Electronic structure of Fe and magnetism in the 3d/5d double perovskites Ca2FeReO6 and Ba2FeReO6

Abstract

The Fe electronic structure and magnetism in (i) monoclinic ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ with a metal-insulator transition at ${T}_{MI}\ensuremath{\sim}140\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and (ii) quasicubic half-metallic ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$ ceramic double perovskites are probed by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). These materials show distinct Fe ${L}_{2,3}$ XAS and XMCD spectra, which are primarily associated with their different average Fe oxidation states (close to ${\mathrm{Fe}}^{3+}$ for ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ and intermediate between ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ for ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$) despite being related by an isoelectronic (${\mathrm{Ca}}^{2+}/{\mathrm{Ba}}^{2+}$) substitution. For ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$, the powder-averaged Fe spin moment along the field direction ($B=5\phantom{\rule{0.28em}{0ex}}\mathrm{T}$), as probed by the XMCD experiment, is strongly reduced in comparison with the spontaneous Fe moment previously obtained by neutron diffraction, consistent with a scenario where the magnetic moments are constrained to remain within an easy plane. For $B=1\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, the unsaturated XMCD signal is reduced below ${T}_{MI}$ consistent with a magnetic transition to an easy-axis state that further reduces the powder-averaged magnetization in the field direction. For ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$, the field-aligned Fe spins are larger than for ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ ($B=5\phantom{\rule{0.28em}{0ex}}\mathrm{T}$) and the temperature dependence of the Fe magnetic moment is consistent with the magnetic ordering transition at ${T}_{C}^{Ba}=305\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Our results illustrate the dramatic influence of the specific spin-orbital configuration of Re $5d$ electrons on the Fe $3d$ local magnetism of these Fe/Re double perovskites.