Tuning Magnetic and Electronic Properties of Double Perovskite La$_2$CoIr$_{1-x}$Ti$_x$O$_6$

TL;DR

This work probes how spin–orbit coupling (SOC) and electronic correlations govern magnetism and transport in La$_2$CoIrO$_6$ by substituting nonmagnetic Ti at the Ir site to form La$_2$CoIr$_{1-x}$Ti$_x$O$_6$, effectively applying chemical pressure and diluting Ir–SOC. A combined experimental approach (XRD, magnetization, hydrostatic pressure, and thermal expansion) and first-principles DFT (GGA+SOC+U) reveals a gradual FM-like to AFM transition with $x$, lattice contraction with doping, and a steadily increasing band gap from $0.28$ eV ($x=0$) to $1.01$ eV ($x=1$). The calculations show that SOC is decisive for the insulating state and Ir moments, which are markedly reduced by Ti substitution, consistent with disrupted Co–Ir exchange pathways. The results highlight a robust route to tailor SOC-driven magnetism and electronic structure via chemical pressure, with potential piezomagnetic responses and implications for functional oxides design.

Abstract

The La$_2$CoIr$_{1-x}$Ti$_x$O$_6$ double perovskite series serves as an effective platform for investigating the evolution of magnetic and electronic properties as a function of chemical pressure (doping) or hydrostatic pressure due to the interplay between the electrons correlation and spin-orbit coupling. In this study, the substitution of nonmagnetic Ti$^{4+}$ at the magnetic Ir$^{4+}$-site leads to a systematic decrease in unit cell volume keeping the monoclinic symmetry throughout, reflecting the effect of chemical pressure along with a gradual suppression of magnetic interactions. The parent compound ($x =$ 0) exhibits a ferromagnetic-like state with a Curie temperature of 92 K, which continuously evolves into an antiferromagnetic ground state upon full Ti substitution ($x =$ 1) with a Neel temperature of 14.6 K. Isothermal magnetization measurements reveal a hysteresis behavior with step-like feature at zero field, indicative of a noncollinear magnetic ordering. Additionally, the enhancement of magnetization under hydrostatic pressure on La$_2$CoIrO$_6$ suggests the presence of piezomagnetic behavior. Thermal expansion measurements on La$_2$CoIrO$_6$ highlight a coupling between spin and lattice degrees of freedom. The pressure dependence of the transition temperature in the zero-pressure limit, calculated using Ehrenfest's relation, shows good agreement with magnetization data under applied pressure. First-principles density functional theory (DFT) calculations preformed for $x =$ 0, 0.5 and 1, further reveal that strong SOC associated with Ir plays a decisive role in shaping the electronic band structure, with the insulating gap progressively widening as Ti content increases from 0.28 eV ($x =$ 0), 0.44 eV ($x =$ 0.5), and 1.01 eV ($x =$ 1). The magnetic moment decreased more than 50\% for $x =$ 0.5, showing the decrease in magnetic exchange pathways.

Figures (8)

• Figure 1: Rietveld-refined powder XRD patterns at room temperature for (a) $x = 0$, (b) $x = 0.5$, (c) $x = 0.85$, and (d) composition dependence of the unit cell volume of compounds fitted with linear Vegard's law. The black line, the red line, the vertical marks (green line), and the blue lines correspond to the experimental data, the calculated data, Bragg peaks, and the difference between the observed and calculated curves, respectively. The other concentrations ($x$ = 0.25, 0.65, and 0.75) are also synthesized in pure form and their lattice parameters are given in table \ref{['tab:table 1']}. The detailed structural details of $x = 1$ has already been reported in our earlier paper Nandi2024.
• Figure 2: (a) Crystal and magnetic structure of LCIO, where the structure is formed by corner-sharing CoO$_6$ and IrO$_6$ octahedra with La ions at the A sites. The Co$^{2+}$ and Ir$^{4+}$ spins are antiferromagnetically aligned within their sublattices and also with respect to each other, resulting in an overall AFM ground state. This magnetic structure is employed for the DFT calculations, and (b) the intensity of various Bragg's positions with the angle of La$_2$CoIr$_{1-x}$Ti$_x$O$_6$. The phenomenon of peak shifting in the XRD patterns is observed across all concentrations, revealing significant insights into the structural variations within the material. As the concentrations vary, the diffraction peaks gradually shift towards higher angles, indicating alterations in lattice parameters and atomic arrangements. In this figure, the ($hkl$) values are mentioned only for the parent compound (LCIO).
• Figure 3: (a) Temperature dependent magnetic susceptibilities of La$_2$CoIr$_{1-x}$Ti$_x$O$_6$, data taken upon FC with field of 1 T, (b) temperature derivative of magnetic susceptibility where the dip in the curves showing the transition temperature, (c) field dependence of magnetization for La$_2$CoIr$_{1-x}$Ti$_x$O$_6$ well below the magnetic ordering temperature, (d) temperature dependent magnetic susceptibility of LCIO at various constant hydrostatic pressure values for FC in the presence of field 0.1 T, and inset shows the derivative of magnetic susceptibility with temperature where the peak for different pressure shows the transition temperature. Note that the sample used for magnetic measurement under pressure (see Fig. \ref{['fig:Fig2_exp.pdf']}(d)) and the sample used for other magnetic measurements (see Fig. \ref{['fig:Fig2_exp.pdf']}(a) and (c)) are taken from two batches, indicating a sample dependency.)
• Figure 4: (a) The relative change in the magnetization from the paramagnetic phase calculated using the relation $\Delta M$ = $M$($T$) - $M$(150 K), as a function of pressure at 62, 75, and 89 K. The dotted line indicates the linear fit using the function $\Delta M = M_0 + QP$, where $M_0$ stands for the spontaneous magnetization, $Q$ the powder-averaged piezomagnetic tensor, and $P$ the applied hydrostatic pressure. (b) Temperature dependence of the powder-averaged piezomagnetic tensor ($Q$).
• Figure 5: Temperature dependence of $\alpha/T$ and $C_\textrm{P}/T$Narayanan2010_thesis of LCIO. A broad transition is visible around the magnetic phase transition, indicating the coupling of lattice degrees of freedom. Entropy conserved equal-area construction for the thermal expansion (red solid line) and heat capacity (green solid line), respectively) was employed in order to determine the transition temperature.