Multiferroic materials and devices have attracted intensified recent interests due to the demonstrated strong magnetoelectric (ME) coupling in new multiferroic materials and devices with unique functionalities and superior performance characteristics. Strong ME coupling has been demonstrated in a variety of multiferroic heterostructures, including bulk magnetic on ferro/piezoelectric multiferroic heterostructures, magnetic film on ferro/piezoelectric slab multiferroic heterostructures, thin film multiferroic heterostructures, etc. Different multiferroic devices have been demonstrated, which include magnetic sensors, energy harvesters, and voltage tunable multiferroic RF/microwave devices which are compact, lightweight, and power efficient. In this progress report, we cover the most recent progress on multiferroic heterostructures and devices with a focus on voltage tunable multiferroic heterostructures and devices with strong converse ME coupling. Recent progress on magnetic-field tunable RF/microwave dev...

Voltage control of magnetism in multiferroic heterostructures and devices

Controlled in-plane rotation of the magnetic easy axis in manganite heterostructures by tailoring the interface oxygen network could allow the development of correlated oxide-based magnetic tunnelling junctions with non-collinear magnetization, with possible practical applications as miniaturized high-switching-speed magnetic random access memory (MRAM) devices. Here, we demonstrate how to manipulate magnetic and electronic anisotropic properties in manganite heterostructures by engineering the oxygen network on the unit-cell level. The strong oxygen octahedral coupling is found to transfer the octahedral rotation, present in the NdGaO3 (NGO) substrate, to the La2/3Sr1/3MnO3 (LSMO) film in the interface region. This causes an unexpected realignment of the magnetic easy axis along the short axis of the LSMO unit cell as well as the presence of a giant anisotropic transport in these ultrathin LSMO films. As a result we possess control of the lateral magnetic and electronic anisotropies by atomic-scale design of the oxygen octahedral rotation.

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Controlled lateral anisotropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling

The book by David Vanderbilt, Berry Phases in Electronic Structure Theory, is a very pedagogical introduction to the role played by Berry phases in our understanding of the electronic properties of matter. It is indeed written by one of the prominent contributors to the field. Since their discovery in 1984, Berry phases have been used to understand or reinterpret a variety of phenomena such as charge pumping, polarization, orbital magnetization and Hall effects, as well as topological insulators. They are all discussed in the book, step by step, from an elementary level to a more detailed understanding. Throughout the book a Python package, PythTB, is used to provide numerical examples that support the theory given in the book. The code of the examples is given in the appendices so that it can be investigated by the reader. In the first chapter D. Vanderbilt gives an overview of the physics covered in the book, and explains on physical grounds several of the results obtained in later chapters. The second chapter reviews elementary concepts in solid-state physics. A rather lucid and self-contained discussion of density functional theory, Bloch functions and tightbinding Hamiltonians is given. The PythTB package is also introduced. This package, developed at Rutgers University, allows one to build and solve tight-binding models and compute Berry phases related properties. It is most useful to understand the concepts newly introduced in later chapters. From the mathematical point of view, the third chapter is the most important in the book. The Berry phase is first introduced as a Pancharatnam phase. A parameter space is considered, and the phase of a state vector is accumulated along a closed path which is covered in a finite number of steps. Later, the continuous limit is taken, and important concepts such as Berry connection and curvature are defined. The Chern theorem, which is that the integral of the Berry curvature on a 2-surface is quantized, is then stated. With regard to its importance for later applications, one may regret that a more mathematical derivation is not given, although convincing arguments are presented. After the discussion of Berry phases in general terms, several applications are considered. Adiabatic dynamics is reviewed, making apparent the role played by the Berry phase. Then, instead of considering a generic parameter space for a defined Berry phase, the Brillouin zone is chosen, and the parameters are the components of the electron wavevectors. This is used in most of the following chapters. Finally, the chapter is concluded with a discussion of Wannier functions. In particular, it is shown how the gauge freedom on the Bloch functions affects the average value of the position operator, the Wannier centres, and how they are related to Berry phases. This is very useful in later chapters to obtain a physical picture of the phenomenon under consideration. Chapters 4, 5 and 6 are devoted to applications of the general concepts developed so far. The modern theory of polarization is presented in Chapter 4, a theory developed by David Vanderbilt himself in the 1990s. In insulators, the textbook approach to polarization is based on a separation of the charge into free and bound charges, which then define polarizable entities. In solids, the separation is far from obvious, and this approach prevented practical computations for many years. The arguments given in the first chapter of the book, advocating for a multivalued polarization, are derived here. To ISSN 2053-2733

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Berry Phases in Electronic Structure Theory. Electric Polarization, Orbital Magnetization and Topological Insulators. By David Vanderbilt. Cambridge University Press, 2018. Hardback, pp. x+384. Price GBP 59.99. ISBN 9781107157651.

We have measured electromagnetic energy harvesting properties of all-thin-film magnetoelectric (ME) heterostructures on Si cantilevers. The devices are built on a silicon oxide/nitride/oxide stack, and the ME layers consist of a magnetostrictive Fe0.7Ga0.3 thin film and a Pb(Zr0.52Ti0.48)O3 piezoelectric thin film. The harvested peak power at 1 Oe is 0.7 mW/cm3 (RMS) at the resonant frequency (3.8 kHz) with a load of 12.5 kΩ. The resonant frequency was found to display DC bias magnetic field dependence indicative of a magnetization canting with respect to the cantilever easy axis as a result of interplay between the anisotropy and Zeeman energies.

Energy harvesting properties of all-thin-film multiferroic cantilevers

We use a combination of symmetry arguments and first-principles calculations to explore the connection between structural distortions and ferroelectricity in the perovskite family of materials. We explain the role of octahedral rotations in suppressing ferroelectricity in these materials and show that, as the tolerance factor decreases, rotations alone cannot fully suppress ferroelectricity. Our results show that it is cation displacements (`hidden' in Glazer notation) that accompany the rotations, rather than the rotations themselves, that play the decisive role in suppressing ferroelectricity in these cases. We use the knowledge gained in our analysis of this problem to explain the origin of ferroelectricity in $R3c$ materials such as FeTiO$_3$ and ZnSnO$_3$ and to suggest strategies for the design and synthesis of new perovskite ferroelectrics. Our results have implications not only for the fundamental crystal chemistry of the perovskites but also for the discovery of new functional materials.

Why Are There So Few Perovskite Ferroelectrics

Nonequilibrium atomic structures can host exotic and technologically relevant properties in otherwise conventional materials. Oxygen octahedral rotation forms a fundamental atomic distortion in perovskite oxides, but only a few patterns are predominantly present at equilibrium. This has restricted the range of possible properties and functions of perovskite oxides, necessitating the utilization of nonequilibrium patterns of octahedral rotation. Here, we report that a designed metastable pattern of octahedral rotation leads to robust room-temperature ferroelectricity in CaTiO3, which is otherwise nonpolar down to 0 K. Guided by density-functional theory, we selectively stabilize the metastable pattern, distinct from the equilibrium pattern and cooperative with ferroelectricity, in heteroepitaxial films of CaTiO3. Atomic-scale imaging combined with deep neural network analysis confirms a close correlation between the metastable pattern and ferroelectricity. This work reveals a hidden but functional pattern of oxygen octahedral rotation and opens avenues for designing multifunctional materials. Previous studies are limited to modulate oxygen octahedral rotation angles within the same pattern, and engineering the pattern remains a challenge. Here, the authors demonstrate a designed nonequilibrium oxygen octahedral rotation pattern leads to room-temperature ferroelectricity in CaTiO3.

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Stabilizing hidden room-temperature ferroelectricity via a metastable atomic distortion pattern.

We report on a giant tunable enhanced resonant magnetoelectric (ME) coupling in multiferroic magnetostrictive/piezoelectric composite bimorph structures. The approach uses a magnetic/electric field assisted stress-reconfigurable resonance to produce frequency tuning of up to 100%. The studies were performed by laser Doppler spectroscopy. We also show that this principle of a continuously tuned resonance might be used to improve sensitivity for ME magnetic sensors.

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Enhanced resonant magnetoelectric coupling in frequency-tunable composite multiferroic bimorph structures

Recent years have seen significant developments in the study of strong light-matter coupling including the control of chemical reactions by altering the vibrational normal modes of molecules. In the vibrational strong coupling regime the normal modes of the system become hybrid modes which mix nuclear, electronic, and photonic degrees of freedom. First principles methods capable of treating light and matter degrees of freedom on the same level of theory are an important tool in understanding such systems. In this work, we develop and apply a generalized force constant matrix approach to the study of mixed vibration-photon (vibro-polariton) states of molecules based on the cavity Born-Oppenheimer approximation and quantum-electrodynamical density-functional theory. With this method vibro-polariton modes and infrared spectra can be computed via linear response techniques analogous to those widely used for conventional vibrations and phonons. We also develop an accurate model that highlights the consistent treatment of cavity coupled electrons in the vibrational strong coupling regime. These electronic effects appear as new terms previously disregarded by simpler models. This effective model also allows for an accurate extrapolation of single and two molecule calculations to the collective strong coupling limit of hundreds of molecules. We benchmark these approaches for single and many CO$_2$ molecules coupled to a single photon mode and the iron-pentacarbonyl Fe(CO)$_5$ molecule coupled to a few photon modes. Our results are the first ab-initio results for collective vibrational strong coupling effects. This framework for efficient computations of vibro-polaritons paves the way to a systematic description and improved understanding of the behavior of chemical systems in vibrational strong coupling.

Ab initio linear-response approach to vibro-polaritons in the cavity Born-Oppenheimer approximation

The switching polarization of a ferroelectric is determined by the current that flows as the system is switched between two variants. Computation of the switching polarization in crystal systems has been enabled by the modern theory of polarization, where it is expressed in terms of a change in Berry phase from the initial state to the final state. It is straightforward to compute this change of phase modulo $2\ensuremath{\pi}$, thus requiring a branch choice to specify the predicted switching polarization. The measured switching polarization depends on the actual path along which the system is switched, which in general involves nucleation and growth of domains and is therefore quite complex. In this work we present a first-principles approach for predicting the switching polarization that requires only knowledge of the initial and final states based on the empirical observation that for most ferroelectrics, the observed polarization change is the same as that for a path involving minimal evolution of the state. To compute the change along a generic minimal path, we decompose the change of Berry phase into many small contributions, each much less than $2\ensuremath{\pi}$, allowing for a natural resolution of the branch choice. We show that for typical ferroelectrics, including those that would have otherwise required a densely sampled path, this technique allows the switching polarization to be computed without any need for intermediate sampling between oppositely polarized states.

Berry flux diagonalization: Application to electric polarization

In the first-principles bulk-layer model the superlattice structure and polarization are determined by first-principles computation of the bulk responses of the constituents to the electrical and mechanical boundary conditions in an insulating superlattice In this work the model is extended to predict functional properties, specifically dielectric permittivity and piezoelectric response A detailed comparison between the bulk-layer model and full first-principles calculations for three sets of perovskite oxide superlattices, ${\mathrm{PbTiO}}_{3}/{\mathrm{BaTiO}}_{3}$, ${\mathrm{BaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$, and ${\mathrm{PbTiO}}_{3}/{\mathrm{SrTiO}}_{3}$, is presented The bulk-layer model is shown to give an excellent first approximation to these important functional properties and to allow for the identification and investigation of additional physics, including interface reconstruction and finite-size effects Technical issues in the generation of the necessary data for constituent compounds are addressed These results form the foundation for a powerful data-driven method to facilitate discovery and design of superlattice systems with enhanced and tunable polarization, dielectric permittivity, and piezoelectric response

John Bonini

Papers

Stabilizing hidden room-temperature ferroelectricity via a metastable atomic distortion pattern.

Enhanced resonant magnetoelectric coupling in frequency-tunable composite multiferroic bimorph structures

Ab initio linear-response approach to vibro-polaritons in the cavity Born-Oppenheimer approximation

Berry flux diagonalization: Application to electric polarization

First-principles bulk-layer model for dielectric and piezoelectric responses in superlattices