Spin-singlet dimerization in La${}_2$RuO${}_5$ investigated using magnetic susceptibility and specific heat measurements

The origin of spin-dimerization and concomitant spin-gap opening in the triclinic phase of poly- and single-crystalline La${}_2$RuO${}_5$ at unusually high temperatures was investigated using magnetic susceptibility and specific-heat measurements. From the low-temperature crystal structure the formation of antiferromagnetically coupled Ru${}^{4+}$ ($S=1$) dimers within the quasi-two-dimensional magnetic system can be deduced, resulting in a nonmagnetic singlet state. It was found that the antiferromagnetic coupling within the dimers is much stronger than the interaction with neighboring dimers. La${}_2$RuO${}_5$ exhibits a step-like change in the magnetic susceptibility at 161 K, indicating a first-order transition of combined magnetic and structural character. The size of the spin-gap has been estimated from the thermally activated behavior in the low-temperature dimerized phase and was found to be significantly different in the polycrystalline sample when compared to the results obtained from the single crystals. The magnetic entropy obtained from specific-heat measurements amounts to roughly $0.5R\ln \left(3\right)$, reflecting solely the contribution of spin degrees of freedom to the entropy change during the phase transition.

Figure 1

(a) Crystal structure of La${}_2$RuO${}_5$ ($2\times 2$ unit cells) viewed along the $c$ axis. La is represented by turquoise spheres, oxygen by red spheres, and RuO${}_6$ octahedra are drawn in light green. The alternating layering of LaO and LaRuO${}_4$ along the $a$ axis is indicated by dashed lines. (b) RuO${}_6$ octahedra network (transparent) in the $bc$ plane of the LaRuO${}_4$ layers. One ruthenium (green spheres) dimer is marked by a red ellipse and the exchange paths are denoted $J_0$ to $J_3$. For details see text.

Figure 2

Top frame: Magnetic susceptibility of polycrystalline La${}_2$RuO${}_5$ from sol-gel reaction (open circles). Bottom frame: Susceptibility of flux grown La${}_2$RuO${}_5$ single crystals (open triangles). The solid red lines (a) mark the 2D Heisenberg model, and the solid blue lines (b) indicate the lt-Curie-Weiss fits of the low-temperature Curie tails. The inset in the lower frame shows the thermal hysteresis of the dimerization transition in the susceptibility of single crystalline La${}_2$RuO${}_5$.

Figure 3

Magnetization hysteresis data of a batch of single crystalline La${}_2$RuO${}_5$ for selected temperatures.

Figure 4

Derivatives of the magnetic susceptibilities for poly- (open circles) and single-crystalline (open triangles) La${}_2$RuO${}_5$. The susceptibilities have been corrected for the low-temperature Curie-Weiss contributions as indicated in Fig. 2. The solid lines correspond to Lorentzian-fits, centered at the phase transition temperature.

Figure 5

Arrhenius plots of the corrected susceptibilities in the low-temperature phase for poly- (open circles) and single-crystalline (open triangles) La${}_2$RuO${}_5$. The solid lines are linear fits assuming an Arrhenius behavior, resulting in spin-gaps of ${\Delta }_{\mathrm{pc}}=48$ meV and ${\Delta }_{\mathrm{sc}}=112$ meV for poly- and single-crsytalline samples, respectively. The Arrhenius behavior characteristic for a spin-gap of $\Delta =40$ meV as observed by inelastic neutron scattering is indicated as dashed line.

Figure 6

Top frame: $C_p/T$ of polycrystalline La${}_2$RuO${}_5$ (symbols) and Einstein-Debye fit (solid red line). Bottom frame: Data after subtraction of the Einstein-Debye fit $((C_p-C_{\mathrm{Lattice}})/T$; symbols$)$ and $S_{\mathrm{mag}}$ obtained from integration of the residual curve (solid red line).