of the blocking and irreversibility temperature. These two temperatures give important information on the nanoparticles distribution and their dispersion together with information on the diameter of the nanoparticles. In particular, the blocking temperature TB can be identified by taking the maximum of the ZFC curve which, in the case of superparamagnetic samples, has a dome shape behavior. The maximum M value of the ZFC curve can be considered as the blocking temperature of the entire sample, which, of course, differs from the blocking temperatures of the various nanoparticles of different sizes present in the sample. It can be considered as a sort of average blocking temperature of the sample useful when these materials are considered in view of applications. On the other hand, to estimate the TB distribution of the sample it is possible to use the procedure reported in our previous paper7. In this work, the attention has been focused only on the TB obtained from the maximum of ZFC curves. In Fig. 1, an example of M(T) measurement at H = 700 Oe has been reported. The panel(a) and (b) show the M(T) behavior of Fe3O4-OA and Fe3O4-CA, respectively. It can be noted in the insets that both the ZFC curves show a dome shape behavior with the presence of the maximum which identifies the TB of the samples. For the sample Fe3O4-OA, TB ≈ 160 K while for the sample Fe3O4-CA TB ≈ 145 K. In this framework, it is worth to underline that, for H = 700 Oe, the samples are in a superparamagnetic state at room temperature since Troom > TB in both the cases.The temperature value where the ZFC and FC curves separate identifies the irreversibility temperature Tirr. In an ideal case where the nanoparticles are of the same dimensions, the TB and the Tirr are equal. Usually, this is quite uncommon and, in the real case, the Tirr represents the temperature at which all the nanoparticles are certainly unblocked, as the reversible phase occurs after it. Specifically, Tirr represents the unblocking moment of the particles with larger dimensions, which, given the bigger volume than the particles with lower dimensions, unblock at higher temperatures. It is evident that in our case TB ≠ Tirr for both the samples which indicates that the nanoparticles have different diameters. In particular, the temperature difference ΔT between Tirr and TB for Fe3O4-OA is lower respect to the Fe3O4-CA one. In fact, Fe3O4-OA has Tirr ≈ 197 K while Fe3O4-CA has Tirr ≈ 282 K, so giving ΔT(Fe3O4-OA) = 37 K and ΔT(Fe3O4-CA) = 137 K indicating a bigger diameters difference among the nanoparticles of the Fe3O4-CA than the Fe3O4-OA. Finally, performing M(T) measurements at several magnetic fields, the magnetic field dependence of TB and Tirr has been determined and fitted with power and exponential equations extracting interesting samples peculiarities. **