Overview of the research 2010-2014
The Laboratory of magnetic oxides focused its interest on fundamental research of cobalt oxides exhibiting spin state transitions, including materials prospective for thermoelectric energy conversion, and perovskite and spinel type magnetic nanomaterials targeted for medical and biological applications. The research methods of the laboratory comprise an advanced synthesis of the materials, their structural characterization, measurements and analysis of magnetic, electric and thermal properties.
The research of Laboratory of magnetic oxides in the field of thermoelectricity comprises both the routes to optimise the thermoelectric oxide based materials aimed to high temperature applications and possibilities of the thermoelectric waste heat recovery from the exhaust gas of automobile heat engine. Due to the lack of commercial thermoelectric characterisation tools which are either unavailable or not trustworthy we focused essentially on measurement automation and substantial thermoelectric metrology refinement in the evaluation of thermal, thermoelectric and electric power characteristics of newly developed materials and thermoelectric modules . This was achieved mainly due to more accurate thermometry (error determining the temperature of ~0.05 K) and the stabilization of the cooling bath. Detail of the measuring apparatus including thermography sensing surface temperature is shown in Fig.1.
Novel layered cobaltates Ln0.30CoO2 (Ln=La,Pr,Nd,...) were prepared from NaxCoO2 precursor by a solid-state ionic exchange . The compound consists of hexagonal sheets of edge-sharing CoO6 octahedra, alternating with planes of Ln3+ cations with the trigonal prismatic coordination, where Ln3+ ions occupy only one third of available sites and forms a 2-dimensional superstructure, see Fig.2. The system displays interesting thermoelectric properties and, in comparison with the parent system NaxCoO2, it allows study of the mutual influence of magnetic ion Ln3+ and the CoO2 layer.
Fig.1. Automated characterization equipment for testing of thermoelectric modules and materials.
Fig.2. The structure of Ln0.30CoO2.
As concerns the cobalt oxides with perovskite structure the studies were directed to magnetic, electric and thermal phenomena in the LaCoO3
-type perovskites and structurally related compounds with variable Co3+/4+
valence. The interest in these systems was stimulated by a possibility of the octahedrally coordinated cobalt ions to attain different electronic configurations, characterized as the low, high or intermediate spin states. Such spin-state degree of freedom is at the root of very complex and often contradicting behaviors in dependence of temperature and actual composition. In order to shed light into this issue, numerous cobaltites were prepared and investigated by experimental and theoretical means, mainly those where lanthanum was substituted by magnetic rare earths. This research included both the single valence LnCoO3
(Ln=Pr,Nd,Sm,Tb,Dy) and mixed valence Ln1-x
(Ln=Pr,Nd) systems (see e.g. [3-5]).
The attention was given to a peculiar metal-insulator (M-I) transition found for the first time in the calcium “half-doped” Pr0.5Ca0.5CoO3 and later on also in less doped systems like (Pr1-yYy)0.7Ca0.3CoO3. Our studies unveil that the transition is not due to a mere change in cobalt ions from the intermediate to the low-spin states, as previously speculated, but is associated with a significant electron transfer between Pr3+ and Co3+/Co4+ sites. The Pr ions thus occur below TM-I in a mixed Pr3+/Pr4+ valence, and the significance of our work lies in an original method that evidences the Pr4+ states and quantifies their amount in rather simple and physically clear way [6-8]. The method is based on the observation of Schottky peak that governs specific heat in 1 K range and arises due to Zeeman splitting of the ground-state Kramers doublet of Pr4+, while Pr3+ ions in singlet ground state do not contribute. The detailed form of the peak and its shift with external field bear also an important information on the ground doublet g-factor anisotropy and on the magnitude of internal magnetic fields experienced by Pr4+ ions. The conclusions deduced from the specific heat measurements are supported by X-ray absorption spectroscopy  and band structure calculations . Challenging issue still represents the origin of the non-zero internal field, which is clearly evident in the specific heat experiments but not visible in magnetization measurements down to 2 K.
As regards the oxide magnetic nanomaterials, major efforts were directed towards finding novel synthetic methods for magnetic nanoparticles and for modification of their surface by complex shells. At the same time, thorough characterizations were carried out addressing, magnetic and structural peculiarities of nanoparticles. Principal studies were focussed on monodisperse ferrite nanoparticles (Fig.3.) and nanocrystalline phases of ferromagnetic manganites
La1-xSrxMnO3 (Fig.4.), with respect to their use as labelling and contrast agents in biological research and heating agents in hyperthermia applications .
Following these aims, magnetic cores were synthesized either in a flux, by sol-gel methods, or via thermal decomposition of metalo-organic precursors, whereas elaborate fluorescent shells based on hybrid silica, gold nanolayers and biocompatible polymers
Fig.3. Examples of bare and silica coated cobalt-zinc ferrite nanoparticles possessing different size, the scale bar is 50 nm.
New method for the synthesis of La1-xSrxMnO3 nanoparticles with impressive yield was developed using the growth of nanocrystals in the flux of NaNO2 at temperature as low as ~500°C. An exciting advantage of this facile method is the morphology and size distribution of the resulting nanoparticles that do not require subsequent mechanical processing compared to the tedious preparation via sol-gel . Complex structural and physical investigations pointed to a decisive role of the surface. The uppermost surface layer is generally over-oxygenated since Mn ions at the surface of manganite particles tend to complete the octahedral coordination . This oxygen excess is responsible for a suppression of hole charge carrier doping close to surface, which diminishes the ferromagnetic double exchange interactions and is thus in the root of so-called magnetically dead layer in
Fig.4. Magnetic nanoparticles La1-xSrxMnO3 as synthesised in flux (a) and modified with fluorescent coating based on two-ply silica shell (b), which may be observed by fluorescent microscopy (c). It shows primary human fibroblasts with nanoparticles (red) localised in lysosomes (green) outside cell nucleus (blue).
We addressed also the fundamental questions associated with Mn3+/Mn4+ ordering in „half-doped“ systems Pr0.5Ca0.5MnO3 and La0.5Ca0.5MnO3. By means of neutron diffraction and magnetic measurements, the particle size effects on the structure and low-temperature spin arrangement have been investigated . The study shows that the Mn3+/Mn4+ charge ordering and CE-type antiferromagnetic structure characteristic for bulk are completely suppressed when particle size is decreased down to 20 nm, and a ferromagnetic state is stabilized instead. The reason is not in a lower energy of the latter state, but in the hindering of displacive processes through which the charge ordering develops. Our results are of general importance for the perovskite manganites. In particular, the room temperature crystal structures in the particle cores are found not to deviate from the bulk material, disproving thus former speculations about enormous structural distortions due to surface effects. Another issue is the changing character of charge carriers in the particle shell, which is at the root of the size-dependent reduction of magnetization observed commonly in manganites possessing ferromagnetic state.
1. J. Hejtmánek, K. Knížek, V. Švejda, P. Horna, M. Sikora, Journal of Electronic Materials, 2014, DOI: 10.1007/s11664-014-3084-7,
2. K. Knížek, J. Hejtmánek, M. Maryško, Z. Jirák, J. Buršík, K. Kirakci, P. Beran, J. Solid State Chem. 184, 2231 (2011).
3. P. Novák, K. Knížek, M. Maryško, Z. Jirák and J. Kuneš, J. Phys.-Condens. Mat. 25 (2013) 4460001
4. K. Knížek, Z. Jirák, P. Novák, Clarina de la Cruz, Solid State Sciences 28 (2014) 26 - 30
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6. J. Hejtmánek, E. Šantavá, K. Knížek, M. Maryško, and Z. Jirák, T. Naito, H. Sasaki, and H. Fujishiro, Phys. Rev. B 82 (2010) 165107.
7. J. Hejtmánek, Z. Jirák, O. Kaman, K. Knížek, E. Šantavá, K. Nitta, T. Naito, H. Fujishiro, Eur. Phys. J. B 86 (2013) 305.
8. K. Knížek, J. Hejtmánek, M. Maryško, P. Novák, E. Šantavá, Z. Jirák, T. Naito, H. Fujishiro, C. de la Cruz, Phys. Rev. B 88 (2013) 224412.
9. H. Fujishiro, T. Naito, S. Ogawa, N. Yoshida, K. Nitta, J. Hejtmánek, K. Knížek, and Z. Jirák, J. Phys. Soc. Jpn. 81 (2012) 064709.
10. K. Knížek, J. Hejtmánek, P. Novák, Z. Jirák, Phys. Rev. B 81 (2010) 155113.
11. M. Kačenka, O. Kaman, J. Kotek , L. Falteisek, J. Černý, D. Jirák, V. Herynek, K. Zacharovová, Z. Berková, P. Jendelová, J. Kupčík, E. Pollert, P. Veverka, and I. Lukeš,. Journal of Materials Chemistry 21, 157-164 (2011).
12. M. Kačenka, O. Kaman, Z. Jirák, M. Maryško, P. Žvátora, S. Vratislav and I. Lukeš, Journal of Applied Physics 115, 17B525 (2014).
13. P. Žvátora, M. Veverka, P. Veverka, K. Knížek, K. Závěta, E. Pollert, V. Král, G. Goglio, E. Duguet, and O. Kaman, Journal of Solid State Chemistry 204, 373-379 (2013).
14. Z. Jirák, E. Hadová, O. Kaman, K. Knížek, M. Maryško, E. Pollert, M. Dlouhá, and S. Vratislav, Phyical. Review B, 81 024403 (2010).