New Material Combines Magnetism and Electricity, Defying Conventional Physics Principles

Scientists have long sought to create materials exhibiting both ferromagnetism and ferroelectricity simultaneously, a combination with substantial potential for advanced technological applications. In a new study, I. V. Solovyev and colleagues demonstrate a pathway to achieving this through the manipulation of orbital ordering, effectively designing a ferromagnetic ferroelectric state. Their research reveals that antiferro orbital order can favour ferromagnetic coupling and break inversion symmetry, overcoming a key obstacle in realising such materials. By outlining principles governing this phenomenon, including specific structural and electronic requirements, the authors identify van der Waals compounds such as VI as promising candidates for exhibiting this behaviour, potentially paving the way for novel multifunctional materials.

Orbital Control and Symmetry Breaking in Ferromagnetic Ferroelectrics reveal complex magnetoelectric coupling effects

Scientists have achieved a breakthrough in materials science by demonstrating a pathway to create ferromagnetic ferroelectrics, materials simultaneously exhibiting both magnetism and spontaneous electric polarization. This longstanding challenge stems from the inherent difficulty of breaking inversion symmetry using purely magnetic means, a symmetry that prevents the coexistence of these properties.

The research details how activating orbital degrees of freedom within materials can overcome this limitation, designing a system where alternating occupied orbitals promote both ferromagnetic coupling and ferroelectricity. This innovative approach centres on a fundamental principle of interatomic exchange, linking antiferro orbital order to both magnetism and the breaking of inversion symmetry.

The work establishes four key principles for realising this combined state in solids. Magnetic atoms must not reside in inversion centres, as exemplified by the honeycomb lattice structure. Crucially, the orbitals within the material must be flexible enough to adjust their shape and minimise the energy of exchange interactions.

This flexibility is facilitated by intraatomic interactions, specifically Hund’s second rule, which balances the energy landscape against crystal field splitting. Finally, for octahedrally coordinated transition-metal compounds, iodides possessing a d2 electronic configuration and weak d, p hybridization appear most promising.

Researchers predict that the van der Waals compound VI3 embodies these characteristics and is therefore a strong candidate for exhibiting both ferromagnetism and ferroelectricity. The study builds upon the Goodenough-Kanamori-Anderson rules, traditionally focused on antiferromagnetic coupling, to reveal how antiferro orbital order not only encourages ferromagnetic behaviour but also intrinsically breaks inversion symmetry, simultaneously inducing electric polarization.

This discovery moves beyond simply combining separate ferromagnetic and ferroelectric layers, offering a route to a unified material with potentially enhanced cross-control of magnetic and electric properties. Numerical simulations performed on VI3, utilising a model derived from first-principle electronic structure calculations, support these theoretical predictions.

The research highlights the importance of Hund’s second rule in promoting orbital flexibility, allowing occupied orbitals to adapt and minimise energy, ultimately driving the emergence of the desired ferromagnetic ferroelectric state. This material design strategy opens new avenues for developing advanced materials with tailored properties and potential applications in data storage, sensors, and novel electronic devices.

Honeycomb lattice structural and magnetic characterisation of vanadium triiodide reveals interesting properties

Vanadium triiodide (VI3) served as the primary material investigated within this study due to its potential to exhibit both ferromagnetic and ferroelectric properties simultaneously. Researchers initially characterised VI3 as a ferromagnet possessing a relatively high Curie temperature of approximately 50 K.

Detailed structural analysis revealed at least two structural phase transitions occurring around 78 K and 32 K, although the precise nature of these transitions remained a subject of ongoing debate. Investigations focused on determining whether vanadium atoms within the honeycomb lattice become inequivalent, either structurally or magnetically, thereby breaking inversion centres.

The work employed a theoretical framework predicated on the principle that alternating occupied orbitals along a bond promotes ferromagnetic coupling and breaks inversion symmetry. This approach necessitates magnetic atoms not residing in inversion centres, as observed in the honeycomb lattice structure of VI3.

Researchers posited that orbital flexibility, facilitated by intraatomic interactions responsible for Hund’s second rule, is crucial for adjusting orbital shapes and minimising exchange interaction energy. The study highlighted the importance of a relatively small 10Dq value to activate Hund’s second rule effects, inducing antiferro orbital order on the honeycomb lattice.

Furthermore, the research addressed the limitations of conventional density functional theory (DFT) calculations in accurately capturing orbital magnetism. To improve the accuracy of these calculations, a phenomenological term, proportional to the Racah parameter B, was proposed to enhance the calculated orbital magnetic moment (ML).

This term, typically expressed as −BM2L, aims to mimic the effects of Hund’s second rule within the electronic structure calculations, addressing the underestimation often observed when using local spin density approximation (LSDA) or generalized gradient approximation (GGA). The study also considered the contribution of on-site Coulomb repulsion U to the orbital magnetization, recognising its emergence in solid-state systems.

Antiferro orbital order and symmetry breaking induce ferromagnetic ferroelectricity in certain materials

Researchers detail a pathway to realizing ferromagnetic ferroelectricity through the manipulation of orbital degrees of freedom. The work centers on designing materials where an alternation of occupied orbitals along a bond favors ferromagnetic coupling and simultaneously breaks inversion symmetry. This approach circumvents the limitations of achieving ferroelectricity solely through magnetic means, as magnetism alone cannot inherently break inversion symmetry.

The study postulates that antiferro orbital order, achieved by minimizing exchange interaction energy, is crucial for inducing both ferromagnetic and ferroelectric properties. Specifically, the research highlights the importance of magnetic atoms not being located in inversion centers, such as in a honeycomb lattice.

Orbital flexibility, facilitated by intraatomic interactions responsible for Hund’s second rule, is also identified as a key factor in adjusting orbital shapes and minimizing energy. For octahedrally coordinated transition-metal compounds, iodides with a specific electronic configuration and relatively weak metal-ligand hybridization are proposed as promising candidates.

Van der Waals compound VI3 is identified as a material expected to exhibit this ferromagnetic behavior. Calculations reveal that electric polarization can be understood through either k-space Berry connection analysis or r-space Wannier function analysis. The modern theory of electric polarization demonstrates that polarization arises from the expectation value of the position operator, and spontaneous polarization develops when inversion symmetry is broken.

The study elaborates on mechanisms for breaking inversion symmetry, including hybridization between bonding and antibonding states, exemplified by d0 perovskites like BaTiO3 and KNbO3. These materials exhibit strong hybridization between transition-metal d and oxygen p states, creating bonding and antibonding bands, and a polar distortion can mix these states to realize a ferroelectric phase.

Furthermore, the research explores the pseudo-Jahn-Teller effect, an energy gain that is even in the polar distortion, differing from the conventional Jahn-Teller effect which is odd. This energy gain, combined with the energy cost of ion core motion, can lead to a stable ferroelectric phase. The study also considers the role of lone pair electrons, such as those found in PbTiO3 and BiMnO3, in contributing to off-centrosymmetric displacements and the potential for realizing ferromagnetic ferroelectricity in materials like SrMnO3 under epitaxial strain.

Orbital Engineering Principles for Magnetoelectric Coupling in Transition Metal Iodides offer new avenues for spintronic devices

Scientists are pursuing the creation of ferromagnetic ferroelectrics, materials combining both ferroic properties, which presents a longstanding challenge in materials science. Achieving this requires overcoming the limitation that ferromagnetism alone cannot break inversion symmetry, a necessary condition for ferroelectricity.

This work demonstrates a pathway to designing such materials by actively utilizing orbital degrees of freedom, leveraging the principle that alternating occupied orbitals along a bond promotes ferromagnetic coupling and simultaneously breaks inversion symmetry. The research establishes key principles for realizing this state in solids, including the requirement for magnetic atoms not to reside in inversion centres, orbital flexibility to minimize exchange interaction energy, and the role of intraatomic interactions, such as Hund’s second rule, in facilitating this flexibility.

Specifically, the study highlights that octahedral transition-metal iodides with a particular electronic configuration are promising candidates, with vanadium triiodide being a potential example. Numerical simulations performed on vanadium triiodide support these theoretical predictions and suggest the possibility of achieving the desired ferromagnetic ferroelectric state.

The authors acknowledge that realizing this state is complex and requires careful consideration of lattice type, electronic configuration, and ligand atoms. Future research should focus on experimentally verifying these theoretical predictions in vanadium triiodide and exploring other materials that meet the outlined criteria. This approach offers a new strategy for designing multifunctional materials with coupled magnetic and electric properties, potentially impacting areas such as data storage and spintronics, although further investigation is needed to fully understand and optimize these effects.