Session A17: Focus Session: Bulk Properties of Complex Oxides - Manganites I

Enhanced magneto-elastic coupling in hexagonal multiferroic HoMnO$_3$

From ultrasonic velocity measurements, we report a study of the magneto-elastic coupling occurring on elastic moduli C$_{11}$ and C$_{33}$ at the different magnetically induced phase transitions in HoMnO$_3$. Although both the Ho-Mn and Ho-Ho interactions soften the the elastic moduli, the largest softening in observed on C$_{11}$ over a wide temperature range extending well beyond the N\'eel temperature. An in-plane orientation of the magnetic field reduces strongly the softening due to a stabilization of the Mn moments order; concurrently, the Ho magnetic order is destroyed. When the field is rather oriented along the $c$ axis, the elastic softening is enhanced as if the Ho-Mn interactions were reinforced and the Mn order consequently destabilized. The phase diagram deduced from the elastic anomalies observed at the several phase transitions are in agreement with microwave measurements performed on the same sample. An in-plane anisotropy of the diagram is also proposed.   

Multiferroics in Eu$_{1-x}$Tb$_x$MnO$_3$ system

A low-$T$ phase diagram of the Eu$_{1-x}$Tb$_x$MnO$_3$ (0 $\leq$ x $\leq$ 1) is reported. Systematic substation of Tb into the system changes the perovskite lattice structure which further varies the electronic and magnetic behaviors of the system from a paraelectric-canted-AFM to a ferroelectric-spiral-AFM ground state. The Mn$^{3+}$ spins ordered, presumably, in a collinear incommensurate sinusoidal antiferromagnetic structure below $T_N$ = 52-45 K (x = 0 to 1). Then system enters a canted-AFM (weak- ferromagnetic) state below $T_{cant}$ for the $x \leq$ 0.5 compounds, which decreases from 42 K to 25 K with increasing $x$. For the $x \geq$ 0.5 compounds, ferroelectricity was found below $T_C \sim$ 28 K with a presumably spiral spin arrangement as that in TbMnO$_3$. At the boundary, $x$ = 0.5, the multiferroics coexists with the weak-ferromagnetism. The Rietveld refinement shows an Mn-O2-Mn angle of 145.9$^\circ$ for the Eu$_{0.5}$Tb$_{0.5}$MnO$_3$ suggesting a critical Mn-O2- Mn angle of $\sim$146$^\circ$ that multiferroics appears at the smaller angle side.   

Ferromagnetically charge ordered nanoclusters in La$_{0.52}$Ca$_{0.48}$MnO$_{3}$

A charge-ordered (CO) nanoscale phase was reported to appear in coincidence with the well known colossal magnetoresistance (CMR) in a wide doping range in manganites. The competition between the CO nanoscale phase and the surrounding ferromagnetic (FM) phase has been considered as the key to understand the CMR phenomenon. However, the role of this nanoscale phase in the CMR effect is not fully established because the magnetic and physical properties of the CO nanoscale phase remain elusive. In particular, the CO nanoscale phase was hypothesized to be antiferromagnetic, the same as its long range counterpart. Here we report the experimental evidences showing the unexpected magnetism and resistivity in the CO nanoclusters in La$_{0.52}$Ca$_{0.48}$MnO$_{3}$. Correlated with a number of bulk property measurements, the transmission electron microscopic observations strongly suggest that the CO nanoclusters are FM and probably conducting. Such results could substantially alter the role of the CO nanoclusters in the CMR.   

Structural and Magneto-electric properties of substituted $R$MnO$_{3}$ crystals ($R$=\textit{Sm, Gd)}

In order to understand the emergence of multiferroic behaviour in the $R$MnO$_{3}$ type compounds, it is educational to study the relationship between ferroelectricity and magnetoelastically induced lattice modulations. The Mn-O-Mn bond angle is a crucial parameter in these systems and it varies with the ionic radii ($r_{R})$ of the $R$ atoms. Multiferroic behaviour may be induced in large $R $systems by substituting the $R$ site with a smaller ion (e.g. \textit{Y, Lu}). We have studied the effect of substituting $Y$ in \textit{Sm}MnO$_{3}$ and \textit{Lu} in \textit{Gd}MnO$_{3}$ respectively. In the optimally substituted compounds, we observe a strong coupling between the magnetic and dielectric properties. We have investigated the local structural distortions in the MnO$_{6}$ octahedra in both these systems using single crystal X-ray studies. Additionally, neutron powder diffraction has been used to investigate the nature of the low temperature magnetic ordering in the Sm system. Investigations of the dielectric properties of the $Y$ and \textit{Lu} substituted crystals reveal anomalies in the dielectric properties coincident with an additional magnetic transition, indicative of multiferroic behaviour. We present detailed investigations of the magnetic, dielectric and structural properties on single crystals of selected compositions.   

Synthesis and Oxygen Content Dependent Properties of Hexagonal Manganites

Oxygen deficient samples of hexagonal (P6$_{3}$cm) DyMnO$_{3+\delta }(\delta $=-0.04) were synthesized in Ar by intentional decomposition of the perovskite phase obtained in air. Hexagonal samples annealed under oxidizing conditions exhibit unusually large excess oxygen content ($\delta <$0.4) and two new structural phases below 350\r{ }C. We will demonstrate how structural, resistive, magnetic, and thermal expansion properties are sensitively dependent $\delta $. Similar observations were made for other hexagonal manganites RMnO$_{3+\delta }$ indicating that their multiferroic properties can be controlled by the synthesis and annealing conditions.   

Spin and Lattice excitations in Ferromagnetic Insulating Manganites

Though double-exchange interaction has been recognized as a major driving force for the couple magnetic and electronic phase transition, the nature of insulating ground state with ferromagnetic ordering in low-doping manganites is still not fully understood. Here we report on an inelastic neutron scattering study of spin and lattice excitations in the ferromagnetic insulating (FMI) phase of La$_{1-x}$Ca$_{x}$MnO$_{3}$ with x(Ca) = 0.2. Dispersion relations for both phonons and spin waves along high-symmetry directions were obtained for temperatures of 5 and 225 K, respectively. At low temperatures, our results indicate an anomalous softening and broadening of the magnons near the zone boundary, especially when the magnon energy E $\sim $ 20 meV, where a longitudinal optical phonon is present. Additional phonon and magnon branches observed will also be discussed.   

X-ray Absorption Spectroscopy studies of photo-induced and magnetic-field-induced phase transitions in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$

Changes in the electronic structure underpinning the ultrafast photo- and magnetic-field-induced insulator to metal phase transition in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ are compared directly via x-ray absorption near edge spectroscopy (XANES). Static and time-resolved XANES at the O K-edge and Mn L-edge directly monitor the evolution of the density of Mn-3d/O-2p electronic states as the system is driven across phase boundaries. Our results reveal the non-thermal nature of the photoinduced phase transition and show that the CMR magnetic-field-induced and the photoinduced phase-transitions rely on identical rearrangements of the electronic structure.   

Striped Multiferroic Phases in Narrow Bandwidth Hole-Doped Manganites

A novel phase with diagonal charge stripes and a complex spin arrangement that allows for ferroelectricity to develop has been recently reported in a model for hole-quarter-doped manganites (S. Dong et al., Phys. Rev. Lett. {\bf 103}, 107204 (2009)). The study of this ``spin-orthogonal stripe'' (SOS) phase is here generalized to other hole doping fractions $x=1/N$ ($N=3$, $5$, $6$, ...), to search for analogous multiferroic states. In this effort, the two-orbital double-exchange model for manganites is studied, employing variational, Monte Carlo, and zero temperature optimization techniques. The phase diagrams obtained by varying the electron-lattice and superexchange couplings also contains exotic C$_{x}$E$_{1-x}$ phases. A systematic procedure to construct new C$_{x}$E$_{1-x}$/SOS$_x$ phases is discussed. Both the Dzyaloshinskii-Moriya interaction and exchange-striction effect may work in these C$_{x}$E$_{1-x}$/SOS$_x$ phases, giving rise to ferroelectricity. In addition, these SOS$_x$/C$_{x}$E$_{1-x}$ phases can be extended into many other similar states, with (almost) degenerate energies but different multiferroic properties.   

Magnetic imaging of domains and walls in multiferroic ErMnO$_{3}$

Multiferroic hexagonal rare-earth manganities $R$MnO$_{3}$ ($R$ = Ho{\ldots} Lu, Y, Sc) have generated great interest because of the coexistence of ferroelectric and magnetic orders. Herein we conducted low temperature magnetic force microscopy (LT-MFM) studies on flux-grown ErMnO$_{3}$ single crystals. The ferroelectric transition T$_{C}$ is $\sim $1300 K while antiferromagnetic transition T$_{N}$ is $\sim $ 80 K. We observed intriguing behaviors of magnetic domains {\&} walls in ErMnO$_{3}$ from the temperature and magnetic field dependence of local magnetic contrast. In addition, we will present results of comparison between LT-MFM images and room temperature piezoresponse force microscopy (PFM) images of the same sample to understand the mechanism of cross-coupling between ferroelectricity {\&} magnetism in $R$MnO$_{3}$.   

Multiferroic Perovskite Manganites with Symmetric Exchange Striction

Orthorhombic perovskite manganites have been extensively studied as a representative system hosting versatile multiferroic phases such as the cycloidal spin phase and the $E$-type antiferromagnetic phase with an exchange striction mechanism. Recently, the latter phase has been the subject of growing interest for a potentially giant polarization as large as 60000 $\mu $C/m$^{2}$, which might involve a significant contribution from the orbital polarization.\footnote{S. Picozzi \textit{et al.}, Phys. Rev. Lett. \textbf{99}, 227201 (2007).} However, while several groups have reported ferroelectricity in this phase, further experimental progress on the clarification of the multiferroic properties and the microscopic mechanism has been hampered by the difficulty in sample preparation. In this talk, we report a series of multiferroic perovskite $R$MnO$_{3}$ with $R$ = Dy-Yb, Eu$_{1-x}$Y$_{x}$ and Y$_{1-y}$Lu$_{y}$, synthesized under high pressure and show the complete phase diagram.\footnote{S. Ishiwata \textit{et al}., Phys. Rev. B \textbf{81}, 100411(R) (2010).}$^,$\footnote{Y. Takahashi \textit{et al}., Phys. Rev. B \textbf{81}, 100413(R) (2010).} The magnitude of the polarization in the $E$-type phase was estimated to be about 5000 $\mu $C/m$^{2}$ (10 times larger than that of the \textit{bc}-cycloidal phase) and an enhanced magnetoelectric response was discovered near the first-order phase boundary. Furthermore, we have succeeded in synthesizing single crystals of perovskite YMnO$_{3}$ under a high pressure and succeeded in structure refinements for the $E$-type phase with a polar space group of $P$2$_{1}$\textit{nm}.\footnote{D. Okuyama \textit{et al.}, manuscript in preparation.} This work demonstrates for the first time the quantitative estimation of ferroelectric lattice displacements induced by a magnetic order. This work was done in collaboration with D. Okuyama, Y. Kaneko, Y. Takahashi, H. Sakai, K. Sugimoto, K. Yamauchi, S. Picozzi, Y. Tokunaga, R. Shimano, Y. Taguchi, T. Arima and Y. Tokura, and in part supported by JSPS FIRST program.   

Dynamical Magnetoelectric Phenomena in the Multiferroic Mn Perovskites

Electric manipulation of magnetic structures is an urgent issue in the field of spintronics. Concurrently magnetic and ferroelectric materials, i.e., multiferroics offers a promising route to attain this goal, and its dynamical aspects are now attracting a great deal of interest. In this talk, we will discuss the recent progress of theoretical study on the dynamical magnetoelectric phenomena in the multiferroic Mn perovskites $R$MnO$_3$ ($R$=Tb, Dy, Eu$_{1-x}$Y$_x$, ...). In these materials, a spiral order of the Mn spins induces spontaneous electric polarization through breaking the inversion symmetry, and thus the strong coupling between electric and magnetic dipoles is realized. Using an accurate spin Hamiltonian describing $R$MnO$_3$, we first study the electromagnon excitation in these materials at THz frequencies, i.e., collective motion of spins with oscillating electric dipoles activated by the electric-field component of light. The optical spectra with two specific peaks are explained by a symmetric magnetostriction model for the spiral spin order with higher harmonic components. After clarification of its mechanism and nature, we then study the nonlinear dynamical processes of magnetic system caused by the intense electromagnon excitations through the optical pumping. The excitation by the electric field can be more intense and faster than that by the magnetic field. This necessarily leads to novel and intriguing phenomena which can never be expected in the conventional magnetic-field-induced magnon excitation. As one of the most interesting phenomena, we will theoretically propose a picosecond optical switching of spin chirality in $R$MnO$_3$. We will demonstrate that by tuning strength, shape and length of the optical pulse, we can control the spin chirality at will. This proposal will pave a new way to control the magnetism in the picosecond/THz time domain.\\[4pt] [1] M. Mochizuki, N. Furukawa and N. Nagaosa, PRL {\bf 104}, 177206 (2010)\\[0pt] [2] M. Mochizuki and N. Nagaosa, PRL {\bf 105}, 147202 (2010)\\[0pt] [3] M. Mochizuki and N. Furukawa, PRB {\bf 80}, 134416 (2009).