Lilian Prodan - Academia.edu (original) (raw)

Papers by Lilian Prodan

Applied Physics Letters

We report the magnetic anisotropy of kagome bilayer ferromagnet Fe3Sn probed by the bulk magnetom... more We report the magnetic anisotropy of kagome bilayer ferromagnet Fe3Sn probed by the bulk magnetometry and magnetic force microscopy (MFM) on high-quality single crystals. The dependence of magnetization on the orientation of the external magnetic field reveals strong easy-plane magnetocrystalline anisotropy and anisotropy of the saturation magnetization. The leading magnetocrystalline anisotropy constant shows a monotonous increase from K1≈−1.0×106 J/m3 at 300 K to −1.3×106 J/m3 at 2 K. Our ab initio electronic structure calculations yield the value of total magnetic moment of 7.1 μB/f.u. and a magnetocrystalline anisotropy energy density of −0.57 meV/f.u. (−1.62×106J/m3) both being in reasonable agreement with the experimental values. The MFM imaging reveals micrometer-scale magnetic vortices with weakly pinned cores that vanish at the saturation field of ∼3 T applied perpendicular to the kagome plane. The observed vortex-domain structure is well reproduced by the micromagnetic sim...

Scientific Reports, Feb 10, 2023

arXiv (Cornell University), Nov 9, 2022

Magnetization reversal in ferro-and ferrimagnets is a well-known archetype of non-equilibrium pro... more Magnetization reversal in ferro-and ferrimagnets is a well-known archetype of non-equilibrium processes, where the volume fractions of the oppositely magnetized domains vary and perfectly compensate each other at the coercive magnetic field. Here, we report on a fundamentally new pathway for magnetization reversal that is mediated by an antiferromagnetic state. Consequently, an atomic-scale compensation of the magnetization is realized at the coercive field, instead of the mesoscopic or macroscopic domain cancellation in canonical reversal processes. We demonstrate this unusual magnetization reversal on the Zn-doped polar magnet Fe2Mo3O8. Hidden behind the conventional ferrimagnetic hysteresis loop, the surprising emergence of the antiferromagnetic phase at the coercive fields is disclosed by a sharp peak in the field-dependence of the electric polarization. In addition, at the magnetization reversal our THz spectroscopy studies reveal the reappearance of the magnon mode that is only present in the pristine antiferromagnetic state. According to our microscopic calculations, this unusual process is governed by the dominant intralayer coupling, strong easy-axis anisotropy and spin fluctuations, which result in a complex interplay between the ferrimagnetic and antiferromagnetic phases. Such antiferro-state-mediated reversal processes offer novel concepts for magnetization control, and may also emerge for other ferroic orders.

Journal of Alloys and Compounds

Advanced Electronic Materials

wide interfaces, for example, domain walls, have been reported to possess the same inherent elect... more wide interfaces, for example, domain walls, have been reported to possess the same inherent electronic response as existing circuit elements, such as switches [6] and half-wave rectifiers. [7] In addition, ferroelectric domain walls can be reconfigured in situ by a variety of external fields which can lead to exotic bulk responses. Such bulk responses offer the opportunity to both enhance existing technology (e.g., magnetoresistance, [8] colossal dielectric constants, [9] memristive behavior [10]) but also provide next generation functionality like, negative capacitance, [11] above band gap photovoltaic effects, [12] and domain wall nanoelectronics. [13] Such effects have been discussed from both a fundamental and a technological perspective in recent reviews. [14,15] For all of these bulk responses, the key requirement is for the domain walls to exhibit a different conductivity compared to the surrounding material. Therefore, much of the research has focused on ferroelectrics as the build up of screening charges at domain walls with polar discontinuities are known to modify the local conductivity. [13,14] Examples of this, in single crystals, include BaTiO 3 , [16] (Ca,Sr) 3 Ti 2 O 7 , [17] Cu-Cl boracite's, [18] LiNbO 3 , [19] h-RMnO 3 (R representing rare earth metals), [20] and GaV 4 S 8. [8] Because of the energetically costly nature of such polar discontinuities, their spontaneous formations are normally restricted to improper ferroelectrics. [21,22] Indeed, so established is this screening charge mechanism that it is a surprise for an improper ferroelectric material, exhibiting polar discontinuities, not to have conducting domain walls. [23] In this scenario, the type of domain wall that is expected to have enhanced conductivity depends on the electronic structure of the host material: In a p-type (n-type) semiconductor the tail-to-tail (head-to-head) domain walls attract screening holes (electrons) and thus provide enhanced conductivity relative to the bulk. [24-26] The corresponding head-to-head (tail-to-tail) wall in a p-type (n-type) material is then expected to have reduced conductivity compared to the bulk. [13,21,22,27] Note, in ferroelectrics, particularly thin-films, further conductivity mechanisms have been reported. [6,28-32] But it is challenging, especially in oxides, to disentangle intrinsic effects and those associated with, for example, enhanced defect density at domain walls, [33,34] which can change surface Schottky barriers and hence conductivity. [7] There have also been reports of domain wall conductivity and even superconductivity in non-ferroelectrics. Examples of the former case include, the antiferromagnetic insulators with conducting magnetic domain Rewritable nanoelectronics offer new perspectives and potential to both fundamental research and technological applications. Such interest has driven the research focus into conducting domain walls: pseudo-2D conducting channels that can be created, positioned, and deleted in situ. However, the study of conductive domain walls is largely limited to wide-gap ferroelectrics, where the conductivity typically arises from changes in charge carrier density, due to screening charge accumulation at polar discontinuities. This work shows that, in narrow-gap correlated insulators with strong charge-lattice coupling, local strain gradients can drive enhanced conductivity at the domain walls-removing polar-discontinuities as a criteria for conductivity. By combining different scanning probe microscopy techniques, it is demonstrated that the domain wall conductivity in GaV 4 S 8 does not follow the established screening charge model but likely arises from the large surface reconstruction across the Jahn-Teller transition and the associated strain gradients across the domain walls. This mechanism could turn any structural, or even magnetic, domain wall conducting, if the electronic structure of the host is susceptible to local strain gradients-drastically expanding the range of materials and phenomena that may be applicable to domain wall-based nanoelectronics.

arXiv (Cornell University), Mar 1, 2022

The cubic spinel FeCr2S4 has been receiving immense research interest because of its emergent pha... more The cubic spinel FeCr2S4 has been receiving immense research interest because of its emergent phases and the interplay of spin, orbital and lattice degrees of freedom. Despite the intense research, several fundamental questions are yet to be answered, such as the refinement of the crystal structure in the different magnetic and orbital ordered phases. Here, using high-resolution synchrotron powder diffraction on stoichiometric crystals of FeCr2S4 we resolved the long sought-after cubic to tetragonal transition at ~65 K, reducing the lattice symmetry to I41/amd. With further lowering the temperature, at ~9 K, the crystal structure becomes polar, hence the compound becomes multiferroic. The elucidation of the lattice symmetry throughout different phases of FeCr2S4 provides a basis for the understanding this enigmatic system and also highlights the importance of structural deformation in correlated materials.

The Journal of Physical Chemistry Letters, 2022

Spin-orbit quartet ground states are associated with rich phenomenology, ranging from multipolar ... more Spin-orbit quartet ground states are associated with rich phenomenology, ranging from multipolar phases in f1 rare-earth borides to magnetism emerging through covalency and vibronic couplings in d1 transition-metal compounds. The latter effect has been studied since the 1960s on t2g1 octahedral ML6 units in both molecular complexes and extended solid-state lattices. Here we analyze the Jeff = 3/2 quartet ground state of larger cubane-like M4L4 entities in lacunar spinels, composed of transition-metal (M) tetrahedra caged by chalcogenide ligands (L). These represent a unique platform where spin-orbit coupling acts on molecular-like, delocalized t2 orbitals. Using quantum chemical methods, we pin down the interplay of spin-orbit couplings in such a setting and many-body physics related to other molecular-like single-electron levels, both below and above the reference t21. We provide a different interpretation of resonant inelastic X-ray scattering data on GaTa4Se8 and, by comparing magnetic susceptibility data with calculated g factors, valuable insights into the important role of vibronic couplings.

Fe3Sn2 Markus Altthaler, 2, 3, ∗ Erik Lysne, 3, ∗ Erik Roede, Lilian Prodan, Vladimir Tsurkan, 4 ... more Fe3Sn2 Markus Altthaler, 2, 3, ∗ Erik Lysne, 3, ∗ Erik Roede, Lilian Prodan, Vladimir Tsurkan, 4 Mohamed A. Kassem, Stephan Krohns, István Kézsmárki, † and Dennis Meier 3, ‡ Experimental Physics V, University of Augsburg, 86135 Augsburg, Germany Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7043 Trondheim, Norway Center for Quantum Spintronics, Department of Physics, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway Institute for Applied Physics, MD-2028, Chisinau, Moldova Department of Physics, Assiut University, 171516 Assiut, Egypt (Dated: June 17, 2021)

Low Temperature Physics, 2017

We report results of magnetic and ultrasound studies of the sulfide spinel CoCr 2 S 4 , for which... more We report results of magnetic and ultrasound studies of the sulfide spinel CoCr 2 S 4 , for which the multiferroicity has recently been suggested. Clear anomalies in the magnetic and acoustic properties have been observed at T N = 222 K and in applied magnetic fields evidencing the important role of magnetoelastic interactions in this material. In contrast, no anomalies have been detected at T C = 28 K, where a spontaneous electric polarization and isostructural distortions have been reported. We have extracted the H-T phase diagram of CoCr 2 S 4 from our experiments for magnetic fields applied along the <111> direction. We discuss our observations in relation to our earlier results obtained for the oxide multiferroic spinel CoCr 2 O 4 .

Physical Review Letters, 2021

We report the observation of an antipolar phase in cubic GaNb4S8 driven by an unconventional micr... more We report the observation of an antipolar phase in cubic GaNb4S8 driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb4S4 molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at TJT = 31 K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above TJT. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb4S8 may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferrolectric.

physica status solidi (b), 2021

Polarization-and temperature-dependent Raman data along with theoretical simulations are presente... more Polarization-and temperature-dependent Raman data along with theoretical simulations are presented for the Kagome ferromagnet Fe3Sn2. Eight out of nine expected phonon modes were identified. The experimental energies compare well with those from the simulations. The analysis of the line widths indicates relatively strong phonon-phonon coupling in the range 0.1 to 1. The temperaturedependent frequencies of three A1g modes show weak anomalies at approximately 100 K. In contrast, the linewidths of all phonon modes follow the conventional exponential broadening up to room temperature except for the softest A1g mode, whose width exhibits a kink close to 100 K and becomes nearly constant for T > 100 K. These features are indicative of a spin reorientation taking place in the temperature range above 100 K which might arise from spin-phonon coupling. The low-energy part of the electronic continuum in Egsymmetry depends strongly on temperature. The possible reasons include particle-hole excitation tracking the resistivity, a spin-dependent gap or spin fluctuations.

Physical Review B, 2020

We investigated the static and dynamic magnetic properties of the polar ferrimagnet Mn2Mo3O8 in t... more We investigated the static and dynamic magnetic properties of the polar ferrimagnet Mn2Mo3O8 in three magnetically ordered phases via magnetization, magnetic torque, and THz absorption spectroscopy measurements. The observed magnetic field dependence of the spin-wave resonances, including Brillouin zone-center and zone-boundary excitations, magnetization, and torque, are well described by an extended two-sublattice antiferromagnetic classical mean-field model. In this orbitally quenched system, the competing weak easy-plane and easy-axis single-ion anisotropies of the two crystallographic sites are determined from the model and assigned to the tetra-and octahedral sites, respectively, by ab initio calculations.

Physical Review B, 2020

In antiferromagnets, the interplay of spin frustration and spin-lattice coupling has been extensi... more In antiferromagnets, the interplay of spin frustration and spin-lattice coupling has been extensively studied as the source of complex spin patterns and exotic magnetism. Here, we demonstrate that, although neglected in the past, the spin-lattice coupling is essential to ferrimagnetic spinels as well. We performed ultrahigh-field magnetization measurements up to 110 T on a Yafet-Kittel ferrimagnetic spinel, MnCr2S4, which was complemented by measurements of magnetostriction and sound velocities up to 60 T. Classical Monte Carlo calculations were performed to identify the complex high-field spin structures. Our minimal model incorporating spin-lattice coupling accounts for the experimental results and corroborates the complete phase diagram, including two new highfield phase transitions at 75 and 85 T. Magnetoelastic coupling induces striking effects: An extremely robust magnetization plateau is embedded between two unconventional spin-asymmetric phases. Ferrimagnetic spinels provide a new platform to study asymmetric and multiferroic phases stabilized by spin-lattice coupling.

Physical Review B, 2020

The high-field (H,T) phase diagram of the multiferroic lacunar spinel GeV 4 S 8 has been studied ... more The high-field (H,T) phase diagram of the multiferroic lacunar spinel GeV 4 S 8 has been studied by ultrasound, magnetization, and pyrocurrent experiments in magnetic fields up to 60 T. The title compound consists of molecular building blocks, with vanadium V 4 clusters characterized by a unique electron density. These vanadium tetrahedra constitute a Jahn-Teller active entity, which drive an orbital-ordering transition at 30 K with the concomitant appearance of ferroelectricity. Ultrasound and magnetization experiments reveal sharp anomalies in magnetic fields of 46 T, which are associated with a first-order phase transition into an orbitally disordered state characterized by significant field and temperature hystereses. We report a sequence of complex magnetic, polar, and orbitally ordered states, i.e., the appearance of two orbitally ordered phases OO1 and OO2 for μ 0 H < 45 T and T < 30 K. Beyond the paraelectric phase we further evidenced three ferroelectric phases, FE1, FE2, and FE3. Finally, antiferromagnetic (AFM) order (T < 15 K) and fully polarized ferromagnetic order (μ 0 H > 60 T) have been observed in GeV 4 S 8. At low temperatures and for fields below 40 T, AFM order coexists with the polar phase FE3 identifying a multiferroic state. Our results demonstrate a fascinating competition of the different orders, which the material manifests in high magnetic fields and at low temperatures.

Physical Review Letters, 2018

Physical Review B, 2017

We report structural, susceptibility and specific heat studies of stoichiometric and offstoichiom... more We report structural, susceptibility and specific heat studies of stoichiometric and offstoichiometric poly-and single crystals of the A-site spinel compound FeSc 2 S 4. In stoichiometric samples no long-range magnetic order is found down to 1.8 K. The magnetic susceptibility of these samples is field independent in the temperature range 10-400 K and does not show irreversible effects at low temperatures. In contrast, the magnetic susceptibility of samples with iron excess shows substantial field dependence at high temperatures and manifests a pronounced magnetic irreversibility at low temperatures with a difference between ZFC and FC susceptibilities and a maximum at 10 K reminiscent of a magnetic transition. Single crystal x-ray diffraction of the stoichiometric samples revealed a single phase spinel structure without site inversion. In single crystalline samples with Fe excess besides the main spinel phase a second ordered single-crystal phase was detected with the diffraction pattern of a vacancy-ordered superstructure of iron sulfide, close to the 5C polytype Fe 9 S 10. Specific heat studies reveal a broad anomaly, which evolves below 20 K in both stoichiometric and off-stoichiometric crystals. We show that the low-temperature specific heat can be well described by considering the low-lying spin-orbital electronic levels of Fe 2+ ions. Our results demonstrate significant influence of excess Fe ions on intrinsic magnetic behavior of FeSc 2 S 4 and provide support for the spin-orbital liquid scenario proposed in earlier studies for the stoichiometric compound.

Science advances, 2017

Frustrated magnets provide a promising avenue for realizing exotic quantum states of matter, such... more Frustrated magnets provide a promising avenue for realizing exotic quantum states of matter, such as spin liquids and spin ice or complex spin molecules. Under an external magnetic field, frustrated magnets can exhibit fractional magnetization plateaus related to definite spin patterns stabilized by field-induced lattice distortions. Magnetization and ultrasound experiments in MnCr2S4 up to 60 T reveal two fascinating features: (i) an extremely robust magnetization plateau with an unusual spin structure and (ii) two intermediate phases, indicating possible realizations of supersolid phases. The magnetization plateau characterizes fully polarized chromium moments, without any contributions from manganese spins. At 40 T, the middle of the plateau, a regime evolves, where sound waves propagate almost without dissipation. The external magnetic field exactly compensates the Cr-Mn exchange field and decouples Mn and Cr sublattices. In analogy to predictions of quantum lattice-gas models, ...

arXiv (Cornell University), Feb 6, 2023

Applied Physics Letters

We report the magnetic anisotropy of kagome bilayer ferromagnet Fe3Sn probed by the bulk magnetom... more We report the magnetic anisotropy of kagome bilayer ferromagnet Fe3Sn probed by the bulk magnetometry and magnetic force microscopy (MFM) on high-quality single crystals. The dependence of magnetization on the orientation of the external magnetic field reveals strong easy-plane magnetocrystalline anisotropy and anisotropy of the saturation magnetization. The leading magnetocrystalline anisotropy constant shows a monotonous increase from K1≈−1.0×106 J/m3 at 300 K to −1.3×106 J/m3 at 2 K. Our ab initio electronic structure calculations yield the value of total magnetic moment of 7.1 μB/f.u. and a magnetocrystalline anisotropy energy density of −0.57 meV/f.u. (−1.62×106J/m3) both being in reasonable agreement with the experimental values. The MFM imaging reveals micrometer-scale magnetic vortices with weakly pinned cores that vanish at the saturation field of ∼3 T applied perpendicular to the kagome plane. The observed vortex-domain structure is well reproduced by the micromagnetic sim...

Scientific Reports, Feb 10, 2023

arXiv (Cornell University), Nov 9, 2022

Magnetization reversal in ferro-and ferrimagnets is a well-known archetype of non-equilibrium pro... more Magnetization reversal in ferro-and ferrimagnets is a well-known archetype of non-equilibrium processes, where the volume fractions of the oppositely magnetized domains vary and perfectly compensate each other at the coercive magnetic field. Here, we report on a fundamentally new pathway for magnetization reversal that is mediated by an antiferromagnetic state. Consequently, an atomic-scale compensation of the magnetization is realized at the coercive field, instead of the mesoscopic or macroscopic domain cancellation in canonical reversal processes. We demonstrate this unusual magnetization reversal on the Zn-doped polar magnet Fe2Mo3O8. Hidden behind the conventional ferrimagnetic hysteresis loop, the surprising emergence of the antiferromagnetic phase at the coercive fields is disclosed by a sharp peak in the field-dependence of the electric polarization. In addition, at the magnetization reversal our THz spectroscopy studies reveal the reappearance of the magnon mode that is only present in the pristine antiferromagnetic state. According to our microscopic calculations, this unusual process is governed by the dominant intralayer coupling, strong easy-axis anisotropy and spin fluctuations, which result in a complex interplay between the ferrimagnetic and antiferromagnetic phases. Such antiferro-state-mediated reversal processes offer novel concepts for magnetization control, and may also emerge for other ferroic orders.

Journal of Alloys and Compounds

Advanced Electronic Materials

wide interfaces, for example, domain walls, have been reported to possess the same inherent elect... more wide interfaces, for example, domain walls, have been reported to possess the same inherent electronic response as existing circuit elements, such as switches [6] and half-wave rectifiers. [7] In addition, ferroelectric domain walls can be reconfigured in situ by a variety of external fields which can lead to exotic bulk responses. Such bulk responses offer the opportunity to both enhance existing technology (e.g., magnetoresistance, [8] colossal dielectric constants, [9] memristive behavior [10]) but also provide next generation functionality like, negative capacitance, [11] above band gap photovoltaic effects, [12] and domain wall nanoelectronics. [13] Such effects have been discussed from both a fundamental and a technological perspective in recent reviews. [14,15] For all of these bulk responses, the key requirement is for the domain walls to exhibit a different conductivity compared to the surrounding material. Therefore, much of the research has focused on ferroelectrics as the build up of screening charges at domain walls with polar discontinuities are known to modify the local conductivity. [13,14] Examples of this, in single crystals, include BaTiO 3 , [16] (Ca,Sr) 3 Ti 2 O 7 , [17] Cu-Cl boracite's, [18] LiNbO 3 , [19] h-RMnO 3 (R representing rare earth metals), [20] and GaV 4 S 8. [8] Because of the energetically costly nature of such polar discontinuities, their spontaneous formations are normally restricted to improper ferroelectrics. [21,22] Indeed, so established is this screening charge mechanism that it is a surprise for an improper ferroelectric material, exhibiting polar discontinuities, not to have conducting domain walls. [23] In this scenario, the type of domain wall that is expected to have enhanced conductivity depends on the electronic structure of the host material: In a p-type (n-type) semiconductor the tail-to-tail (head-to-head) domain walls attract screening holes (electrons) and thus provide enhanced conductivity relative to the bulk. [24-26] The corresponding head-to-head (tail-to-tail) wall in a p-type (n-type) material is then expected to have reduced conductivity compared to the bulk. [13,21,22,27] Note, in ferroelectrics, particularly thin-films, further conductivity mechanisms have been reported. [6,28-32] But it is challenging, especially in oxides, to disentangle intrinsic effects and those associated with, for example, enhanced defect density at domain walls, [33,34] which can change surface Schottky barriers and hence conductivity. [7] There have also been reports of domain wall conductivity and even superconductivity in non-ferroelectrics. Examples of the former case include, the antiferromagnetic insulators with conducting magnetic domain Rewritable nanoelectronics offer new perspectives and potential to both fundamental research and technological applications. Such interest has driven the research focus into conducting domain walls: pseudo-2D conducting channels that can be created, positioned, and deleted in situ. However, the study of conductive domain walls is largely limited to wide-gap ferroelectrics, where the conductivity typically arises from changes in charge carrier density, due to screening charge accumulation at polar discontinuities. This work shows that, in narrow-gap correlated insulators with strong charge-lattice coupling, local strain gradients can drive enhanced conductivity at the domain walls-removing polar-discontinuities as a criteria for conductivity. By combining different scanning probe microscopy techniques, it is demonstrated that the domain wall conductivity in GaV 4 S 8 does not follow the established screening charge model but likely arises from the large surface reconstruction across the Jahn-Teller transition and the associated strain gradients across the domain walls. This mechanism could turn any structural, or even magnetic, domain wall conducting, if the electronic structure of the host is susceptible to local strain gradients-drastically expanding the range of materials and phenomena that may be applicable to domain wall-based nanoelectronics.

arXiv (Cornell University), Mar 1, 2022

The cubic spinel FeCr2S4 has been receiving immense research interest because of its emergent pha... more The cubic spinel FeCr2S4 has been receiving immense research interest because of its emergent phases and the interplay of spin, orbital and lattice degrees of freedom. Despite the intense research, several fundamental questions are yet to be answered, such as the refinement of the crystal structure in the different magnetic and orbital ordered phases. Here, using high-resolution synchrotron powder diffraction on stoichiometric crystals of FeCr2S4 we resolved the long sought-after cubic to tetragonal transition at ~65 K, reducing the lattice symmetry to I41/amd. With further lowering the temperature, at ~9 K, the crystal structure becomes polar, hence the compound becomes multiferroic. The elucidation of the lattice symmetry throughout different phases of FeCr2S4 provides a basis for the understanding this enigmatic system and also highlights the importance of structural deformation in correlated materials.

The Journal of Physical Chemistry Letters, 2022

Spin-orbit quartet ground states are associated with rich phenomenology, ranging from multipolar ... more Spin-orbit quartet ground states are associated with rich phenomenology, ranging from multipolar phases in f1 rare-earth borides to magnetism emerging through covalency and vibronic couplings in d1 transition-metal compounds. The latter effect has been studied since the 1960s on t2g1 octahedral ML6 units in both molecular complexes and extended solid-state lattices. Here we analyze the Jeff = 3/2 quartet ground state of larger cubane-like M4L4 entities in lacunar spinels, composed of transition-metal (M) tetrahedra caged by chalcogenide ligands (L). These represent a unique platform where spin-orbit coupling acts on molecular-like, delocalized t2 orbitals. Using quantum chemical methods, we pin down the interplay of spin-orbit couplings in such a setting and many-body physics related to other molecular-like single-electron levels, both below and above the reference t21. We provide a different interpretation of resonant inelastic X-ray scattering data on GaTa4Se8 and, by comparing magnetic susceptibility data with calculated g factors, valuable insights into the important role of vibronic couplings.

Fe3Sn2 Markus Altthaler, 2, 3, ∗ Erik Lysne, 3, ∗ Erik Roede, Lilian Prodan, Vladimir Tsurkan, 4 ... more Fe3Sn2 Markus Altthaler, 2, 3, ∗ Erik Lysne, 3, ∗ Erik Roede, Lilian Prodan, Vladimir Tsurkan, 4 Mohamed A. Kassem, Stephan Krohns, István Kézsmárki, † and Dennis Meier 3, ‡ Experimental Physics V, University of Augsburg, 86135 Augsburg, Germany Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7043 Trondheim, Norway Center for Quantum Spintronics, Department of Physics, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway Institute for Applied Physics, MD-2028, Chisinau, Moldova Department of Physics, Assiut University, 171516 Assiut, Egypt (Dated: June 17, 2021)

Low Temperature Physics, 2017

We report results of magnetic and ultrasound studies of the sulfide spinel CoCr 2 S 4 , for which... more We report results of magnetic and ultrasound studies of the sulfide spinel CoCr 2 S 4 , for which the multiferroicity has recently been suggested. Clear anomalies in the magnetic and acoustic properties have been observed at T N = 222 K and in applied magnetic fields evidencing the important role of magnetoelastic interactions in this material. In contrast, no anomalies have been detected at T C = 28 K, where a spontaneous electric polarization and isostructural distortions have been reported. We have extracted the H-T phase diagram of CoCr 2 S 4 from our experiments for magnetic fields applied along the <111> direction. We discuss our observations in relation to our earlier results obtained for the oxide multiferroic spinel CoCr 2 O 4 .

Physical Review Letters, 2021

We report the observation of an antipolar phase in cubic GaNb4S8 driven by an unconventional micr... more We report the observation of an antipolar phase in cubic GaNb4S8 driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb4S4 molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at TJT = 31 K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above TJT. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb4S8 may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferrolectric.

physica status solidi (b), 2021

Polarization-and temperature-dependent Raman data along with theoretical simulations are presente... more Polarization-and temperature-dependent Raman data along with theoretical simulations are presented for the Kagome ferromagnet Fe3Sn2. Eight out of nine expected phonon modes were identified. The experimental energies compare well with those from the simulations. The analysis of the line widths indicates relatively strong phonon-phonon coupling in the range 0.1 to 1. The temperaturedependent frequencies of three A1g modes show weak anomalies at approximately 100 K. In contrast, the linewidths of all phonon modes follow the conventional exponential broadening up to room temperature except for the softest A1g mode, whose width exhibits a kink close to 100 K and becomes nearly constant for T > 100 K. These features are indicative of a spin reorientation taking place in the temperature range above 100 K which might arise from spin-phonon coupling. The low-energy part of the electronic continuum in Egsymmetry depends strongly on temperature. The possible reasons include particle-hole excitation tracking the resistivity, a spin-dependent gap or spin fluctuations.

Physical Review B, 2020

We investigated the static and dynamic magnetic properties of the polar ferrimagnet Mn2Mo3O8 in t... more We investigated the static and dynamic magnetic properties of the polar ferrimagnet Mn2Mo3O8 in three magnetically ordered phases via magnetization, magnetic torque, and THz absorption spectroscopy measurements. The observed magnetic field dependence of the spin-wave resonances, including Brillouin zone-center and zone-boundary excitations, magnetization, and torque, are well described by an extended two-sublattice antiferromagnetic classical mean-field model. In this orbitally quenched system, the competing weak easy-plane and easy-axis single-ion anisotropies of the two crystallographic sites are determined from the model and assigned to the tetra-and octahedral sites, respectively, by ab initio calculations.

Physical Review B, 2020

In antiferromagnets, the interplay of spin frustration and spin-lattice coupling has been extensi... more In antiferromagnets, the interplay of spin frustration and spin-lattice coupling has been extensively studied as the source of complex spin patterns and exotic magnetism. Here, we demonstrate that, although neglected in the past, the spin-lattice coupling is essential to ferrimagnetic spinels as well. We performed ultrahigh-field magnetization measurements up to 110 T on a Yafet-Kittel ferrimagnetic spinel, MnCr2S4, which was complemented by measurements of magnetostriction and sound velocities up to 60 T. Classical Monte Carlo calculations were performed to identify the complex high-field spin structures. Our minimal model incorporating spin-lattice coupling accounts for the experimental results and corroborates the complete phase diagram, including two new highfield phase transitions at 75 and 85 T. Magnetoelastic coupling induces striking effects: An extremely robust magnetization plateau is embedded between two unconventional spin-asymmetric phases. Ferrimagnetic spinels provide a new platform to study asymmetric and multiferroic phases stabilized by spin-lattice coupling.

Physical Review B, 2020

The high-field (H,T) phase diagram of the multiferroic lacunar spinel GeV 4 S 8 has been studied ... more The high-field (H,T) phase diagram of the multiferroic lacunar spinel GeV 4 S 8 has been studied by ultrasound, magnetization, and pyrocurrent experiments in magnetic fields up to 60 T. The title compound consists of molecular building blocks, with vanadium V 4 clusters characterized by a unique electron density. These vanadium tetrahedra constitute a Jahn-Teller active entity, which drive an orbital-ordering transition at 30 K with the concomitant appearance of ferroelectricity. Ultrasound and magnetization experiments reveal sharp anomalies in magnetic fields of 46 T, which are associated with a first-order phase transition into an orbitally disordered state characterized by significant field and temperature hystereses. We report a sequence of complex magnetic, polar, and orbitally ordered states, i.e., the appearance of two orbitally ordered phases OO1 and OO2 for μ 0 H < 45 T and T < 30 K. Beyond the paraelectric phase we further evidenced three ferroelectric phases, FE1, FE2, and FE3. Finally, antiferromagnetic (AFM) order (T < 15 K) and fully polarized ferromagnetic order (μ 0 H > 60 T) have been observed in GeV 4 S 8. At low temperatures and for fields below 40 T, AFM order coexists with the polar phase FE3 identifying a multiferroic state. Our results demonstrate a fascinating competition of the different orders, which the material manifests in high magnetic fields and at low temperatures.

Physical Review Letters, 2018

Physical Review B, 2017

We report structural, susceptibility and specific heat studies of stoichiometric and offstoichiom... more We report structural, susceptibility and specific heat studies of stoichiometric and offstoichiometric poly-and single crystals of the A-site spinel compound FeSc 2 S 4. In stoichiometric samples no long-range magnetic order is found down to 1.8 K. The magnetic susceptibility of these samples is field independent in the temperature range 10-400 K and does not show irreversible effects at low temperatures. In contrast, the magnetic susceptibility of samples with iron excess shows substantial field dependence at high temperatures and manifests a pronounced magnetic irreversibility at low temperatures with a difference between ZFC and FC susceptibilities and a maximum at 10 K reminiscent of a magnetic transition. Single crystal x-ray diffraction of the stoichiometric samples revealed a single phase spinel structure without site inversion. In single crystalline samples with Fe excess besides the main spinel phase a second ordered single-crystal phase was detected with the diffraction pattern of a vacancy-ordered superstructure of iron sulfide, close to the 5C polytype Fe 9 S 10. Specific heat studies reveal a broad anomaly, which evolves below 20 K in both stoichiometric and off-stoichiometric crystals. We show that the low-temperature specific heat can be well described by considering the low-lying spin-orbital electronic levels of Fe 2+ ions. Our results demonstrate significant influence of excess Fe ions on intrinsic magnetic behavior of FeSc 2 S 4 and provide support for the spin-orbital liquid scenario proposed in earlier studies for the stoichiometric compound.

Science advances, 2017

Frustrated magnets provide a promising avenue for realizing exotic quantum states of matter, such... more Frustrated magnets provide a promising avenue for realizing exotic quantum states of matter, such as spin liquids and spin ice or complex spin molecules. Under an external magnetic field, frustrated magnets can exhibit fractional magnetization plateaus related to definite spin patterns stabilized by field-induced lattice distortions. Magnetization and ultrasound experiments in MnCr2S4 up to 60 T reveal two fascinating features: (i) an extremely robust magnetization plateau with an unusual spin structure and (ii) two intermediate phases, indicating possible realizations of supersolid phases. The magnetization plateau characterizes fully polarized chromium moments, without any contributions from manganese spins. At 40 T, the middle of the plateau, a regime evolves, where sound waves propagate almost without dissipation. The external magnetic field exactly compensates the Cr-Mn exchange field and decouples Mn and Cr sublattices. In analogy to predictions of quantum lattice-gas models, ...

arXiv (Cornell University), Feb 6, 2023