Magnetic domain interactions of Fe3O4 nanoparticles embedded in a SiO2 matrix

Currently, superparamagnetic functionalized systems of magnetite (Fe3O4) nanoparticles (NPs) are promising options for applications in hyperthermia therapy, drug delivery and diagnosis. Fe3O4 NPs below 20 nm have stable single domains (SSD), which can be oriented by magnetic field application. Dispersion of Fe3O4 NPs in silicon dioxide (SiO2) matrix allows local SSD response with uniaxial anisotropy and orientation to easy axis, 90° <001> or 180° <111>. A successful, easy methodology to produce Fe3O4 NPs (6–17 nm) has been used with the Stöber modification. NPs were embedded in amorphous and biocompatible SiO2 matrix by mechanical stirring in citrate and tetraethyl orthosilicate (TEOS). Fe3O4 NPs dispersion was sampled in the range of 2–12 h to observe the SiO2 matrix formation as time function. TEM characterization identified optimal conditions at 4 h stirring for separation of SSD Fe3O4 in SiO2 matrix. Low magnetization (Ms) of 0.001 emu and a coercivity (Hc) of 24.75 Oe indicate that the embedded SSD Fe3O4 in amorphous SiO2 reduces the Ms by a diamagnetic barrier. Magnetic force microscopy (MFM) showed SSD Fe3O4 of 1.2 nm on average embedded in SiO2 matrix with uniaxial anisotropy response according to Fe3+ and Fe2+ electron spin coupling and rotation by intrinsic Neél contribution.

The addressable Fe 3 O 4 NPs in functionalized systems are of interest for the development of applications in magnetic hyperthermia, drug delivery and diagnosis agents and alternative cancer treatments due to their high biocompatibility 1-3 , bioactivity 4,5 , driving accumulation 6,7 and magnetic excitation [8][9][10] features. The systems based on superparamagnetic NPs behaviour have either biological or technological potential applications. Fe 3 O 4 NPs have been extensively studied due to their interesting properties, which allow their evaluation and potential application as catalytic agents [11][12][13] , gas sensors 14 , water treatment agents 15 , environmental remediation agents [16][17][18] magnetic resonance contrast agents 1,19 , ferrofluids 20 , data storage devices 21 , and electronic devices 22 . Superparamagnetic Fe 3 O 4 NPs with SSD have crystal anisotropy in the order 1.1, which allows easy reorientation and energy exchange to generate magnetic hyperthermia in presence of a magnetic field AC 23 . However, avoiding Fe 3 O 4 NPs agglomeration due to interactions between neighbours is a challenge for biological applications. One alternative is separating the Fe 3 O 4 SSD considerable distances by embedding them in a negatively charged, amorphous SiO 2 matrix 24 . This method allows a local response orientation of SSD to be obtained, according to Majetich S. A. et al.; if neighbouring superparamagnetic particles with a low anisotropy are coupled by exchange or dipolar interactions, their optimal separation for the maximum coercivity will allow the configuration of individually addressable SSD 25 . The local interactions will be the same, if the Fe 3 O 4 particle size is similar. Then, SSD will have coherent rotation and contribute to the effective energy exchange by hyperthermia.
Current implementation of these systems depends on the superparamagnetic particle size (<20 nm) and SSD anisotropy, which directly influence the stability, dissipated energy and magnetic properties as function of time, such as the magnetic susceptibility and coercive field 26 . For this reason, different investigations have developed different methods to stabilize Fe 3 O 4 NPs while preserving the main SSD properties [27][28][29] . V Reichel et. al. showed that the magnetic properties of magnetite are dependent on Néel relaxation. These depend on the particle size, morphology and interparticle interactions as well as the temperature and time-scale of the measurement in Fe 3 O 4 NPs (9 ± 3 nm) 30 . The SiO 2 media can be used to give Fe 3 O 4 NPs specific accumulation in organs or tissues Scientific REPORTS | (2018) 8:5096 | DOI:10.1038/s41598-018-23460-w through bio-functionalization [31][32][33][34][35] because it is an interaction-capable surface with ligands related to biological molecules.
Successful stabilization of Fe 3 O 4 NPs with SiO 2 can be achieved by using citrate as a dispersing agent. The polymerization time of SiO 2 was considered in the range of 2-12 h. However, Fe 3 O 4 functionalization and interaction media barriers with organic products can reduce M s . The magnetic response will depend on the volume, particle form and SSD population density in the system 36 . Rotation SSD Fe 3 O 4 will depend on the intrinsic structure defined Néel interactions and magnetic moment distribution per volume unit by the easy axis crystal orientation, <100> or <111> and the coercivity terms, which are crucial for SSD uniaxial response 37 . The routes to control the dispersion Fe 3 O 4 NPs embedded in SiO 2 help to consolidate well defined the local SSD and their magnetic interactions. Superparamagnetic Fe 3 O 4 NPs dispersions in an inorganic matrix allow the creation of a functional target or hyperthermia generators with potential applications in nanomedicine 38,39 .

Results and Discussion
The key to obtaining Fe 3 O 4 NPs is the ion relation ½ Fe +2 : Fe +3 by co-precipitation and the additional dispersion reaction by Stöber route. These ions were dispersed with citrate and TEOS to produce Fe 3 O 4 in an amorphous SiO 2 matrix according to the methodology described by L. Yang et al. 40 with some modifications.
XRD results show the main crystalline reflections of Fe 3 O 4 NPs with a cubic inverse spinel structure, forming a close packing fcc (a = 8.396 Å) 41 , Fig. 1a. Ferromagnetic spin interactions are defined by divalent Fe 2+ and trivalent Fe 3+ occupying octahedral sites and a double-exchange mechanism, while Fe 3+ tetrahedral sites form an antiferromagnetic response, Fig. 1b. Because of superexchange, oxygen-mediated coupling, all the magnetic moments of the tetrahedral iron ions are aligned in a specific direction, while all the octahedral iron magnetic moments are aligned in the opposite direction 42 .
The spectra (Fig. 1a  Additionally, wide peaks are related to the nanometric particle size. Some changes in the NPs size were observed as a function of the reaction time, Fig. 1a. Supplementary information (s1) shows the FWHM values and sizes obtained through a modified Scherrer analysis by using a pseudo-Voigt function 43 . In some cases, the size of the particle increased due to agglomeration and the dynamic stirring energy. When the SiO 2 matrix does not complete the polymerization, the uncoated Fe 3 O 4 NPs form aggregates and reassemble 30 . Peaks of smaller The FT-IR spectra of different samples are presented in Fig. 1c. The interactions of the functional group Fe 3 O 4 NPs in the SiO 2 matrix can be observed. Characteristic iron-oxygen bond (Fe-O) signals at approximately 585 cm −1 are in the five spectra. Crystalline Fe 3 O 4 is conserved after the embedding process. Also, it is possible to identify the SiO 2 contributions in Fig. 1c; a broad band centred at 1048 cm −1 is observed. There are also two interactions of interest at 1113 cm −1 , which is attributed to the Si-O-Si bonds, and another at 986 cm −1 , which is due to the Si-OH groups from the matrix. These interactions slightly increased in intensity with the reaction time, which promoted the SiO 2 layer thickness. No additional linkage interactions were observed for the functional groups characteristic of citrate and the stabilizing agents used during the experiment. These were completely removed during the particle washing step. This indicates that the reaction parameters, such as the concentration of the solutions and the reaction times, allow the dispersed particles to be obtained without adsorbed citrate molecules on the surface of the  (311) and (440) 44 . The agglomerates increased in size. (4) Agglomerates plus isolated SSD were obtained in the SiO 2 matrix after 12 h of stirring, Fig. 2(j-l). This phenomenon is associated with the strong interaction between neighbours. SAED shows the rings of the diffraction planes of Fe 3 O 4 , which are very similar in intensity to those in Fig. 2c. It could be said that the matrix of SiO 2 formed at 4 h stirring is a negatively charged film that confines the SSD as individual entities. As the agitation time is increased to 8-12 h, the dynamic movement increases the coalitions between NPs and their interactions. As consequence, the attraction between NPs increases the agglomeration with a long agitation time.
The main challenge in potential applications of Fe 3 O 4 NPs lies in their superparamagnetic properties, uniaxial anisotropy, particle size and form. Typically, bulk Fe 3 O 4 has a M s of 91 emu/g, and its inner structure forms multidomains with magnetic anisotropic axial ratios larger than 5 45,46 . In superparamagnetic NPs, the magnetic anisotropic energy barrier is reduced and allows the rotation of SSD from a spin-up state to spin-down state to readily rotate the magnetic spin direction on the easy axis 47 . The magnetization from Fe 3 O 4 NPs embedded in a SiO 2 matrix shows superparamagnetic, closed hysteresis loops 48 Fig. 4b. The profiles below 3D images are associated to particle size and their agglomerations in both cases.
MFM shows the magnetic moment density per volume unit. The 3D images are generated by the interaction of magnetic tip polarization, which is normal with respect to the NPs surface, and sensing of the attractive and repulsive states 48 . SSD are NPs that can be aligned in parallel by exchanging forces of the electron spins from applied magnetic field. Inner magnetic structure Fe 3 O 4 SSD with Fe +3 and Fe +2 electron spin coupling respond uniaxially because magneto-crystalline anisotropy forces tend to align in a preferred direction based on the easy axis. For superparamagnetic sizes, the relative magnitude of the boundary energy becomes larger than the magnetostatic energy. At a critical size, the spin direction can be oriented by exchange interactions in the SSD 37 .
MFM 3D images showed surface of NPs by magnetization H↑ and demagnetization H = 0 effects from Fe 3 O 4 SSD embedded in the SiO 2 matrix. The magnetic interaction at the tip in the lift mode shows the disorder from the intrinsic structure at H = 0. Under the saturation measurement conditions H~1200 Oe for the Co-Ni tip, the SSD uniaxial orientation of the magnet is shown with the domains oriented at 90° for the majority n↑ and minority n ↓ states with H↑ 48 . Finally, domain disorder is observed by demagnetization under the initial conditions H = 0. When the magnetic field AC is applied to these systems, the magnetic domains will fluctuate with local vibrations and will transform this energy into thermal energy. The orientation was observed in a non-continuous media  Fig. 5(a-c). In the case of NPs agglomeration, the surface interactions showed a global response by addition of all magnetic domains in the 12 h stirring sample, Fig. 5(d-f).
Zoom was performed under the saturation conditions H↑ to show the uniaxial behaviour. Parallel stripes from attractive and repulsive interactions define the magnetic domains. The exchange forces of the parallel alignment are result of the internal magnetic structure in Fe 3 O 4 NPs, which are defined by Neél interactions, Fig. 6. The profiles show the θ (deg) and domains oriented in the normal direction, 90°, with average domain sizes of 1.2 nm and uniaxial anisotropy from main crystalline direction 37 . The anisotropy energy is given by E a = KVsin 2 θ, where K is a typical constant of the material, V is the volume of the particle, θ is the angle between the magnetization and the easy axis. The local response of  Tipically Hc decreases as a function of particle size in Fe 3 O 4 NPs with close hysteresis loop. However, the superparamagnetic single-domains response of Fe 3 O 4 NPs is maintained by local response SSD as agglomeration phenomena is reduced in the case of SiO 2 matrix due to its permeability 47 . According to Dave S., R. Dave and G. Xiaohu, the formation of the domain walls is a process driven by the balance between the magnetostatic energy as a function of the particle size 52 .

Conclusions
The synthesis strategy presented is a fast and effective option to avoid the agglomeration phenomenon of Fe 3 O 4 NPs by stabilizing them in SiO 2 as a function of the stirring time without altering their crystalline features.
The H c is maintained in individual Fe 3 O 4 SSD with uniaxial behaviour at 4 h of stirring. MFM showed SSD and that local NPs in the SiO 2 matrix have similar responses as the NPs concentration increases. However, M s is reduced by the distance between the NPs among the SiO 2 matrix, and this depends on the permeability of the media. Neél interactions produce local fluctuations and energy transference from reorientable SSD by magnetic fields at high frequencies. The challenge is to control the Fe 3 O 4 SSD as individuals in organic media to increase the response velocity and effective magnetic excitation to produce hyperthermia.

Synthesis of Fe 3 O 4 NPs.
The formation of nanostructured magnetite through a co-precipitation method 49 was realized via the following: an aqueous solution of hydrochloric acid (HCl, 0.64 N) was prepared to dissolve the precursor salts of the Fe +2 (ferrous chloride, FeCl 2 ) and Fe +3 ions (ferric chloride, FeCl 3 ).
After the acidic solution was prepared, 6.25 mL was taken, and 2.52 g of FeCl 2 was added. Another 25 mL of the solution was used to dissolve 6.99 g of FeCl 3 , and both solutions were kept under magnetic stirring for 1 h. Then, both solutions were mixed in a round-bottom, three-necked flask and bubbled with Ar for 5 min to remove any dissolved oxygen in the solution. The resultant solutions were stirred at 500 rpm and heated to 70 °C. Once the temperature stabilized, 21 mL of tetramethylammonium hydroxide was added dropwise, providing an alkaline environment for the formation of the magnetite. The solution turned black, which was an indication that iron oxide formed in the solution. After 40 min, the stirring and heating were stopped, and the mixture was allowed to cool to room temperature. The nanoparticles were washed with water and precipitated with the aid of a magnet until the pH of the dispersion was equal to 7. Once the nanoparticles were dispersed in water, they were freeze-dried for 12 h to obtain the magnetite powder. Then, the SiO 2 precursor solution was prepared as follows: 10 mL of water, 50 mL of ethanol, 5 mL of ammonium hydroxide and 0.2 mL of TEOS were mixed by magnetic stirring for 10 min. After that, the solution of magnetite and citrate was mixed with the SiO 2 precursor solution. To modify the thickness of the SiO 2 coating, the mixture reacted for 2, 4, 8 and 12 h under mechanical stirring at 500 rpm. Once the different times passed, the particles were washed with ethanol and precipitated with the help of a magnet. They were dried at room temperature, and the powder was characterized.

Synthesis of Fe
The analysis of the material allowed determination of the optimal reaction time conditions to reduce the agglomeration in the citrate suspension. Additionally, these samples were processed by the Stöber method to embed the magnetite NPs in the matrix of silicon dioxide. This is an easy methodology to reduce agglomeration and to improve the Fe 3 O 4 NPs dispersal in the SiO 2 matrix and the magnetic properties of the magnetite under different stirring conditions.
To determine the properties of the nanostructures, different characterization techniques were used: RIGAKU Smart Lab X-ray diffractometer, XRD, was used in the Bragg-Brentano configuration with 0.02 steps in the range from 25 to 70°, 2 θ with a copper source of 1.5424 Å and wavelength to define Fe 3 O 4 /SiO 2 structures. PowderCell Software was used to analyse experimental diffractograms from XRD 53,54 . Their dispersion and particle sizes were observed by JEOL-JEM 2010 transmission electron microscope, TEM, with a LaB 6 filament at an acceleration voltage of 200 kV. The functional groups of silicon and magnetite were examined with an FT-IR spectrophotometer, NICOLET 6700. The magnetic response was determined using a MPMS3 Magnetometer SQUID of Quantum Design with a sensitivity of 5 × 10 −8 emu.
Magnetic force microscopy (MFM) was performed with a scanning probe microscope (SPM) JEOL JSPM 5200 multimodes. The samples were dispersed in carbon adhesive tape over the holder. An ultra-sharp silicon cantilever NSC14/Co-Cr/15 micro-mesh was exposed to a strong neodymium magnet. The voltage and lift conditions were defined according to the magnetic surface interactions of the sample with the tip lift output [0.030-0.1 V].