Magnetoelectric Multiferroics Research Articles

Magnetoelectric domain engineering from micrometer to Ångstrøm scales

The functionality of magnetoelectric multiferroics depends on the formation, size, and coupling of their magnetic and electric domains. Knowing the parameters guiding these criteria is a key effort in the emerging field of magnetoelectric domain engineering. Here we show, using a combination of piezoresponse-force microscopy, nonlinear optics, and x-ray scattering, that the correlation length setting the size of the ferroelectric domains in the multiferroic hexagonal manganites can be engineered from the micron range down to a few unit cells under the substitution of Mn3+ ions with Al3+ ions. The magnetoelectric coupling mechanism between the antiferromagnetic Mn3+ order and the distortive-ferroelectric order remains intact even at substantial replacement of Mn3+ by Al3+. Hence, chemical substitution proves to be an effective tool for domain-size engineering in one of the most studied classes of multiferroics. Published by the American Physical Society 2024

Ferroelectric polarization and magnetic structure at domain walls in a multiferroic film

Domain walls affect significantly ferroelectric and magnetic properties of magnetoelectric multiferroics. The stereotype is that the ferroelectric polarization will reduce at the domain walls due to the incomplete shielding of depolarization field or the effects of gradient energy. By combining transmission electron microscopy and first-principles calculations, we demonstrate that the ferroelectric polarization of tail-to-tail 180° domain walls in ε-Fe2O3 is regulated by the bound charge density. A huge enhancement (43%) of ferroelectric polarization is observed in the type I domain wall with a low bound charge density, while the ferroelectric polarization is reduced to almost zero at the type II domain wall with a high bound charge density. The magnetic coupling across the type I and type II ferroelectric domain walls are antiferromagnetic and ferromagnetic, respectively. Revealing mechanisms for enhancing ferroelectric polarization and magnetic behaviors at ferroelectric domain walls may promote the fundamental research and potential applications of magnetoelectric multiferroics.

Multiferroic kinks and spin-flop transition in Ni2InSbO6 from first principles

Magnetoelectric multiferroics are key materials for next-generation spintronic devices due to their entangled magnetic and ferroelectric properties. Spiral multiferroics possess ferroelectric polarization and are particularly promising for electric control of magnetism and magnetic control of ferroelectricity. In this work, we uncover long-period incommensurate states characterized by unique multiferroic kinks in corundum nickelate Ni2InSbO6, a member of a promising family of polar magnets. Utilizing a 2-orbital S = 1 model, we derive formulas for Heisenberg and anisotropic magnetic exchanges and magnetically-induced polarization, enabling their calculations from first principles. We use these parameters in Monte Carlo and Landau theory-based calculations to reproduce experimentally observed magnetic structures and polarization dependence on the magnetic field. We predict magnetic phase transitions between flat spiral, conical spiral, canted antiferromagnetic and ferromagnetic states under increasing magnetic fields. Kinks in the spiral phases repel each other through a Yukawa-like potential arising from exchange of massive magnons. We also find that suitably directed electric fields can be used to stabilize the ferromagnetic and spiral states. The findings open a new pathway to predictive first-principles modelling of multiferroics and will inspire experiments and technological applications based on multiferroic kinks.

Electric-field-induced multiferroic topological solitons.

Topologically protected spin whirls in ferromagnets are foreseen as the cart-horse of solitonic information technologies. Nevertheless, the future of skyrmionics may rely on antiferromagnets due to their immunity to dipolar fields, straight motion along the driving force and ultrafast dynamics. While complex topological objects were recently discovered in intrinsic antiferromagnets, mastering their nucleation, stabilization and manipulation with energy-efficient means remains an outstanding challenge. Designing topological polar states in magnetoelectric antiferromagnetic multiferroics would allow one to electrically write, detect and erase topological antiferromagnetic entities. Here we stabilize ferroelectric centre states using a radial electric field in multiferroic BiFeO3 thin films. We show that such polar textures contain flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric field polarity. By tuning the epitaxial strain, quadrants of canted antiferromagnetic domains can also be electrically designed. These results open the path to reconfigurable topological states in multiferroic antiferromagnets.

Dynamic magnetoelectric effect in bismuth-layer structured Aurivillius ceramics

Multiferroic materials have been developed and studied for decades due to their potential applications in microelectronics and spintronics, such as multi-state digital memories, energy storage, and ultra-fast electric field-controlled magnetic sensors. Among these materials, magnetoelectric multiferroics exhibit a magnetoelectric (ME) interaction allowing for the manipulation of polarization by a magnetic field and magnetization by an electric field through internal strain and/or charge. However, strain-mediated ME effect is time-consuming and inefficient at high frequencies (∼ GHz). To address this issue and pave the way toward high frequency applications of ME multiferroics, we have systematically designed and characterized layered Bi5FeTi3O15-type Aurivillius ceramics for a strong dynamic ME coupling in the GHz band. By introducing cobalt and niobium ions into Bi4.3Gd0.7FeTi3O15, a phase evolution from a four-layered structure to a mixed structure consisting of four- and three-layered phases was revealed. The Co/Nb co-doping was found to enhance the magnetization at room temperature while preserving ferroelectricity. The understanding of the substitution-enhanced dynamic ME coupling at satellite and radar communication frequencies is expected to broaden the range of applications for ME multiferroics.

Sensing multiferroic states non-invasively using optical second harmonic generation

The current boost in the search for energy-efficient device paradigms motivates the integration of materials with coexisting and coupled electric and magnetic order parameters into application-relevant architectures. In the so-called multiferroic magnetoelectrics, the understanding of switching events, however, is most of the time obstructed by the complex physics involved and in the non-trivial domain and domain wall configurations. This perspective offers an insightful overview of the most recent progress in the non-invasive optical probe of technology-significant ferroelectricity and antiferromagnetic order: the optical second harmonic generation (SHG). Over the last decade, its use in materials science has evolved, and SHG now enables the monitoring of the emergence of polarization in thin films, even during the epitaxial deposition process. Its long working distance further expedites the probe of multiple order parameters in various environments and under multi-stimuli excitations. The potential for the probe of complex electric dipole textures, such as ferroelectric skyrmions and time-resolved measurements, using SHG-based approaches in the most recent materials systems will be discussed.

Robust multiferroicity and magnetic modulation of the ferroelectric imprint field in heterostructures comprising epitaxial Hf0.5Zr0.5O2 and Co.

Magnetoelectric multiferroics, either single-phase or composites comprising ferroelectric/ferromagnetic coupled films, are promising candidates for energy efficient memory computing. However, most of the multiferroic magnetoelectric systems studied so far are based on materials that are not compatible with industrial processes. Doped hafnia is emerging as one of the few CMOS-compatible ferroelectric materials. Thus, it is highly relevant to study the integration of ferroelectric hafnia into multiferroic systems. In particular, ferroelectricity in hafnia, and the eventual magnetoelectric coupling when ferromagnetic layers are grown atop of it, are very much dependent on quality of interfaces. Since magnetic metals frequently exhibit noticeable reactivity when grown onto oxides, it is expected that ferroelectricity and magnetoelectricity might be reduced in multiferroic hafnia-based structures. In this article, we present excellent ferroelectric endurance and retention in epitaxial Hf0.5Zr0.5O2 films grown on buffered silicon using Co as the top electrode. The crucial influence of a thin Pt capping layer grown on top of Co on the ferroelectric functional characteristics is revealed by contrasting the utilization of Pt-capped Co, non-capped Co and Pt. Magnetic control of the imprint electric field (up to 40% modulation) is achieved in Pt-capped Co/Hf0.5Zr0.5O2 structures, although this does not lead to appreciable tuning of the ferroelectric polarization, as a result of its high stability. Computation of piezoelectric and flexoelectric strain-mediated mechanisms of the observed magnetoelectric coupling reveal that flexoelectric contributions are likely to be at the origin of the large imprint electric field variation.

First-Principles Approaches to Magnetoelectric Multiferroics

Magnetoelectric multiferroics, which display both ferroelectric and magnetic orders, are appealing because of their rich fundamental physics and promising technological applications. The revival of multiferroics since 2003 led to a comprehensive understanding of the mechanisms that facilitate the coexistence of electric and magnetic orders and conceptually new design strategies for device architectures, which brought us an important step closer to multiferroic-based technology. In the past thirty years, first-principles calculations based on the laws of quantum mechanics played a crucial role in understanding the electronic, magnetic, and structural properties of multiferroics and guided the design of new multiferroics with improved properties. In this review, we provide a comprehensive overview of first-principles approaches to magnetoelectric multiferroics, especially in low-dimensional forms. In particular, we discuss methods to build an effective Hamiltonian from first principles for magnets, ferroelectrics, and multiferroics. The recently developed machine learning potential approach for multiferroics is also outlined. Furthermore, we present the unified model for spin-induced ferroelectricity and methods for computing the linear magnetoelectric coupling tensor. Finally, recent progress in multiferroic systems and the applications of first-principles approaches to these systems are reviewed.

Engineering materials that could change the world

2022 Europhysics Prize and Hamburg Prize for Theoretical Physics winner Nicola Spaldin talks to Laura Hiscott about her work on magnetoelectric multiferroics, the importance of finding fun in research and how physics can benefit from a more flexible idea of excellence.

Dielectric and room temperature ferromagnetism in vacuum annealed Bi(Zn0.5Zr0.5)O3 modified PbTiO3 ceramics

Polycrystalline solid solutions of (1-x)PbTiO3-(x)Bi(Zn0.5Zr0.5)O3[x = 0.10, 0.20, 0.30] have been synthesized by using solid state reaction method and followed by air annealing (AA)/ vacuum annealing (VA). All the samples show single phase perovskite structure with tetragonal symmetry (P4mm). Structural parameters are calculated using the Rietveld refinement technique. With the help of Scanning Electron Microscope (SEM), the surface morphology of all the samples is recorded at room temperature. The influence of VA on dielectric parameters such as dielectric constant and tangent loss of the synthesized samples are studied and compared with that of AA samples. These results unfold superior dielectric properties of VA samples as compared to AA samples. Intriguingly, the VA samples exhibit exotic ferromagnetism in non-magnetic PbTiO3–Bi(Zn0.5Zr0.5)O3 samples, transforming these into room temperature magneto-electric multiferroics. Besides, both the ferroelectric and ferromagnetic order parameters were found to increase with the increase in Bi(Zn0.5Zr0.5)O3 doping in PbTiO3. The enhanced physical properties witnessed in VA PbTiO3–Bi(Zn0.5Zr0.5)O3 samples localized hopping of charge carriers confined to the grain.

Coexistence of Ferroelectricity and Ferromagnetism in Fullerene-Based One-Dimensional Chains.

One-dimensional (1D) magnetoelectric multiferroics are promising multifunctional materials for miniaturized sensors, actuators, and memories. However, 1D materials with both ferroelectricity and ferromagnetismare quite rare. Herein, using first-principles calculations, a series of fullerene-based 1D chains, namely U2 C@C80 -M (M=Cr, Mn, Mo, and Ru) 1D chains with both ferroelectric (FE) and ferromagnetic (FM)properties is designed. Compared to individual U2 C@Ih (7)-C80 , the spontaneous polarization (Ps) in 1D chains is enhanced by about two to four times owing to the interaction between U2 C@Ih (7)-C80 fullerene and M (M=Cr, Mn, Mo, and Ru) atoms. Meanwhile, the introduction of transition metal atoms dopes electrons into U's 5f orbitals, leading to numerous intriguing magnetic properties, such as U2 C@C80 -Cr and U2 C@C80 -Mo as 1D ferromagneticsemiconductors, U2 C@C80 -Ru as 1D ferrimagnetic (FiM) semiconductor, and U2 C@C80 -Mn as 1D antiferromagnetic (AFM) semiconductor. Excitingly, it is found that magnetic ordering and electrical polarization can be modulated independently by linking different transition metal atoms. These findings not only broaden the range of 1D multiferroic materials, but also provide promising candidates for novel electronic and spintronic applications.

Simple 4-segment thermal cycling pyroelectric measurement protocol for differentiating between ferroelectric and non-ferroelectric materials

The rare-earth chromates (RECrO$_3$) and manganites (REMnO$_3$) where, RE = Eu, Y, Dy, Ho, Gd are constantly under scrutiny in search of room temperature magnetoelectric multiferroics. However, the artefacts and undesirable signal in some of the measurements pose a severe challenge in confirming the ferroelectric (FE) phase, especially in reference to pyroelectric current measurement technique. In this regard, we propose a simple modified approach to pyroelectric current measurement named as 4-segment thermal cycling protocol. This protocol assists in isolating the elusive, irreversible thermally stimulated current from the currents associated with spontaneous and reversible nature of the electric polarization in FE phase. In order to explain working principle of the protocol, we have compared simulated response of two hypothetical materials; an FE material free of space charges and a paraelectric material possessing only space charges. Further, we experimentally verify these new protocols in a single crystal of prototype ferroelectric material, Glycine Phosphite. This report primarily focuses on detailed investigation of ferroelectricity using the proposed protocol in two polycrystalline materials, HoCrO$_3$ and DyFe$_{0.5}$Mn$_{0.5}$O$_3$ where, the former has been reported to be multiferroic earlier. Our elaborative and careful approach to pyroelectric studies expound on the absence of reversible spontaneous electric polarization at temperature ranges tested in both, HoCrO$_3$ and DyFe$_{0.5}$Mn$_{0.5}$O$_3$.

Room temperature multiferroicity of hexagonal LuFeO3 and its enhancement by co-doping in Lu0.9Co0.1Fe0.9Ti0.1O3 nanoparticle system

Antiferromagnetic LuFeO3 can be good multiferroic by having ferroelectricity in its non-centrosymmetric hexagonal phase. But, it is hard to stabilize this metastable phase, preventing the stable orthorhombic phase. In this work, metastable hexagonal LuFeO3 (LFO) nanoparticle was stabilized in chemical sol-gel route in pure phase and co-doped with Co and Ti in the same route to synthesize Lu0.9Co0.1Fe0.9Ti0.1O3 (LCFTO) nanoparticles. Room temperature multiferroicity of bare LFO was established through relevant characterization. And motive behind the co-doping is to enhance the magnetoelectric behavior of the bare system to synthesize a new monophasic type II magnetoelectric multiferroic. Structural investigation by thorough Rietveld analyses of the recorded X-ray diffractograms, confirmed the formation of pure hexagonal (P63cm) phase of both bare and doped LFO. However, deviations in various structural & microstructural parameters were observed in the doped system, which is mainly responsible for the enhancement of magnetic and electric properties of the sample. Presence of antiferromagnetic transition at ∼604 K confirmed the room temperature magnetic ordering of bare LFO. Interestingly, LCFTO shows a drastic enhancement of magnetic property than the bare one in all concerns, where the maximum magnetization at the maximum applied field is enhanced by nearly 36 times at room temperature. Detailed high-temperature dielectric investigation shows, good dielectric strength (∼261) of LFO gets enhanced highly in LCFTO (∼1053) and a high relaxation time having negligible loss factor with an indication of ferro to paraelectric transition above room temperature. Current density vs. electric field (J-E) curve suggests the presence of polarization at room temperature with negligible leakage loss. Direct measurement of ferroelectric loop shows the ferroelectricity (Pmax∼0.064 µc/cm2) of bare LFO at room temperature and a well improvement in the doped LCFTO (Pmax∼0.151 µc/cm2). The presence of room temperature magnetoelectric coupling, confirmed by magnetocapacitance measurements, results in a high value (∼5 %) of magnetocapacitance in the doped system which is also much higher than the bare one (<∼1 %) as expected. All these properties confirm the magnetoelectric multiferroicity of bare h-LuFeO3 at room temperature. And, co-doping in hexagonal LuFeO3 nanoparticle system results in a considerable improvement in its magnetoelectric behavior, so this co-doped system can be a promising and potential magnetoelectric multiferroic for the future generation magnetoelectric devices.

Grain size and piezoelectric effect on magnetoelectric coupling in BFO/PZT perovskite-perovskite composites

Magnetoelectric multiferroics are provoking much research activity for their potential applications in novel multifunctional devices. Compared with single-phase multiferroics, magnetoelectric composites have competitive advantages. The magnetoelectric coupling of composites is heavily influenced by composition and grain size. We prepared BFO/PZT composites with separate phases via the solid-state sintering method. We show that equal molar ratio composition has a better magnetoelectric coupling response than other compositions. The composites with nano bismuth ferrite have better magnetoelectric coupling performance, and these with both nano grain sizes present the highest magnetoelectric voltage coefficients (αE = 16.9 mV cm−1 Oe−1). The strain-mediated mechanism was confirmed via a comparison between strain response and polarization of nano/micro type composites with the composition of 3:7. We found that the magnetoelectric voltage coefficient αE positively correlates with strain response instead of polarization in BFO/PZT composites, which provides experimental evidence for the strain-mediated ME composite theory.

Helimagnets by disorder: Its role on the high-temperature magnetic spiral in the YBaCuFeO5 perovskite

Most of the spiral magnetoelectric multiferroics investigated in recent years are geometrically or exchange-frustrated magnets, where the presence of triangular or other frustrated spin networks produce low magnetic transition temperatures. This critically limits their potential uses. The exceptional stability of the spiral magnetic order (at ${T}_{S}$) in the layered structure of the ${\mathrm{YBaCuFeO}}_{5}$ double perovskite involves a nonconventional mechanism: spiral order by disorder. The model has been theoretically developed by Scaramucci et al. [Phys. Rev. Res. 2, 013273 (2020)] after the discovery of a huge impact of cation disorder on ${T}_{S}$ [M. Morin et al., Nat. Commun. 7, 13758 (2016)]. In this work the influence of disorder (and only disorder) on the magnetic phase diagram is studied on a quantitative basis extending the range of previous studies. We thoroughly investigate the impact of frustration due to $B$-site disorder (${n}_{d}$) on the magnetic spirals in the reference composition ${\mathrm{YBaCuFeO}}_{5}$. The interplay between disorder, stability, and the detailed features of the incommensurate spiral magnetic orders were systematic, quantitative, and methodically investigated in samples of identical composition, spanning a wide range of ${n}_{d}$ values. Three different regimes are distinguished in the ${\mathrm{YBaCuFeO}}_{5}$ phase diagram versus disorder. A triple point is found in ${\mathrm{YBaCuFeO}}_{5}$ driven by Fe/Cu disorder that sets limits to ${T}_{S}$ and the cycloidal component of the helicoidal order. These layered materials appear as a very efficient realization of the avenue ``spiral order by disorder'' to supply functional helimagnets at normal working temperatures.

Room-temperature multiferroicity in GaFeO3 thin film grown on (100)Si substrate

Room-temperature magnetoelectric multiferroicity has been observed in c-axis oriented GaFeO3 thin films (space group Pna21), grown on economic and technologically important (100)Si substrates by a pulsed laser deposition technique. Structural analysis and comprehensive mapping of the Ga:Fe ratio across a length scale range of 104 reveals coexistence of epitaxial and chemical strain. It induces formation of finer magnetic domains and large magnetoelectric coupling—a decrease in remanent polarization by ∼21% under ∼50 kOe. Magnetic force microscopy reveals the presence of both finer (&amp;lt;100 nm) and coarser (∼2 μm) magnetic domains. Strong multiferroicity in epitaxial GaFeO3 thin films, grown on a (100)Si substrate, brighten the prospect of their integration with Si-based electronics and could pave the way for development of economic and more efficient electromechanical, electrooptic, or magnetoelectric sensor devices.

Room-temperature multiferroicity and magnetoelectric couplings in (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 spinel films

The electric field manipulation of a magnetic order is a key feature of advanced information technologies and the development of room-temperature multiferroic magnetoelectrics remains an important task. In this work, we report novel single-phase magnetoelectric (ME) films consisting of a (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 spinel ferrite. The (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 thin films display a switchable polarization and magnetization with a remnant polarization of about 2.1 µC cm–2 and a remnant magnetization of about 82.0 emu cm–3 at 300 K. Electric field control of magnetism is detected, the magnetization modulation reaches 8.5 % at room temperature with an electric field of 500 kV cm−1, revealing the existence of magnetoelectric coupling. We believe that the substitutions of Al3+ and Mg2+ cations at the spinel sublattices might induce local distortions and local electric field and modulate the superexchange interactions between magnetic ions, which give rise to the ferroelectricity and ME effect in the (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 thin films. The electric field-induced charge modulation may also contribute to the observed magnetoelectric effect, which likely results from the migration and redistribution of oxygen vacancies in the prepared nanofilms. The findings of this work can stimulate research studies on the development of room temperature multiferroics and magnetoelectrics based on the spinel ferrite compounds.

Chiral magnetic structure of spin-ladder multiferroic BaFe2Se3

A complex magnetic structure with specific symmetry properties is a relevant ingredient for magnetoelectric multiferroicity. In BaFe${}_{2}$Se${}_{3}$, a member of the Fe spin-ladder family of pressure-induced superconductors, the authors reveal this complexity. Using polarized and unpolarized single-crystal neutron diffraction, they evidence a chiral, umbrella-like magnetic order made of blocks of quasiferromagnetic spins, antiferromagnetically ordered along the ladder. Such accurate information on the magnetism will help to understand the magnetoelectric coupling in this family and its puzzling phase diagram.

Terahertz spectroscopy of spin excitations in magnetoelectric LiFePO4 in high magnetic fields

Magnetoelectric multiferroics feature crosscoupling between magnetic and charge order. Here, the authors study the absorption of electromagnetic radiation by spin waves in magnetoelectric LiFePO${}_{4}$ in the terahertz (THz) spectral range. Using linearly polarized THz radiation, they establish that there are spin waves that interact with magnetic $a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d$ electric components of radiation. This is unusual for electron spins since they are susceptible to magnetic fields only. Another striking observation is that the number of spin-wave modes in the THz spectrum exceeds four times the number of modes expected for a four-spin system. Using the mean-field model and the magnetic field dependence of the THz spectra, the authors identify four conventional spin-wave modes in LiFePO${}_{4}$. It is proposed that other THz-active spin-wave modes are more exotic quadrupolar excitations, such as spin-stretching modes and two-magnon excitations.

Study of the role of dysprosium substitution in tuning structural, optical, electrical, dielectric, ferroelectric, and magnetic properties of bismuth ferrite multiferroic

Magnetoelectric multiferroics, which combine ferroelectric and magnetic characteristics, have potential use in a variety of electronic devices. In this work, Dy 3+ substituted bismuth ferrites with the chemical formula Bi 1−x Dy x FeO 3 ( x = 0.0, 0.15, 0.30, 0.45, and 0.60) were synthesized using the sol-gel auto combustion process. The effect of Dysprosium substitution in BiFeO 3 (BFO), on its structural, surface morphology, optical, electrical, dielectric, ferroelectric, and magnetic properties were studied. The rhombohedral perovskite structure of the space group ( R3c ) was confirmed via X-ray diffraction (XRD) analysis. Moreover, the crystallite size had a maximum value of 59.57 nm for x = 0.30. XRD and FTIR confirmed the substitution of Dy 3+ into BFO ferrite. Further, the structural change and absorption bands confirmed the substitution of Dy 3+ ions into the lattice. For x = 0.30, the energy bandgap of 2.81 eV was found. The resistivity and activation energy were minimum and drift mobility was maximum at x = 0.30 as compared to Dy 3+ doped BFO samples. At low frequency, the dielectric loss was reduced, while at high frequency, the dielectric loss increased with increasing frequency. The saturated polarization ( P S ), electric polarization ( E C ), and remnant polarization ( P r ) have values of 6.95 µC/cm 2 , 3.49 µC/cm 2 , and 1.53 kV/cm for x = 0.30, respectively. The maximum saturation magnetization and microwave frequencies were 10.89 emu/g and 2.41 GHz, respectively at Dy 3+ concentration x = 0.30. These materials are suitable for electronic and microwave devices. • Bi 1- x Dy x FeO 3 ( x = 0.0, 0.15, 0.30, 0.45, and 0.60) multiferoaic were successfully prepared via simple sol-gel auto-combustion. • The optical bandgap energy has values between 2.71.71 eV and 2.93 eV. • The ferroelectric nature of the samples was confirmed using the P - E loop. • Maximum magnetization saturated is observed ~ 13 emu/g. • The maximum microwave frequency has values between 1.39.39 GHz and 2.88 GHz.