Laboratory of magnetic oxides

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 [1].

The detail of the measuring apparatus including thermography sensing surface temperature is shown in Fig. 1.

Fig. 1: Automated characterization equipment for testing of thermoelectric modules and materials.

As concerns the cobalt perovskites with spin state transitions the studies were directed to probe the magnetic, electric and thermal phenomena in the LaCoO₃ -type cobaltites and structurally related compounds with variable Co³⁺/Co⁴⁺ 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 behaviours in dependence of temperature and actual composition. 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 Ln_1-xCa_xCoO₃ (Ln=Pr, Nd) systems (see e. g. [2-4]). The attention was given to a peculiar metal-insulator (M-I) transition found for the first time in the calcium “half-doped” cobaltite Pr_0.5Ca_0.5CoO₃ and, later on, to less doped systems like (Pr_1-yY_y)_0.7Ca_0.3CoO₃. 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 [5-7]. 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 conclusions deduced from the specific heat measurements are supported by X-ray absorption spectroscopy [8] and band structure calculations, performed also in our department [9] managed to explain why, in the absence of magnetic ordering is observed splitting ion exchange Pr⁴⁺ [1]. The challenging issue appearing from the studies is the exchange splitting on Pr sites, clearly evident in the specific heat experiments, though no magnetic ordering is detected down to 2 K. Applying the dynamical mean-field model calculations, a new kind of order parameter which physics is related to the excitonic condensation has been suggested for Pr_0.5Ca_0.5CoO₃ and other systems in the proximity of spin-state transition [10].

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, foremost, magnetic and structural peculiarities of nanoparticles. Principal studies were focused on monodisperse ferrite nanoparticles (Fig. 24-4) and nanocrystalline phases of ferromagnetic manganites La_1-xSr_xMnO₃ (Fig. 2), with respect to their use as labeling and contrast agents in biological research and heating agents in hyperthermia applications [11].

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 were prepared for subsequent biological studies.

Fig. 2: Examples of bare and silica coated cobalt-zinc ferrite nanoparticles possessing different size, the scale bar is 50 nm.

Fig. 3: Magnetic nanoparticles La1-xSrxMnO3, as synthesised (a) and modified with bilayer fluorescent coating based on SiO2 (b), which may be observed by fluorescent microscopy (c). It shows human desmocytes with nanoparticles (red) localised in lysosomes (green) outside cell nucleus (blue).

New method for the synthesis of La_1-xSr_xMnO₃ 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 [12]. Importantly, complex structural and physical investigations performed on these products 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 [13]. 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 La_1-xSr_xMnO₃ particles.

We addressed also the fundamental questions associated with Mn³⁺/Mn⁴⁺ ordering in „half-doped“ systems Pr_0.5Ca_0.5MnO₃ and La_0.5Ca_0.5MnO₃. By means of neutron diffraction and magnetic measurements, the particle size effects on the structure and low-temperature spin arrangement have been investigated [14]. 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.

Literature

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   Test System for Thermoelectric Modules and Materials
   J. Electron. Mater. 43 (2014) 3726 - 3732.
2. P. Novák, K. Knížek, M. Maryško, Z. Jirák and J. Kuneš
   Crystal field and magnetism of Pr³⁺ and Nd³⁺ ions in orthorhombic perovskites
   J. Phys.-Condens. Mat. 25 (2013) 4460001(1) - 4460001(8).
3. K. Knížek, Z. Jirák, P. Novák, C. de la Cruz
   Non-collinear magnetic structures of TbCoO₃ and DyCoO₃
   Solid State Sci. 28 (2014) 26 - 30.
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   Ground-state properties of the mixed-valence cobaltites Nd_0.70Sr_0.3CoO₃, Nd_0.7Ca_0.3.CoO₃ and Pr_0.7.Ca_0.3CoO₃
   J. Phys.-Condens. Mat. 25 (2013) 216006(1) - 216006(12).
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    Dual imaging probes for magnetic resonance imaging and fluorescence microscopy based on perovskite manganite nanoparticles
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