Room Temperature Electric‐Field Control of Magnetism in Layered Oxides with Cation Order

Abstract

Searching for materials with room‐temperature electric‐field control of magnetism has interested researchers for many years with three‐dimensional perovskite BiFeO3‐based compounds as the main focus. Here we choose the layered hybrid improper ferroelectric Ruddlesden‐Popper oxides as a platform from which to realize electric field controllable magnetism, leveraging a recently identified strain tunable polar‐to‐nonpolar (P‐NP) transition. We first propose a design principle for selecting the required A and B cation chemistries that will ensure (001) A3B2O7 films exhibit P‐NP transitions, which we substantiate with density functional calculations. By extending the guideline to B‐site ordered A3BB′O7 oxides, we identify more compounds exhibiting P‐NP transitions marked by the disappearance of an in‐plane polarization that can be functionalized. We then demonstrate that weak ferromagnetism can be tuned by an electric field at the boundary of the P‐NP transition in B‐site ordered (001) A3BB′O7 magnetic films, based on which we predict that cation ordered Ca3TcTiO7 may be a viable candidate for room‐temperature electric‐field control of magnetism.

Ferroelectric Sr3Zr2O7: Competition between Hybrid Improper Ferroelectric and Antiferroelectric Mechanisms

“…Most remarkably, we discover a metastable paraelectric polymorph (space group Pnab ), which competes with the equilibrium ferroelectric phase ( A 2 1 am ), and an unusual first‐order ferroelectric–paraelectric phase transition involving a change in the “sense” of the OOR while the OOT persists at all temperatures. We show that the paraelectric Pnab phase appears through a trilinear coupling of OOR and OOT modes interacting with an antipolar mode, a recently predicted hybrid improper “antiferroelectric” mechanism . In addition to being of technological significance for the development of new ferroelectrics, our results highlight the importance of multimode anharmonic interactions in producing functional acentric material properties.…”

“…An interesting direction for possible future investigations is understanding how to control the relative energetics of these two structures. In this respect, it will be interesting to explore experimentally the possibility of epitaxial‐strain‐induced polar‐to‐nonpolar ( A 2 1 am ‐to‐ Pnab ) transition, according to the recent theoretical prediction . The synthesis routes, annealing conditions, and cation substitutions should also activate new modes through the change in OOT and OOR preferences.…”

“…Thus, the unusual ferroelectric phase transition in Sr 3 Zr 2 O 7 is understood to occur from the interplay between the multimode anharmonic interactions active in the layered oxide. The form of hybrid improper “antiferroelectricity,” , was recently predicted by first‐principles calculations, but has not been demonstrated experimentally. Our results represent the first experimental verification corroborating this prediction, and show the importance of understanding anharmonic interactions among lattice degrees of freedom using state‐of‐the‐art X‐ray and neutron diffraction as well as nonlinear optical methods.…”

Hybrid Improper Ferroelectricity in (Sr,Ca)3Sn2O7 and Beyond: Universal Relationship between Ferroelectric Transition Temperature and Tolerance Factor in n = 2 Ruddlesden–Popper Phases

“…The experimental studies, complemented by first-principles density functional theory (DFT) calculations, allow us to identify the role played by the two nonpolar structural distortions-oxygen octahedral rotations (OOR, rotational modes about c axis) and oxygen octahedral tilts (OOT, rotational modes about the a and/or b axis)-in stabilizing the ferroelectric A2 1 am and paraelectric Pnab structures. Our results demonstrate that the stable ferroelectric A2 1 am phase appears through a trilinear coupling of OOR and OOT modes to a polar mode (i.e., hybrid improper ferroelectric mechanism) and that the metastable Pnab polymorph, which competes with the equilibrium A2 1 am phase, emerges from a trilinear coupling of OOR and OOT modes interacting with an antipolar mode (i.e., hybrid improper antiferroelectric mechanism) 37,50,51 . Further, we observe that the T C s increase linearly with the substitution of Ca 2+ for Sr 2+ .…”

Electric-Field Control of Magnetization, Jahn-Teller Distortion, and Orbital Ordering in Ferroelectric Ferromagnets

“…For instance, the weak ferromagnetism and Dzyaloshinskii-Moriya (DM) vector in BiFeO 3 can be switched by 180°under an electric field by a two-step sequential rotation of the polarization [10]. Moreover, the weak ferromagnetism of hybrid improper ferroelectric superlattices, such as BiFeO 3 =NdFeO 3 [11] and BiFeO 3 =LaFeO 3 [12], and of Ruddlesden-Popper oxides with B-site cation ordered A 3 BB 0 O 7 [13] was also predicted to be controllable by an electric field because the polarization is coupled with other nonpolar lattice distortions. However, all of the latter multiferroics possess a predominant antiferromagnetic alignment; that is, they "only" have weak ferromagnetism and/or are ferrimagnetic [14,15].…”