A. Günther

Universität Augsburg, Augsburg, Bavaria, Germany

Are you A. Günther?

Claim your profile

Publications (38)

  • Source
    S. Widmann · E. Ruff · A. Günther · [...] · A. Loidl
    [Show abstract] [Hide abstract] ABSTRACT: The lacunar spinel GaV4S8 exhibits orbital ordering at 44 K and shows a complex magnetic phase diagram below 12.7 K, which includes ferromagnetic and cycloidal spin order. At low but finite external magnetic fields, N\'eel-type skyrmions are formed in this material. Skyrmions are whirl-like spin vortices that have received great theoretical interest because of their non-trivial spin topology and that are also considered as basic entities for new data-storage technologies. Interestingly, we found that the orbitally ordered phase shows sizable ferroelectric polarization and that excess spin-driven polarizations appear in all magnetic phases, including the skyrmion-lattice phase. Hence, GaV4S8 shows simultaneous magnetic and polar order and belongs to the class of multiferroics, materials that attracted enormous attention in recent years. Here, we summarize the existing experimental information on the magnetic, electronic, and dielectric properties of GaV4S8. By performing detailed magnetic susceptibility, resistivity, specific heat, and dielectric experiments, we complement the low-temperature phase diagram. Specifically, we show that the low-temperature and low-field ground state of GaV4S8 seems to have a more complex spin configuration than purely collinear ferromagnetic spin order. In addition, at the structural Jahn-Teller transition the magnetic exchange interaction changes from antiferromagnetic to ferromagnetic. We also provide experimental evidence that the vanadium V4 clusters in GaV4S8 can be regarded as molecular units with spin 1/2. However, at high temperatures deviations in the susceptibility show up, indicating that either the magnetic moments of the vanadium atoms fluctuate independently or excited states of the V4 molecule become relevant.
    Full-text Article · Jun 2016
  • S. Riegg · S. Widmann · B. Meir · [...] · A. Loidl
    [Show abstract] [Hide abstract] ABSTRACT: Rare d-electron-derived heavy-fermion properties of the solid-solution series LaCu3RuxTi4−xO12 were studied for 1≤x≤4 by resistivity, susceptibility, specific-heat measurements, and magnetic-resonance techniques. Our results suggest the existence of a coherent Kondo lattice formed by localized Ru 4d electrons leading to strongly enhanced effective electron masses. Pure ruthenate (x=4) is a heavy-fermion metal characterized by a resistivity proportional to T2 at low temperatures T. By increasing titanium substitution the coherent Fermi-liquid state is disturbed, yielding single-ion Kondo-type properties as in the paradigm 4f-based heavy-fermion compound CexLa1−xCu2.05Si2 [M. Ocko et al., Phys. Rev. B 64, 195106 (2001)]. In LaCu3RuxTi4−xO12 the heavy-fermion behavior finally breaks down upon crossing the metal-to-insulator transition close to x=2.
    Article · Mar 2016
  • S. Riegg · S. Widmann · A. Günther · [...] · A. Loidl
    [Show abstract] [Hide abstract] ABSTRACT: In this work we investigate the solid-solution series Lay Cu3Rux Ti4−x O12. The titanate La2/3Cu3Ti4O12 (x = 0) is an antiferromagnetic insulator exhibiting colossal dielectric constants, while the ruthenate LaCu3Ru4O12 (x = 4) is known as a rare d-electron derived heavy-fermion compound. Detailed structural investigations, AC- and DC-magnetization measurements, resistivity, specific-heat, and magnetic-resonance investigations have been performed for all polycrystalline compounds prepared by solid-state synthesis. These experiments have been accompanied by band-structure calculations. Close to the Ru concentration x = 2 we identify a quantum-critical point coinciding with a metal-to-insulator transition. The quantum-critical point separates an insulating spin glass from a paramagnetic metal. Interestingly, there is no evidence for a divergence of the effective mass upon reaching the quantum-critical point from the metallic side. In the paramagnetic metal, Ru behaves like a canonical Kondo ion. While the Ru oxidation state remains stable at + 4 for the whole concentration regime, the Cu valence seems to decrease from + 2 in the insulating antiferromagnet with localized copper spins to a significantly lower value in the metallic heavy-fermion compounds.
    Article · Jul 2015 · The European Physical Journal Special Topics
  • [Show abstract] [Hide abstract] ABSTRACT: Materials with broad absorption bands are highly desirable for electromagnetic filtering and processing applications, especially if the absorption can be externally controlled. Here, a new class of broadband-absorption materials is introduced. Namely, layered metamagnets exhibit an electromagnetic excitation continuum in the magnetic-field-induced mixed ferro- and anti­ferromagnetic phase. Employing a series of complementary experimental techniques involving neutron scattering, muon spin relaxation, specific heat, ac and dc magnetization measurements, and electron magnetic resonance, a detailed magnetic phase diagram of Cu3Bi(SeO3)2O2Br is determined and it is found that the excitations in the mixed phase extend over at least ten decades of frequency. The results, which reveal a new dynamical aspect of the mixed phase in metamagnets, open up a novel approach to controllable microwave filtering.
    Article · May 2015 · Advanced Functional Materials
  • G M Kalvius · O Hartmann · A Günther · [...] · R Scheuermann
    [Show abstract] [Hide abstract] ABSTRACT: The metallic ruthenium perovskites ACu3Ru4O12 with A = Ca, Pr,Nd were investigated by zero field (ZF) and weak transverse field (TF) muon spin rotation/relaxation (μSR) spectroscopy. The ZF spectra for CaCu3Ru4O12 show pure static Gaussian Kubo-Toyabe relaxation arising from the interaction between the muon spin and the nuclear moments of the Cu ions. This confirms that no atomic magnetic moment exists. A sudden increase of the lattice parameter a when heating above ~150 K had previously been detected by neutron diffraction. The root mean square field at the muon site Brms was found to be 0.15 mT independent of temperature, in particular around 150 K. Also, no change of spectral parameters is seen in the weak TF data in that temperature range. Those findings imply that the structural change around 150 K takes place without noticeably shifting atomic positions. The spectra for PrCu3Ru4O12 and NdCu3Ru4O12 are dominated by the interaction with the dynamic rare earth moments. The analysis requires the use of the electron-nuclear double relaxation formalism. The electronic part shows simple exponential relaxation typical for a paramagnet. It features, with reducing the temperature a steep increase of paramagnetic relaxation rate as is characteristic for an approach to a magnetic spin freezing transition from above. That result suggests that the magnetic ground states of PrCu3Ru4O12 and NdCu3Ru4O12 are spin frozen states, although bulk magnetic data give no direct evidence in that direction.
    Article · Dec 2014 · Journal of Physics Conference Series
  • V Felea · S Yasin · A Günther · [...] · A Loidl
    [Show abstract] [Hide abstract] ABSTRACT: We report on ultrasound studies of FeCr2S4 in static and pulsed magnetic fields exhibiting an orbital-order transition at 9 K. A longitudinal acoustic mode exhibits distinct features in the phase space of temperature and magnetic field due to magnetic and structural transformations. Pulsed-field measurements show significant differences in the sound velocity below and above the orbital-ordering transition as well as the spin-reorientation transition at 60 K. Our results indicate a reduction of the magnetocrystalline anisotropy on entering the orbitally ordered phase.
    Article · Nov 2014 · Journal of Physics Condensed Matter
  • M. Hemmida · H. -A. Krug von Nidda · A. Günther · [...] · J. Sichelschmidt
    [Show abstract] [Hide abstract] ABSTRACT: The evolution of the electronic density of states at the Fermi level of intermetallic ${\mathrm{EuFe}}_{2$-${}x}{\mathrm{Ru}}_{x}{\mathrm{As}}_{2}$ in the whole concentration range $(0$\le${}x$\le${}2)$ is reported. Systematic electron spin resonance investigations of the Korringa-type Eu spin relaxation in combination with the results of band-structure calculations and magnetic susceptibility measurements can provide a clear picture of how the density of states is reduced by substituting Fe with Ru in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$. Moreover, the strength of the exchange coupling between the local Eu and the itinerant spin systems can be estimated consistently with the weak coupling expected for this class of layered compounds.
    Article · Nov 2014 · Physical Review B
  • Source
    J Bertinshaw · C Ulrich · A Günther · [...] · J Deisenhofer
    [Show abstract] [Hide abstract] ABSTRACT: We report on neutron diffraction, thermal expansion, magnetostriction, dielectric, and specific heat measurements on polycrystalline FeCr2S4 in external magnetic fields. The ferrimagnetic ordering temperatures TC ≈ 170 K and the transition at TOO ≈ 10 K, which has been associated with orbital ordering, are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the state below TOO. The magnetic moments of the Cr- and Fe-ions are reduced from the spin-only values throughout the magnetically ordered regime, but approach the spin-only values for fields >5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample below about 60 K. Below TOO this contraction is followed by a moderate expansion of the sample for fields larger than ~4.5 T. The transition at TOO is accompanied by an anomaly in the dielectric constant. The dielectric constant depends on both the strength and orientation of the external magnetic field with respect to the applied electric field for T < TOO. A linear correlation of the magnetic-field-induced change of the dielectric constant and the magnetic-field dependent magnetization is observed. This behaviour is consistent with the existence of a ferroelectric polarization and a multiferroic ground state below 10 K.
    Full-text Article · Aug 2014 · Scientific Reports
  • [Show abstract] [Hide abstract] ABSTRACT: Magnetic properties as well as the specific heat of the spin-dimer system La2Ru1-yMnyO5 were investigated for manganese concentrations 0 \leq y \leq 0.25. The magnetic (DC-) susceptibility of the unsubstituted (y = 0) system shows a step-like decrease close to 160 K reflecting a magneto-structural transition into a dimerized ground state. With increasing manganese concentration this behavior (typical for singlet formation) becomes continuously suppressed and the susceptibility bears the signatures of the emergence of new magnetic ground states. The high-temperature Curie-Weiss susceptibility can be described by Ru^4+ (S = 1) and Mn^4+ (S = 3/2) spin moments, with a dramatic decrease of the Curie-Weiss temperatures by almost 30\% close to y = 0.1, indicating significant changes in the average mean magnetic exchange. Field cooled and zero-field cooled experiments as well as AC-susceptibility measurements provide clear evidence for the formation of a spin-glass state, well below the characteristic dimerization temperature. The relaxation dynamics can be described by a Vogel-Fulcher-Tammann behavior and indicates high fragility when characterized in terms of glassy dynamics of canonical super-cooled liquids. Additional electron-spin resonance experiments indicate different spin-glass regimes and a rather dynamic nature of the dimerized phase. In the Mn-substituted compounds a linear contribution to the heat capacity at low temperatures can be ascribed to the spin-glass formation. With increasing manganese concentration the anomaly in the specific heat caused by the spin-singlet formation is shifted to lower temperatures and becomes continuously suppressed and smeared out. On the basis of these results we propose a (y,T)-phase diagram indicating the competition of the spin-glass and the dimerized state. We stress the similarities with doped CuGeO3, the canonical inorganic spin-Peierls system.
    Article · Jul 2014 · Physical Review B
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: We report results of a muon spin rotation and relaxation ($\mu$SR) study of dilute Pd$_{1-x}$Ni$_x$ alloys, with emphasis on Ni concentrations $x =$ 0.0243 and 0.025. These are close to the critical value $x_\mathrm{cr}$ for the onset of ferromagnetic long-range order (LRO), which is a candidate for a quantum critical point. The 2.43 and 2.5 at.% Ni alloys exhibit similar $\mu$SR properties. Both samples are fully magnetic, with average muon local fields $\langle B^\mathrm{loc}\rangle =$ 2.0 and 3.8 mT and Curie temperatures $T_C =$ 1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively, at $T = 0$. The temperature dependence of $\langle B^\mathrm{loc}\rangle$ suggests ordering of Ni spin clusters rather than isolated spins. Just above $T_C$ a two-phase region is found with separate volume fractions of quasistatic short-range order (SRO) and paramagnetism. The SRO fraction decreases to zero with increasing temperature a few kelvin above $T_C$. This mixture of SRO and paramagnetism is consistent with the notion of an inhomogeneous alloy with Ni clustering. The measured values of $T_C$ extrapolate to $x_\mathrm{cr}$ = 0.0236 $\pm$ 0.0027. The dynamic muon spin relaxation in the vicinity of $T_C$ differs for the two samples: a relaxation-rate maximum at $T_C$ is observed for $x$ = 0.0243, reminiscent of critical slowing down, whereas for $x =$ 0.025 no dynamic relaxation is observed within the $\mu$SR time window. The data suggest a mean-field-like transition in this alloy.
    Full-text Article · Feb 2014 · Physical Review B
  • Source
    J. Bertinshaw · C. Ulrich · A. Günther · [...] · J. Deisenhofer
    [Show abstract] [Hide abstract] ABSTRACT: We report on neutron diffraction, thermal expansion, dielectric and specific heat measurements on polycrystalline FeCr2S4 samples in external magnetic fields. The ferrimagnetic and orbital ordering temperatures, T_C = 170K and T_OO = 10 K are only weakly shifted in magnetic fields up to 9 T. The cubic lattice parameter is found to decrease when entering the orbitally ordered state below T_OO. This contraction does not significantly change in external fields up to 7.5 T. The magnetic moments of the Cr and Fe ions are reduced from the spin-only values throughout the magnetically ordered regime. The moments start to increase in the orbitally ordered regime when the magnetic fields become larger than 4.5 T and approach the expected spin-only values above about 5.5 T. Thermal expansion in magnetic fields and magnetostriction experiments indicate a contraction of the sample due to magnetic domains below about 60 K. In the orbitally ordered state this contraction is followed by a moderate expansion of the sample for fields larger than about 4.5 T, coinciding with the onset of the increase of the magnetic moments. The orbital ordering transition is accompanied by an anomaly in the dielectric constant. Below T_OO the dielectric constant depends on both the strength of the external magnetic field as well as the orientation of the external magnetic field with respect to the applied electric field.
    Full-text Article · Sep 2013
  • [Show abstract] [Hide abstract] ABSTRACT: Powder samples of CdCr2S4 and HgCr2S4 were investigated with respect to their magnetic behavior using muon spin rotation/relaxation spectroscopy (μSR). In CdCr2S4, a second order magnetic transition at 83 ± 1 K is confirmed. No influence on the μSR magnetic parameters due to the simultaneous ferroelectric transition was detected. In the limit T → 0, persistent spin fluctuation exists as a consequence of magnetic frustration. The proposed structural transition around 150 K is barely visible in the μSR spectra, indicating that the change in local symmetry is subtle. In HgCr2S4, we see in zero field at 27 ± 2 K a second order transition from the paramagnetic into a long-range ordered (LRO) magnetic state. This LRO state is characterized in the μSR spectra by a spontaneous spin precession signal. Applications of moderate magnetic fields (up to 450 mT) destroy the precession signal in favor of a rapidly decaying exponential signal. It reflects short-range order (SRO) of strongly correlated dynamic spins. The magnitude of the field required to induce this change is dependent on temperature. The observation of an increase of magnetic spin fluctuations with applied field suggests that spin dynamics play a key role in the field induced changes of the magnetic properties.
    Article · Apr 2013 · Physics of Condensed Matter
  • S Riegg · S Widmann · A Günther · [...] · S G Ebbinghaus
    [Show abstract] [Hide abstract] ABSTRACT: La2RuO5 shows a magneto-structural phase transition at 161 K with spin dimerization and concomitant formation of a non-magnetic singlet ground state. To gain a deeper insight into the origin of this transition systematic substitution of Ru by Ti has been carried out. Polycrystalline samples have been synthesized by thermal decomposition of citrate precursors leading to La2Ru1-yTiyO5 (0 ≤ y ≤ 0.45). The crystal structure was investigated by x-ray powder diffraction at room temperature and at 100 K. The valences of Ti and Ru were obtained from x-ray absorption near edge structure spectroscopy at the Ti-K and the Ru-LIII absorption edges, respectively. The magnetic phase transition was investigated by magnetic susceptibility measurements as a function of Ti substitution, revealing a decreasing transition temperature on increasing the level of substitution. The step-like feature in the magnetic susceptibility reflecting the Ru-Ru spin dimerization transition becomes smeared out close to y = 0.3 and completely vanishes at y = 0.45, indicating complete suppression of spin-dimer formation. Additional specific-heat measurements show a continuous decrease of the magnetic entropy peak with increasing Ti substitution mirroring the reduced number of spin dimers due to the magnetic dilution. A magnetic anomaly of the dimerization transition can hardly be detected for y ≥ 0.3. Density functional theory calculations were carried out to study changes of the electronic band structure caused by the substitution. A possibly preferred distribution of Ti and Ru and the magnetic interactions as well as the change of the density of states close to the Fermi level are investigated. Based on these experimental results a detailed (y,T) phase diagram is proposed.
    Article · Feb 2013 · Journal of Physics Condensed Matter
  • Source
    S. Riegg · A. Günther · H. -A. Krug von Nidda · [...] · S.G. Ebbinghaus
    [Show abstract] [Hide abstract] ABSTRACT: The Ru-Ru spin-singlet formation in La2 − x L n x RuO5 (Ln = Pr, Nd, Sm, Gd, Dy) was investigated by measurements of the specific heat and magnetic susceptibility. After subtraction of the lattice contribution from the specific heat (C p ), similar excess entropy values were obtained for all compounds. These entropies can be explained by the formation of antiferromagnetic Ru-spin dimers at low temperatures and provide a lower estimate for the intradimer exchange strength. Pronounced changes in the transition temperatures and a broadening of the corresponding peak in C p were observed. These changes depend on the rare-earth element and are due to local structural changes and heterogeneities caused by the substitution. The magnetic susceptibilities can be described by the sum of a rare-earth paramagnetic moment and the susceptibility of the unsubstituted La2RuO5. Density functional theory (DFT) calculations were performed for various compounds to investigate the origin of the magnetic transition and the relationship between structural changes and the spin-dimerization temperature. The combination of the present results with previous structural investigations supports the model of a spin-pairing of the Ru moments which occurs as a reason of the structural phase transition in La2 − x L n x RuO5.
    Full-text Article · Dec 2012 · Physics of Condensed Matter
  • V. Felea · S. Yasin · A. Günther · [...] · A. Loidl
    [Show abstract] [Hide abstract] ABSTRACT: Ultrasound and magnetization studies of the frustrated spinel ZnCr2Se4 are performed as a function of temperature and magnetic field up to 14 T. In zero field, the sound velocity and attenuation reveal significant anomalies at the antiferromagnetic transition at TN≈ 21 K indicating strong spin-lattice coupling. External magnetic fields shift these anomalies to lower temperatures concomitantly with the reduction of the Néel temperature. At 2 K, the sound velocity as a function of magnetic field manifests three pronounced anomalies: a deep minimum at 5.4 T related to an inflection point of the magnetization followed by two plateaus with distinct stiffness at fields above 7 and 10 T. The first plateau is ascribed to a transformation from a tetragonal to a cubic phase, while the second one corresponds to a state with fully polarized magnetization. The evolution of magnetic and structural states is discussed within a H-T phase diagram and compared with related frustrated magnetic spinels with strong spin-lattice coupling.
    Article · Sep 2012 · Physical review. B, Condensed matter
  • Zhe Wang · M. Schmidt · A. Günther · [...] · J. Deisenhofer
    Article · Jul 2012 · Physical review. B, Condensed matter
  • Zhe Wang · M. Schmidt · A. Günther · [...] · J. Deisenhofer
    [Show abstract] [Hide abstract] ABSTRACT: We report on the phonon spectrum of the spin-gapped quantum magnet Ba3Cr2O8 determined by infrared spectroscopy, and on specific heat measurements across the Jahn-Teller transition in magnetic fields up to 9 T. Phonon modes split below the Jahn-Teller transition, which occurs at TJT=70 K as detected by specific-heat measurements. The field-dependent specific heat data is analyzed in terms of the contributions from lattice, magnetic and orbital degrees of freedom. In contrast to the isostructural compound Sr3Cr2O8, our analysis does not indicate the existence of orbital fluctuations below the Jahn-Teller transition in Ba3Cr2O8.
    Article · Jun 2012 · Physical review. B, Condensed matter
  • Zhe Wang · M. Schmidt · A. Günther · [...] · J. Deisenhofer
    [Show abstract] [Hide abstract] ABSTRACT: We report on the phonon spectrum of Ba3Cr2O8 determined by infrared spectroscopy, and on specific heat measurements across the Jahn-Teller transition in magnetic fields up to 9 T. Phonon modes split below the Jahn-Teller transition, which occurs at T_{JT} = 70 K as detected by specific heat measurements. The field-dependent specific heat data is analyzed in terms of the contributions from lattice, magnetic and orbital degrees of freedom. In contrast to the isostructural compound Sr3Cr2O8 our analysis does not indicate the existence of orbital fluctuations below the Jahn-Teller transition in Ba3Cr2O8.
    Article · May 2012
  • Source
    M. Pregelj · O. Zaharko · A. Günther · [...] · S. Guerrero
    [Show abstract] [Hide abstract] ABSTRACT: Anisotropic magnetic properties of a layered kagome-like system Cu3Bi(SeO3)2O2Br have been studied by bulk magnetization and magnetic susceptibility measurements as well as powder and single-crystal neutron diffraction. At TN=27.4 K the system develops an alternating antiferromagnetic order of (ab) layers, which individually exhibit canted ferrimagnetic moment arrangement, resulting from the competing ferro- and antiferro-magnetic intralayer exchange interactions. A magnetic field BC∼0.8 T applied along the c axis (perpendicular to the layers) triggers a metamagnetic transition, when every second layer flips, i.e., resulting in a ferrimagnetic structure. Significantly higher fields are required to rotate the ferromagnetic component towards the b axis (∼7 T) or towards the a axis (∼15 T). The estimates of the exchange coupling constants and features indicative of an XY character of this quasi-2D system are presented.
    Full-text Article · Mar 2012 · Physical review. B, Condensed matter
  • M. Pregelj · O. Zaharko · A. Günther · [...] · S. Guerrero
    Article · Mar 2012