Control of Chiral Magnetism Through Electric Fields in Multiferroic Compounds above the Long-Range Multiferroic Transition

Organic radical ferroelectric crystals with martensitic phase transition

Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.

Anhydrous copper tellurite disulfate Cu3TeO3(SO4)2 featuring the coexistence of spin singlets and a long-range antiferromagnetic order

Anhydrous copper tellurite sulfate, Cu₃TeO₃(SO₄)₂, has been synthesized via vapor transport reactions in sealed silica glass ampoules. In measurements of magnetization M, magnetic susceptibility χ, specific heat C_p and X-band electron spin resonance, a long-range antiferromagnetic order at T_N = 13 K and an H-T magnetic phase diagram have been established. One-third of Cu²⁺ ions were found to form magnetically silent dimers. A peak in dielectric permittivity ε, which accompanies the Néel order, allows considering Cu₃TeO₃(SO₄)₂ as a magnetoelectric multiferroic material of the second type. Density functional theory calculations provided estimations of leading exchange interaction parameters.

A first-principles study on the multiferroicity of semi-modified X2M (X = C, Si; M = F, Cl) monolayers

The research of two-dimensional multiferroic materials has attracted extensive attention in recent years. In this work, we systematically investigated the multiferroic properties of semi-fluorinated and semi-chlorinated graphene and silylene X₂M (X = C, Si; M = F, Cl) monolayers under strain using first principles calculations based on density functional theory. We find that the X₂M monolayer has a frustrated antiferromagnetic order, and a large polarization with a high reversal potential barrier. When increasing the applied biaxial tensile strain, the magnetic order remains unchanged, but the polarization flipping potential barrier of X₂M gradually decreases. When the strain increases to 35%, although the energy required to flip the fluorine and chlorine atoms is still very high in the C₂F and C₂Cl monolayers, it goes down to 312.5 meV and 260 meV in unit cells of the Si₂F and Si₂Cl monolayers, respectively. At the same time, both semi-modified silylenes exhibit metallic ferroelectricity with a band gap of at least 0.275 eV in the direction perpendicular to the plane. The results of these studies show that Si₂F and Si₂Cl monolayers may become a new generation of information storage materials with magnetoelectric multifunctional properties.

High-Magnetic-Sensitivity Magnetoelectric Coupling Origins in a Combination of Anisotropy and Exchange Striction

Magnetoelectric (ME) coupling is highly desirable for sensors and memory devices. Herein, the polarization (P) and magnetization (M) of the DyFeO₃ single crystal were measured in pulsed magnetic fields, in which the ME behavior is modulated by multi-magnetic order parameters and has high magnetic-field sensitivity. Below the ordering temperature of the Dy³⁺-sublattice, when the magnetic field is along the c-axis, the P (corresponding to a large critical field of 3 T) is generated due to the exchange striction mechanism. Interestingly, when the magnetic field is in the ab-plane, ME coupling with smaller critical fields of 0.8 T (a-axis) and 0.5 T (b-axis) is triggered. We assume that the high magnetic-field sensitivity results from the combination of the magnetic anisotropy of the Dy³⁺ spin and the exchange striction between the Fe³⁺ and Dy³⁺ spins. This work may help to search for single-phase multiferroic materials with high magnetic-field sensitivity.

Combined Arrhenius-Merz Law Describing Domain Relaxation in Type-II Multiferroics

Electric fields were applied to multiferroic TbMnO_{3} single crystals to control the chiral domains, and the domain relaxation was studied over 8 decades in time by means of polarized neutron scattering. A surprisingly simple combination of an activation law and the Merz law describes the relaxation times in a wide range of electric field and temperature with just two parameters, an activation-field constant and a characteristic time representing the fastest possible inversion. Over the large part of field and temperature values corresponding to almost 6 orders of magnitude in time, multiferroic domain inversion is thus dominated by a single process, the domain wall motion. Only when approaching the multiferroic transition other mechanisms yield an accelerated inversion.

Electric field control of natural optical activity in a multiferroic helimagnet

Controlling the chiral degree of freedom in matter has long been an important issue for many fields of science. The spin-spiral order, which exhibits a strong magnetoelectric coupling, gives rise to chirality irrespective of the atomic arrangement of matter. Here, we report the resonantly enhanced natural optical activity on the electrically active magnetic excitation, that is, electromagnon, in multiferroic cupric oxide. The electric field control of the natural optical activity is demonstrated through magnetically induced chirality endowed with magnetoelectric coupling. These optical properties inherent to multiferroics may lead to optical devices based on the control of chirality.

Rational magnetic modification of N,N-dioxidized pyrazine ring expanded adenine and thymine: a diradical character induced by base pairing and double protonation

Rational modification of biomolecules especially DNA base pairs for the theoretical design of molecular magnets has attracted extensive interest. In this work, we report a modification strategy for adenine/thymine-based magnets through introducing a N,N-dioxidized pyrazine ring to either adenine or thymine to form ring-expanded bases (noA/noT) based on their experimentally synthesized derivatives. Further functionalization is conducted by double protonation and pairing with a normal complementary base (nohA-T/nohT-A), respectively. The diversity of protonation sites in noA generates totally six nohA-Ts, together with nohT-A forming seven two-step modified topic base pairs. DFT calculations are performed to characterize the magnetic properties and the diradical character, which indicate three diamagnetic (DM) nohA-Ts and three antiferromagnetic (AFM) nohA-Ts with extremely large magnetic coupling constants J ranging from -1279.7 to -2807.4 cm-1, while a relatively mild AFM nohT-A with a J of -194.6 cm-1. The electron separation effect induced by attraction of positive charges originating from protonation is proposed to explain the diradicalization, which is different from the traditional radical-coupler-radical coupling mode. In addition, atomic natural charges and spin densities, and H-bond and molecular orbital analyses are further discussed for verification and deep understanding of the observed unique phenomena. It should be noted that our designed seven topic base pairs have excellent characters including a good synthetic basis, a large scope of the |J| values, and the AFM-DM magnetic conversion or AFM strength modulation controlled by protonation/deprotonation, prototropic tautomerization, base pairing/dissociation, single proton transfer, and even the applied electric field. All these indicate the promising applications in the field of magnetic information storage or switch control. This work highlights the magnetic modification schemes and possible modulation methods of double positive charge doped DNA base pairs by utilizing their potential spin coupling modes.