Structure, magnetism, and electronic properties in 3d−5d based double perovskite ( Sr1−xCax)2FeIrO6 ( 0≤x≤1 )

Abstract

The $3d\text{\ensuremath{-}}5d$ based double perovskites offer an ideal playground to study the interplay between electron correlation ($U$) and spin-orbit coupling (SOC) effect, showing exotic physics. The ${\mathrm{Sr}}_{2}{\mathrm{FeIrO}}_{6}$ is an interesting member in this family with ionic distribution of ${\mathrm{Fe}}^{3+}$ ($3{d}^{5}$) and ${\mathrm{Ir}}^{5+}$ ($5{d}^{4}$) where the later is believed to be nonmagnetic under the picture of strong SOC. Here we report a detailed investigation of structural, magnetic, and electronic transport properties along with electronic structure calculations in ${({\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x})}_{2}{\mathrm{FeIrO}}_{6}$ series with $x$ from 0 to 1. While the basic interactions such as $U$ and SOC are unlikely to be modified, a structural modification is expected due to ionic size difference between ${\mathrm{Sr}}^{2+}$ and ${\mathrm{Ca}}^{2+}$ which would influence other properties such as crystal field effect and bandwidths. While nonmonotonic changes in lattice parameters are observed across the series, the spectroscopic investigations reveal that $3+/5+$ charge state of Fe/Ir continue till the end of the series. An analysis of magnetic data suggests similar nonmonotonic evolution of magnetic parameters with doping. Temperature dependent crystal structure as well as low temperature (5 K) magnetic structure have been determined from neutron powder diffraction measurements which further indicate site ordered moments for both Fe and Ir. The whole series shows insulating behavior with a nonmonotonic variation in resistivity where the charge transport follows the three-dimensional variable range hopping model. The electronic structure calculations show, SOC enhanced, a noncollinear antiferromagnetic and a Mott-type insulating state is the stable ground state for the present series with a substantial amount of orbital moment, but less than the spin magnetic moment, at the Ir site and the magnetocrystalline anisotropy. The calculations further show the evolution of the spin and orbital magnetic moment components across the series along with the magnetization density. The obtained results imply that the local structural modification with introduction of lower size ${\mathrm{Ca}}^{2+}$ has a large influence on the magnetic and transport properties, further showing a large agreement between experimental results as well as theoretical calculations.