Lattice Instability and Competing Spin Structures in the Double Perovskite InsulatorSr2FeOsO6

Abstract

The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic iron-osmium layers along the tetragonal c axis. Neutron powder diffraction experiments, 57Fe Mössbauer spectra, and density functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating iron-osmium distances fine-tunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example of a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbital, and lattice degrees of freedom.

Fragility of ferromagnetic double exchange interactions and pressure tuning of magnetism in3d−5ddouble perovskiteSr2FeOsO6

“…In fact, two different AFM configurations (AF1 and AF2, Fig. 6c) are observed by neutron diffraction to coexist at low temperature, the AF2 phase appearing on cooling below 67 K and becoming dominant below 55 K (85% at 2 K) 27,28 . The AF1 phase consists of FM Fe-Os chains along the c-axis, while half of FeOs bonds become AFM coupled in the AF2 phase.…”

“…Experimental and theoretical reports on the nature of exchange interactions in Sr 2 FeOsO 6 have addressed the origin of AFM order in this structure at ambient pressure 27,28,33 . In fact, two different AFM configurations (AF1 and AF2, Fig.…”

“…The AF1 phase consists of FM Fe-Os chains along the c-axis, while half of FeOs bonds become AFM coupled in the AF2 phase. The change in magnetic structure on cooling may be associated with a lattice distortion in which short (AFM) and long (FM) Fe-Os distances are created 27 .…”

“…27 . High pressure single crystal diffraction experiments will be needed to detect whether this modulation, if present, persists under pressure.…”

“…The orbital magnetization at Os sites is opposite to the spin magnetization, as expected for a less than half-filled 5d orbital occupancy (calculated number of 5d holes is n h =6.38). 27,28 ). The orbitalto-spin moment ratio is ten times larger at Os sites due to the stronger S-O interaction.…”