Variable blocking temperature of a porous silicon/Fe3O4 composite due to different interactions of the magnetic nanoparticles
© Rumpf et al.; licensee Springer. 2012
Received: 30 April 2012
Accepted: 26 July 2012
Published: 8 August 2012
In the frame of this work, the aim was to create a superparamagnetic nanocomposite system with a maximized magnetic moment when magnetized by an external field and a blocking temperature far below room temperature. For this purpose, iron oxide nanoparticles of 3.8-, 5- and 8-nm size have been infiltrated into the pores of porous silicon. To fabricate tailored magnetic properties of the system, the particle size and the magnetic interactions among the particles play a crucial role. Different concentrations of the particles dispersed in hexane have been used for the infiltration to vary the blocking temperature TB, which indicates the transition between the superparamagnetic behavior and blocked state. TB is not only dependent on the particle size but also on the magnetic interactions between them, which can be varied by the particle-particle distance. Thus, a modification of the pore loading on the one hand and of the porous silicon morphology on the other hand results in a composite material with a desired blocking temperature. Because both materials, the mesoporous silicon matrices as well as the Fe3O4 nanoparticles, offer low toxicity, the system is a promising candidate for biomedical applications.
In recent years, magnetic nanoparticles attracted high attention in various fields of nanoscience. Not only is the change of the physical properties of low-dimensional materials compared to their bulk materials of interest, but their applicability in various fields is also a developing subject. Magnetic materials in the nanoscale range are utilized in magnetic data storage  and GMR devices , and also in biological and medical applications, magnetic particles are employed . The magnetic properties of magnetite nanoparticles embedded in a nonmagnetic matrix have been figured out, which show the dependence of the magnetic behavior on the utilized matrix material . Furthermore, magnetite nanoparticles are extensively used for potential biomedical applications such as imaging of diseases (e.g., cancer and diabetes) or in cellular therapy . One precondition of magnetic particles for the utilization in biomedicine is the vanishing magnetic remanence when the external magnetic field is switched off, which requires that particles be superparamagnetic and do not magnetically interact. Thus, the blocking temperature should exhibit low values, but in any case, it has to be far below room temperature. Porous silicon, a versatile material which is also biodegradable, can be used as a matrix. Recently, works have been carried out, for example, by the groups of Sailor and Ferrari, concerning a combination of porous silicon and iron oxide nanoparticles applied in biomedicine [6, 7]. In the following work, the optimization of the magnetic properties, meaning the magnetic moment, to be as high as possible and the transition temperature between superparamagnetic and ferromagnetic-like behaviors of the porous silicon/iron oxide nanocomposites to be sufficiently low will be figured out to show that the system is applicable for biomedicine.
The magnetic properties of the nanocomposite system have been investigated by SQUID-magnetometry (S600, Cryogenic Ltd., London, UK) and with a vibrating sample magnetometer (PPMS, Quantum Design Inc., San Diego, CA, USA). During these measurements, the temperature has been varied from 4 to 300 K to gain zero-field-cooled (ZFC)/field-cooled (FC) magnetization curves. For field-dependent magnetization measurements, a magnetic field within ±6 T has been applied.
Results and discussion
The variation of the distance between the particles within the pores or between adjacent pores results in a modification of the strength of the dipolar coupling among the iron oxide nanoparticles. In general, one can say that the blocking temperature of the system depends on the particle size on the one hand. On the other hand, for a constant particle size, the TB can be modified by changing the distance between the particles within the pores, and a dramatic shift can be observed (see Figure 3). Also, in the case of varying the morphology of the porous silicon matrices, the TB can be modified in a smaller regime (see Figure 4) due to the change of the distance between the pores.
The blocking temperature is a crucial factor that has to be addressed in the case of biomedical applications because of the importance of the superparamagnetic behavior of the particles and the porous silicon/iron oxide composite system. Above TB, the nanoparticles do not magnetically interact due to the randomization of their magnetization, whereas below TB, the particles do interact and offer a ferromagnetic-like behavior. To facilitate the utilization of the particles in biomedicine, the blocking temperature has to be below room temperature to guarantee the disappearance of the remanence when the magnetic field is switched off.
In the frame of this work, the magnetic properties of Fe3O4 nanoparticles infiltrated within the pores of porous silicon have been investigated with respect to the transition between superparamagnetic behavior and blocked state. To fabricate distinct composite systems suitable, e.g., for magnetic field-guided drug delivery, the porous silicon template, as well as the loading of the pores with the particles, has to be adjusted. The nanocomposite should be superparamagnetic at room temperature but also offer a magnetic moment as big as possible. To ensure that there is no remanence after the external field has been switched off, magnetic coupling between the SPM particles has to be sufficiently low. A low coupling is in competition with the particle size and the maximized magnetic moment of individual particles, respectively, which means that a decrease of the size of the core particle results not only in weaker coupling but also in lower magnetic moments. The blocking temperature is decreased due to less coupling between smaller particles whereas the minimum distance is constant due to the same organic coating of about 2 nm in thickness for all particle sizes. To achieve systems with distinct magnetic properties, a variation of the particle size, as well as of the concentration of the particle solution for the pore filling, is appropriate.
This work is supported by the Austrian Science Fund FWF under project P21155.
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