The theoretical prediction of intrinsic magnetoelectric behavior in solid crystals dates back to Pierre Curie in 1894, based on symmetry considerations. The coexistence of ferromagnetism and ferroelectricity in a single material offers potential applications in magnetoelectric random access memories, magnetic field sensors, and energy harvesting devices.1
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Understanding Multiferroic Properties
Ferromagnetism is the core process responsible for the formation of permanent magnets. In non-magnetic compounds, permanent magnetic dipoles typically line up antiparallel, canceling each other out. However, in ferromagnetic materials, the dipoles align parallel to each other spontaneously. This alignment results from the movement of electron pairs within their atomic or molecular orbitals.2
Ferromagnetism occurs when atoms are arranged in a lattice, allowing their atomic magnetic moments to interact and align parallel to each other. According to classical theory, this process is attributed to the molecular field present within the ferromagnetic material.
Ferroelectricity, in contrast, is a crystal property where a permanent dipole can be reoriented with an electric field. Ferroelectricity is linked to specific bonding environments within a crystal. These materials, which have two non-zero spontaneous polarization states switchable by an external electric field, are popular for non-volatile memories.3
Multiferroics are materials that exhibit two or more 'ferro' properties (ferroelectric, ferromagnetic, ferroelastic) combined in a single entity. They are extensively studied among other new smart materials due to their broad potential in sensing, energy transformation, and harvesting technologies.
The most intriguing feature of multiferroics is their ability to perform magnetoelectric conversion, either through the direct magnetoelectric effect (electrical polarization under a magnetic field) or the inverse effect (magnetization change under an electric field).4
The magnetoelectric effect (MEE) in composite multiferroics surpasses that in natural multiferroic materials. The highest MEE values are achieved in solid composite multiferroics, such as layered materials or nanocomposites, which include both electrostrictive and magnetostrictive (piezoelectric) phases.
A notable example is a composite made of a closely packed and sintered mixture of ferroelectric (FE) and ferromagnetic (FM) grains. Since the grains are in tight contact, the restriction of one component by an external field causes mechanical stress, which acts on the other component and induces a piezoelectric effect.5
Magnetoelectric Interactions in Multiferroics and Their Impact on Cross-Coupling
Early research on multiferroics focused on combining ferroelectricity and magnetism in a single material. This proved challenging because these two contrasting order parameters are often mutually exclusive.
The uniqueness of these materials lies not in the strength of the magnetoelectric coupling or the high magnitude of electric polarization but in the high sensitivity of multiferroics to any magnetic field. This is caused by the complex spinning electronic arrangement in the material.
Complex magnetic structures and phase diagrams are observed in all multiferroics, showing a strong interplay between magnetic and dielectric phenomena. These materials are 'frustrated' magnets, where competing interactions between spins prevent simple magnetic orders.
These magnetic ferroelectrics can create unprecedented cross-coupling effects, including the high tunability of magnetically induced electric polarization and dielectric constant by applied magnetic fields, making them useful for various applications.6
Major Applications of Multiferroics
Researchers aiming to develop smart and highly efficient materials have increasingly focused on multiferroics due to their unique properties.
The presence of magnetoelectric coupling (MEC) interactions in multiferroics refers to a linear magnetoelectric effect. This effect can be used to produce magnetoelectric sensors, data storage devices, spintronic devices, and electrically tunable microwave devices with low power consumption.
In particular, future-generation four-state logic devices in the form of electrically controlled (±E) non-volatile magnetic storage bits (±M) are currently a scientific focus to increase computing capacity.7
Recent Progress in Multiferroics
Multiferroics are being researched extensively as they can transform modern memory storage devices and make them much more efficient. A prime candidate is Barium monoferrite (γ-BaFe2O4 or BaFeO), which has become popular due to its advantageous multiferroic attributes, particularly at room temperature.
BaFeO thin films, engineered for grain orientation, composition, elastic strain, and magnetic interlayer couplings, could pave the way for innovative devices. However, fabricating the stoichiometric "1–2–4" phase in the BaO-Fe2O3 multiferroic system remains a critical challenge.
A recent study focused on depositing pure and crystalline γ-BaFe2O4 thin films using the Pulsed Electron Deposition (PED) method. Analysis of the plasma plume revealed two main mechanisms govern BaFeO deposition dynamics: a congruent ablation process far from thermodynamic equilibrium and an incongruent low-energy evaporation process. These findings highlight the potential of PED for producing high-quality γ-BaFe2O4 polycrystalline thin films.8
In another significant discovery, researchers from China and Australia demonstrated the layer Hall effect (LHE) by integrating a coupling layer concept with multiferroics using symmetry analysis. Due to time-reversal symmetry breaking and valley physics, the Bloch electrons in one valley experience a large Berry curvature.
This, combined with inversion symmetry breaking, creates a layer-polarized Berry curvature that can cause electrons to deflect in one direction within a given layer, generating the LHE.9
The researchers demonstrated that the resulting LHE is ferroelectrically controllable and reversible. Using first-principles calculations, they verified this mechanism and its predicted phenomena in the multiferroic material bilayer Co2CF2. This finding opens a new direction for LHE study and the modern application of multiferroic 2D materials research.9
Progress in Room Temperature Multiferroics
Room-temperature multiferroics are the ultimate goal in materials science. Due to its high curie temperature, BiFeO3 is the most highly anticipated material. However, BiFeO3 suffers from poor magnetoelectric coupling, which is difficult to detect in bulk form.
In contrast, hexagonal rare-earth ferrites (h-RFeO3) are promising candidates for single-phase room-temperature multiferroics with strong intrinsic magnetoelectric coupling and large ferroelectric polarization.
However, these particular multiferroic materials face several challenges. Achieving a stable hexagonal structure at room temperature is difficult. Characterizing their ferroelectric and magnetoelectric properties at room temperature is also challenging due to their generally poor resistivity.10
In a recent article, researchers prepared stable multiferroic h-Yb1−xInxFeO3 ceramics using a standard solid-state sintering method and examined their structure evolution along with ferroelectric, magnetic, and magnetoelectric coupling characteristics. By introducing chemical pressure, they achieved a stable hexagonal single-phase structure in Yb1−xInxFeO3 (x = 0.5–0.8) ceramics.
The symmetry of these ceramics varied from P63cm (x = 0.5 and 0.6) to P63/MMC (x = 0.7 and 0.8) as x increased. Variable temperature synchrotron X-Ray diffraction analysis showed that the Curie point decreases monotonically from 723 K as x increases. These findings provide new insights into the bulk magnetoelectric coupling effect near room temperature, which could also apply to other hexagonal ferrites and similar materials.
Future Directions
Although multiferroics are not new, many challenges remain. Discovering new room-temperature multiferroics with strong magnetoelectric coupling is crucial. The voltage required for ferroelectric/magnetoelectric switching should be less than 100 mV to make a significant impact.
A multiferroic device controlled by an electric field, especially at low voltages, at room temperature, and with rapid switching, remains a primary target. With continuous advancements, multiferroics will undoubtedly become an essential material of the future.
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References and Further Reading
[1] Jana, B., et al. (2022). Recent progress in flexible multiferroics. Frontiers in Physics. doi.org/10.3389/fphy.2021.822005
[2] Spain, E., Venkatanarayanan, A. (2014). Ferromagnetism. Comprehensive Materials Processing. doi.org/10.1016/B978-0-08-096532-1.01302-9
[3] Said, SM., et al. Ferroelectricity. Reference Module in Materials and Materials Engineering. doi.org/10.1016/B978-0-12-803581-8.04143-6
[4] Spaldin, N., et al. (2019). Advances in magnetoelectric multiferroics. Nature Mater. doi.org/10.1038/s41563-018-0275-2
[5] Makarova, L., et al. (2022). Multiferroic Coupling of Ferromagnetic and Ferroelectric Particles through Elastic Polymers. Polymers. doi.org/10.3390/polym14010153
[6] Cheong, S., et al. (2007). Multiferroics: a magnetic twist for ferroelectricity. Nature Mater. doi.org/10.1038/nmat1804
[7] Gupta, R., et al. (2022). A review on current status and mechanisms of room-temperature magnetoelectric coupling in multiferroics for device applications. J Mater Sci. doi.org/10.1007/s10853-022-07377-4
[8] Casappa, M., et al. (2024). Growth of multiferroic γ-BaFe2O4 thin films by Pulsed Electron Deposition technique. Journal of Alloys and Compounds. Available at: doi.org/10.1016/j.jallcom.2024.174193
[9] Feng, Y., et al. (2023). Layer Hall Effect in Multiferroic Two-Dimensional Materials. Nano Letters. doi.org/10.1021/acs.nanolett.3c01651
[10] Liu, M., et al. (2021). Room‐temperature multiferroic characteristics and unique vortex domain structures of h‐Yb1− x In x FeO3 solid solutions. Journal of the American Ceramic Society. doi.org/10.1111/jace.17987
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