Session J30: Focus Session: Multiferroic Manganites

Order Parameters and Phase Diagram of Multiferroic RMn2O5

\def\rhov{{\mbox{\boldmath{$\rho$}}}} \def\tauv{{\mbox{\boldmath{$\tau$}}}} \def\Lambdav{{\mbox{\boldmath{$\Lambda$}}}} \def\sigmav{{\mbox{\boldmath{$\sigma$}}}} \def\xiv{{\mbox{\boldmath{$\xi$}}}} \def\chiv{{\mbox{\boldmath{$\chi$}}}} \def\oh{{\scriptsize 1 \over \scriptsize 2}} \def\ot{{\scriptsize 1 \over \scriptsize 3}} \def\of{{\scriptsize 1 \over \scriptsize 4}} \def\tf{{\scriptsize 3 \over \scriptsize 4}} Recently there has been great interest in systems which display phase transitions at which incommensurate magnetic order and a spontaneous polarization develop simultaneously. Perhaps the most puzzling and seemingly complicated behavior occurs in the series of compounds RMn$_2$O$_5$, where R=Y, Ho, Er, Tb, Tm, and Dy. (For references to experimental data, see [1].) The sequence of magnetoelectric phases of the type I systems R=Tb, Ho, and Dy is slightly different from that of the type II systems R= Y, Tm, and Er. At about 45K both types develop essentially collinear modulated magnetic order into a ``high-temperature ordered" (HTO) phase with a wave vector ${\bf q} = (1/2-\delta , 0, 1/4 + \epsilon)$ where $\delta$ and $|\epsilon|$ are of order 0.01 and the spontaneous polarization is zero. There is a lower-temperature phase transition to a ferroelectric phase in which transverse magnetic order appears and produces a magnetic spiral with $\delta=\epsilon=0$. In type I systems, this transition occurs directly from the HTO phase, whereas for type II systems, there is an intervening ferroelectric phase in which $\epsilon=0$, but $\delta$ remains nonzero. %At low ($<10$K) temperature the classification into types I and II %breaks down and each system requires its own specific description. I will discuss a Landau free energy[1] which allows both type I and type II sequences of phase transitions. This theory is couched in terms of the uniform polarization vector ${\bf P}$ and two complex-valued magnetic order parameters $\sigma_1({\bf q})$ and $\sigma_2 ({\bf q})$ whose symmetry follows from the magnetic structure analyses.[2] The magnetoelectric coupling and the competition between commensurate and incommensurate phases are analyzed. \\[4pt] [1] A. B. Harris, A. Aharony, and O. Entin-Wohlman, Phys. Rev. Lett. {\bf 100}, 217202 (2008) and J. Phys. Condens. Mat. {\bf 20}, 434202 (2008). \\[0pt] [2] A. B. Harris, Phys. Rev. {\bf 76}, 054447 (2007); A. B. Harris, M. Kenzelmann, A. Aharony, and O. Entin-Wohlman, Phys. Rev. B {\bf 78}, 014407 (2008).   

Local Structure Investigation of ReMn$_{2}$O$_{5}$

The temperature dependent structure of the ReMn$_{2}$O$_{5}$ (Re=rare earth) system has been examined by the x-ray pair distribution function method based on high-q data. Temperature dependent measurements reveal anomalies in the short range structure involving oxygen atoms. Comparison with Rietveld and XAFS analysis will be made. The detailed temperature dependent structure on multiple length scales will be presented with implications for the observed low temperature ferroelectric properties. This work is supported by DOE Grant DE-FG02-07ER46402.   

Magnetic Field Dependent Changes in the Local Structure of ReMn$_{2}$O$_{5}$

The low temperature structure of ReMn$_{2}$O$_{5}$ systems has been studied by x-ray absorption spectroscopy. Temperature dependent measurements indicate that Re-O correlations play an important role in the low temperature properties. While no variation in the Mn-O distribution is observed with magnetic fields, we find evidence for direct coupling of the Re-O distribution to an external magnetic field. The results suggest that polarization of the Re-O bonds may contribute significantly to magnetic field induced electrical polarization. This work is supported by DOE Grant DE-FG02-07ER46402.   

Studies of Competing Order in Multiferroic RMnO$_{3}$

Unconventional magnetic order, a ferroelectric background and the possibility of coupled ground states, together with competing spin, charge, lattice and orbital degrees of freedom, give systems such as $R$MnO$_{3}$ ($R$=Rare Earth), an ABO$_{3}$-type compound, a rich and fascinating phenomenology. The possibility of using these materials in switching, in spin based electronics, and as materials with negative refractive index, make them important candidates for device applications. Here we present a review of our studies of the detailed phenomenology of a series of single crystals of $R$MnO3 grown from a floating zone, for different Rare Earths $R$, and by inducing structural distortions and charge disproportionation through substitutions at both the A and the B sites.   

Enhanced local lattice distortions with the antiferromagnetic transition in the multiferroic LuMnO$_{3}$

The ferroelectric hexagonal manganite, LuMnO$_{3}$, has been investigated via neutron scattering and the pair density function analysis to determine the nature of the local atomic distortions with the antiferromagnetic transition, T$_{N}$, of the Mn ions. While in previously reported neutron diffraction data, it was shown that all atomic coordinates changed based on symmetry considerations with T$_{N}$, we hereby show that it is the ferroelectric motion of the Lu ions coupled with O distortions that exhibits a strong temperature dependence below T$_{N}$ as reflected in the Lu-O bonds. This suggests an enhancement of the net electric polarization below T$_{N}$. At the same time, the motion of the apical O1 and O2 ions distorts the MnO$_{5}$ bipyramids, leading to more buckling of the ab-layers. However, the Mn ions do not appear to distort significantly away from their equilibrium position. The oxygen distortions induced with the spin reorientations below T$_{N}$ may be the cause for the Lu ion displacements through electrostatic interactions and this in turn produces coupling to the electric dipole moments.   

Pressure Dependence of Structure Stability of Multiferroic Hexagonal-RMnO$_{3}$

We present high pressure IR and X-ray diffraction measurements of the hexagonal multiferroic systems HoMnO$_{3}$, YMnO$_{3}$ and LuMnO$_{3}$. Measurements were conducted over the pressure range ambient to $\sim $20 GPa. No phase changes were observed over this broad range of hydrostatic pressures. These suggest that the hexagonal structure is stable at higher pressures. The thermal treatment is necessary to overcome the barrier (breaking and reconnection of bonds) to achieve the hexagonal to orthorhombic phase change. A discussion of the effect of hydrostatic pressure on the ferroelectric properties of these systems will be given based on comparisons with density functional calculations.   

X-ray absorption spectroscopy studies of YMnO$_{3}$, HoMnO$_{3}$, and Y$_{.4}$Ho$_{.6}$MnO$_{3}$

We have investigated three hexagonal perovskites, YMnO$_{3}$, HoMnO$_{3}$, and Y$_{.4}$Ho$_{.6}$MnO$_{3 }$by O K$_{1}$ and Mn L$_{2,3}$ edge X-ray absorption spectroscopy. In YMnO$_{3}$ and HoMnO$_{3}$ the lowest energy features are predominantly Mn 4p and 3d states with a least five distinct states occurring at approximately the same X-ray energies in both samples. We associate this portion of electronic structure with the trigonal bipyramid bonding symmetry of a five-fold coordinated Mn. Higher energy transitions in the XAS OK$_{1}$ spectra are broader and associated with Ho 5d and Y 4d orbitals. Compared with YMnO$_{3}$ and HoMnO$_{3 }$Mn 3d, and Ho 5d and Y 4d spectral features, the corresponding features in theY$_{.4}$Ho$_{.6}$MnO$_{3}$ O K$_{1}$ spectrum exhibit broader features fewer in number. These are consistent with random alloy bonding in which the Ho and Y are randomly distributed on the A-atom sub-lattice. We will discuss the electronic structure of these empty states in the context of symmetry adapted linear combinations of molecular orbital O 2p*, and Mn 3d*, Ho 5d* and Y 4d* nearest neighbor states.   

Competing Magnetic Interactions in Magnetoelectric YbMnO$_{3}$

The (\textit{RE)}MnO$_{3}$ (\textit{RE }= \textit{Rare Earth}) series of magnetoelectrics exist as both hexagonal and orthorhombic lattice structures. These have recently attracted much attention due to possible applications in spintronics, in switching, and as media with negative refractive index. YbMnO$_{3}$ is hexagonal with ferroelectricity (Tc$\sim $ 970K) and antiferromagnetism (T$_{N}$: Mn$\sim $80K, Yb$\sim $5K) in the same phase. Here, we report detailed studies of the H-T phase diagram using a high-quality single crystal of YbMnO$_{3}$ grown by floating zone. We examine the magnetically ordered phases of Yb: Yb3+ (2$a)$ via Yb-Yb, and Yb3+ (4$b)$ via Yb-Mn interactions within the hexagonal YbMnO$_{3}$ structure, and report several new features in the magnetic phase diagram. Contrary to recent reports, we observe that the magnetic moment of Yb does not become fully suppressed with external magnetic field, but rather directly transitions from the A$_{1}$ order into the A$_{2 }$order at low temperature.   

Resonant Soft X-ray Scattering Study on Multiferroic TbMnO$_{3}$

TbMnO$_{3}$ has been extensively studied both experimentally and theoretically about its fascinating properties (e.g., magneto-electric coupling, spiral magnetic order). Comprehensive resonant x-ray scattering at absorption edges of Mn and Tb, mainly at Mn $L$-edge, were performed on single crystals of TbMnO$_{3}$ to understand the fascinating properties. Under \textit{Pbnm} space group of TbMnO$_{3}$, we found the forbidden reflections, such as (0 q 0), (0 1-2q 0), and (0 2q 0). Each reflection was also investigated by dependency on temperature, photon energy, photon polarization, and etc, which gives us a clue to unveil hidden properties of TbMnO$_{3}$. Detailed description will be dealt with this presentation.   

High Temperature XPS Studies of a Single Crystal of Magnetoelectric TbMnO$_{3}$

Recent interest in magnetoelectric and unconventional magnetic phenomena in materials such as the RMnO$_{3}$ series (R=Rare Earth) reveals competing electronic ground states, together with electronic and crystal structure phase transitions both above and below room temperature[1]. A complete elucidation of these phenomena calls for detailed studies of electronic properties at varying temperatures. Using a new high temperature insert with a resistive heater stage fitted to a Cylindrical Mirror Analyzer at the Synchrotron Radiation Center in Stoughton, WI, we have performed x-ray photoemission studies at variable temperatures (200-1000K) in a large single crystal of magnetoelectric orthorhombic TbMnO$_{3}$ (0.5 x 0.5 x 1cm) grown by us from a floating zone. We observe peak splitting in the Mn 3p and Tb 4d peaks near a structural distortion observed by us. Together with resistivity and detailed Rietveld analysis of powder x-ray diffraction, we ascribe this to possibly an orbital order-disorder transition around 900K. [1]J.S.Zhou,Phys.Rev.Let. 96,247202(2006)   

Emergence of the multiferroic state in $R$MnO$_{3}$ ($R$ = Sm and Nd) crystals

In order to understand the emergence of multiferroic behaviour in the $R$MnO$_{3}$ compounds, it is educational to study the relationship between ferroelectricity and magnetoelastically induced lattice modulations. Lattice modulations in $R$MnO$_{3}$ are strongly dependent on the Mn-O-Mn bond angle ($\Phi )$, which in turn is determined by the ionic radii ($r_{R})$ of the $R$ atoms. Multiferroic properties have been observed in the orthorhombic RMnO$_{3}$ (R = Tb,Dy) compounds, in which $\Phi $ is close to 145\r{ }. In order to induce multiferroic behaviour in other magnetic members of the orthorhombic $R$MnO$_{3}$, and to tune the structure to be in the same region of the phase diagram as Tb/DyMnO$_{3}$, it is necessary to substitute at the $R$ site with a suitable (smaller) atom. We have achieved this in SmMnO$_{3}$ and NdMnO$_{3}$ by substitutions at the Sm and Nd sites with smaller $R$ ions. In the optimally substituted compounds (40 to 50{\%}), we observe an additional magnetic transition. Investigations of the dielectric properties of the crystals reveal anomalies in the dielectric properties coincident with this magnetic transition, analogous to those exhibited by Tb/DyMnO$_{3}$, indicative of multiferroic behaviour. We present detailed investigations of the magnetic, dielectric and structural properties in single crystals of selected compositions.