Electronic and magnetic ground state of 4$d^3$ double perovskite ruthenates A$_2$LaRuO$_6$ (A $=$ Ca, Sr, Ba)

TL;DR

This study clarifies the electronic and magnetic ground state of 4d$^3$ double perovskite ruthenates A$_2$LaRuO$_6$ (A = Ca, Ba) by combining bulk measurements and density functional theory. The results show a conventional, spin-only Mott insulator with Ru$^{5+}$ ($4d^3$) ions displaying AFM order and strong frustration, while SOC plays a negligible role in determining the ground state. The calculated exchange constants and the observed low-temperature spin dynamics point to a frustrated spin system with unusual spin-wave behavior, and the dynamical instability of the Sr variant explains its nonexistence. Overall, the work demonstrates that Ca$_2$LaRuO$_6$ and Ba$_2$LaRuO$_6$ realize a non-relativistic Mott AFM ground state, enriching the understanding of 4d$^3$ DP ruthenates and guiding future explorations of SOC- vs correlation-driven physics in related materials.

Abstract

4$d$ transition metal oxide (TMO) offers an intriguing puzzle for their electronic and magnetic ground state. They are in the cross-over regime of strong spin orbit interaction (SOI) and electron-electron correlation ($U$) with quenched orbital angular momentum. Our work unravels the electronic and magnetic ground state of the less investigated 4$d^{3}$ double perovskite ruthenates A$_{2}$LaRuO$_6$ (A = Ca, Ba). The negligible effect of SOI is evident from the bulk magnetic, specific heat measurements and density functional theory (DFT) calculations, indicating a classical spin-only magnetic ground state (${S}$ = 3/2) for the materials. Magnetization measurements show that both materials have long range antiferromagnetic order with high degree of magnetic frustration ($f$ $\approx$13 -15). Interestingly, a near $T^2$- behavior is observed in low-$T$ magnetic heat capacity measurement, indicating the presence of low-dimensional spin-wave exciation and magnetic frustration in both materials. The temperature dependent resistivity measurements and electronic band structure calculations confirm a conventional Mott insulating ground state in these two systems. Moreover, our experimental investigation and DFT calculations highlight the reason for the nonexistence of Sr$_2$LaRuO$_6$.

Figures (12)

• Figure 1: Reitveld refinement of PXRD pattern a) BLRO and b) CLRO. The measured data, the calculated pattern from the refinement, the background and the difference between the measured data and the calculated pattern are referred to as obs, calc, bkgd, and diff, respectively. The vertical blue lines refer to the location of Bragg peaks for the respective phases. The crystal structure of BLRO and CLRO, their RuO$_6$ octahedron with Ru-O bond lengths and the corresponding Ru sublattice are shown in the respective insets, which are visualized using the VESTA softwarevesta.
• Figure 2: Temperature dependent inverse magnetic molar susceptibility data for a) BLRO and b) CLRO at an external magnetic field, $H$ = 10 kOe with the CW fit (red line). Inset (i) shows the molar susceptibility ($\chi_M$) and $\frac{\mathrm{d}(\chi_{M}T)}{\mathrm{d} T}$$vs.$$T$ and inset (ii) shows the zero field cooled (ZFC) and field cooled (FC) $\chi_M$$vs.$$T$ at low field, $H$ = 100 Oe near magnetic phase transitions for both the systems in their respective panels.
• Figure 3: Field dependent magnetization ($M$$vs.$$H$) data for BLRO (closed symbols ) and CLRO (open symbols) at different temperatures. Inset (i) shows the $\frac{\mathrm{d} M}{\mathrm{d} H}$$vs.$$H$ plot of BLRO for the same temperatures.
• Figure 4: Temperature dependent resistivity measurements ($\rho$$vs.$$T$) of BLRO and CLRO. Inset shows 3D variable range hopping (VRH) fit for the same.
• Figure 5: Temperature dependent heat capacity measurements of a) BLRO and b) CLRO at $H$ = 0 kOe along with the Debye, Einstein and Debye - Einstein fit. Insets (i) display the temperature dependence of heat capacity at different fields.