Glasslike ordering and spatial inhomogeneity of magnetic structure in Ba${}_3$FeRu${}_2$O${}_9$: Role of Fe/Ru site disorder

Several doped 6$H$ hexagonal ruthenates, having the general formula Ba${}_{3}M$Ru${}_2$O${}_9$, have been studied over a significant period of time to understand the unusual magnetism of ruthenium metal. However, among them, the $M=\text{Fe}$ compound appears different since it is observed that unlike others, the 3$d$ Fe ions and 4$d$ Ru ions can easily exchange their crystallographic positions, and as a result many possible magnetic interactions become realizable. The present study involving several experimental methods on this compound establishes that the magnetic structure of Ba${}_3$FeRu${}_2$O${}_9$ is indeed very different from all other 6$H$ ruthenates. Local structural study reveals that the possible Fe/Ru site disorder further extends to create local chemical inhomogeneity, affecting the high-temperature magnetism of this material. There is a gradual decrease of ${}^{57}\mathrm{Fe}$ Mössbauer spectral intensity with decreasing temperature (below 100 K), which reveals that there is a large spread in the magnetic ordering temperatures, corresponding to many spatially inhomogeneous regions. However, finally at about 25 K, the whole compound is found to take up a global glasslike magnetic ordering.

Figure 1

The ordered crystal structure of Ba${}_3$MRu${}_2$O${}_9$.

Figure 2

Experimental (black) and refined (red) XRD patterns (a) and neutron powder diffraction patterns (b) for Ba${}_3$FeRu${}_2$O${}_9$ at room temperature.

Figure 3

Field-cooled and zero-field-cooled magnetization curve vs $T$ under $H$ $=$ 100 Oe is shown in (a). The inverse susceptibility as a function of temperature for $H$ $=$ 5 kOe is shown in (b), while the solid line expresses the Curie-Weiss behavior. The deviation from the Curie-Weiss nature is observed below 80 K. The relaxation of IRM as a function of time at selected temperatures is shown in (c). The ZFC as well as FC (cooled under 50 kOe applied field) $M$ vs $H$ loops at 2 K are shown in (d), while an expanded view is shown in (e).

Figure 4

The variation of heat capacity $C$ with temperature ($T$) is shown in (a) while C/T vs $T$ is plotted in (b). (c) shows the experimental (black) and refined (red) neutron powder diffraction patterns for Ba${}_3$FeRu${}_2$O${}_9$ collected at 2 K.

Figure 5

The $M$ vs $H$ at different temperatures are shown in (a), while an expanded view near zero field is plotted in (b). (c) shows the variation of IRM with time ($t$) at 80 K.

Figure 6

${}^{57}\mathrm{Fe}$ Mössbauer data collected at different temperatures.

Figure 7

(a) and (b) show the Ru $K$-edge XAFS data and its Fourier transform along with the respective best-fit spectra.