Session Q23: Focus Session: Bilayer Manganites

Doping dependence of the bilayered colossal magnetoresistive manganites La(2-x)Sr(1+2x)Mn(2)O(7): Angle Resolved Photoemission studies

We have measured the doping dependence of the bilayered colossal magnetoresistive manganites La(2{\-}x)Sr(1+2x)Mn(2)O(7) with Angle Resolve Photoemission (ARPES). Our measurements reveal profound differences in the spectral features depending on the doping levels. Surprisingly, the spectra corresponding to x = 0.4 exhibit more similarities to those corresponding to x = 0.6 than the ones with x = 0.36 and x = 0.38. Further aspects of these data in relation to the physics of layered manganites will be discussed.   

Non-monotonic Fermi surface geometry and its correlation with real-space ordering in the bilayer magnetoresistive oxide La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$

Angle-resolved photoemission spectroscopy was used to study the Fermi surface and nesting effects for a wide range of doping levels (x) of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$. While band structure calculations indicate a monotonic trend in the size of each piece of the Fermi surface, our data indicates non-monotonic trends which are consistent with the evolution of the charge modulations, unequivocally confirming the direct connection between the nesting and the charge correlation vectors. This may be classified as a key example of a system with a strongly k-dependent self-energy.   

Doping evolution of bilayered colossal magnetoresistive manganites: bilayer splitting and c-axis coupling

We present a detailed momentum, doping and temperature dependent study of the electronic properties of bilayer manganites, La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, by means of angle-resolved photoemission spectroscopy. In particular, we will address both the in-plane bilayer splitting, as well as the out-of-plane band, as a function of doping. We will discuss possible implications for the role of inter-plane coupling in the CMR manganites.   

Calculated properties of the La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ bilayer manganites, $0.30\leq x\leq0.50$

The low temperature properties of the La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$, $0.30\leq x\leq0.50$ bilayer manganites have been studied in the past using a host of experimental techniques in order to understand the outstanding phenomena they exhibit. To complement these investigations, we present a systematic study of their calculated ground state properties as a function of doping level, using the highly precise full-potential linearized augmented plane wave (FLAPW) method,\footnote{Wimmer, Krakauer, Weinert, Freeman, Phys.Rev.B, {\bf 24}, 864 (1981).} and focusing on magnetic order and optical and transport properties. Our results, which are in very good agreement with experiment in several respects, underline the correlation between the structural and orbital degrees of freedom and also shed light on many of the unexpected observations at low temperature in angle-resolved photoemission spectroscopy, optical conductivity, and resistivity measurements.   

Magnetic State of La$_{1.36}$Sr$_{1.64}$Mn$_{2}$O$_{7}$Probed by Magnetic Force Microscopy

We have investigated the evolution of the ferromagnetic (FM) domain structure of a single-crystal La$_{1.36}$Sr$_{1.64}$Mn$_{2}$O$_{7}$ with temperature and external magnetic field by using low-temperature magnetic force microscopy. We observed that the FM domains form stable treelike patterns with out-of-plane magnetization below 65 K. As the temperature increases, the FM domains begin to change gradually. Around 88 K, the magnetization changes from the out-of-plane to an in-plane direction. The in-plane FM domains almost completely disappear near the Curie temperature of this sample ($T_{C} \quad \approx $ 110$K)$. Interestingly, the evolution of the FM domain patterns with temperature was seen to coincide with the change in resistivity. We also observed large changes in the magnetic structures upon thermal cycling. We concluded that the formation of FM domains is determined by the competition between the magnetostatic energy and domain-wall energy and is also associated with the 2D character of the system.   

Thermal Expansion, Heat Capacity and Magnetization Measurements of La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$

La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ is a bi-layered ferromagnet of perovskite structure with two dimensional magnetic and electrical properties. In this presentation, we will show measurements of the specific heat, magnetization, and thermal expansion for single crystalline La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ in the temperature range 300 K $< T <$ 5 K. The thermal expansion of this tetragonal crystal along the $a$ and $c$ directions will be presented, the anisotropy will be discussed, and the critical behavior near the magnetic transition will be evaluated.   

Thermal conductivity of layered La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ has many similarities to underdoped cuprates: highly anisotropic transport, strong disorder in a charge reservoir layer, and even claims for Fermi arcs. Thus we measured the thermal and charge transport of this system to examine whether the Wiedemann-Franz law is violated in La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ as it is in several cuprate systems in the T=0 limit. One significant difference to cuprates, however, is that the low temperature state of La$_{1.2} $Sr$_{1.8}$Mn$_2$O$_7$ is a ferromagnet. A consequence of this is that magnons in addition to phonons and electrons can transport heat. By using an applied magnetic field to gap out the magnon spectrum, we have also found clear evidence for the transport of 2D ferromagnetic magnons.   

Temperature Dependent Raman Scattering in Layered Manganites

La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (LSMO) is a manganite system built up of perovskite bilayers of corner linked MnO$_{6} $ octahedra forming infinite sheets and separated by a (La,Sr)O layer along the c-axis. For 0.3 $<$ x $<$ 0.5, there is a transition from a low temperature ferromagnetic metallic state at T$_{c}$ $\sim$ 110-130 K to a high temperature paramagnetic insulating state. Due to its layered structure, this system provides a unique opportunity to explore the interplay between spin, charge and lattice degrees of freedom in reduced dimensions. In this talk we present our results on temperature dependent Raman scattering measurements on single crystals of LSMO for x in the range 0.3-0.45. We observe four modes in the frequency range 300-800 cm$^{-1}$ and track the changes in the Raman bands line shape parameters as a function of temperature. We correlate our results with other known transport and magnetic measurements on these systems.   

Optical Measurements of Transition from Insulator to Metallic Phases in LaxMnO3:Evidence for Mixed Phases Below Tp

Infrared measurements were conducted over the range 100 to 8000 cm-1 on film of LaxMnO3 (x$\sim $0.8). From these measurements we determined the frequency dependent conductivity, the effective number of carriers and the frequency dependence of specific phonon modes. The Drude model conductivity matches well with the measured DC resistance measurements. The bulk magnetization noset and resistance peak coincide near $\sim $270 K. However, the onset of the increase in the free carrier number lags these curves and occurs $\sim $ 25 K lower in temperature and saturates at 200 K. This indicates that indicates that a significant volume of the insulating phase exists below Tp.   

Experimental Evidence for an Energy Gap in the Magnetic Insulator SrRu$_{.06}$Mn$_{.94}$O$_3$

$^{99}$Ru M\"{o}ssbauer Effect and magnetic measurements have been made on an enriched $^{99}$Ru sample of SrRu$_{.06}$Mn$_{.94}$O$_3$ which are compared with a series of measurements on compounds SrRu$_x$Mn$_{1-x}$O$_3$ made with natural Ru completed earlier. SrRu$_x$Mn$_{1-x}$O$_3$ is an antiferromagnet for x=.06 and a ferromagnet for x=.9. The change from the ferromagnet to the antiferromagnet is accompanied by a change in the hyperfine magnetic field from 33T to 50T at 4.2K and also an isomer shift from a +4 to close to a +5 charge state. The measurements of the hyperfine field as a function of temperature show a change from 50T at 4.2K to 48T at 105K. Since the N\'{e}el temperature is about 160K for this compound, these small changes indicate that the thermal excitation of spin waves, which lead to the decrease of the hyperfine field with temperature, are strongly suppressed by a gap of order 100K in the spin wave spectrum. This indicates the presence of strong magnetic anisotropy in the material.   

Elastic properties of the transition metal oxides Ca$_{2-x}$Sr$_{x}$RuO$_{4}$

Layered perovskite ruthenates have attracted considerable interest since the discovery of superconductivity in Sr$_{2}$RuO$_{4}$, the only copper-free superconductor isostructural to the cuprates. Among the doped varieties of Sr$_{2}$RuO$_{4}$, the Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ series is heavily studied, as it connects the Mott insulator Ca$_{2}$RuO$_{4}$ with the superconductor Sr$_{2}$RuO$_{4}$ and exhibits a variety of physical properties. The current work focuses on the elastic properties of Ca$_{2-x}$Sr$_{x}$RuO$_{4. }$Resonant Ultrasound Spectroscopy (RUS) has been used to study the elastic response of the samples, and results are presented for single crystals with x = 0.2, 0.3, 0.4, 0.5, 1, 1.5, 1.9 and 2.0. The temperature-dependence of the elastic behavior is found to be quite unusual and reflects the rich phase diagram of these materials. Almost all measured Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ samples show a soft phonon mode at low temperatures, which is believed to be associated with the dynamics of the RuO$_{6}$ octahedra.   

Synthesis and Physical Properties of Double Perovskite Pb$_{2}$FeReO$_{6}$

A double perovskite Pb$_{2}$FeReO$_{6}$, in which Fe and Re are ordered in a rock-salt type configuration, was prepared at a high pressure (6GPa) and high temperature (1000 $^{\circ}$C) condition. The crystal structure determined by the synchrotron X-ray powder diffraction was centrosymmetric with a space group $I$4/$m$ despite the presence of Pb$^{2+}$ ion at the $A$ -site. The lattice parameters are $a$ = 5.6159(1) \AA and $c$ = 7.9455(3) \AA. No structural transition to a lower symmetry was observed down to 23 K. The resistivity showed semiconducting behavior. This compound exhibited a ferrimagnetic transition at 420 K with saturation magnetizations depending on the degree of Fe$^{3+}$/Re$^{5+}$ ordering controlled by the cooling rate after the heat treatment.