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Register a new userNature of the magnetic order and origin of induced ferroelectricity in TbMnO$_3$
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The magnetic structures which endow TbMnO$_3$ with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn $L_2$ edge, Mn 3d moments; Tb M$_4$ edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a non-collinear magnetic structure, as has previously been supposed, it is shown to occur between two non-collinear structures.
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This work shows an unconventional route for spin-driven ferroelectricity originating from a metastable magnetic field-induced canting of chromium sublattice in the presence of gadolinium moments in GdCrTiO5 at low temperatures. Compared to the isostructural neodymium compound, significant differences of magnetism and magnetoelectric effects are seen. We present the results of thorough investigations of temperature and magnetic field dependent magnetization as well as ac and dc magnetic susceptibility. These bulk measurements are complemented by local-probe spectroscopy utilizing electron-spin resonance and muon-spin rotation/relaxation for probing the chromium moments. Ferroelectric order is inferred from pyro- and magnetocurrent measurements. GdCrTiO5 shows a pyrocurrent signal around 10 K, only if the system is cooled in an applied magnetic field exceeding 10 kOe. A distinct spin-driven ferroelectric order is revealed in this state for temperatures below 10 K, which can be switched by changing magnetic-field direction and the polarity of the electric field. But, the magnetic measurements reveal no clear signature of long-range magnetic ordering. The presence of such meta-magnetoelectric-type behaviour in the absence of any meta-magnetic behavior is rare in the literature. Our microscopic spectroscopy results indicate significant changes of the magnetic properties around 10 K. Probably there is an exchange frustration between Gd and Cr moments, which prevents long-range magnetic ordering at further high temperature. Below 10 K, weak magnetic ordering occurs by minimizing frustration due to lattice distortion, which helps in magnetodielectric coupling. However, the non-polar distortion attains appreciable values after application of magnetic fields above 10 kOe to break the spatial inversion symmetry, which creates ferroelectricity.
Magnetic and magnetoelectric excitations in the multiferroic TbMnO_3 have been investigated at terahertz frequencies. Using different experimental geometries we can clearly separate the electro-active excitations (electromagnons) from the magneto-active modes, i.e. antiferromagnetic resonances (AFMR). Two AFMR resonances were found to coincide with electromagnons. This indicates that both excitations belong to the same mode and the electromagnons can be excited by magnetic ac-field as well. In external magnetic fields and at low temperatures distinct fine structure of the electromagnons appears. In spite of the 90^o rotation of the magnetic structure, the electromagnons are observable for electric ac-fields parallel to the a-axis only. Contrary to simple expectations, the response along the c-axis remains purely magnetic in nature.
We demonstrate that small but finite ferroelectric polarization ($sim$0.01 $mu$C/cm$^2$) emerges in orthorhombic LuFeO$_3$ ($Pnma$) at $T_N$ ($sim$600 K) because of commensurate (k = 0) and collinear magnetic structure. The synchrotron x-ray and neutron diffraction data suggest that the polarization could originate from enhanced bond covalency together with subtle contribution from lattice. The theoretical calculations indicate enhancement of bond covalency as well as the possibility of structural transition to the polar $Pna2_1$ phase below $T_N$. The $Pna2_1$ phase, in fact, is found to be energetically favorable below $T_N$ in orthorhombic LuFeO$_3$ ($albeit$ with very small energy difference) than in isostructural and nonferroelectric LaFeO$_3$ or NdFeO$_3$. Application of electric field induces finite piezostriction in LuFeO$_3$ via electrostriction resulting in clear domain contrast images in piezoresponse force microscopy.
We calculate spectra of magnetic excitations in the spin-spiral state of perovskite manganates. The spectra consist of several branches corresponding to different polarizations and different ways of diffraction from the static magnetic order. Goldstone modes and opening of gaps at zero and non-zero energies due to the crystal field and the Dzyaloshinski-Moriya anisotropies are discussed. Comparing results of the calculation with available experimental data we determine values of effective exchange parameters and anisotropies. To simplify the spin-wave calculation and to get a more clear physical insight in the structure of excitations we use the {sigma}-model-like effective field theory to analyze the Heisenberg Hamiltonian and to derive the spectra.
We report on an extensive investigation of the multiferroic compound TbMnO$_3$. Non-resonant x-ray magnetic scattering (NRXMS) revealed a dominant $A$-type domain. The temperature dependence of the intensity and wavevector associated with the incommensurate magnetic order was found to be in good agreement with neutron scattering data. XRS experiments were performed in the vicinity of the Mn $K$ and Tb $L_3$ edges in the high-temperature collinear phase, the intermediate temperature cycloidal/ferroelectric phase, and the low-temperature phase. In the collinear phase resonant $E1-E1$ satellites were found at the Mn $K$ edge associated with $A$-type but also $F$-type peaks. The azimuthal dependence of the $F$-type satellites (and their absence in the NRXMS experiments) indicates that they are most likely non-magnetic in origin. We suggest instead that they may be associated with an induced charge multipole. At the Tb $L_3$ edge resonant $A$- and $F$-type satellites ($E1-E1$) were observed in the collinear phase. These we attribute to a polarisation of the Tb 5$d$ states by the ordering of the Mn sublattice. In the cycloidal/ferroelectric phase a new set of resonant satellites appear corresponding to $C$-type order. These appear at the Tb $L_3$ edge only. In addition to a dominant $E1-E1$ component in the $sigma-pi^prime$ channel, a weaker component is found in the pre-edge with $sigma-sigma^prime$ polarization. Calculations of the XRS were performed using the $FDMNES$ code showing that the unrotated $sigma-sigma^prime$ component of the Tb $L_3$ $C$-type peaks appearing in the ferroelectric phase contains a contribution from a multipole that is odd with respect to both space and time, known in various contexts as the anapole.
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