Strong orbital correlations in a Fe-substituted spin-glass-manganite

The compound ${\mathrm{La}}_{0.66}{\mathrm{Ba}}_{0.40}{\mathrm{Mn}}_{0.61}{\mathrm{Fe}}_{0.33}{\mathrm{O}}_3$ shows anisotropic magnetic correlations with no long-range order. Specific heat measurements suggest these correlations represent the bulk. Orbital correlations of ${\mathrm{Mn}}^{3+}{e}_g$ electrons, surviving in an environment of largely disordered exchange interactions, are invoked to account for this magnetic state. These results argue in favor of a strain-field mechanism for orbital ordering in manganites.

Figure 1

(Color online) (a) Resistivity of ${\mathrm{La}}_{0.66}{\mathrm{Ba}}_{0.40}{\mathrm{Mn}}_{0.61}{\mathrm{Fe}}_{0.33}{\mathrm{O}}_3$ as a function of the inverse $T$ (symbols), and a fit to a semiconducting behavior with activation energy $ϵ=0.11\mathrm{eV}$ (solid line). (b) $T$ dependence of the specific heat $C(H=0)$ (thick solid line) and dc-magnetic susceptibility ${\chi }_{\mathit{dc}}(H=0.01\mathrm{T})$ taken on field cooling (FC) and warming after zero field cooling (WZFC) (thin solid lines). (c) $C∕T$ vs. $T$ in the low $T$ limit (symbols), highlighting the dominant contribution due to the lowest power term in $T$, and tentative fits to square-root and linear behaviors due to gapless spin excitations in a ferromagnet (dashed curve) and a layered $A$-type antiferromagnet (solid line) (Ref. 9), respectively. The estimated phonon contribution, using ${\theta }_{\mathit{\text{Debye}}}\sim 275\mathrm{K}$ (Ref. 9), is also given. (d) Out-of-phase ac-magnetic susceptibility for several frequencies in a selected $T$ interval, taken with a field amplitude of $1\mathrm{mT}$.

Figure 2

(Color online) (a) Inelastic neutron scattering of ${\mathrm{La}}_{0.66}{\mathrm{Ba}}_{0.40}{\mathrm{Mn}}_{0.61}{\mathrm{Fe}}_{0.33}{\mathrm{O}}_3$ at fixed $\mathbf{Q}=(0.9,0,0)$ (cubic notation), taken at $10\mathrm{K}$ (open symbols) and $200\mathrm{K}$ (filled symbols), as a function of neutron energy loss. (b) $T$ dependence of the scattering at $Q=(0.9,0,0)$ and $\delta E=1.5\mathrm{meV}$. (c) Elastic transverse scans across the reciprocal lattice point $(1∕2,0,0)$ at selected $T$. The $\text{instrumental}+\text{mosaic}$ width is indicated by short vertical bars. Data at different $T$ were vertically translated for clarity. (d) $T$ dependence of the integrated intensity of the $(1∕2,0,0)$ reflection and $A$-AFM correlation length $\xi$. (e) $T$ dependence of lattice parameters $a^{*}=a∕\sqrt{2}=c∕\sqrt{2}$ (open circles), $b^{*}=b∕2$ (filled circles), and $b^{*}-a^{*}$ (open triangles). Data below and above room temperature were obtained from the analysis of neutron and synchrotron x-ray powder diffraction data, respectively. (f) $(\mathrm{Mn},\mathrm{Fe})\mathrm{O}$ crystallographic distances along the $b$ direction (apical, filled circles), and $ac$ plane (basal, open circles) below room temperature. Solid lines in (a)–(f) are guides to the eyes.

Figure 3

(Color online) (a) Fe $2p$, (b) Mn $2p$, and (c,d) Mn $1s$ x-ray absorption near-edge structure (XANES) spectra of ${\mathrm{La}}_{0.66}{\mathrm{Ba}}_{0.40}{\mathrm{Mn}}_{0.61}{\mathrm{Fe}}_{0.33}{\mathrm{O}}_3$ (symbols). The solid line in (a) displays a crystal-field multiplet simulation (Ref. 12), assuming a high-spin ${\mathrm{Fe}}^{3+}:t_{2g}^3{e}_g^2$ ground state. The solid line in (b) is the Mn $2p$ XANES spectrum of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$. The Mn $1s$ XANES spectra of $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_3$ and $\mathrm{Ca}\mathrm{Mn}{\mathrm{O}}_3$ reference samples are given in (c,d) as thin solid and dashed lines, respectively, while the combination $0.55\times \mu \left(\mathrm{Ca}{\mathrm{Mn}}^{4+}{\mathrm{O}}_3\right)+0.45\times \mu \left(\mathrm{La}{\mathrm{Mn}}^{3+}{\mathrm{O}}_3\right)$ is given as thick solid lines.