Session X16: Manganites: Perovskite and Bilayer

Persistence of Jahn-Teller Distortion up to the Insulator to Metal Transition in LaMnO$_{3}$

High pressure, low temperature Raman measurements performed on LaMnO$_{3}$ up to 34~GPa provide the first evidence for the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase. This result resolves the ongoing debate about the nature of the pressure driveninsulator to metal transition (IMT), demonstrating that LaMnO$_{3}$ is not a classical Mott insulator. The formation of domains of distorted and regular octahedra, observed from 3 to 34~GPa, suggests that LaMnO$_{3}$ becomes metallic when the fraction of undistorted octahedra domains increases beyond a critical threshold. In this scenario, it is interesting to consider whether or not the CMR effect may be induced in LaMnO$_{3}$ by applying pressure. Preliminary results obtained performing high pressure resistivity measurements in a magnetic field will be reported. \\[4pt] [1] I. Loa, et al., \textit{Phys. Rev. Lett.} 87, 125501 (2001). \\[0pt] [2] A.Y. Ramos et al., \textit{Phys. Rev. B} 75, 052103(2007). \\[0pt] [3] A.Y. Ramos et al., \textit{J. Phys. Conf. Ser}. 190, 012096 (2009). \\[0pt] [4] A. Yamasaki et al., \textit{Phys. Rev. Lett.} 96, 166401 (2006). \\[0pt] [5]J. D. Fuhr et al., \textit{Phys. Rev. Lett.} 100, 216402 (2008). \\[0pt] [6] M. Baldini et al.\textit{, Phys. Rev. Letter} 106, 066402 (2011).   

Influence of Gd-doping in La0.7Ca0.3MnO3 on its structural and Magneto-Electrical Properties

We present a study of the structural and electrical properties of lanthanum-based manganite, La0.7Ca0.3MnO3 with x=0.0 and 0.1. The samples synthesized by the conventional solid state reaction method. The samples are characterized by X-ray diffraction, scanning electron microscope and energy dispersive X-ray spectrometer. The electrical and magneto-transport properties of bulk samples have been investigated in the temperature range 5-300 K and a magnetic field up to 7 T. Although the replacement of La ion by Gd results a decrease in metal-insulator transition temperature TMI, the magnetoresistance and resistivity are found to be increased. The electrical resistivity in the entire temperature range fit well with the phenomenological percolation model, which is based upon an approach that the system consists of the phase separated ferromagnetic metallic and paramagnetic insulating regions.   

Size control of Magnetism, Charge and Orbital Order in Half-Doped Manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$

Motivated by recent experimental results, we study the effect of size reduction on half-doped manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$, using the combination of density functional theory (DFT) and dynamical mean field theory (DMFT). We find that upon size reduction, the charge-ordered antiferromagnetic phase, observed in bulk, to be destabilized, giving rise to the stability of a ferromagnetic metallic state. Our theoretical results, carried out on defect-free nanocluster in isolation, establish the structural changes that follow upon size reduction to be responsible for this. Our study further points out the effect of size reduction to be distinctively different from application of hydrostatic pressure. Interestingly, our DFT+DMFT study, additionally, reports the correlation-driven stability of charge-orbitally ordered state in bulk La$_{0.5}$Ca$_{0.5}$MnO$_3$, even in absence of long range magnetic order.   

Measuring Coexisting Phases in La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$

Manganite compounds in the La$_{0.625-y}$Pr$_y$Ca$_{0.375}$MnO$_3$ series are known for exhibiting phase separation over a large temperature range. We combined the x-ray photoemission electron microscopy (PEEM) and resonant elastic soft x-ray scattering (RSXS) techniques to study the interplay between the low-temperature ferromagnetic and intermediate temperature charge-ordered/antiferromagnetic phases, respectively, in La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$. We found that the system is driven by glassy polarons, which are present above the curie temperature $T_C$ in many ferromagnetic metallic manganites. They stunt the growth of the ferromagnetism on cooling: we clearly observe the onset of small, strained ferromagnetic domains almost $30\,\mathrm{K}$ above the temperature where ferromagnetism fully sets in, and the ferromagnetism has a very unconventional temperature dependence even below $T_C$. This relationship could explain the need for such high magnetic fields to induce colossal magnetoresistance.   

Oxygen Vacancy Induced Metal Insulator Transition in Epitaxial Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ Thin Films

We report the metal-insulator transition in epitaxial Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ thin film by introducing oxygen vacancies, which assist the nucleation of ferromagnetic metallic domains in a antiferromagnetic insulating matrix. The hysteresis of the resistivity indicates the transition is first-order, and covers a broad temperature range from 80K to 220K. Such novel transport properties of the x=0.3 doped manganite may result from strong spin-lattice coupling which stabilizes the system to a metallic metastable state at low temperature.   

Search for Multi-Ferroic Manganites with Elongated Mn-O bonds

Development of multi-ferroic materials, where magnetism and ferroelectricity are strongly coupled near room temperature, is of fundamental technological and theoretical importance. Typically, both phenomena tend to be mutually exclusive because ferroelectricity is usually present for d$^{0}$ and magnetism for non-d$^{0}$ transition metals. By analogy to perovskite Ba$^{2+}$Ti$^{4+}$O$_{3}$ (d$^{0})$ for which [Ti-O] bonds are highly elongated beyond their equilibrium lengths resulting in Ti distortion out of the center of the TiO$_{6}$ octahedral unit, resulting in T$_{F}\sim $400 K ferroelectricity, we have projected that similar effect should be observed for the non-d$^{0}$ insulating and antiferromagnetic (T$_{N}\sim $240 K) perovskites of Mn$^{4+}$ (d$^{3})$. I will describe our search for such compounds guided by our ``tolerance factor design rules'' in the (Sr,Ba)Mn O$_{3}$ system for which strong multi-ferroic behavior was achieved near room temperature.   

Formation of mesoscopic metallic filaments in manganite thin films imaged by microwave impedance microscopy

We study the ferromagnetic metallic domains from the charge-order insulating background at mesoscopic length scale in a Pr0.55Ca0.75Sr0.25MnO3 thin film using a variable temperature microwave impedance microscope (MIM). The metallic state in this compound can be easily induced at a moderate magnetic field as low as 2 T observed by both the transport and MIM. The temperature dependent transport under 1.2 T shows a large hysteresis loop. MIM allows us to observe the formation and melting of metallic domains at different temperatures during the cooling and warming processes. At higher temperatures, the metallic domains first emerge in small isolated filaments along certain crystal axes of the LSAT(110) substrate, suggesting that the local strain plays an important role. Surprisingly, small insulating islands remain in the metallic ground state and persist up to very high magnetic fields, indicating strong pining sites. Lastly, the sizes of the insulating islands at the ground state increase when the film is field cooled at lower speeds, suggesting s glassy order in this compound.   

Imaging of nano domain fluctuations in manganites

We present a soft x-ray resonant diffraction study of the electronic spatial nano-scale domains present in manganites. We explain the details of our new set up that allow us to perform this diffraction contrast microscopy in the nano regime. We will present preliminary results on the nanoscale domains in the bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$. The structural reflection (002) and the A-type antiferromagnetic reflection (001) have been investigated with soft x-rays in the vicinity of the manganese $\mathit{L}$$_{3}$ edge. A resolution of $\sim$ 150-200 nm has been achieved by implementing a zone plate focusing optic in our diffraction set up. These two reflections have been mapped in the same region of the sample. This region has dimensions of 20 x 20 microns. We will present measurements comparing the magnetic nano scale domains in LaSr$_{2}$Mn$_{2}$O$_{7}$ probed directly by measuring the (001) antiferromagnetic reflection, with crystallographic inhomogeneities that are observed in the mapping of the (002) crystallographic reflection. Finally, future directions using this setup will be discussed for the study of strongly correlated systems.   

Fragile Magnetic Ground State in Half-Doped Manganite LaSr$_2$Mn$_2$O$_7$: Orbital Instability

Recently, a careful doping control study, however, reported that the La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ material has an exotic phase diagram very near the half doping with extremely narrow antiferromagnetic phase boundaries at x$\simeq$ 0.5 $\pm$ 0.005 -- the $CE$-type within the boundaries but the $A$-type outside.\footnote{Q. Li $et~al.$, Phys. Rev. Lett. 98, 167201 (2007).} To understand a complexity on this material's phase diagram, we investigated the orbital and antiferromagnetic ordering behaviors of the half-doped bilayer manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (x $\simeq$ 0.5) by using Mn $L_{2,3}$-edge resonant soft x-ray scattering. We confirmed the predicted $CE$-type antiferromagnetic order for the true half-doped (x = 0.5) case. Moreover, we found that such a narrow phase boundary is due to the close competition of the two antiferromagnetic ordering phases via $3d$ Mn $e_g$ orbital instability. Our study reveals the spin and orbital orders of electrons in the sample as well as information about their ground states.\footnote{J.-S. Lee $et~al.$, Phys. Rev. Lett. 107, 037206 (2011).}   

Bilayer manganites reveal polarons in the midst of a metallic breakdown

Just what tips the balance between the wealth of competing phases and textures in spin, charge and orbital degrees of freedom in the CMR manganites such as La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (LSMO)? Combining ARPES and STM/S measurements on bilayered LSMO (0.30$\le $x$<$0.5), we arrive at a compelling explanation for the seemingly contradictory data on the electronic structure of bilayered LSMO that has appeared in the literature. We show that the true signature of bilayered (N=2) LSMO is that of a gapped non-metal on the verge of a metallic breakdown. All the former confusion stems from the intrinsic presence of stacking faults leading to either more gapped single layer (N=1) LSMO or sharply peaked, N$>$2 slabs displaying true metallic double-exchange.   

The intrinsic electronic structure of bilayer manganites from Angle Resolved Photoemission

The Colossal MagnetoResistant (CMR) manganites are one of the most studied condensed matter physics systems since decades. Yet, the mechanism behind the CMR effect and their electronic structure are still under hot debate. Recent angle resolved photoemission (ARPES) studies on the bilayer manganite La$_{(2-2x)}$Sr$_{(1+2x)}$Mn$_3$O$_7$, LSMO327, reported contradictory results [1]. Here we present an ARPES study unveiling the intrinsic $k$-- and temperature dependent electronic structure of LSMO327, while carefully steering away from the recently reported sample inhomogeneities [2] that have caused all the confusion. \\[4pt] [1] N. Mannella, Nature (2005); S. Sun Nature Phys. (2007); S. de Jong, PRB (2007)\\[0pt] [2] F. Massee, Nature Phys(2011)   

Unconventional response of acoustic phonons to the onset of charge order in a bilayer manganite

The acoustic phonons in the 50\% doped bilayer manganite LaSr$_2$Mn$_2$O$_7$ exhibiting CE type charge order were investigated using inelastic neutron scattering. At the onset of charge ordering, we observe an abrupt increase of the energies and a decrease of the linewidts of the transverse mode along (1,1,0), which crosses the CE ordering wavevector. This effect is however not localized to the CE ordering wavevector, but is observed over an extended range of momentum transfers,for which the phonon energy is lower than 15 meV. These observations indicate a reduced electron-phonon coupling due to a partial removal of the Fermi surface and provide direct evidence for a link between electron-phonon coupling and charge order in manganites. However, the observed response of the phonons is not consistent with a standard CDW mechanism, clearly showing that the transition is unconventional. \\ \\ Work supported by US DOE BES-DMS DE-AC02-06CH11357.   

Magnetoelastic Spin Flip in La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$

The magnetoelastic coupling in a bilayer manganite was investigated by using x-ray absorption spectroscopy (XAS) and resonant soft x-ray scattering (RSXS) at Mn $L_{2,3}$-edge. Huge occupation reversal of $e_g$ level from $d_{3z^2-r^2}$ to $d_{x^2-y^2}$ was observed at the temperature and magnetic field induced phase transition in La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$. The CI model calculation indicated that the direction of magnetocrystalline anisotropy is affected by the configuration of $e_g$ level, and the sharp spin flip transition was expected. The field dependent RSXS measurements demonstrated a strong magnetoelastic coupling in La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$, where the AFM spin axis was changed from out-of-plane to in-plane as a result of the field induced change of $e_g$ orbital occupation. Finally, we discuss the spin-orbital-lattice coupling in bilayer manganites.