Multiferroic State Research Articles

Flexible strained membranes of multiferroic TbMnO3

The multiferroic properties of TbMnO3 demonstrate high versatility under applied pressure, making the material potentially suitable for use in flexible electronics. Here, we report on the preparation of elastic freestanding TbMnO3 membranes with dominant (001) or (010) crystallographic out-of-plane orientation. Membranes with a thickness of 20 nm display orthorhombic bulk-like relaxed lattice parameters with strong suppression of twinning for the (010) oriented membranes. Strain in flexible membranes was tuned using a commercial strain cell device and characterized by Raman spectroscopy. The B1g out-of-phase oxygen-stretching mode, representative of the Mn–O bond distance, systematically shifts to lower energy with increasing strain (εmax ≈ 0.5%). The flexibility and elastic properties of the membranes allow for specific manipulation of the multiferroic state by strain, whereas the choice of the crystallographic orientation gives the possibility of an in- or out-of-plane electric polarization.

Strain engineering the spin-valley coupling of the R-stacking sliding ferroelectric bilayer 2H-VX2 (X = S, Se, Te)

The emergence of magnetic transition metal dichalcogenides has significantly advanced the development of valleytronics due to the spontaneous breaking of time-reversal symmetry and space-inversion symmetry. However, the lack of regulation methods has prevented researchers from exploring their potential applications. Herein, we propose to use strain engineering to control the spin-valley coupling in the sliding ferroelectric bilayer 2H-VX2 (X = S, Se, Te). Four multiferroic states are constructed by combining the sliding ferroelectricity and antiferromagnetism in the R-stacking bilayer VX2, where the spin and valley polarizations are coupled together from the layer-dependent spin-polarized band structures. By applying a small external strain or pressure on the out-of-plane van der Waals direction, we predicted that there is an antiferromagnetic to magnetic transition in the bilayer VX2, leading to the interesting spin-polarized and chiral circularly polarized radiation at K+ and K- valleys, similar to those found in the magnetic monolayer. To comprehend the coupling between various degrees of freedom in these multiferroic systems, we have developed an effective k·p model. This model unveils a linear relationship between the electric polarization generated by interlayer sliding and the energy difference of the valence band maximum at K+ and K- valleys. Thus, providing an alternate method to measure the electric polarization in the sliding ferroelectrics. Based on the strong coupling between the strain, spin-valley, and electric polarization, it is likely to use the strain to control the interesting emerging properties of 2H-VX2 such as the anomalous valley Hall effect.

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.

Onset of Multiferroicity in Prototypical Single-Spin Cycloid BiFeO3 Thin Films.

In the room-temperature magnetoelectric multiferroic BiFeO3, the noncollinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single-domain ferroelectric and spin cycloid state in epitaxial BiFeO3 (111) thin films grown on orthorhombic DyScO3 (011). Comparing them with films grown on SrTiO3 (111), we identify anisotropic in-plane strain as a powerful handle for tailoring the single antiferromagnetic state. In this single-domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nm. This as-grown single-domain multiferroic configuration in BiFeO3 thin films opens an avenue both for fundamental investigations and for electrically controlled noncollinear antiferromagnetic spintronics.

Room temperature multiferroicity in a transition metal dichalcogenide

The coexistence of multiple ferroic orders, i.e., multiferroicity, is a scarce property to be found in materials. Historically, this state has been found mainly in 3-dimensional complex oxides, but so far, this state has still been elusive for the most widely studied and characterized family of 2-dimensional compounds, the transition metal dichalcogenides. In this study, we report the experimental realization of multiferroic states in this family of materials, at room temperature, in bulk single crystals of Te-doped WSe2. We observe the coexistence of ferromagnetism and ferroelectricity, evidenced in the presence of magnetization and piezoresponse force microscopy hysteresis loops. These findings open the possibility of widening the use and study of van der Waals-based multifunctional devices for nanoelectronics and spintronics applications.

Evidence for a single-layer van der Waals multiferroic.

Multiferroic materials have attracted wide interest because of their exceptional static1-3 and dynamical4-6 magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order that directly induces ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect3,7. This intrinsic coupling between the magnetic and dipolar order parameters results in high-strength magnetoelectric effects3,8. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for to enable the harnessing of magnetoelectric coupling in nanoelectronic devices1,9,10. Here we report the discovery of type-II multiferroic order in a single atomic layer of the transition-metal-based van der Waals material NiI2. The multiferroic state of NiI2 is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Combining birefringence and second-harmonic-generation measurementswith theoretical modelling and simulations, we detect a highly anisotropic electronic state that simultaneously breaks three-fold rotational and inversion symmetry, and supports polar order. The evolution of the optical signatures as a function of temperature and layer number surprisingly reveals an ordered magnetic polar state that persists down to the ultrathin limit of monolayer NiI2. These observations establish NiI2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the two-dimensional limit.

Dynamics of multiferroic LiCuVO4 influenced by electric field

We present the electron spin resonance study of the influence of an electric field on the low-field multiferroic magnetic state in LiCuVO4. The shift of the magnetic resonance spectra in the electric field has been observed experimentally. Symmetry analysis has been conducted in order to describe the static properties of the magnetic system. The low-frequency dynamics of LiCuVO4 in magnetic and electric fields was considered in the framework of hydrodynamic approach. It was shown that the application of the external electric field leads to the change of the configuration of the magnetic system before and after spin-flop reorientation. Satisfactory agreement was obtained between the results of experimental studies and theoretical consideration.

Ferromagnetic and ferroelectric 5d1 insulator Ba5(OsO5)3Cl

Poly- and single-crystalline samples of ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$ have been successfully synthesized. ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$ crystallizes in the noncentrosymmetric space group $P{6}_{3}cm$. ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$ shows a ferromagnetic order below 5 K and a possible ferroelectric order below 150 K. A multiferroic state with both ferromagnetic and ferroelectric ordering is established, although the ferromagnetic and ferroelectric orders are not coupled. The experimental effective moment and the saturated magnetization for ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$ are close to the spin-only value, indicating that the spin-orbit coupling does not play an important role, which is due to the large crystal field and spin splitting in ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$.

Writing of strain-controlled multiferroic ribbons into MnWO4

Local and low-dimensional structures, such as interfaces, domain walls and structural defects, may exhibit physical properties different from the bulk. Therein, a wide variety of local phases were discovered including conductive interfaces, sheet superconductivity, and magnetoelectric domain walls. The confinement of combined magnetic and electric orders to spatially selected regions may be particularly relevant for future technological applications because it may serve as basis of electrically controllable magnetic memory devices. However, direct observation of magnetoelectric low-dimensional structures cannot readily be done partly because of the lack of experimental techniques locally probing their physical nature. Here, we report an observation of multiferroic ribbon-like domains in a non-multiferroic environment in MnWO4. Using optical second harmonic generation imaging, we reveal that a multiferroic phase is stabilized by locally generated strain while the bulk magnetic structure is non-multiferroic. We further find that the confined multiferroic state retains domains with different directions of electric polarization and we demonstrate deterministic writing of a multiferroic state embedded in a non-multiferroic environment.

Ferromagnetoelectric phases of hexaferrites: a group-theoretical analysis

A symmetry-guided approach for designing and tailoring magnetoelectric and multiferroic hexaferrites is proposed. A group-theoretical study of the magnetoferroelectric structures of a hexagonal ferrite was carried out. The results were applied to M-, W- and Z-type hexaferrites. It is shown that the magnetic structure cannot be collinear in the main magnetic phase, with the direction of the magnetic moment along the hexagonal axis. Magnetic sublattices are shown in which cation substitution should lead to the formation of a multiferroic state. A thermodynamic model of a hexagonal ferroelectric ferrimagnet was constructed, in which isostructural phase transitions were observed.

Magnetoelectricity in Jahn–Teller Elastics

The magnetoelectric effects in Jahn–Teller crystals are discussed on the basis of phenomenology and microscopic theory. New magnetoelectric effects—metamagnetoelectricity—are analyzed. Formation of multiferroic crystal states as the consequence of the cooperative Jahn–Teller effect is discussed.

Possible Persistence of Multiferroic Order down to Bilayer Limit of van der Waals Material NiI2.

Realizing a state of matter in two dimensions has repeatedly proven a novel route of discovering new physical phenomena. Van der Waals (vdW) materials have been at the center of these now extensive research activities. They offer a natural way of producing a monolayer of matter simply by mechanical exfoliation. This work demonstrates that the possible multiferroic state with coexisting antiferromagnetic and ferroelectric orders persists down to the bilayer flake of NiI2. By exploiting the optical second-harmonic generation technique, both magnitude and direction of the ferroelectric order, arising from the cycloidal spin order, are successfully traced. The possible multiferroic state's transition temperature decreases from 58 K for the bulk to about 20 K for the bilayer. Our observation will spur extensive efforts to demonstrate multifunctionality in vdW materials, which have been tried mostly by using heterostructures of singly ferroic ones until now.

Multiferroic ground states in free standing perovskite-based nanodots: a density functional theory study

We use density functional theory to investigate the possibility of polar and multiferroic states in free-standing, perovskite-based nanodots at the atomic limit of miniaturization: single unit cells with terminations which allow centro-symmetry. We consider both A-O and B-O2 terminations for three families of nanodots: (i) A = Ba with B = Ti, Zr, and Hf; (ii) A = Ca and Sr with B = Ti; and (iii) A = Na and K with B = Nb. We find all A–O terminated dots to be non-polar and to exhibit cubic symmetry (except for K8NbO6), regardless of the presence of ferroelectricity in the bulk. In contrast, all the B–O2 terminated nanodots considered relax to a non-cubic ground state. Rather surprisingly, all of these structures exhibit polar ground states (except NaNb8O12). We propose a new structural parameter, the cluster tolerance factor (CTF), to determine whether a particular chemistry will result in a polar ground state nanodot, analogous to the Goldschmidt factor for bulk ferroelectrics. In addition, we find that all A–O terminated (except Ca8TiO6) and all polar B–O2 terminated nanodots are magnetic, where none show magnetism in the bulk. As with bulk systems, multiferroicity in the B–O2 terminated dots originates from separation between spin density in peripheral B atoms and polarity primarily caused by the off-center central A atom. Our findings stress that surface termination plays a crucial role in determining whether ferroelectricity is completely suppressed in perovskite-based materials at their limit of miniaturization.

Imaging Ferroelectric Nanodomains in Strained BiFeO3 Nanoscale Films Using Scanning Low-Energy Electron Microscopy: Implications for Low-Power Devices

Precise control of ferroelectric and multiferroic domain states at the nanoscale is of considerable interest due to the potential to boost the development of next-generation low-energy-consumption ...

Role of magnetic ordering in the phase coexistence at the structural instability of the multiferroic BiFeO3–PbTiO3

A promising approach for obtaining a large magnetoelectric response at room temperature in single-phase materials is to make use of phase-change phenomena at structural instabilities of a multiferroic state. This is the case of perovskites showing the morphotropic phase boundary (MPB) between ferroelectric polymorphs with differentiated magnetic orderings, like in BiFeO3–PbTiO3. Indeed, distinctive magnetoelectric effects were recently shown in Mn-modified BiFeO3–PbTiO3, associated with the electric field-induced phase transition between the coexisting MPB phases. We report here analogous effects under the magnetic field in 0.7BiFeO3–0.3PbTiO3, which shed light on the reversibility of the phase-change phenomena and the role of the magnetic ordering in the phase coexistence at the structural instability characteristic of the MPB. Results demonstrate magnetic field-driven phase-change phenomena.

Magnetoelectric properties and low-energy excitations of multiferroic FeCr2S4

We report on the low-frequency optical excitations in the multiferroic ground state of polycrystalline FeCr2S4 in the frequency range 0.3-3~THz and their changes upon applying external magnetic fields up to 7~T. In the ground state below the orbital-ordering temperature T_OO=9K we observe the appearance of several new modes. By applying the external magnetic field parallel and perpendicular to the propagation direction of the THz radiation, we can identify the strongest absorptions to be of predominantly electric-dipole origin. We discuss these modes as the low-energy electronic excitations of the Fe^{2+} ions (3d^6, S=2) in an tetrahedral environment. The eigenfrequencies and relative intensities of these absorption lines are satisfactorily reproduced by our calculation assuming an effective exchange field of 12.8 cm-1 at the Fe^{2+}-ions sites. The direction of the exchange field is found to be slightly tilted out of the ab-plane. With our approach we can also describe previously reported results from M\"ossbauer studies and the order of magnitude of the electric polarisation induced by orbital and non-collinear spin ordering.

Mössbauer Study of Rare-earth Ferroborate NdFe3(BO3)4

An 57Fe Mossbauer spectroscopic study has been conducted on the single-crystal NdFe3(BO3)4 to investigate its magnetic structure and multiferroicity. In the multiferroic state below the Neel temper...

High-field phase transitions in the orbitally ordered multiferroic GeV4S8

The high-field (H,T) phase diagram of the multiferroic lacunar spinel $\mathrm{Ge}{\mathrm{V}}_{4}{\mathrm{S}}_{8}$ has been studied by ultrasound, magnetization, and pyrocurrent experiments in magnetic fields up to 60 T. The title compound consists of molecular building blocks, with vanadium ${\mathrm{V}}_{4}$ clusters characterized by a unique electron density. These vanadium tetrahedra constitute a Jahn-Teller active entity, which drive an orbital-ordering transition at 30 K with the concomitant appearance of ferroelectricity. Ultrasound and magnetization experiments reveal sharp anomalies in magnetic fields of 46 T, which are associated with a first-order phase transition into an orbitally disordered state characterized by significant field and temperature hystereses. We report a sequence of complex magnetic, polar, and orbitally ordered states, i.e., the appearance of two orbitally ordered phases OO1 and OO2 for ${\ensuremath{\mu}}_{0}H<45\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and $T<30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Beyond the paraelectric phase we further evidenced three ferroelectric phases, FE1, FE2, and FE3. Finally, antiferromagnetic (AFM) order $(T<15\text{ K})$ and fully polarized ferromagnetic order $({\ensuremath{\mu}}_{0}H>60\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ have been observed in $\mathrm{Ge}{\mathrm{V}}_{4}{\mathrm{S}}_{8}$. At low temperatures and for fields below 40 T, AFM order coexists with the polar phase FE3 identifying a multiferroic state. Our results demonstrate a fascinating competition of the different orders, which the material manifests in high magnetic fields and at low temperatures.

Evidence of ferromagnetic ordering at low temperatures in epitaxial TbMnO3 thin films grown by means of DC magnetron sputtering

Thin films of the archetypal multiferroic TbMnO3 were epitaxially deposited on (0 0 1)-oriented SrTiO3 substrates by means of DC magnetron sputtering. A substrate temperature of 1053 K and an oxygen pressure of 300 Pa were determined to be appropriate growing conditions for obtaining high-quality, epitaxial TbMnO3 films using this physical growing technique. The structural and magnetic properties of ~100 nm thin TbMnO3 films were investigated. X-ray diffraction patterns showed that the films are single-phase and (00ℓ)-oriented. Rocking curves of the (0 0 2) peak confirm the good crystalline quality of the films. X-ray photoelectron spectroscopy analysis confirmed that the nominal valence of the Mn ions is 3+. The films grown on the SrTiO3 substrates displayed substantial epitaxial strain, which decidedly changed the magnetic ground state of the bulk-like counterpart. In particular, the films displayed ferromagnetic-like interactions below a temperature close to the magnetic ordering temperature of the Mn3+ spins (TN ≈ 42 K). Well-defined hysteresis loops with coercive fields as high as 0.1 T were observed at 5 K. The variation of the coercive field with the temperature clearly indicated the presence of low-temperature ferromagnetism in the TMO films. The origin of the anomalous ferromagnetism in the films is discussed in terms of the strain-induced distortion generated by the lattice mismatch between the film and substrate. The reported results can give insight into accurate strategies for controlling single-phase spin-driven multiferroic states.

Size-dependent bistability in multiferroic nanoparticles

Most multiferroic materials with coexisting ferroelectric and magnetic order exhibit cycloidal antiferromagnetism with wavelength of several nanometers. The prototypical example is bismuth ferrite (${\mathrm{BiFeO}}_{3}$ or BFO), a room-temperature multiferroic considered for a number of technological applications. While most applications require small sizes such as nanoparticles, little is known about the state of these materials when their sizes are comparable to the cycloid wavelength. This work describes a microscopic theory of cycloidal magnetism in nanoparticles based on Hamiltonian calculations. It is demonstrated that magnetic anisotropy close to the surface has a huge impact on the multiferroic ground state. For certain nanoparticle sizes the modulus of the ferromagnetic and ferroelectric moments are bistable, an effect that may be used in the design of ideal memory bits that can be switched electrically and read out magnetically.