Session Q9: Focus Session: Complex Bulk Oxide: Doped and Undoped Manganites

Weak ferromagnetism in lightly electron-doped CaMnO$_3$

The origin of the weak ferromagnetism, that is observed in lightly electron-doped CaMnO$_3$, is studied by means of the non-collinear spin density functional theory, including the spin-orbit interaction. We show that the spin-canting in the G-type antiferromagnetic structure is realized by the electron-doping, and the canting angle becomes larger with increase of the doping amount. The estimated canting angle is in a good agreement with the experimental value of the Ce-doped CaMnO$_3$ [1] The spin-canted state is stabilized by the double-exchange interaction, and the spin-orbit interaction does not play a crucial role in this phenomenon. In addition, the spin-canted state shows metallic behavior by the double-exchange transfer that gives a reasonable interpretation for the experimental electronic transport property [2] We also clarify a possibility of the antiferromagnetic-ferromagnetic phase separation [3] that will be realized when doped-electrons are strongly localized.\\[4pt] [1] E.N. Caspi et al., Phys. Rev. B 69, 104402 (2004).\\[0pt] [2] P.-H. Xiang et al., Appl. Phys. Lett. 94, 062109 (2009).\\[0pt] [3] E. Dagotto et al., Phys.Rep. 344, 1 (2001).   

Synthesis, Structural and Magnetic Characterization of Rare Earth Doped SrMnO$_{3}$ Compounds

We report on our ongoing work on the synthesis, characterization and magnetic studies of Rare Earth (RE) containing RE-SrMnO$_{3}$ compounds. The aim of the study is to determine how different RE elements influence the magnetic properties of said materials. We have so far investigated the incorporation of Dy and Yb into the SrMnO$_{3}$ unit cell. To this effect we have employed solid state reaction synthesis methods to fabricate said compounds. The reaction products evolution were monitored as a function of time by XRD, TGA and SEM. Measurements of lattice parameters and Rietveld refinement of the XRD spectra, indicate that Dy and Yb are incorporated substitutionally in SrMnO$_{3}$. Temperature dependent magnetic measurements, on the other hand, reveal a common transition temperature around 41 K for both Dy and Yb doped SrMnO$_{3}$. We are in the process of synthesizing additional materials containing additional RE elements to investigate how the electronic properties of said RE may influence the magnetic properties of these compounds.   

Far-IR spectra of magnons, crystal field transitions, and phonons in hexagonal \textit{RE}MnO$_{3}$ (\textit{RE}=Er, Tm, Yb, Lu) single crystals

Far-IR spectra of hexagonal \textit{RE}MnO$_{3}$ (\textit{RE}=Er, Tm, Yb, Lu) single crystals have been studied between $T$=1.6 K and 300 K using transmission in high magnetic field and rotating analyzer ellipsometry. The symmetry of the IR optical phonons and their oscillator strengths were determined for compounds with different \textit{RE} ions. The temperature dependence of the phonon frequencies revealed a strong spin-phonon interaction in the temperature range below $T_{N}\sim $70 K. The effective g-factors have been determined for the AFM resonances and crystal field transitions using external magnetic fields up to 10 T. The frequency of the AFM resonances around 50 cm$^{-1}$ increases systematically with a decrease of the \textit{RE} ion radius. The observed effects are analyzed taking into account main magnetic interactions in the system including exchange of the Mn$^{3+}$ spins with \textit{RE}$^{3+}$ paramagnetic moments. The magnetic ordering of \textit{RE} ions was observed at low temperatures $T<$3.5 K and in strong magnetic fields.   

Direct observation of collective magnetism at ferroelectric domain walls in multiferroic ErMnO$_{3}$

Multiferroic hexagonal manganities \textit{RE}MnO$_{3}$ (\textit{RE} = Ho, Er, Lu, etc.) have been of great interest because of the coexistence of ferroelectric and magnetic orders. Herein we report cryogenic magnetic force microscopy (MFM) studies of flux-grown ErMnO$_{3}$ single crystals with vortex ferroelectric domain pattern. By uniquely correlating ambient piezo-response force microscopy and low temperature MFM images at the same location, we identified alternating uncompensated magnetic moments at ferroelectric domain walls that correlate over entire vortex network, suggesting collective magnetism at ferroelectric vortex domain walls.   

Maximally localized Wannier functions in LaMnO$_3$ within PBE$+U$, hybrid functionals, and GW: an efficient route to construct ab-initio tight-binding parameters for $e_g$ perovskites

Using the newly developed VASP2WANNIER90 interface we have constructed maximally localized Wannier functions [1] (MLWFs) for the $e_g$ states of the prototypical Jahn-Teller magnetic perovskite LaMnO$_3$ at different levels of approximation for the exchange-correlation kernel. These include conventional density functional theory (DFT) with and without additional on-site Hubbard $U$ term, hybrid-DFT, and single shot GW. By suitably mapping the MLWFs onto an effective $e_g$ tight-binding (TB) Hamiltonian [2,3] we have computed a complete set of TB parameters providing the band dispersion in excellent agreement with the underlying {\em ab initio} and MLWF bands. The method-dependent changes of the TB parameters and their interplay with the electron-electron interaction term are discussed and interpreted, outlining a guidance for more elaborate treatments of correlation effects in effective Hamiltonian-based approaches. \newline [1]\,I.\,Souza,\,N.\,Marzari,\,and\,D.\,Vanderbilt,\,Phys.Rev.B\,65,\,035109\,(2001).\newline [2]\,R.\,Kov\'a\v{c}ik\,and\,C.\,Ederer,\,Phys.Rev.B\,81,\,245108\,(2010).\newline [3]\,R.\,Kov\'a\v{c}ik\,and\,C.\,Ederer,\,Phys.Rev.B\,84,\,075118\,(2011).   

Reverse Monte Carlo Modeling of Pair Distribution Function Data as a Tool for Separating the Coordination Environments of Multiple Atoms Disordered Over a Single Site

The local structures of 8 perovskite compounds which contain equal concentrations of 2 transition metal cations disordered over the $B$-sites have been investigated using reverse Monte Carlo (RMC) modeling of neutron pair distribution function (PDF) data. Such compounds are known to display a number of interesting magnetic and electronic properties which, due to the cation disorder, cannot be correlated with the average long range structure and so remain poorly understood. In compounds with $B$=Mn/Ru there exists a valence degeneracy between Mn$^{3+}$/Ru$^{5+}$ and Mn$^{4+}$/Ru$^{4+}$. We demonstrate that the RMC method can be used as an effective tool to separate out the individual coordination environments of these cations and also to monitor the relative concentrations of the different oxidation states. We find that the valency ratio is governed by the size of the $A$-site cations. In a different series of Sr$_{2}$FeMnO$_{6-x}$ perovskites we find that locally the structures are quite different from the average cubic structures, with the local coordination environments more closely resembling those of the brownmillerite structure. In all compounds the octahedra containing Mn$^{3+}$ are Jahn-Teller distorted, even if this distortion is not evident in the average structure.   

Direct Visualization of Electric-Field-Driven Migration and Decay of Oxygen Vacancy-induced Stripes in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$

We report on the microscopic evidence of electric-field driven migration and decay of oxygen vacancy stripes in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO30). A local lattice stripe phase associated with oxygen vacancy migrating along the applied electric field was imaged in real time by using \textit{in-situ} imaging with high-resolution transmission electron microscopy (TEM). Such a field-driven dynamic oxygen migration process should be responsible to the transport for the resistance switching effects observed in many metal-oxide-metal structures, thus providing a direct microscopic evidence for the oxygen migration model. A decay of oxygen vacancy stripes with a characteristic decay time has been observed, consistent with measurement of resistance relaxation in the materials.   

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

Time resolved dynamical studies of multiferroic materials help unravel the fundamental interactions between various degrees of freedom. We report an 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 pump-probe reflectance in this compound using 400- and 800-nm pump and probe pulses. We found that the amplitude of the photoinduced reflectance increases dramatically with the development of local and long-range spin-spin correlations as the temperature is lowered toward the magnetic ordering transition. We also observe a dramatic increase in the rise time, up to 10s of picoseconds, of the photoinduced reflectance. This time scale is consistent with the long response times of the spin system in manganites. We suggest that the modification of the local exchange coupling around the photoinduced electrons and holes is responsible for the observed reflectance behavior, as the optical properties of manganites are known to couple strongly to the local and long-range spin correlations [1]. \\[4pt] [1] N.N. Kovaleva et al., Phys. Rev. Lett. \textbf{93}, 147204 (2004)   

Multiple Magnetic Transitions and Magnetocaloric Effect in (La,Pr,M)MnO$_{3}$ (M = Ca, Sr, Ba) Mixed Phase Manganites

The manganite compound (La,Pr,Ca)MnO$_{3}$ is a well-studied system that is known to exhibit a complex phase diagram featuring ``strain liquid'' and ``strain glass'' regions in combination with competing ferromagnetic (FM) and charge-ordered antiferromagnetic (CO/AFM) phases. The balance of these phases is sensitive to various perturbations including magnetic and electric field, strain, bandwidth, and A-site cation disorder. The A-site disorder and bandwidth of this compound can be tuned through the replacement of Ca with larger Sr and Ba ions. We report here a systematic study of the influence of cation substitution on the magnetic and magnetocaloric properties of La$_{0.35}$Pr$_{0.275}$M$_{0.375}$MnO$_{3}$ (M = Ca, Sr, Ba). Structural properties, including lattice parameters and Mn--O--Mn bond angles, were determined from X-ray diffraction patterns. DC magnetometry studies reveal multiple magnetic transitions in each sample which are probed by magnetocaloric effect (MCE) and transverse susceptibility (TS) experiments. Increasing the average A-site cationic radius is found to strongly impact the magnetic properties and phase behavior of the system.   

Evolution of antiferromagnetic order during the colossal magnetoresistive transition

Pr$_{0.7}$Ca$_{0.3}$MnO$_3$ (PCMO) displays one of the largest colossal magneto-resistances (CMR) among manganites. Magneto-transport data suggest that an insulating antiferromagnetic to metallic ferromagnetic transition occurs as the system goes through a CMR transition with the application of a high magnetic field (~3-6T). However, the nature of this transition is a mystery. We report the first high magnetic field resonant soft x-ray scattering (RSXS) experiments on PCMO which follow the evolution of the antiferromagnetic superlattice order through the CMR transition. We find that the antiferromagnetic order is first enhanced by several orders of magnitude before melting away into the metallic ferromagnetic phase. Additional high field x-ray magnetic circular dichroism (XMCD) measurements allow us to track the spin and orbital moments and construct a microscopic picture of the competing forces at the heart of CMR.   

Low energy magnetic excitations in phase-separated La5/$8-y$Pr$y$Ca3/8MnO3

La5$/$8-$y$Pr$y$Ca3$/$8MnO3 (LPCMO) ($y= 0.$4) is one of the prototype materials for the study of phase separation. The end members of the series, La5/8Ca3/8MnO3 and Pr5/8Ca3/8MnO3, have a robust low temperature FM metallic and charge ordered insulating states, respectively. Various experimental techniques have shown evidence of two-phase coexistence for intermediate Pr contents. However, a clear understanding of some basic macroscopic signature of phase separation is still lacking. The states of the coexistence phases are different when the system goes through different thermodynamic paths with the application of magnetic field. The zero-field-cooled (ZFC) procedure results in a dominant CO-OO state with little FM clusters at low temperature. Such insulating state is robust against external magnetic field. On the other hand, a zero-field cooled, field warming (ZFC-FW) procedure causes a sudden increase in FM intensity near the glassy transition temperature (TG $\sim $ 25K). In this talk we will present the results of recent elastic and inelastic neutron scattering experiments on a single crystal specimen of LPCMO, which reveal the nature of the complex phase coexistence at low temperatures.   

Anisotropic field-induced melting of orbital ordering phase in Pr0.6Ca0.4MnO3

The orbital degree of freedom in correlated electron systems plays an essential role in creating versatile phenomena via its coupling with charge, spin, and lattice. One extraordinary phase in doped oxide compounds is the charge/orbital-ordered (OO) antiferromagnetic (AF) insulating state, which, interestingly, can be melted to an orbital-liquid metallic state by external magnetic field. By measuring the field-dependent transport behavior of Pr0.6Ca0.4MnO3 single crystal at different temperatures, we have found a field-orientation dependent melting of the charge-orbital ordered state, regardless whether or not the system is in AF phase. The field-induced melting is found stronger when the applied field is in theab (basal)-plane. This can be understood as suppression of the CE-type OO state via spin-orbit coupling induced preference of in-plane orbital direction.   

Paramagnetic Spin Correlations in Colossal Magnetoresistive La$_{0.7}$Ca$_{0.3}$MnO$_{3}$

Inelastic neutron scattering measurements, taken on the ARCS time-of-flight spectrometer, reveal dynamic spin correlations throughout the Brillouin zone in the colossal magnetoresistive system La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ at 265~K ($\approx$1.03~$T_{C}$). Well defined correlations are observed in constant-$E$ scans. The long-wavelength behavior can be attributed to spin diffusion, qualitatively consistent with dynamical scaling theory, with a correlation length of $\approx$10~{\AA}. Dynamic correlations are observed at energies up to at least 28~meV, suggesting persistent short range spin correlations in the paramagnetic phase. An additional and unexpected component of the scattering is observed at lower energies which takes the form of ridges of strong scattering running along ($H$~0~0) and equivalent directions.   

Existence of a New Phase in the Intermediate $J_{\rm AF}$-coupling Regime of the Two-Orbital Model for Manganites

We report the existence of a new exotic state in the two-orbital model for manganites in the intermediate $J_{\rm AF}$ coupling regime, analyzed using the standard Monte Carlo technique based on the exact diagonalization of the electronic sector. At density $x=1/4$, this state shows diagonal ferromagnetic (FM) chains with uniform charge order (CO), stacked in between CE-like zig-zag patterns of spin with less populated staggered CO. The new state exists in a narrow range between the well-known FM metallic and C$_{x}$E$_{1-x}$ insulating states, and provide a realization in the clean limit of the nanoscale phase separation scenario of CMR manganites. The existence of many other competing exotic states in this range of couplings will also be discussed.