One of the most fascinating aspects of condensed matter is its ability to conduct electricity, which is particularly pronounced in conventional metals such as copper or silver. Such behavior stems ...from a strong tendency of valence electrons to delocalize in a periodic potential created by ions in the crystal lattice of a given material. In many advanced materials, however, this basic delocalization process of the valence electrons competes with various processes that tend to localize these very same valence electrons, thus driving the insulating behavior. The two such most important processes are the Mott localization, driven by strong correlation effects among the valence electrons, and the Anderson localization, driven by the interaction of the valence electrons with a strong disorder potential. These two localization processes are almost exclusively considered separately from both an experimental and a theoretical standpoint. Here, we offer an overview of our long-standing research on selected organic conductors and manganites, that clearly show the presence of both these localization processes. We discuss these results within existing theories of Mott-Anderson localization and argue that such behavior could be a common feature of many advanced materials.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
We report on the electrodynamic properties of the single crystalline lead-substituted M-type barium hexaferrite, Ba0.3Pb0.7Fe12O19, performed in the broad frequency range including radio-frequency, ...terahertz and sub-terahertz bands, which are particularly important for the development of microelectronic devices. We demonstrate how changing on a molecular level the chemical characteristics (composition, intermolecular interaction, spin-orbital interaction) of lead-substituted M-type hexaferrite influences its radio-frequency and terahertz electrodynamic response. Our results indicate a critical temperature range, 50 K < T < 70 K, where significant changes of the electrodynamic response occur that are interpreted as freezing of dynamical oscillations of bi-pyramidal Fe(2b) ions. In the range 5-300 K, the heat capacity shows no sign of any phase transition and is solely determined by electron and phonon contributions. An anomalous electrodynamic response is detected at 1-2 THz that features a rich set of absorption resonances which are associated with electronic transitions within the fine-structured Fe2+ ground state and are visualized in the spectra due to magnetostriction and electron-phonon interaction. We show that lead substitution of barium in barium hexaferrite, BaFe12O19, leads to the emergence of a pronounced dielectric and magnetic relaxational dynamics at radio-frequencies and that both dynamics have the same characteristic relaxation times, thus evidencing the bi-relaxor-like nature of Ba0.3Pb0.7Fe12O19. We associate the origin of the relaxations as connected with the motion of magnetic domain walls. In order to unveil crucial influence of chemical substitution on electrodynamic characteristics of the compound, we analyze our results on substituted compound in comparison with the data available for pristine barium (BaFe12O19) and pristine lead (PbFe12O19) hexaferrites. The obtained spectroscopic data on the dielectric properties of Ba0.3Pb0.7Fe12O19 provide insight into fundamental phenomena responsible for the absorption mechanisms of the compound and demonstrates that chemical ionic substitution is an effective tool to tune the dielectric properties of the whole family of hexaferrites.
Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie ...temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La1−xCaxMnO3 ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases x=0.5. Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles.
Herein, carbon distribution and grain‐boundary diffusion processes are studied in a variety of FeCr alloys and steels with different chromium content and microstructure, based on magnetic ...after‐effect measurements performed in a broad temperature range, from 100 to 1000 K. The existence of three metastable carbon positions in the lattice is revealed in the FeC alloys. Carbon as interstitial in the lattice, segregated at the dislocations and grain boundaries, is represented by the relaxation peaks at about 269, 432, and 607 K, respectively. The relaxation process due to the grain‐boundary self‐diffusion is clearly distinguished from those mentioned earlier, and is observed to be at about 643 and 681 K in low and high carbon Fe, respectively. Addition of Cr has a twofold effect: a) for Cr concentrations higher than about 3 wt% carbon‐related relaxation processes completely disappear from the spectra, most probably due to formation of carbides, b) the solute grain‐boundary diffusion relaxation peak appears in the spectra at about 800 K, with an activation energy that is directly dependent on the Cr content. Activation energy of the solute grain‐boundary diffusion is found to be generally smaller in ferrite–martensite microstructure in comparison with fully ferrite alloys.
One of the main parameters governing the formation of the alloy microstructure is dissolved carbon distribution. Herein this work, carbon distribution and grain‐boundary diffusion processes are studied in a variety of FeCr alloys and steels with different chromium content and microstructure, based on magnetic after‐effect measurements performed in a broad temperature range, from 100 to 1000 K.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
•Discovery of correlated activation energies governing magnetic relaxation and electrical transport in M-type hexaferrites.•Correlation persists with Aluminum substitution.•Magnetic response is ...similar to that of uniaxial antiferromagnets.•Correlation is interpreted as the motion of the charged magnetic domain walls that are screened by free charge carriers.
In this work, we present frequency-dependent magnetic susceptibility and dc electric transport properties of three different compositions of hexaferrite Ba1−xPbxFe12−yAlyO19 single crystals. We found a correlation between the activation energies of the dc electric transport and the ac magnetic susceptibility which persists over the whole studied range of aluminum substitution y = 0–3.3. This result is discussed in the context of charged magnetic domain walls, the pinning of which is determined by charge carriers activated across the transport gap. Our work points toward a general relaxation mechanism in ferrimagnetic semiconductors that directly affects the dynamic magnetic properties via electrical transport.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The results indicating magnetic flux pinning in MgB2 wires doped with three types of magnetic nanoparticles (MNP) are reported. The magnetic state of MNPs, both as-prepared and inside the MgB2 core, ...was determined with magnetization and ac susceptibility measurements. The competition between detrimental influence of doping (reduced connectivity, pair breaking) and enhanced flux pinning leads to deterioration of electromagnetic properties of doped wires at high MNP content, whereas light doping causes an enhancement of critical current density, Jc, and/or irreversibility field, Birr, for all our MNPs. For Ni and dextrin coated NiFe2O4 MNPs the enhancement of Jc was comparable to that achieved with the best nonmagnetic dopands. Detailed analysis indicates the contribution of magnetic flux pinning including the matching effects in flux pinning on MNPs.
•Rigorous study of the effects of magnetic particles on superconductivity in MgB2 wires.•Detailed characterisation of magnetic state of nanoparticles embedded in MgB2 matrix.•Matching effects in pinning of vortices on magnetic nanoparticles•Consistent indications of possible contribution of magnetic pinning of vortices.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
beta-TeVO4 is a frustrated spin-1/2 zig-zag chain system, where spin-density-wave (SDW), vector-chiral (VC) and an exotic dynamic spin-stripe phase compete at low temperatures. Here we use torque ...magnetometry to study the anisotropy of these phases in magnetic fields of up to 5 T. Our results show that the magnetic-field-induced spin reorientation occurs in the SDW and in the spin-stripe phases for mu H-0 >= 2 T. The observed spin reorientation is another element of the anisotropic phase diagram for the field directions in the ac and a*b crystallographic planes. The presented results should help establishing the model of anisotropic magnetic interactions, which are responsible for the formation of complex magnetic phases in beta-TeVO4 and similar low-dimensional quantum spin systems.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
β−TeVO4 is a frustrated spin-1/2 zig-zag chain system, where spin-density-wave (SDW), vector-chiral (VC) and an exotic dynamic spin-stripe phase compete at low temperatures. Here we use torque ...magnetometry to study the anisotropy of these phases in magnetic fields of up to 5 T. Our results show that the magnetic-field-induced spin reorientation occurs in the SDW and in the spin-stripe phases for μ0H ≥ 2 T. The observed spin reorientation is another element of the anisotropic phase diagram for the field directions in the a c and a ∗ b crystallographic planes. The presented results should help establishing the model of anisotropic magnetic interactions, which are responsible for the formation of complex magnetic phases in β−TeVO4 and similar low-dimensional quantum spin systems.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
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