Oxygen-vacancy induced magnetic phase transitions in multiferroic thin films

Abstract

Multiferroics in which giant ferroelectric polarization and magnetism coexist are of tremendous potential for engineering disruptive applications in information storage and energy conversion. Yet the functional properties of multiferroics are thought to be affected detrimentally by the presence of point defects, which may be abundant due to the volatile nature of some constituent atoms and the high temperatures involved in the synthesis of materials. Here, we demonstrate with theoretical methods that oxygen vacancies may enhance the functionality of multiferroics by radically changing their magnetic interactions in thin films. Specifically, oxygen vacancies may restore missing magnetic super-exchange interactions in large axial ratio phases, leading to full antiferromagnetic spin ordering, and induce the stabilization of ferrimagnetic states with considerable net magnetizations. Our theoretical study should help to clarify the origins of long-standing controversies in bismuth ferrite and improve the design of technological applications based on multiferroics.

Ab initio description of oxygen vacancies in epitaxially strained $$\hbox {SrTiO}_{{3}}$$ at finite temperatures

“…For simulation of the stoichiometric systems, we employed a 20-atoms simulation cell that allows to reproduce the usual ferroelectric and antiferrodistortive (AFD) distortions in perovskite oxides 8 , 16 (Fig. 1 ).…”

“…Either compressive or tensile biaxial stress can thus be introduced in thin films upon the condition of coherent elastic coupling with the substrate. The ferroelectric 14 – 18 , magnetic 8 , 19 , optical 20 , 21 and catalytic 12 , 21 properties of TMO thin films can be drastically changed by strain engineering due to the existing strong couplings between their structural and electronic degrees of freedom.…”

Breakdown of ion-polarization-correspondence and born effective charges: Algebraic formulas of accurate polarization under field

“…Born effective charges: Practical Born effective charges are not accurate as Berry phase calculations [20], because they are intended to estimate many different materials. However, Born effective charges can agree perfectly with Berry phase calculations, by incorporating u-dependence of Z * , and other effects such as defects specific to the target FE.…”

“…Because of the similarity of the physics or spirit, we relate P S iso-bulk with Born effective charges, although P S by Born effective charges may deviate from a Berry phase P S even for E = 0 [20] and, hence, from P S iso-bulk . However, P S estimated by an optimized Z * (u)u could agree almost perfectly with Berry phase P S for a wide range, while the difference between P S 's estimated with Berry phase using different exchange correlation functionals was visible (Appendix).…”

Giant Thermal Enhancement of the Electric Polarization in FerrimagneticBiFe1−xCoxO3Menéndez1, Cazorla2 2020Phys. Rev. Lett.Self Cite150120View full textAdd to dashboardCite

“…Furthermore, phase transitions involving T phases typically exhibit colossal volume changes of ∼10% (e.g., PbVO 3 and related solid solutions), which can be exploited in mechanical degradation [11,12] and solidstate cooling [13][14][15] applications. Examples of T multiferroic materials are bulk BiCoO 3 (BCO) and BiFeO 3 (BFO) thin films [16][17][18].…”