Spin-wave dispersion and magnon chirality in multiferroic TbMnO3

Inelastic neutron scattering experiments combining time-of-flight and polarized techniques yield a comprehensive picture of the magnon dispersion in multiferroic TbMnO3 including the dynamic chirality. Taking into account only Mn3+ moments, spin-wave calculations including nearest-neighbor interactions, frustrating next-nearest neighbor exchange as well as single-ion anisotropy and antisymmetric terms describe the energy dispersion and the distribution of neutron scattering intensity in the multiferroic state very well. Polarized neutron scattering reveals strong dynamic chirality of both signs that may be controlled by external electric fields in the multiferroic phase. Also above the onset of long-range multiferroic order in zero electric field, a small inelastic chiral component can be inverted by an electric field. The microscopic spin-wave calculations fully explain also the dynamic chirality of magnetic excitations, which is imprinted by the static chirality of the multiferroic phase. The ordering of Tb3+ moments at lower temperature reduces the broadening of magnons but also renders the magnon dispersion more complex.Comment: 20 pages, 19 figure

Low-temperature magnetism of KAgF3

KAgF3 is a quasi-one-dimensional quantum antiferromagnet hosting a series of intriguing structural and magnetic transitions. Here we use powder neutron diffraction, μSR spectroscopy, and density functional theory calculations to elucidate the low-temperature magnetic phases. Below TN1 = 29 K we find that the material orders as an A-type antiferromagnet with an ordered moment of 0.52 μ B . Both neutrons and muons provide evidence for an intermediate phase at temperatures TN1 < T < TN2 with TN2 ≈ 66 K from a previous magnetometry study. However, the evidence is at the limit of detection and its nature remains an open problem

Multiferroic domain relaxation in ( NH 4 ) 2 [ FeCl 5 ( H 2 O ) ]

The molecular compound (NH4)2[FeCl5(H2O)] is a type-II multiferroic material, in which incommensurate cycloidal order directly induces ferroelectric polarization. The multiferroic domain kinetics in (NH4)2[FeCl5(H2O)] were studied by time-resolved neutron-diffraction experiments utilizing neutron polarization analysis. The temperature- and electric-field-dependent multiferroic relaxation obeys the simple combined Arrhenius-Merz law, which was reported to describe domain kinetics in the prototype multiferroics TbMnO3 and NaFeGe2O6. However, the characteristic time scale of the multiferroic relaxation is considerably larger than those in TbMnO3 or NaFeGe2O6. Temperature-dependent diffraction on (NH4)2[FeCl5(H2O)] reveals the emergence of higher-order and commensurate magnetic contributions upon cooling in the multiferroic phase in zero field. The good agreement with studies of higher-harmonic contributions in the deuterated material indicates that the isotopes only possess a minor impact on the magnetic ordering. However, in contrast to similar observations in multiferroic MnWO4, this anharmonic modification of magnetic ordering does not depin multiferroic domain walls or alter the temperature dependence of the multiferroic relaxation

Intermediate valence and Kondo effect in Ce_1−xSc_xAl₂

Results of lattice parameter, thermal expansion, thermopower and magnetic susceptibility measurements are reported for Ce1 − xScxAl2, a system exhibiting a solubility gap over a wide composition range. Two isostructural phases are identified within the gap. For the Sc-rich systems, both phases contain intermediate valent (IV) Ce ions, whereas in the Ce-rich systems (x < 0.5), a low-volume IV phase coexists with a high-volume phase containing Ce3+ ions. The temperature-induced overall valence change of the IV ions amounts to 15–20%. A factor-of-three increase of the spinfluctuation temperature is found upon increasing the Sc concentration from x = 0.5 to 0.95. For the Ce-rich systems, we have discovered a hitherto not known “Kondo anomaly” in the temperature dependence of the thermal expansivity

Structural dimerization in the commensurate magnetic phases of Na Fe ( WO 4 ) 2 and Mn WO 4

International audienceThe structural distortion and magnetoelastic coupling induced through commensurate magnetism has been investigated by neutron diffraction in structurally related MnWO4 and NaFe(WO4)2. Both systems exhibit a competition of incommensurate spiral and commensurate spin up-up-down-down ordering along the magnetic chains. In the latter commensurate phases, the alternatingly parallel and antiparallel arrangement of Fe 3+ respectively Mn 2+ moments leads to sizeable bond-angle modulation and thus to magnetic dimerization. For NaFe(WO4)2 this structural distortion has been determined to be strongest for the low-field up-up-down-down arrangement, and the structural refinement yields a bond-angle modulation of ±1.15(16) degrees. In the commensurate phase of MnWO4, superstructure reflections signal a comparable structural dimerization and thus strong magneto-elastic coupling different to that driving the multiferroic order. Pronounced anharmonic second-and third-order reflections in the incommensurate and multiferroic phase of MnWO4 result from tiny commensurate fractions that can depin multiferroic domains

Chiral order and multiferroic domain relaxation in Na Fe Ge 2 O 6

The magnetic structure and the multiferroic relaxation dynamics of NaFeGe2O6 were studied by neutron scattering on single crystals partially utilizing polarization analysis. In addition to the previously reported transitions, the incommensurate spiral ordering of Fe3+ moments in the ac plane develops an additional component along the crystallographic b direction below T≈5K, which coincides with a lock-in of the incommensurate modulation. The quasistatic control of the spin-spiral handedness, respectively of the vector chirality, by external electric fields proves the invertibility of multiferroic domains down to the lowest temperature. Time-resolved measurements of the multiferroic domain inversion in NaFeGe2O6 reveal a simple temperature and electric-field dependence of the multiferroic relaxation that is well described by a combined Arrhenius-Merz relation, as it has been observed for TbMnO3. The maximum speed of domain wall motion is comparable to the spin-wave velocity deduced from an analysis of the magnon dispersion

Combined Arrhenius-Merz Law Describing Domain Relaxation in Type-II Multiferroics

Electric fields were applied to multiferroic TbMnO3 single crystals to control the chiral domains, and the domain relaxation was studied over 8 decades in time by means of polarized neutron scattering. A surprisingly simple combination of an activation law and the Merz law describes the relaxation times in a wide range of electric field and temperature with just two parameters, an activation-field constant and a characteristic time representing the fastest possible inversion. Over the large part of field and temperature values corresponding to almost 6 orders of magnitude in time, multiferroic domain inversion is thus dominated by a single process, the domain wall motion. Only when approaching the multiferroic transition other mechanisms yield an accelerated inversion

Control of Chiral Magnetism Through Electric Fields in Multiferroic Compounds above the Long-Range Multiferroic Transition

Polarized neutron scattering experiments reveal that type-II multiferroics allow for controlling the spin chirality by external electric fields even in the absence of long-range multiferroic order. In the two prototype compounds TbMnO3 and MnWO4, chiral magnetism associated with soft overdamped electromagnons can be observed above the long-range multiferroic transition temperature T-MF, and it is possible to control it through an electric field. While MnWO4 exhibits chiral correlations only in a tiny temperature interval above T-MF, in TbMnO3 chiral magnetism can be observed over several kelvin up to the lock-in transition, which is well separated from T-MF