Multiferroic Manganites Research Articles

Lattice dynamics of manganites RMnO3 (R =Sm, Eu or Gd): instabilities and coexistence of orthorhombic and hexagonal phases

The lattice dynamical and allied properties of the multiferroic manganites SmMnO3, EuMnO3 and GdMnO3 were investigated in this work by means of a shell model with transferable pairwise interionic interaction potential. This shell-model potential is able to reproduce the available crystal structure and phonon frequencies. A zone center imaginary Au mode is observed in these lattice dynamics calculations that indicates metastability of the perovskite phase. Comparison of the Gibbs free energies in the orthorhombic and hexagonal phases points to the possible coexistence of the two phases of these manganites under ambient conditions.

Mechanism of Lattice-Distortion-Induced Electric-Polarization Flop in the Multiferroic Perovskite Manganites

We theoretically study a competition between two types of spin cycloids ( a b -plane and b c -plane ones) in the multiferroic perovskite manganites, which is an origin of intriguing magnetoelectric phenomena in these compounds. Analysis of a microscopic model using the Monte-Carlo method reveals that their competition originates from a conflict between the single-ion anisotropy and the Dzyaloshinsky–Moriya interaction. It is demonstrated that the conflict can be controlled by tuning the second-neighbor spin exchanges through the GdFeO 3 -type distortion, which leads to a cycloidal-plane flop from a b to b c observed in the solid-solution systems like Eu 1- x Y x MnO 3 and Gd 1- x Tb x MnO 3 with increasing x .

Microwave investigation of the phase diagram of hexagonal multiferroicHoMnO3

Using a microwave cavity perturbation technique at 16.5 GHz, we have investigated the complex dielectric and magnetic susceptibilities of the hexagonal multiferroic manganite ${\text{HoMnO}}_{3}$. The $c$-axis and $ab$-plane magnetic susceptibilities both appear to be dominated by the contribution of the ${\text{Ho}}^{3+}$ moments below 100 K. Along the same crystal orientations, Debye relaxation effects are found on the dielectric and magnetic-susceptibility functions above 100 K in a temperature range where unconventional magnetic fluctuations associated to the ${\text{Mn}}^{3+}$ moments are known to exist. At low temperatures, anomalies at the various phase transitions are observed on both susceptibility functions. These were investigated in magnetic fields up to 16 T and an $H\text{\ensuremath{-}}T$ phase diagram is deduced for the two crystal directions. Along the $c$ axis, the diagram is compared with the one found in the literature.

Polarized reflectivity and lattice dynamics calculation of multiferroicYMnO3

Temperature-dependent reflectivity spectra of a multiferroic hexagonal ${\text{YMnO}}_{3}$ single crystal were measured with polarized light in the ferroelectric phase. The phonon parameters for both $E\ensuremath{\parallel}c$ and $E\ensuremath{\perp}c$ polarizations are obtained by fitting the dielectric function to experimental data. Phonon assignment is done on the basis of lattice dynamics calculations. Phonon anomalies are found at the antiferromagnetic transition ${T}_{N}$. Mean Born effective charges are calculated from phonon fit parameters and discussed in the framework of the different mechanisms proposed for the origin of ferroelectricity in hexagonal multiferroic manganites.

Origin of multiferroic spiral spin order in theRMnO3perovskites

The origin of the spiral spin order in perovskite multiferroic manganites $R{\text{MnO}}_{3}$ ($R=\text{Tb}$ or Dy) is here investigated using a two ${e}_{g}$-orbital double-exchange model. Our main result is that the experimentally observed spiral phase can be stabilized by introducing a relatively weak next-nearest-neighbor superexchange coupling ($\ensuremath{\sim}10%$ of the nearest-neighbor superexchange). Moreover, the Jahn-Teller lattice distortion is also shown to be essential to obtain a realistic spiral period. Supporting our conclusions, the generic phase diagram of undoped perovskite manganites is obtained using Monte Carlo simulations, showing phase transitions from the $A$-type antiferromagnet, to the spiral phase, and finally to the $E$-type antiferromagnet, with decreasing size of the $R$ ions. These results are qualitatively explained by the enhanced relative intensity of the superexchanges.

Magnetic order and ferroelectricity inRMnO3 multiferroic manganites: coupling between R- and Mn-spins

Combining polarized and unpolarized neutron scattering techniques with x-ray resonant magneticscattering we have studied the coupling between the Mn- and R-spin-ordering in the multiferroicRMnO3,R = Tb and Dy. Polarized neutron diffraction reveals the moment orientation associatedwith the various modes describing the complex magnetic ordering observed inTbMnO3, while neutron diffraction in high magnetic fields allows the identification of theorigin (Mn versus Tb) of the various modes. In this way we identify significantCx and Fz contributions from Tb arising from the coupling of Tb moments to the Mn cycloidal ordering.The x-ray studies give further insight into this coupling. In the ferroelectric phase, bothTbMnO3 and DyMnO3 show an induced ordering of the R-ion with a propagation vector clamped to the Mn ordering. Whilein TbMnO3 this clamping leads to a ground state in which the two propagation vectorsτTb andτMn obey therelation 3τTb−τMn = 1, in DyMnO3 the ferroelectric polarization is effectively enhanced. The theoreticalanalysis of these effects not only explains the observed behavior forR = Tb and Dy but can also be applied to understand the Mn–R interaction in the related compounds withR = Gd and Ho. Finally we show both experimentally and theoretically how the Mn–Rcoupling can enhance the ferroelectric polarization in this manganite’s multiferroics.

Spin lattice coupling in multiferroic hexagonal YMnO3

Aiming to shed light on the possible existence of hybrid phonon-magnon excitations in multiferroic manganites, neutron scattering measurements have been undertaken at LLB and ILL on the particular case of hexagonal YMnO3. Our experiments focused on a transverse acoustic phonon mode polarized along the ferroelectric axis. The neutron data show that below the magnetic transition, this particular phonon mode splits in two different branches. The upper branch is found to coincide with a spin wave mode. This manifestation of a strong spin-lattice coupling is discussed in terms of a possible hybridization between the two types of elementary excitations, the phonon and magnons.

Electronic Correlations Decimate the Ferroelectric Polarization of MultiferroicHoMn2O5

We show that electronic correlations decimate the intrinsic ferroelectric polarization of multiferroic manganites RMn2O5, where R is a rare earth element. Such is manifest from ab initio band structure computations that account for the Coulomb interactions between the manganese 3d electrons--the root of magnetism in RMn2O5. Including these leads to an amplitude and direction of polarization of HoMn2O5 that agree with experiment. The decimation is caused by a near cancellation of the ionic polarization induced by the lattice and the electronic one due to valence charge redistributions.

Multiferroic response and clamped domain structure in a two-dimensional spiral magnet: Monte Carlo simulation

The Monte Carlo simulation on multiferroic behaviors of the two-dimensional $\mathrm{Mn}{\mathrm{O}}_{2}$ lattice in multiferroic manganites is performed based on the model [I. A. Sergienko and E. Dagotto, Phys. Rev. B 73, 094434 (2006)] associated with the Dzyaloshinskii-Moriya interaction. The simulated ferroelectric polarization induced by the spiral spin ordering and its response to external magnetic field agree well with reported experimental observations. Furthermore, the coexistence of clamped ferroelectric domains and spiral spin domains is revealed in our simulation.

Electronic structures of hexagonalRMnO3(R=Gd, Tb, Dy, and Ho) thin films: Optical spectroscopy and first-principles calculations

We investigated the electronic structure of multiferroic hexagonal RMnO3 (R = Gd, Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations. Using artificially stabilized hexagonal RMnO3, we extended the optical spectroscopic studies on the hexagonal multiferroic manganite system. We observed two optical transitions located near 1.7 eV and 2.3 eV, in addition to the predominant absorption above 5 eV. With the help of first-principles calculations, we attribute the low-lying optical absorption peaks to inter-site transitions from the oxygen states hybridized strongly with different Mn orbital symmetries to the Mn 3d3z2-r2 state. As the ionic radius of the rare earth ion increased, the lowest peak showed a systematic increase in its peak position. We explained this systematic change in terms of a flattening of the MnO5 triangular bipyramid.

Coexistence and Coupling between Dielectric Order and Magnetic Order in Multiferroic Material of RMn2O5 (R=rare-earth, Bi, Y)

Microscopic magnetism and dielectric properties in the multiferroic rare-earth manganites of RMn2O5 studied by neutron diffraction and dielectric measurements are reviewed.It has been revealed that RMn2O5 shows successive dielectric and magnetic phase transitions, which concomitantly take place and are induced by temperature, magnetic field, and hydrostatic pressure.This coexistence and mutual coupling between dielectric and magnetic order, as well as the induced 4f magnetic moment on rare-earth ions, are ingredients essential for realizing a rich variety of colossal magnetoelectric effect.

Multiferroic phases ofEu1−xYxMnO3

We report on structural, magnetic, dielectric, and thermodynamic properties of (Eu:Y)MnO3 for Y doping levels 0 <= x < 1. This system resembles the multiferroic perovskite manganites RMnO3 (with R= Gd, Dy, Tb) but without the interference of magnetic contributions of the 4f-ions. In addition, it offers the possibility to continuously tune the influence of the A-site ionic radii. For small concentrations x <= 0.1 we find a canted antiferromagnetic and paraelectric groundstate. For higher concentrations x <= 0.3 ferroelectric polarization coexists with the features of a long wavelength incommensurate spiral magnetic phase analogous to the observations in TbMnO3. In the intermediate concentration range around x = 0.2 a multiferroic scenario is realized combining weak ferroelectricity and weak ferromagnetism, presumably due to a canted spiral magnetic structure.

Multiferroic BiFeO3 films: domain structure and polarization dynamics

BiFeO3 simultaneously shows antiferromagnetic and ferroelectric order with high transition temperatures, i.e. T N ∼ 370°C and T C ∼ 830°C, respectively. Naturally, it has been inferred that coupling exists between the magnetic and ferroelectric order parameters like in the multiferroic manganites with low transition temperatures. A thorough investigation of the ferroelectric properties of BiFeO3 is therefore in line with the understanding of its multiferroic behaviour. Here, we review the ferroelectric properties of epitaxial (001) oriented BiFeO3 films grown by different techniques on several substrates. Structural characterization along with ferroelectric quantitative analysis point at the high quality of the films. Emphasis is put on identifying the various polarization variants and domain dynamics under an applied bias. In these studies, to unravel the intricate ferroelectric domain structure, piezo-force microscopy scans have been taken along the principal crystallographic directions. Two cases have been analysed. First, a 600 nm thick film grown on SrTiO3 (001) with a thin SrRuO3 underlayer exhibits a mosaic domain pattern due to the presence of both up and down polarization domains. Mainly four polarization domains have been identified in this case, which correspond to two structural domains. Second, epitaxial BiFeO3 films grown on DyScO3 (110) and miscut SrTiO3 (001) with a thin SrRuO3 underlayer show stripe patterns, with mainly two down polarization domains. A single structural domain of orthorhombic SrRuO3 epitaxial underlayer induces this changes in the domain structure of BiFeO3. The suppression of up domains by changing the substrate conditions prove the possibility of ferroelectric domain engineering. The three possible polarization switching mechanisms, namely 71 and 109° rotations, as well as 180° rotation, have been identified by following the domain dynamics in a two-domain epitaxial BiFeO3 film. Interestingly, 180° polarization reversal seems to be the most favorable switching mechanism in epitaxial films under an applied bias along [001]. The observation of both ferroelastic and ferroelectric switching processes open exciting possibilities for the optimization of BiFeO3’s ferroelectric properties and investigation of magnetoelectric coupling in epitaxial films. A recent photoemission study using linearly polarized X-rays proved the coupling between the ferroelectric and antiferromagnetic domain structures.

Correlations between magnetic and electrical orderings in multiferroic manganites (invited)

The multiferroic rare-earth compounds RMnO3 with R=Ho–Yb are shown to be the source of “gigantic” magnetoelectric effects. Application of static magnetic or electric field induces a phase transition with antiferromagnetic reordering of the Mn3+ sublattice and ferromagnetic ordering in the rare-earth sublattices. An imbalance of the Mn3+–R3+ superexchange induced by the ferroelectric distortion is revealed as the microscopic origin of the magnetoelectric effect.

Possible evidence for electromagnons in multiferroic manganites

Magnetodielectric materials are characterized by a strong coupling of magnetic and dielectric properties and in rare cases simultaneously exhibit both, magnetic and polar order. Among other multiferroics, TbMnO3 and GdMnO3 reveal a strong magneto-dielectric (ME) coupling and as a consequence fundamentally new spin excitations exist: Electro-active magnons, or electromagnons, i. e. spin waves which can be excited by ac electric fields. Here we show that these excitations appear in the phase with an incommensurate (IC) magnetic structure of the manganese spins. In external magnetic fields this IC structure can be suppressed and the electromagnons are wiped out, thereby inducing considerable changes in the index of refraction from dc up to THz frequencies. Hence, besides adding a new creature to the zoo of fundamental excitations, the refraction index can be tuned by moderate magnetic fields, which allows the design of a new generation of optical switches and optoelectronic devices.

Magnetic field and external pressure control of ferroelectricity in multiferroic manganites

We have investigated dielectric properties in a series of crystals of RMnO 3 (R is a rare-earth ion) under magnetic fields and quasihydrostatic pressure. We have found that a ferroelectric phase appeared in GdMnO 3 crystal below 13 K. We have confirmed that a small spontaneous polarization exists along a-axis ( P a ) in the orthorhombic Pbnm setting and that P a can be reversed by the DC electric field. The dielectric anomaly due to the ferroelectric transition accompanied thermal hysteresis and lattice striction. The ferroelectric transition temperature decreased with quasihydrostatic pressure. These results indicate that the ferroelectric transition is improper and is of the first-order displacement type. P a was easily collapsed by the application of a magnetic field of 0.4 T parallel to the spin-canting direction ( H ∥ c ) while it was enhanced parallel to the easy axis ( H ∥ b ).

Magnetoelectric effects in multiferroic manganites

A variety of bulk as well as local magnetoelectric effects are observed in R MnO 3 compounds with R = Sc , Y , In , Ho , Er , Tm , Yb , Lu due to the coexistence of ferroelectric and multiple magnetic ordering. On the one hand, ‘gigantic’ magnetoelectric bulk effects, where magnetic phase control is exerted by applied electric or magnetic fields, are revealed. On the other hand, interaction of domain walls leads to a coupling of ferroelectric and antiferromagnetic domains, and a contribution to the linear magnetoelectric effect that is induced in the antiferromagnetic domain walls is identified. The effects are observed by linear and nonlinear magnetooptical techniques and explained microscopically by the interplay of magnetic exchange, wall magnetization, and ferroelectric distortion.