Session S31: Focus Session: Multiferroics I: 113 and 125

Dynamics and Phase Transitions in Multiferroic Helimagnets

The strong coupling between magnetism and ferroelectricity in multiferroics has recently been attracting much attention due to the fundamental physics involved and promising applications. The representative materials are helical magnets $R$MnO$_3$ ($R=$Gd,Tb,Dy) and they have been extensively studied experimentally. We theoretically study the dynamics and phase transitions in cycloidal helical magnets showing the multiferroic behavior. Our approach reproduces several novel features such as the anomalous dielectric response revealed by recent experiments on $R$MnO3 [1,2]. We also study the nature of the phase transition from collinear to helical spin structure. [1]N. Kida, Y. Ikebe, Y. Takahashi, J. P. He, Y. Kaneko, Y. Yamasaki, R. Shimano, T. Arima, and Y. Tokura, [arXiv:0711.2733]. [2]A. Pimenov, A. Loidl, A. A. Mukhin, V. D. Travkin, V. Yu. Ivanov, and A. M. Balbashov, [arXiv:0707.3614].   

Novel coupling of Tb- and Mn-magnetic orders in multiferroic TbMnO$_{3}$

We report on diffraction measurements on multiferroic TbMnO$_{3}$ which demonstrate that the Tb- and Mn-magnetic orders are coupled below the ferroelectric transition $T_{FE}=$28 K. For $T [Preview Abstract]

Electric polarization reversal in multiferroic TbMnO$_{3}$ with rotating magnetic field direction.

Recent extensive studies of magneto-electric multiferroics have revealed that the magnitude and direction of electric polarization can be considerably modified by the application of a magnetic field. TbMnO$_{3}$ is a prototypical multiferroic which shows the electric polarization flop from $P$//$c$ to $P$//$a$ by the application of magnetic field along the $b$ or $a$ axis. We have found that the direction of the magnetic-field induced polarization along the $a$-axis ($P_{a})$ is memorized even in the zero field where $P_{a}$ is absent. The polarization direction can be reversed by rotating the magnetic field direction in the \textit{ab} plane. For the memory application of the multiferroic material, such a bistability in zero field and a switching between the bistable states with some noneverlasting stimulus are essential.   

Spiral magnetic order in the ferroelectric phase of Gd$_{0.7}$Tb$_{0.3}$MnO$_3$

Perovskite manganite Gd$_{0.7}$Tb$_{0.3}$MnO$_3$ possesses a ferroelectric phase with an electric polarization along the a axis ($P||a$) in zero magnetic field, while $R$MnO$_3$ ($R$=Tb and Dy) undergo ferroelectric transitions with P along the c axis ($P||c$). The $P||a$ phase emerges upon the incommensurate to commensurate transition of the lattice modulation in a similar way of the magnetic field induced $P||a$ phase of TbMnO$_3$. The polarized neutron diffraction and the magnetic structure analysis of the $^{160}$Gd$^{3+}$-enriched single crystal of Gd$_{0.7}$Tb$_{0.3}$MnO$_3$ were performed to uncover the coupling between the magnetic order and the ferroelectric polarization $P||a$ on a microscopic level. We found that the ferroelectric transition occurs concomitantly with the collinear to spiral spin transformation and the spin helicity can be controlled by the electric field applied on cooling. Namely, the ferroelectric polarization in the $P||a$ phase can be explained by the spin current model as well as the $P||c$ phases known for TbMnO$_3$.   

Reentrant electromagnons in multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$ in the H-T phase diagram

The electromagnon spectra of Eu$_{0.75}$Y$_{0.25}$MnO$_3$ has been measured as a function of magnetic field $H\| c$ up to 8 T and temperature between 5 and 300 K. Three magnetic induced electric dipole features reported earlier\footnote{Valdes Aguilar, et al. PRB \textbf{76}, 060404(R) (2007)} are observed to weaken simultaneously but not shift for increasing field. These electromagnon features all show reentrant behavior as a function of temperature for $H >$ 6 T, and track with the anomalies in the static dielectric constant, confirming their electromagnon origin. While the magnetic structure of Eu$_{0.75}$Y$_{0.25}$MnO$_3$ is unknown, it is assumed that it is a cycloidal magnet where the spins lie in the crystallographic \textit{a-b} plane, based on the behavior of the magnetic susceptibility and the direction of static polarization \textbf{P}. Therefore, it appears that the electromagnon selection rule, $e\|$\textit{a}, in all the multiferroic $R$MnO$_3$ manganites is independent of the spin plane and polarization direction. We will compare the phase diagrams of Eu$_{0.75}$Y$_{0.25}$MnO$_3$ and TbMnO$_3$ where similar anomalies are observed.   

Ultrafast pump-probe reflectance study of multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$

Dynamical studies of multiferroic materials help unravel the fundamental interactions between various degrees of freedom and answer technological questions such as achievable switching speeds in multiferroic-based memory elements. We report the results of the ultrafast pump-probe reflectance study of multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$. The material undergoes antiferromagnetic ordering and, upon further cooling, ferroelectric ordering that strongly couples to the material's magnetic state. We measured the relaxation time of the pump-probe reflectance in this compound using 800-nm pump and probe pulses. The temperature dependence of the relaxation time follows that of the low-energy spectral weight that includes phonons and electro-active magnons [1]. This suggests a strong coupling between electronic (1.55 eV) and low-energy electro-active excitations in Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$ that can be tuned by magnetic field. The relaxation time increases upon the application of magnetic field along the crystal's $c$-axis in the ferroelectric phase, but exhibits no change in the paraelectric phase. Our results indicate the importance of multiple energy scales (electronic, lattice, and magnetic) for the multiferroicity of Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$. 1. R. Valdes Aguilar et al, Phys. Rev.B \textbf{76}, 060404(R) (2007)   

Theory of terahertz absorption spectra due to two-magnon processes in cycloidal spin magnets

Ferroelectric perovskite magnets $R$MnO$_{3}$ have an attraction both experimentally and theoretically after the discovery of the ferroelectric polarization and its flop by external magnetic field. Recently the measurements by the terahertz time-dominant spectroscopy have been investigated with the several sets of light polarization, and novel magnon states induced by the electric field are observed. Such a spin excitation is called an electromagnon. However, the origin of the electric-dipole active absorption is not clarified yet. We calculated the absorption of light due to the electric dipole transitions associated with two magnon excitations in cycloidal spin magnets. The theory is applied to the ferroelectric magnets $R$MnO$_{3,}$ and absorption peaks in terahertz time domain spectroscopy, which correspond to electromagnons, are interpreted with reasonable parameter sets.   

Resonant magnetic scattering of multiferroic HoMnO$_{3}$ in an applied electric field

The multiferroic material HoMnO$_{3}$ displays electrical polarization $P_{c}$~=~5.6~$\mu $C~cm$^{-2}$ along the hexagonal \textbf{c} axis below the Curie temperature $T_{C}$ = 875 K and antiferromagnetic Mn$^{3+}$ ordering at the N\'{e}el temperature, $T_{N}$~= 75 K. The recently reported ferromagnetic response of Ho$^{3+}$ by an applied electric field opens up the possibility of electric field controlled magnetic data storage. However, both the role of Ho$^{3+}$ in magnetism and details of the magnetic structure of Ho$^{3+}$ have been topics of significant debate. Using element specific x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, we have focused on resolving this controversy. Both quadrupole and dipole Ho $L_{III}$ resonances were observed below 40 K. In zero field, Ho$^{3+}$ orders antiferromagnetically with moments along the c direction below 40 K and undergoes a transition to a different magnetic order below 4.5 K. The role of Ho$^{3+}$ upon the application of an external electric field in the temperature range 1.7-80~K will be discussed. -- The support by U.S. DOE (DE-AC02-07CH11358 and DE-AC02-06 CH11357) is acknowledged.   

Enhanced ferroelectric polarization by induced Dy spin-order in multiferroic DyMnO$_{3}$

Neutron powder diffraction and single crystal x-ray resonant magnetic scattering measurements suggest that Dy plays an active role in enhancing the ferroelectric polarization in multiferroic DyMnO$_{3}$ above $T_{N}^{\rm Dy}$=~6.5~K. We observe the evolution of an incommensurate ordering of Dy moments with the same periodicity as the Mn spiral ordering. It closely tracks the evolution of the ferroelectric polarization. Below $T_{N}^{\rm Dy}$, where Dy spins order commensurately, the polarization decreases to values similar for those of TbMnO$_{3}$. The higher $P_{s}$~found just above $T_{N}^{\rm Dy}$~arises from the contribution of Dy-spins so as to effectively increase the amplitude of the Mn spin-spiral.   

Electrically driven spin excitation in a ferroelectric magnet DyMnO$_{3}$

In multiferroic manganites, there have been recent experimental and theoretical arguments on the possibility of the presence of the low-lying spin excitation, called $electromagnon$, where the spin excitation electrically becomes active [1]. Here we report on a complete set of low-energy (1--10 meV) electrodynamics of spin excitations for a multiferroic DyMnO$_3 $ with variations of the light polarization in a variety of phases tuned by both temperature (5--250 K) and magnetic field (0--70 kOe) [2]. We identify the pronounced absorption continuum (1--8 meV) with a peak feature around 2 meV, which is electric-dipole active only for the light $E$-vector along the $a$-axis. This absorption band grows in intensity with lowering temperature from the spin-collinear paraelectric phase above the ferroelectric transition, but irrespectively of the direction of the $bc$ or $ab$ spiral spin plane. The possible origin of this electric-dipole active band is argued in terms of the gigantic fluctuations of spins and spin-current. [1] A. Pimenov {\it et al.}, Nat. Phys. {\bf 2}, 97 (2006). [2] N. Kida {\it et al.}, arXiv:0711.2733   

Spin structures of magnetic phases in YMn2O5

A magnetic ferroelectric material, YMn2O5, undergoes several magnetic phase transitions at low temperatures and develops spontaneous electric polarization along the b-axis in a commensurate magnetic phase with a characteristic wave vector of (0.5,0,0.25). We have determined the commensurate spin structure by performing four circle neutron diffraction (FCD) and three-dimensional polarization analysis (CRYOPAD) on a single crystal of YMn2O5. In the spin structure, Mn4+ moments form a transverse (cycloidal) spiral structure along the c-axis that can induce the spontaneous electric polarization along the b-axis.   

Magnetic domains in multiferroic YMn$_$2O$_5$ probed by Spherical Neutron Polarimetry under electric field

Precise determination of the magnetic structures in multiferroics RMn$_2$O$_5$ (R: Y, Ho, Bi) have been obtained by single crystal neutron diffraction. The analysis shows the presence of zig-zag antiferromagnetic chains in the ab-plane. An additional weak magnetic component parallel to the c-axis was detected which is modulated in phase quadrature with the a-b components. The nature and population of the coexisting antiferromagnetic domains in YMn$_2$O$_5$ have been determined by Spherical Neutron Polarimetry under an external electric field. We have proved that reversing the electrical polarity results in the inversion of the population of two types of antiferromagnetic domains, with opposite in-plane spin components. This analysis strongly supports theories in which the coupling of the magnetic configuration to the ferroelectric polarisation is due to magnetic exchange striction and likely not related to the small cycloidal modulation in the bc-plane.   

Electromagnon H-T phase diagram of multiferroic TbMn$_2$O$_5$

We report the results of infrared (5--250 cm$^{-1}$) transmission study of multiferroic TbMn$_2$O$_5$ as a function of temperature (3--300~K) and H$\|$a for magnetic fields up to 8 T. Our major observation is that the main electromagnon feature softens without splitting with increasing field at T=5 K. This observation is in contrast with the gradual suppression of electromagnons without shifting by magnetic field in RMnO$_3$ compounds. Also, it is in agreement with the theoretical prediction of Fang and Hu \footnote{C. Fang and J. Hu, condmat/0703487} for this system. In this talk, we will also discuss other features of the magnetic field dependence of the low energy electromagnon excitations in multiferroic TbMn$_2$O$_5$.   

Far Infrared Anomalies in Orthorhombic Multiferroics (Bi,Pr)Mn$_{2}$O$_{5}$

We report on far infrared reflectivity between 4 K and 300 K of polycrystalline BiMn$_{2}$O$_{5}$ y PrMn$_{2}$O$_{5}$ known to sustain orbital, lattice, charge, and spin interactions. After conventional temperature mode stiffening band profiles undergo a relative intensity raise with maximum in the interval $\sim $40K to $\sim $30 K (magnetic ordering temperature, T$_{N}$, and the onset of ferroelectricity, T$_{C})$ indicative of a global electric polarization. We do not observe on cooling, in agreement with our high resolution neutron diffraction patterns, new phonon activity that might be associated to structural changes. Below 30 K there is a weak reflectivity attenuation at about the temperature in which the spin glass sets in. In contrast with BiMn$_{2}$O$_{5}$, in PrMn$_{2}$O$_{5}$ we found a Drude shaped overdamped band centered at zero frequency that, active at all temperatures, develops substructure below 15 K. We associate this feature to collective modes responsible for ferroelectricity of magnetic origin ascribed to coupling spins with electronic polarization without atomic displacement In overall, our spectra suggest a qualitative agreement with magnetoferroelectricity originating in spin dislocation and commensuration.   

Magnetic X-ray scattering in multiferroic HoMn2O5

The commensurate magnetic phase of multiferroic HoMn2O5 has been studied by x-ray magnetic scattering off resonance and at the L3 edge of Holmium. The magnetic ordering of the Manganese ions below 40 K induces a magnetic order of Ho. Due to the element selectivity of the technique we were able to extract the temperature dependence of the Ho ordering up to temperature very close to TN. Azimuthal scans confirm the recent model of the magnetic structure determined from single crystal neutron diffraction data, for both the Manganese and Holmium ions. The d-f coupling is discussed in the light of these results.