Electronic and magnetic phase transitions, optimized MAE/ TC, and high thermoelectric response in Y2NiIrO6: Strain effects

Abstract

We explore the biaxial ([110]) strain consequences on the distinct features of the pristine (prist.) Y${}_2$NiIrO${}_6$ motif using ab-initio calculations. The anomalous $J_{eff.}=\frac{1}{2}$ state of Ir${}^{+4}$, leads the system into a Mott-insulating (MI) state attaining an energy gap ($E_g$) of 0.21 eV with a ferrimagnetic (FiM) phase, which is ultimately caused by anti-ferromagnetic (AFM) coupling between the half-filled Ni${}^{+2}3d^8$ and partially-filled Ir${}^{+4}5d^5$ orbitals, via oxygen 2$p^2$ states. Remarkably, lattice thermal conductivity (${\kappa }_L$) is computed utilizing the Slack model, which significantly lowers the figure of merit (ZT) from 0.75 (excluding ${\kappa }_L$) to 0.34 (including ${\kappa }_L$) at 300 K. Interestingly, a reasonable ZT of 0.58 is achieved above room temperature (500 K). Moreover, the computed partial spin moments ($m_s$) for the Ni${}^{+2}$ and Ir${}^{+4}$ ions holding high spin and low spin states of S $=1$ and $\frac{1}{2}$ are ＋ 1.67 and $-0.53{\mu }_B$, respectively. The easy magnetic axis is determined to be the $b$-axis, which produces a significant magnetic anisotropy energy (MAE) constant of $1.7\times 10^8$ erg/cm³ keeping a Curie temperature ($T_C$) of 198 K. Strikingly, the system demonstrated magnetic transitions from FiM to FM and FiM to AFM at the critical ＋ 5% and ＋ 4% tensile strains within the GGA$+U$ and GGA$+U+$SOC methods, respectively. Likewise, a Mott-insulator-to-half-metal transition is obtained at a crucial compressive strain of $-$6% with a robust FiM state. Moreover, our results revealed that compressive strain enhances the structural distortions, which substantially improved the MAE and $T_C$ values up to 28.9 meV and 265 K, respectively, indicating the system to be the best candidate for magnetic memory devices as well as for thermoelectric applications.