[Paper Review] Understanding the Magnetic Puzzles of Double Perovskites A2FeOsO6 (A=Ca, Sr)

Yusheng Hou, Hongjun Xiang+1 more|arXiv (Cornell University)|Oct 16, 2014

Magnetic and transport properties of perovskites and related materialsMaterials Science31 references18 citations

TL;DR

This study uses density functional theory and maximally localized Wannier functions to explain the contrasting magnetic behaviors in A2FeOsO6 (A = Ca, Sr): Ca2FeOsO6 exhibits high-temperature ferrimagnetism, while Sr2FeOsO6 is a low-temperature antiferromagnet. Lattice distortion modulates superexchange pathways, weakening antiferromagnetic coupling through Os-O-O-Os and Fe-O-Os-O-Fe paths, which relieves magnetic frustration and enables the observed transition with decreasing A-site cation radius.

ABSTRACT

Double perovskites Sr2FeOsO6 and Ca2FeOsO6 show puzzling magnetic properties, the former a low-temperature antiferromagnet while the later a high-temperature insulating ferrimagnet. Here, in order to understand the underlying mechanism, we have investigated the frustrated magnetism of A2FeOsO6 by employing density functional theory and maximally-localized Wannier functions. We find that lattice distortion enhances the antiferromagnetic nearest-neighboring Fe-O-Os interaction but weakens the antiferromagnetic interactions through the Os-O-O-Os and Fe-O-Os-O-Fe paths, which is responsible for the magnetic transition from the low-temperature antiferromagnetism to the high-temperature ferrimagnetism with the decrease of the radius of the A2+ ions. We also discuss the 5d3-3d5 superexchange and propose such superexchange is intrinsically antiferromagnetic instead of the expected ferromagnetic. Our work illustrate that the magnetic frustration can be effectively relieved by lattice distortion, which provides another dimension to tune the complex magnetism in other 3d-5d (4d) double perovskites.

Motivation & Objective

• To resolve the puzzling magnetic behavior in double perovskites A2FeOsO6 (A = Ca, Sr), where Ca-based compounds show high-temperature ferrimagnetism while Sr-based ones exhibit low-temperature antiferromagnetism.
• To investigate the role of lattice distortion in modulating magnetic interactions in 3d-5d double perovskites.
• To clarify the nature of the 5d3-3d5 superexchange interaction, which is found to be intrinsically antiferromagnetic despite expectations of ferromagnetism.
• To explore how structural distortions relieve magnetic frustration and influence the magnetic ground state in these complex oxides.

Proposed method

• Employed density functional theory (DFT) with spin-polarized calculations to model electronic and magnetic properties of A2FeOsO6.
• Used maximally localized Wannier functions to map the electronic structure and extract effective magnetic interactions.
• Analyzed superexchange pathways involving Fe-O-Os, Os-O-O-Os, and Fe-O-Os-O-Fe to determine the strength and sign of exchange coupling.
• Compared the magnetic interactions in Ca2FeOsO6 and Sr2FeOsO6 to correlate structural differences with magnetic behavior.
• Evaluated the role of lattice distortion in modifying the Fe-O-Os bond angles and distances, affecting the strength of antiferromagnetic coupling.
• Proposed a mechanism where lattice distortion suppresses competing antiferromagnetic paths, favoring ferrimagnetic order in smaller A-site cations.

Experimental results

Research questions

• RQ1 Why does Ca2FeOsO6 exhibit high-temperature ferrimagnetism while Sr2FeOsO6 shows low-temperature antiferromagnetism despite similar electronic configurations?
• RQ2 How does lattice distortion influence the relative strength of antiferromagnetic superexchange pathways in 3d-5d double perovskites?
• RQ3 Is the 5d3-3d5 superexchange interaction intrinsically ferromagnetic or antiferromagnetic, and what determines its sign?
• RQ4 To what extent can magnetic frustration in A2FeOsO6 be relieved by structural distortions?
• RQ5 What is the role of the Os-O-O-Os and Fe-O-Os-O-Fe pathways in determining the overall magnetic ground state?

Key findings

• Lattice distortion in Ca2FeOsO6 enhances antiferromagnetic Fe-O-Os coupling but weakens competing antiferromagnetic paths through Os-O-O-Os and Fe-O-Os-O-Fe pathways.
• The weakening of long-range antiferromagnetic interactions due to structural distortion leads to a stabilization of ferrimagnetic order in Ca2FeOsO6.
• The 5d3-3d5 superexchange is found to be intrinsically antiferromagnetic, contrary to the commonly assumed ferromagnetic character.
• The magnetic transition from antiferromagnetism in Sr2FeOsO6 to ferrimagnetism in Ca2FeOsO6 is attributed to the reduction in A-site cation radius, which induces lattice distortion.
• Lattice distortion effectively relieves magnetic frustration by suppressing competing antiferromagnetic pathways, enabling a net ferrimagnetic ground state.
• The study provides a general mechanism for tuning complex magnetism in 3d-5d (4d) double perovskites through structural control.