Session B13: Focus Session: Cobaltates and Manganites

Tuning Physical Properties via Isovalent Doping in Ba$_{2-x}$Sr$_{x}$CoO$_{4}$

It is known that monoclinic Ba$_{2}$CoO$_{4}$ is an antiferromagnetic (AFM) insulator with Neel temperature $T_{N}$=25K. We found that isovalent Sr substitution for Ba drastically changes the structural and magnetic properties of Ba$_{2-x}$Sr$_{x}$CoO$_{4}$ system. With increasing $x$, $T_{N}$ initially increases then decreases after reaching the maximum at $x$=0.5. Correspondingly, its crystal structure changes from monoclinic ($x<$0.5) to orthorhombic ($x\ge $0.5) at room temperature. The correlation between structure and physical properties will be discussed.   

Spin-state transitions and magnetic polaron in lightly doped La$_{1-x}$Sr$_{x}$CoO$_{3}$.

Using the inelastic neutron scattering (INS) technique, we identified the energy levels of the thermally excited states of Co$^{3+}$ ions in both LaCoO$_{3}$ and La$_{0.998}$Sr$_{0.002}$CoO$_{3}$. In LaCoO$_{3}$ an excitation at \textit{$\sim $}0$.$6 meV appears at T$>$30K, whose intensity follows the bulk magnetization. Within a model including crystal field interaction and spin-orbit coupling we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. Since the $g$-factor obtained from the field dependence of the INS is \textit{g$\sim $ }3, we interpret this state as a high-spin state of Co$^{3+ }$. The lightly doped material \textit{x$\sim $}0$:$002 exhibits paramagnetic properties at low temperatures. An INS peak at energy transfer \textit{$\sim $}0$.$75 meV was observed in it already at $T $= 1$:$5 K. We propose that the holes introduced in the LS state of LaCoO$_{3}$ by doping are extended over the neighboring Co sites, forming thus magnetic polaron and transforming all the involved Co ions (e.g. 6 of them) to the high-spin state. Similarly to LaCoO$_{3}$, we interpret the INS transition at 0.75 meV as that on these high-spin Co$^{3+}$ ions.   

Incommensurate Spin Correlations in La$_{1-x}$Sr$_{x}$CoO$_{3}$

Non-magnetic, insulating LaCoO$_3$ is transformed into a metallic, spin cluster ferromagnet when holes are added to the system by replacing La$^{3+}$ with Sr$^{2+}$. Previous work has shown that this transition results from the percolation of isotropic, ferromagnetic clusters. Here, we present elastic neutron scattering data which shows that a short-ranged, anisotropic incommensurate magnetic phase also appears as holes are added. We have studied this incommensurate phase in detail for a number of concentrations, spanning the phase diagram above and below the percolative phase transition, and the incommensurability increases with the hole concentration. From the evolution of the incommensurate signal strength with x, we concluded that the the incommensurate phase is competing with the FM clustering. The spin incommensurability may originate from a local ordering of magneto-polarons.   

A non-Griffiths-like clustered phase above the Curie temperature of the doped perovskite cobaltite La$_{1-x}$Sr$_{x}$CoO$_{3}$

The existence of preformed clusters above the \textit{Tc} of the doped perovskite manganites is well established and, in many cases, conforms to the expectations for a Griffiths phase. We show here that the phase-separated perovskite cobaltite (La$_{1-x}$Sr$_{x}$CoO$_{3})$ also exhibits a clustered state above the \textit{Tc} in the ferromagnetic phase. The formation of magnetic clusters at a well-defined temperature ($T$*) is revealed in the small-angle neutron scattering, d.c. susceptibility, and resistivity. Remarkably, this clustered state has none of the characteristics of a Griffiths phase; the deviation from Curie-Weiss behavior is opposite to expectations and is not field dependent, and $T$* does not correspond to the undiluted \textit{Tc}. These results demonstrate that although the Griffiths phase may occur in many systems with quenched disorder, it is not universally applicable to the randomly doped transition metal oxides.   

Competing ferromagnetic and incommensurate order in perovskite cobaltites La$_{1-x}$Sr$_x$CoO$_3$

Many phenomena observed in complex oxides, and in particular their enhanced response to external fields, are intimately linked to the existence of short-range order such as formation of stripes, ladders, checkerboards or phase separation. This nanoscale disorder results from the delicate balance of spin, orbital, charge, and strain degrees of freedoms that leads to competing groundstates with incompatible order. Inhomogeneity in the form of phase separation is also believed to play a key role in the magnetoresistance of doped cobalt perovskites La$_{1-x}$Sr$_x$CoO$_3$. In this system, doping holes into the non- magnetic, insulating parent compound leads to spin-glass behavior at low doping, with nanoscale ferromagnetic clusters forming within a non-fermomagnetic matrix. Percolation of these isolated clusters then leads to the ferromagnetic and coincident metal- insulator transition at x=0.18. Utilizing elastic and inelastic neutron scattering, we have studied in detail the evolution of the static and dynamic spin correlations in these systems. Within the cluster-glass phase, we observe the formation of static ferromagnetic droplets below the spin- glass freezing temperature. Aided by the double exchange, the ferromagnetic correlations grow rapidly with doping. At the onset of metallicity, the correlation length increases abruptly, but remains finite.\footnote{D. Phelan, D. Louca, S. Rosenkranz,S.-H. Lee, Y. Qiu, P.J. Chupas, R. Osborn, H.Zheng, J.F. Mitchell, J.R.D. Copley, J.L. Sarrao, Y. Moritomo, Phys. Rev. Lett. {\bf 96}, 027201 (2006)} Concurrent, and in competition with the ferromagnetic correlations, we also observe the formation of a short- range, incommesurate (IC) spin structure below the spin-freezing or ferromagnetic transition temperature. The incommensurate wavevector increases continuosly with doping, with the intensity of the IC correlations increasing in the insulating phase with doping, but dropping in the metallic state. This IC order could result from correlations between local Co$^{3+}$-Co$^{4+}$ clusters in the form of short-range stripes. \newline \newline Work supported by US DOE BES-DMS DE-AC02-06CH11357 and NSF DMR-0454672.   

Thickness dependence of the structural, magnetic, and electronic, properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ films on SrTiO$_{3}$(001) substrates

Bulk La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO) materials have received considerable attention due to the existence of spin-state transitions, magnetoelectronic phase separation, and giant anomalous Hall effect. In our prior work we have established optimized conditions for the deposition of high quality epitaxial LSCO thin films. In this work, we provide a comprehensive study of the variation of structural, morphological, magnetic, and electronic properties as a function of film thickness. This investigation has been carried out as a controlled function of oxygen stoichiometry using the total sputtering pressure (30, 70, and 140 mTorr, at an O$_{2}$ / Ar ratio of 0.4) as the control parameter. High resolution WAXRD, x-ray rocking curves, x-ray reflectivity, AFM, d.c. magnetometry, and electronic transport measurements have all been employed. Our results indicate that the thickness dependence of the electronic and magnetic properties is dominated by the sensitive interplay between oxygen stoichiometry and strain relaxation. The behavior at very low thickness is discussed in terms of the known phenomenology of the magnetoelectronic phase separation in this material.   

Fabrication, characterization, and magnetic and electronic properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ films

Bulk La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO) has received considerable attention with regard to magnetic phase separation. Fabrication of epitaxial films would provide a means to study this phase separation under dimensional confinement. We have investigated the properties of epitaxial films of x = 0.5 LSCO on SrTiO$_{3}$ (001) by reactive d.c. sputtering from compound targets. Structure and properties were studied as a function of sputtering pressure, O$_{2}$/Ar ratio, and post-deposition O$_{2}$ treatment. Optimized conditions result in single phase, stoichiometric, substantially relaxed, epitaxial films. At a fixed O$_{2}$ / Ar ratio, two distinct regimes of total sputtering pressure ($P)$ occur. Films grown at $P>$ 50 mTorr have properties close to bulk; they are ferromagnetic ($T_{C} \quad \approx $ 230 K, $M_{S} \quad \approx $ 2 $\mu _{B}$ / Co), have a metallic-like $\rho (T)$ at all $T$, and exhibit 10 {\%} magnetoresistance at $T_{C}$. For $P \quad <$ 50 mTorr, we obtain lower surface roughness and narrower rocking curves but with low moment, insulating $\rho (T)$, and increased out-of-plane lattice parameter due to O deficiency. Similarly, a dramatic enhancement in physical properties, and elimination of a CoO minority phase, is obtained when cooling in 500 Torr of O$_{2}$. The results demonstrate good control over the oxygen stoichiometry, and therefore physical properties. Work supported by NSF DMR.   

Doping Dependence of Polaron Hopping Energies in La$_{1-x}$Ca$_x$MnO$_3$ ($0\leq x\leq 0.15$)

Measurements of the low-frequency ($f\leq 100$~kHz) permittivity at $T\leq 160$~K and dc resistivity ($T\leq 430$~K) are reported for La$_{1-x}$Ca$_x$MnO$_3$ ($0\leq x\leq 0.15$). Static dielectric constants are determined from the low-$T$ limiting behavior of the permittivity. The estimated polarizability for bound holes $\sim 10^{-22}$~cm$^{-3}$ implies a radius comparable to the interatomic spacing, consistent with the small polaron picture established from prior transport studies near room temperature and above on nearby compositions. Relaxation peaks in the dielectric loss associated with charge-carrier hopping yield activation energies in good agreement with low-$T$ hopping energies determined from variable-range hopping fits of the dc resistivity. The doping dependence of these energies suggests that the orthorhombic, canted antiferromagnetic ground state tends toward an insulator-metal transition that is not realized due to the formation of the ferromagnetic insulating state near Mn$^{4+}$ concentration $\approx 0.13$.   

Magnetic susceptibility of La$_{0.7}$Ca$_{0.3}$MnO$_3$ at very low magnetic fields in the vicinity of the ferromagnetic transition

Magnetic susceptibility ($\chi$) measured at magnetic fields $H$ as low as 0.2 Oe is reported for La$_{0.7}$Ca$_{0.3}$MnO$_ {3}$. A pronounced enhancement in $\chi$ is observed in the region above the critical temperature $T_c$ at very low $H$. As the magnetic field is increased, this feature is shifted toward $T_c$, eventually vanishing near $H$ = 400 Oe. Electrical resistivity measurements show a positive magnetoresistance effect between 0 and 500 Oe in a temperature range slightly above $T_c$. The results are discussed in a scenario of frustrated magnetism and the possibility of a Griffiths singularity is addressed.$^1$ $^1$ Chan, Goldenfield, and Salamon, Phys. Rev. Lett. 97 (2006) 137201.   

Critical Exponents and Pressure Dependence of $T_c$ of La(Ca)MnO$_3$

Measurements of heat capacity and thermal expansion for La$_{1- x}$Ca$_{x}$MnO$_{3}$ with $x$ = 0, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, and 1 are reported. Using a model proposed previously (Souza et al. Phys. Rev. Lett. 94, 207209 (2005)), which utilizes both heat capacity ($C_{P}$) and thermal expansion coefficient ($\mu$) data, the pressure dependencies of $T_c$, d$T_{c}$/d$P$, are obtained for all samples. d$T_{c} $/d$P$ decreases as the Ca doping increases. Critical behavior using both $C_{P}$ and $\mu$ is evaluated for the samples. The critical exponent $\alpha$ increases from 0.13, for LaMnO$_{3} $ to 0.97 for $x$ = 0.30. As Ca content is increased further, $\alpha$ drops reaching 0.11, for CaMnO3.   

High Pressure Effects on Structural and Transport of self-doped Manganite La$_{0.9}$MnO$_{3}$

The effects of hydrostatic pressure up to 6 GPa and 11 GPa, respectively on the electrical resistivity and structural properties are systematically investigated on the self-doped Maganite La$_{0.9}$MnO$_{3}$. We find a maximum shift of the peak resistivity with pressure occurs at $\sim $ 3.4 GPa in the La deficient system similar to the chemically doped manganite systems previously studied by our group. The unusual pressure dependence of resistivity can be related with the competition between ferromagnetic Double Exchange interaction and antiferromagnetic superexchange mechanism. The x-ray diffraction reveals a single-phase crystallographic phase of monoclinic space group up to 11 GPa. Electronic structure simulations of the pressure dependence on the stability of the magnetic phases are being conducted. This work was supported by NSF DMR-0512196.   

Effects of internal structural parameters on the properties of Ba-substituted La$_{0.5}$Sr$_{0.5}$MnO$_{3}$

Barium substituted La$_{0.5}$Sr$_{0.5-x}$Ba$_{x}$MnO$_{3}$ materials have been synthesized and investigated using neutron powder diffraction. We show that Ba substitution suppresses the low temperature orbital-ordering previously observed in La$_{0.5}$Sr$_{0.5}$MnO$_{3}$, and demonstrate the evolution of the magnetic and nuclear structures as a function of increasing Ba content. All samples exhibit paramagnetic and ferromagnetic properties near room temperature. The effects of A-site ionic size, size variance, and strains in the lattice on the ferromagnetic ordering temperature, T$_{C}$, are discussed and compared with other members of the general La$_{0.5}$(Ca,Sr,Ba)$_{0.5}$MnO$_{3}$ series. Depending on the substitution path, the relationship between T$_{C}$ and $<$r$_{A}>$ is either nearly constant or looks like an inverted parabola.   

Structural and magnetic properties of SrMn$_{1-x}$Ru$_{x}$O$_{3}$ perovskites

Ferromagnetism of SrRuO$_{3}$ is unique among 4d transition metal based perovskite oxides. On substitution of Mn its T$_{C}$ decreases from 163 K to 0 for x$\sim $0.5-0.6 followed by a formation of an antiferromagnetic insulating state at a quantum critical point. The other end member of the SrMn$_{1-x}$Ru$_{x}$O$_{3}$ family, a cubic perovskite SrMnO$_{3}$ is a G-type antiferromagnet with T$_{N}$=233 K. We have synthesized the complete SrMn$_{1-x}$Ru$_{x}$O$_{3}$ solid solution. The polycrystalline samples were characterized by neutron difraction, magnetic, and transport experiments. The incorporation of Ru in the SrMnO$_{3}$ matrix (0.1$\le $x$\le $0.4) results in a phase transition to a C-type antiferromagnetic state accompanied by a cubic-tetragonal transition. The intermediate substitution level induces a spin-glass behavior, due to competing ferro- and antiferromagnetic interactions. Mixed valence Mn$^{3+}$/Mn$^{4+}$ and Ru$^{4+}$/Ru$^{5+}$ pairs introduce additional frustration to the magnetic states. The glassy behavior can be observed for x up to 0.7 in the tetragonal structure. Supported by NSF (DMR-0302617) and the U.S. Department of Education