Ba-substitution induced evolution of structural and magnetic properties of La2-xBaxCoIrO6 double perovskites

TL;DR

This study probes how Ba substitution at the La site tunes structure, valence balance, and magnetism in the La_2−xBa_xCoIrO_6 double perovskites, where Ir bears strong spin-orbit coupling. Using solid-state synthesis, X-ray diffraction with Rietveld refinement, and dc magnetometry, the authors map structural evolution from monoclinic to triclinic, unit-cell expansion, and anti-site disorder across x=0, 0.5, 0.75, 1.0, linking these changes to Co–Ir exchange and oxidation states. The magnetic ground state shifts from ferrimagnetic behavior due to AFM Co–Ir coupling for x=0, 0.5, 0.75 to a predominantly antiferromagnetic arrangement on Co when Ir becomes nonmagnetic Ir^5+ at x=1, with Tc/TN values around 94, 70, 66, and 61 K, respectively. The results underscore the pivotal role of Ir SOC and valence tuning in controlling exchange interactions in 3d–5d double perovskites, suggesting routes to engineer complex magnetic states via A-site substitution.

Abstract

The Iridium-based oxides are the subject of great recent interest due to the non-conventional physics that may emerge from the strong spin-orbit coupling present in 5d ions. Here, we explore the coupling between Ir and Co in the La2-xBaxCoIrO6 perovskites (x = 0, 0.5, 0.75 and 1.0), where the structural, electronic, and magnetic properties of the series are investigated by means of x-ray powder diffraction and magnetometry. The system's crystal structure evolves from the monoclinic P2_1/n to the triclinic I-1 space group as the Ba concentration increases. Measurements of magnetization revealed ferrimagnetic behavior in x = 0, 0.5 and 0.75 compounds, possibly resulting from antiferromagnetic coupling between Co2+/3+ and Ir4+. In contrast, for x = 1.0 a clear collinear antiferromagnetic character is observed for the Co2+ ions, resulting from the quenching of the Ir5+ magnetic moment. The evolution of the magnetic properties of the series is discussed in terms of the structural and electronic changes, as well as the spin-orbit coupling in Ir.

Figures (3)

• Figure 1: (a) Rietveld refinement fitting of the $x$ = 1.0 crystal structure from XRD. The inset compares the XRD patterns of the $x$ = 0.5 and 1.0 samples, where the vertical lines represent the Bragg reflections for the $P2_{1}/n$ and $I\overline{1}$ space groups, highlighting the distinct diffraction peaks. (b) Main diffraction peaks for $x$ = 0, 0.5, 0.75 and 1.0, where the inset shows the evolution of the unit cell volume. (c) Co/Ir ASD and $\langle$Co-O-Ir$\rangle$ bond angle as a function of Ba content.
• Figure 2: (a) ZFC-FC $M(T)$ curves measured with $H$ = 0.1 T. The inset shows a magnified view of the low temperature region of $x$ = 1.0, highlighting its $T_N$. (b) $\chi^{-1}$ curves, where the straight lines represent the best fits with the CW law.
• Figure 3: $M(H)$ curves measured at 5 K. The inset shows a magnified view of the low field region.