Fyzikální ústav Akademie věd ČR

In the field of magnetic nanomaterials targeting biomedical theragnosis and nanosensing, low-dimensional magnetic materials attracted enormous attention due to promising applications in various fields. Among the variety of nanomagnets, nanoparticles of iron oxide are of high importance due to their size-dependent magnetic properties and biocompatibility, which already initiated their exploitation as MRI contrast agents, hyperthermia mediators and drug carriers. One of emergent objectives of nowadays nanomedicine is to develop multimodal nanoparticles with simultaneous detection and treatment abilities following the so-called theragnosis concept – a goal of our 7^thFP project MULTIFUN.

However, design and tuning of particles for proposed applications requires deep understanding of their magnetic properties with respect to their internal crystallographic and spin structure (see Fig. 1), and mutual interactions in dense systems. Therefore; we focus on in-deep characterization of nanoparticles with impact on limits of real systems including surface spin canting [1, 2], inter-particle interactions [3], size distribution and collective response of nanoparticle ensembles [4, 5].

Fig.1: Illustration of the limiting cases of the internal structure of nanoparticles with particle size determined from transmission electron microscopy (TEM), X-ray diffraction (XRD) and magnetic measurements (mag). The large arrow represents the particle superspin, while the small arrows correspond to individual magnetic moments per unit cell. The figure on right represent TEM images of particles with identical TEM size, but very different XRD and magnetic sizes together with schematic response of the spin structure to the applied magnetic field. The core-shell particles (left) show huge SAR value in contrast to almost no heating effect of the later ones.

For reliable classification of the surface spin canting phenomena in iron oxide nanoparticles, we proposed to determine the evolution of the hyperfine/effective field with respect to the applied magnetic field by using Mössbauer spectroscopy in order to obtain complex response of spins in individual sublattices. Finally, we have broken the myth that the spin canting is a general feature for all nanoparticles; it was recognized as negligible in highly crystalline particles with sizes larger than 6.5 nm [2].

We also observed experimentally, that some level of internal particle spin and/or structural disorder is crucial for enhancement of the so-called specific absorption rate (SAR) value – a measure of the particle performance in magnetic fluid hyperthermia [6]. Therefore, we proposed a model connecting the internal structure of individual nanoparticles with the SAR disproving the correlation of SAR to the particle size obtained from transmission electron microscopy. Hence the approach is a powerful tool for prediction of particle response in any application where the functional property depends on the single particle anisotropy.

Deep understanding of magnetic behavior of interacting superparamagnets enabled us to also decouple response of residual metal catalyst in single-wall carbon nanotubes and hence optimize their purification procedure [7-9].

In cooperation with the Faculty of Science, Charles University; we were interested in the characterization of multifunctional nanomaterials that exhibit multiple functional properties simultaneously, such as nanocomposites with magnetic and photocatalyst component (e.g. CoFe₂O₄/TiO₂, Fe₂O₃-CeO₂/SiO₂) [10-12].

Furthemore; we investigated the properties of the nanomaterials that has not been prepared at the nanoscale yet (e.g. ACr₂O4, where A=Fe, Mg, Co) [13,14] or whose properties (such as magnetic ground state) has not been explained yet sufficiently (ε-Fe₂O₃).

The standard macroscopic magnetic measurements are performed in the Joint Laboratory for Magnetic Studies. Determination of the phase composition and crystallographic structure of the nanoparticles is done by x-ray diffraction in collaboration with Faculty of Science. In the Joint Low Temperature Laboratory, we use the Mössbauer spectroscopy (applicable only for materials containing iron atoms) for studies of the valence state of compounds, local symmetry, defects of crystal lattice, magnetic ordering and the local orientation of the magnetic moments with respect to the externally applied magnetic field. Scanning Probe Microscopy (SPM), especially AFM and MFM, is used for imaging of the nanoparticles distributed on the substrate, for imaging of the single particle magnetic moment and for visualization of the magnetic field in the nanoparticle clusters or aggregates. Furthermore, we deal with the methods based on utilization of the synchrotron or neutron scattering techniques such as neutron diffraction, X-ray magnetic circular dichroism (XMCD) in the large synchrotron and neutron facilities.

References:
[1] S. Burianova, J. Vejpravova, P. Holec, J. Plocek, D. Niznansky, Surface spin effects in La-doped CoFe₂O₄ nanoparticles prepared by microemulsion route , J. Appl. Phys. 110 (2011) 073902 - 073902.
[2] S. Kubickova, D. Niznansky, M. P. Morales Herrero, G. Salas, J. Vejpravova, Structural disorder versus spin canting in monodisperse maghemite nanocrystals, Appl. Phys. Lett. 104 (2014) 223105.
[3] B. Pacakova, A. Mantlikova, D. Niznansky, S. Kubickova, J. Vejpravova, Understanding particle size and distance driven competition of interparticle interactions and effective anisotropy, J.Phys.: Condens. Matter, submitted.
[4] B. Bittova, J. Vejpravova, M.P. Morales, A.G. Roca, A. Mantlíkova, Relaxation phenomena in ensembles of CoFe₂O₄ nanoparticles, J. Magn. Magn. Mater. 324 (2012) 1182.
[5] B. Bittova, J. Vejpravova, M.P. Morales, A.G. Roca, D. Niznansky, A. Mantlíkova, INFLUENCE OF AGGREGATE COATING ON RELAXATIONS IN THE SYSTEMS OF IRON OXIDE NANOPARTICLES, Nano 7 (2012) 1250004.
[6] B. Pacakova, S. Burianova, M.P. Morales, G. Salas, J. Vejpravova, Universal parameter of spin and structural disorder of single-domain nanoparticles for biomedical applications, prepared for publication.
[7] B. Bittova, J. Vejpravova, M. Kalbac, S. Burianova, A. Mantlikova, S. Danis, S. Doyle, Magnetic Properties of Iron Catalyst Particles in HiPco Single Wall Carbon Nanotubes, J. Phys. Chem. C 115 (2011) 17303.
[8] B. Pacakova Bittova, M. Kalbac, S. Kubickova, A. Mantlikova, S. Mangold, J. Vejpravova, Structure and magnetic response of a residual metal catalyst in highly purified single walled carbon nanotubes, Phys. Chem. Chem. Phys. 15 (2013) 5992.
[9] B.Pacakova, Z.Kominkova, J. Vejpravova, A. Mantlikova, M. Kalbac, Analysis of metal catalyst content in magnetically filtered SWCNTs by SQUID magnetometry, J. Mater. Sci. 50 (2015) 2544.
[10] A.Mantlíková, J.Poltierová Vejpravová, B.Bittová, S.Burianová, D.Nižňanský, A.Ardu, C.Cannas, Stabilization of the high coercivity ϵ-Fe₂O₃ phase in the CeO₂–Fe₂O₃/SiO₂ nanocomposites, J. Solid State Chem. 191 (2012) 136.
[11] S.Kubickova, J.Plocek,A. Mantlikova, J. Vejpravova, Nanocomposites of monodisperse nanoparticles embedded in high-K oxide matrices – a general preparation strategy , RSC Adv. 4 (2014) 5113.
[12] A. Mantlikova, J. Plocek, B. Pacakova, S. Kubickova, O. Vik, D. Niznansky, J. Vejpravova, Nanocomposite of CeO₂ and high-coercivity magnetic carrier with large specific surface area, prepared for publication.
[13] D. Zakutna, A. Repko,I. Matulkova,D. Niznansky,A. Ardu,C. Cannas, A. Mantlikova, J. Vejpravova, Hydrothermal synthesis, characterization, and magnetic properties of cobalt chromite nanoparticles, J. Nanopart. Res. 16 (2014) 2251.
[14] I. Matulkova, P. Holec, B. Pacakova, S. Kubickova , A. Mantlikova, J. Plocek, I. Nemec, D. Niznansky, J. Vejpravova, On preparation of nanocrystalline chromites by co-precipitation and autocombustion methods, Mat. Sci. Eng. B 195 (2015) 66.

Research team: J.Vejpravová, S.Kubíčková, B.Pacáková, A.Mantlíková