Session B60: Multiferroics I

Electric Field Control of Magnetism

Complex perovskite oxides exhibit a rich spectrum of functional responses, including magnetism, ferroelectricity, highly correlated electron behavior, superconductivity, etc.   The basic materials physics of such materials provide the ideal playground for interdisciplinary scientific exploration with an eye towards real applications.  Over the past decade the oxide community has been exploring the science of such materials as crystals and in thin film form by creating epitaxial heterostructures and nanostructures.  Among the large number of materials systems, there exists a small set of materials which exhibit multiple order parameters; these are known as multiferroics, particularly, the coexistence of ferroelectricity and some form of ordered magnetism (typically antiferromagnetism). The scientific community has been able to demonstrate electric field control of both antiferromagnetism and ferromagnetism at room temperature. Current work is focused on ultralow energy (1 attoJoule/operation) electric field manipulation of magnetism as the backbone for the next generation of ultralow power electronics. We are exploring many pathways to get to this goal. In this talk, I will describe our progress to date on this exciting possibility.  The talk will conclude with a summary of where the future research is going.   

Probing the Magnetic Order of BiFeO3 using Spin Hall Magnetoresistance

With Néel and Curie temperatures well above room temperature, multiferroic BiFeO₃ (BFO) is an ideal candidate material for antiferromagnetic (AFM) spintronic device applications. The limited coupling of AFMs to external magnetic field perturbations challenges investigations of the magnetic domain structure. Recently, spin Hall magnetoresistance (SMR) has been used to probe the magnetic order in AFM materials such as Fe₂O₃ and NiO. Here we show that SMR can probe the magnetic order in BFO thin films. To measure SMR, a BFO/Pt heterostructure was patterned into a Hall bar, and the longitudinal and transverse resistance measured as a function of applied magnetic field. In monodomain films, we deduce the BFO spin cycloid propagation vector from the dependence of the SMR signal on spin current orientation.   

Aluminium-doped BiFeO3 as a potential multiferroic material

Epitaxial films of BiAl_xFe_1-xO₃ (xBAFO) are a candidate for alternate multiferroic materials. xBAFO films are grown on SrTiO₃ (001) (STO) and SrRuO₃ buffered STO substrates using pulsed laser deposition technique. The X-ray diffraction patterns show that phase-pure xBAFO can be synthesized for x ≤ 0.35 without any secondary phases. xBAFO retains  rhombohedral structure as BiAlO₃ and BiFeO₃ are isotypic. The ferroelectric properties of xBAFO are explored by analyzing P-V hysteresis loops. The shape of the loops changes from being square to increasingly slanted with higher Al doping. The domain growth mode, morphology, size, dimension and nucleation have a crucial role on the switching behavior. To analyze the switching behavior, domain images have been studied using piezo-response force microscopy. With increasing concentration of Al, the films exhibit higher resistance enabling the application of higher voltage. We demonstrate a reduction of piezoelectric coercive voltage is achieved by increasing substitution of Fe by Al on BFO. Since the spin ordering of Fe atoms is modified by Al, enhancement in magnetic properties is also expected. These results suggest that xBAFO is a potential multiferroic material to realize BFO-based next generation non-volatile memory devices.   

Microscopy of the magnetoelectric coupling in hexagonal manganites: multiferroic domains and emergent topological defects

When studying magnetoelectric coupling effects in materials with simultaneous magnetic and electric order, the so-called multiferroics, fundamental understanding of the cross coupling between ferroic orders at the level of domains and domain walls is paramount. Domains and domain walls reflect the different interdependence of magnetic and electric order in multiferroics. For example in type-II multiferroics, domain patterns are one-to-one coupled whereas in type-I, magnetic and electric domain configurations can be different and their coupling no longer mandatory. We show in experiment and theory, that multiferroics with separately emerging magnetic and electric order (type-I) can exhibit a strong bulk coupling. We show, taking h-ErMnO3 as an example, that a strong bulk coupling between ferroelectric and antiferromagnetic orders is realized even though the linear magnetoelectric effect is symmetry forbidden. We show that the structural distortions that lead to the ferroelectric polarization break the balance of the competing superexchange contributions leading to this hidden, yet strong phenomenon. The non-rigidity of this bulk coupling leads to uncommon types of magnetoelectric domain walls and vortex-like singularities.   

Manipulating ferroic orders in BiFeO3 using BiFeO3/BiFe0.5Co0.5O3 multilayer heterostructures

           Multiferroic materials have recently attracted wide interest due to the coexistence of multiple order parameters, typically ferroelectricity and magnetism, coupled to each other and are promising materials for spintronics, memory, and sensing devices. BiFeO3 (BFO) is one of the most important magnetoelectric multiferroic materials with unique high ferroelectric Curie (T_C = 1083 K) and antiferromagnetic Neel (T_N = 643K) temperatures.¹ In this work, we will explore the tuneability of the ferroic orders in BFO epitaxial thin films using heterostructures with Fe-site Co-substituted BFO. Specifically, we will report on the growth of {[BiFeO₃]_{4 UC} + [BiFe_0.5Co_0.5O₃]_{n UC}; where UC = unit cells} multilayer structures grown using pulsed laser deposition and examine the effect of increasing BFCO layer thickness (n) in the multiferroicity of BFO.   

Modeling Scalable Logic Devices Based on Multiferroic and Ferroelectric Materials with the ARTEMIS Framework

Fundamental understanding and design of ferroelectric/multiferroic materials and devices for logic-in-memory and transistor enhancement is essential to enable low switching energies and thereby revolutionary power reductions in computing. Modelling these devices is challenging due to coupling between different physical phenomena at multiple spatial and temporal scales. We have developed a massively parallel code framework, ARTEMIS, to model the next generation of beyond-CMOS microelectronic devices. ARTEMIS contains a finite-difference time-domain (FDTD) algorithm for Maxwell’s equations coupled with a magnetization model described by the Landau-Lifshitz-Gilbert (LLG) equation. ARTEMIS has been developed in the framework of Exascale Computing Project software, AMReX, and leverages some key functionalities from the WarpX electromagnetic Particle-In-Cell code. Support for dispersive material properties, user-defined excitations and boundary conditions, and heterogeneous physical coupling that can be present in next-generation devices has been implemented. Code’s structure, capabilities, and performance will be summarized and it's application in modeling ferroelectrics in transistor logic and multiferroic logic-in-memory devices will be presented.   

Theoretical study of two-step polarization switching in BiFeO3

BiFeO₃ (BFO) is a multiferroic material in which electric polarization, P, coexists with weak magnetization at room temperature. An electric-field induced reversal of the magnetization occurs in BFO as a result of a two-step P switching process (109^o out-of-plane then 71^o in-plane P rotation or vice versa) [1]. However, the origin of this is still not understood, which hampers its optimization necessary for the development of magnetoelectric memory and logic devices [2].

We combine density functional theory (DFT) and Landau-Khalatnikov (LK) simulations to elucidate the origin of the two-step P switching in BFO and to study its dynamics. First, we introduce the BFO thermodynamic potential and compute its parameters using DFT. Then, we extend our model by introducing the constraints that account for the presence of the substrate and multidomain configuration observed experimentally. Finally, we run LK dynamical simulations to investigate the role of these constraints on the two-step P switching, and to identify potential ways to optimize it.

[1] Heron J. et al., Nature 516, 370 (2014).

[2] Manipatruni S. et al, Nature 565, 35 (2019).   

Phase field predictions of the domain wall topology of BiFeO3

Recently, BiFeO₃ has been shown to display interesting coupling between the multiferroic order parameters leading to the formation of chiral antiferromagnetic polar domain walls [1]. Utilizing the phase field method and a free energy parameterized by first principles calculations [2], we explore the zoology of the different domain walls that can arise in this material. Specifically, we focus on the predictions of the coupled polarization, oxygen octahedral tilt, and strain tensor components at these regions of interest. Our calculations reveal the presence of Bloch-like components which may influence the system’s noncollinear magnetism or the formation of topologically non-trivial states. We investigate the sensitivity of these features and corresponding domain wall thicknesses on the Lifshitz invariants (gradient terms) and also compare the results to predictions of domain wall profiles found from DFT calculations and observations from experiments.

[1] Chauleau, J. Y. et al., Nature Mater. 19, 386 (2020)

[2] Fedorova, N, Nikonov, D., Young, I. and Iniguez, J. in preparation. (2021)   

Disentangling the phase sequence and correlated critical properties in Bi0.7La0.3FeO3: a combined neutron diffraction and Raman scattering study.

This work addresses the study of the high temperature phase sequence of Bi_0.7La_0.3FeO₃ by undertaking temperature dependent high resolution neutron powder diffraction (NPD) and Raman spectroscopy measurements. The analysis revealed that Bi_0.7La_0.3FeO₃ exhibits an incommensurate modulated orthorhombic Pn2₁a(00??)000 structure at room temperature, with a weak ferromagnetic behavior, likely arising from a canted antiferromagnetic (c-AFM) ordering. Above T₁ = 543 K, the low temperature modulated Pn2₁a(00??)000 evolves monotonically into a fractionally growing Pnma structure up to T_N = 663 K. At 663 K, the low temperature c-AFM phase is suppressed concurrently with the switching of the former into a non-modulated Pn2₁a structure that continues to coexist with the Pnma one, until the latter is expected to reach the 100% fraction of the sample volume at high temperatures above 733 K. The Pn2₁a space group is obtained from the Pnma one through the  polar distortion. Neutron diffraction and Raman spectroscopy results provide evidence for the emergence of a noteworthy linear spin-phonon coupling. In this regard, a magnetostructural coupling is observed below T_N, revealed by the relation between the weak ferromagnetism of the canted iron spins and the FeO₆ octahedra symmetric stretching mode. The correlation between magnetization and structural results from NPD provides definite evidence for the magnetic origin of the structural modulation. The analysis of the temperature dependent magnetization and the magnetic peak intensity as well, yield a critical exponent (β) value of 0.38. The lower limit of the phase coexistence temperature T₁ = 543 K, marking the emergence of the Pnma phase, is also associated with the temperature whereupon the modulation magnitude starts to decrease.   

Anti-Ruddlesen-Popper A4X2O, a new family of (anti-)ferroelectric and magnetoelectric multiferroic materials discovered through high-throughput computing

Ferroelectric materials are of great fundamental and applied interests. For decades, most applications have relied on ferroelectric oxide perovskites. However, the need to combine ferroelectricity with other properties such as visible light absorption or long-range magnetic order is driving the search for materials and structural classes beyond perovskites.

We show here how the search for new ferroelectrics can be accelerated using high-throughput computing and a recently built database of more than 2,000 phonons. Browsing the phonon database, we identify materials exhibiting dynamically unstable polar phonon modes, a signature of a potential ferroelectric. We focus on a new family of ferroelectric materials discovered through this high-throughput screening approach: the anti-Ruddlesden-Popper phases of formula A₄X₂O with A: a +2 alkali-earth or rare-earth element and X: a −3 anion Bi, Sb, As and P. We show that Ba₄Sb₂O is ferroelectric and in fact hyperferroelectric. The polar instability involves the movement of an anion in a cationic cage and is geometrically-driven. Introducing a magnetic element in this structure as Eu₄Sb₂O leads to a magnetoelectric multiferroic combining coupled ferroelectricity and ferromagnetism.