The chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with ... more The chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with a strong potential for multiferroicity. We report neutron scattering measurements for the multichiral Ba3M Fe3Si2O14 (M = Nb, Ta) langasites revealing new important features of the magnetic order of these systems: the bunching of the helical modulation along the c-axis and the in-plane distortion of the 120 • Fe-spin arrangement. We discuss these subtle features in terms of the microscopic spin Hamiltonian, and provide the link to the magnetically-induced electric polarization observed in these systems. Thus, our findings put the multiferroicity of this attractive family of materials on solid ground.
Proceedings of the 2006 International Conference on Nanoscience and Nanotechnology, ICONN, 2006
Carbon nanoclusters produced by high-repetition-rate laser ablation of graphite and glassy carbon... more Carbon nanoclusters produced by high-repetition-rate laser ablation of graphite and glassy carbon in Ar exhibits para-and ferromagnetic behavior at low temperature. Magnetic susceptibility and electron paramagnetic resonance of carbon nanofoam samples produced at different Ar pressures demonstrate ferromagnetic behaviour at low temperatures. The results show that the degree of remanent order is strongly dependent on the magnetic history, i.e. whether the samples were cooled under zero-field or field conditions. Such behaviour is typical for a spin glass picture where the system can exist in many different roughly equivalent spin configurations. Detailed EPR studies identified three different types of independent spin systems with relaxation times 100 ns, 0.1-1 µs, and 1 ms. We conclude that unpaired spins exist in the nanofoam in three quite different structural environments. Their role in the magnetic ordering is discussed.
In this report we combine synchrotron powder X-ray diffraction, 7Li nuclear magnetic resonance (N... more In this report we combine synchrotron powder X-ray diffraction, 7Li nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to study the structural evolution of Li4C60 and how its electronic ground state depends on the crystal symmetry. The compound in the two-dimensional polymer phase with mixed interfullerene bonding motifs is a band gap insulator. EPR however reveals the presence of intrinsic centers originating from broken C60-C60 bonds and local Li off-stoichiometry that create states in the band gap and account for the complex temperature dependence of the spin susceptibility as well as the residual temperature dependence of the 7Li NMR shift. Li+ ions are at low temperatures on the 7Li NMR timescale statically disordered. The observed 7Li NMR line narrowing T > 200 K is ascribed to the Li+ diffusion dynamics and above room temperature the polymer phase is already a good ionic conductor. Heating the sample to temperatures above ∼470 K results ...
We present electron spin resonance (ESR) investigation of the acentric Ba3NbFe3Si2O14, featuring ... more We present electron spin resonance (ESR) investigation of the acentric Ba3NbFe3Si2O14, featuring a unique single-domain double-chiral magnetic ground state. Combining simulations of the ESR line-width anisotropy and the antiferromagnetic-resonance modes allows us to single out the Dzyaloshinsky-Moriya (DM) interaction as the leading magnetic anisotropy term. We demonstrate that the rather minute out-of-plane DM component dc = 45 mK is responsible for selecting a unique ground state, which endures thermal fluctuations up to astonishingly high temperatures.
The structural, electronic, and magnetic properties of the superconducting NdFeAsO1−xFx phases (T... more The structural, electronic, and magnetic properties of the superconducting NdFeAsO1−xFx phases (TC = 43 K for x = 0.15) have been investigated experimentally by high-resolution synchrotron x-ray powder diffraction, magnetization and 75 As NMR measurements. Density-functional calculations were performed to calculate and analyze the electric field gradient and the density of states. Compared to LaFeAsO family, the NdFeAsO family shows a contraction of the lattice parameters with shorter rare-earth (RE)-As bond distances, an increased thickness of the As-Fe2-As layer, and less distorted Fe-As4 tetrahedra. The 75 As quadrupole frequencies are enhanced with respect to the La analogs. This is due to a more prolate As 4p electron distribution mainly caused by the reduced lattice parameters and not by the presence of Nd 4f electrons. A non-negligible hyperfine coupling between the 75 As nuclei and the Nd 4f states indicates a weak coupling between the REO and FeAs layer and possibly opens the channel for a Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction between localized Nd 4f moments mediated by itinerant Fe 3d and/or Nd 5d states.
We report a comprehensive 29 Si NMR and 69,71 Ga NMR/NQR study of the largespin magnetically anis... more We report a comprehensive 29 Si NMR and 69,71 Ga NMR/NQR study of the largespin magnetically anisotropic kagomé compound Nd3Ga5SiO14. We find a sizable transferred hyperfine coupling between the nuclear and electron moments, proving that these nuclei can be utilized as local probes which provide useful information about the magnetic Nd 3+ ions. Our results give evidence of unexpectedly broad distributions of local environments on all nuclear sites under investigation, implying also exchange disorder.
Elementary excitations in condensed matter capture the complex many-body dynamics of interacting ... more Elementary excitations in condensed matter capture the complex many-body dynamics of interacting basic entities in a simple quasiparticle picture. In magnetic systems the most established quasiparticles are magnons, collective excitations that reside in ordered spin structures, and spinons, their fractional counterparts that emerge in disordered, yet correlated spin states. Here we report on the discovery of elementary excitation inherent to spin-stripe order that represents a bound state of two phason quasiparticles, resulting in a wiggling-like motion of the magnetic moments. We observe these excitations, which we dub “wigglons”, in the frustrated zigzag spin-1/2 chain compound β-TeVO4, where they give rise to unusual low-frequency spin dynamics in the spin-stripe phase. This result provides insights into the stripe physics of strongly-correlated electron systems.
The chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with ... more The chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with a strong potential for multiferroicity. We report neutron scattering measurements for the multichiral Ba3M Fe3Si2O14 (M = Nb, Ta) langasites revealing new important features of the magnetic order of these systems: the bunching of the helical modulation along the c-axis and the in-plane distortion of the 120 • Fe-spin arrangement. We discuss these subtle features in terms of the microscopic spin Hamiltonian, and provide the link to the magnetically-induced electric polarization observed in these systems. Thus, our findings put the multiferroicity of this attractive family of materials on solid ground.
Proceedings of the 2006 International Conference on Nanoscience and Nanotechnology, ICONN, 2006
Carbon nanoclusters produced by high-repetition-rate laser ablation of graphite and glassy carbon... more Carbon nanoclusters produced by high-repetition-rate laser ablation of graphite and glassy carbon in Ar exhibits para-and ferromagnetic behavior at low temperature. Magnetic susceptibility and electron paramagnetic resonance of carbon nanofoam samples produced at different Ar pressures demonstrate ferromagnetic behaviour at low temperatures. The results show that the degree of remanent order is strongly dependent on the magnetic history, i.e. whether the samples were cooled under zero-field or field conditions. Such behaviour is typical for a spin glass picture where the system can exist in many different roughly equivalent spin configurations. Detailed EPR studies identified three different types of independent spin systems with relaxation times 100 ns, 0.1-1 µs, and 1 ms. We conclude that unpaired spins exist in the nanofoam in three quite different structural environments. Their role in the magnetic ordering is discussed.
In this report we combine synchrotron powder X-ray diffraction, 7Li nuclear magnetic resonance (N... more In this report we combine synchrotron powder X-ray diffraction, 7Li nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) experiments to study the structural evolution of Li4C60 and how its electronic ground state depends on the crystal symmetry. The compound in the two-dimensional polymer phase with mixed interfullerene bonding motifs is a band gap insulator. EPR however reveals the presence of intrinsic centers originating from broken C60-C60 bonds and local Li off-stoichiometry that create states in the band gap and account for the complex temperature dependence of the spin susceptibility as well as the residual temperature dependence of the 7Li NMR shift. Li+ ions are at low temperatures on the 7Li NMR timescale statically disordered. The observed 7Li NMR line narrowing T > 200 K is ascribed to the Li+ diffusion dynamics and above room temperature the polymer phase is already a good ionic conductor. Heating the sample to temperatures above ∼470 K results ...
We present electron spin resonance (ESR) investigation of the acentric Ba3NbFe3Si2O14, featuring ... more We present electron spin resonance (ESR) investigation of the acentric Ba3NbFe3Si2O14, featuring a unique single-domain double-chiral magnetic ground state. Combining simulations of the ESR line-width anisotropy and the antiferromagnetic-resonance modes allows us to single out the Dzyaloshinsky-Moriya (DM) interaction as the leading magnetic anisotropy term. We demonstrate that the rather minute out-of-plane DM component dc = 45 mK is responsible for selecting a unique ground state, which endures thermal fluctuations up to astonishingly high temperatures.
The structural, electronic, and magnetic properties of the superconducting NdFeAsO1−xFx phases (T... more The structural, electronic, and magnetic properties of the superconducting NdFeAsO1−xFx phases (TC = 43 K for x = 0.15) have been investigated experimentally by high-resolution synchrotron x-ray powder diffraction, magnetization and 75 As NMR measurements. Density-functional calculations were performed to calculate and analyze the electric field gradient and the density of states. Compared to LaFeAsO family, the NdFeAsO family shows a contraction of the lattice parameters with shorter rare-earth (RE)-As bond distances, an increased thickness of the As-Fe2-As layer, and less distorted Fe-As4 tetrahedra. The 75 As quadrupole frequencies are enhanced with respect to the La analogs. This is due to a more prolate As 4p electron distribution mainly caused by the reduced lattice parameters and not by the presence of Nd 4f electrons. A non-negligible hyperfine coupling between the 75 As nuclei and the Nd 4f states indicates a weak coupling between the REO and FeAs layer and possibly opens the channel for a Ruderman-Kittel-Kasuya-Yosida (RKKY)-type interaction between localized Nd 4f moments mediated by itinerant Fe 3d and/or Nd 5d states.
We report a comprehensive 29 Si NMR and 69,71 Ga NMR/NQR study of the largespin magnetically anis... more We report a comprehensive 29 Si NMR and 69,71 Ga NMR/NQR study of the largespin magnetically anisotropic kagomé compound Nd3Ga5SiO14. We find a sizable transferred hyperfine coupling between the nuclear and electron moments, proving that these nuclei can be utilized as local probes which provide useful information about the magnetic Nd 3+ ions. Our results give evidence of unexpectedly broad distributions of local environments on all nuclear sites under investigation, implying also exchange disorder.
Elementary excitations in condensed matter capture the complex many-body dynamics of interacting ... more Elementary excitations in condensed matter capture the complex many-body dynamics of interacting basic entities in a simple quasiparticle picture. In magnetic systems the most established quasiparticles are magnons, collective excitations that reside in ordered spin structures, and spinons, their fractional counterparts that emerge in disordered, yet correlated spin states. Here we report on the discovery of elementary excitation inherent to spin-stripe order that represents a bound state of two phason quasiparticles, resulting in a wiggling-like motion of the magnetic moments. We observe these excitations, which we dub “wigglons”, in the frustrated zigzag spin-1/2 chain compound β-TeVO4, where they give rise to unusual low-frequency spin dynamics in the spin-stripe phase. This result provides insights into the stripe physics of strongly-correlated electron systems.
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Papers by A. Zorko