Crystal structures and magnetic order of ${\mathrm{La}}_{0.5+\delta }{A}_{0.5-\delta }{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5-ϵ}{\mathrm{O}}_3(A=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba})$: Possible orbital glass ferromagnetic state

The crystallographic and magnetic properties of ${\mathrm{La}}_{0.5+\delta }{A}_{0.5-\delta }{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5-ϵ}{\mathrm{O}}_3(A=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba})$ were investigated by means of neutron powder diffraction. All studied samples show the orthorhombic perovskite crystal structure, space group $Pnma$, with regular $(\mathrm{Mn},\mathrm{Ru}){\mathrm{O}}_6$ octahedra and no chemical ordering of the ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Ru}}^{4+}$ ions. Ferromagnetic spin structures were observed below $T_c\sim 200–250\mathrm{K}$, with an average ordered moment of $\sim 1.8–2.0{\mu }_B∕(\mathrm{Mn},\mathrm{Ru})$. The observation of long-range ferromagnetism and the absence of orbital ordering are rationalized in terms of a strong $\mathrm{Mn}\mathrm{Ru}$ hybridization, which may freeze the orbital degree of freedom and broaden the $e_g$ valence band, leading to an orbital glass state with carrier-mediated ferromagnetism.

Figure 1

Observed (cross symbols) and calculated (solid line) high-resolution neutron-diffraction intensities of ${\mathrm{La}}_{0.5+\delta }{\mathrm{Ca}}_{0.5-\delta }{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5-ϵ}{\mathrm{O}}_3$ at $300\mathrm{K}$, taken with $\lambda =1.5402\left(1\right)\mathrm{\AA }$. The difference profile is also given. Short vertical lines correspond to Bragg peak positions for the main phase (upper) and for the $\mathrm{La}{\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_6$ impurity phase (lower).

Figure 2

Temperature dependence of the lattice parameters $a$ (filled circles), $b∕\sqrt{2}$ (open circles), and $c$ (filled tringles), and unit-cell volume (solid line) of (a) ${\mathrm{La}}_{0.5+\delta }{\mathrm{Ca}}_{0.5-\delta }{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5-ϵ}{\mathrm{O}}_3$, and (b) ${\mathrm{La}}_{0.5+\delta }{\mathrm{Sr}}_{0.5-\delta }{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5-ϵ}{\mathrm{O}}_3$.

Figure 3

Temperature-dependence of (a) $B\text{−}\mathrm{O}$ bond lengths $(B=\mathrm{Mn},\mathrm{Ru})$ and (b) $B\text{−}\mathrm{O}\text{−}B$ bonding angles for ${\mathrm{La}}_{0.5+\delta }{\mathrm{Ca}}_{0.5-\delta }{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5-ϵ}{\mathrm{O}}_3$.

Figure 4

Low angle portion of the observed high-resolution neutron-diffraction intensities of ${\mathrm{La}}_{0.5+\delta }{\mathrm{Ca}}_{0.5-\delta }{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5-ϵ}{\mathrm{O}}_3$ (cross symbols) at (a) $300\mathrm{K}$ and (b), (c) $10\mathrm{K}$, taken with the Ge(311) monochromator. Calculated intensities (solid lines) are also given, for a nuclear-only model (a), (b), and for a $\text{nuclear}+\text{ferromagnetic}$ model for the Mn and Ru spins (c). The difference profiles are also shown. Short vertical lines correspond to Bragg peak positions for the main phase (upper) and for the $\mathrm{La}{\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_6$ impurity phase (lower).

Figure 5

Temperature dependence of the summed integrated intensity of the (101), (020), (200), (002), and (121) Bragg reflections which have significant contributions from a ferromagnetic spin arrangement of Mn and Ru spins. Solid curves are guides to the eye.

Figure 6

High-intensity difference neutron profiles for ${\mathrm{La}}_{0.5+\delta }{\mathrm{Ca}}_{0.5-\delta }{\mathrm{Mn}}_{0.5+ϵ}{\mathrm{Ru}}_{0.5-ϵ}{\mathrm{O}}_3$: (a) $I\left(13\mathrm{K}\right)–I\left(300\mathrm{K}\right)$, and (b) $I\left(230\mathrm{K}\right)–I\left(300\mathrm{K}\right)$, highlighting the magnetic scattering well below and slightly above $T_c$, respectively. The smooth solid line in (b) is a two-Gaussian fit to the experimental data.