Deformable Permanent Ferroelectric or Ferromagnetic Media

In the framework of continuum mechanics, one of possible consistent definitions of deformable permanent magnets is introduced and explored. Similar model can be used for ferroelectric substances. Based on the suggested definition, we establish the key kinematic relationship for the deformable permanent magnet and suggest the simplest master system, allowing to analyze behavior of such substances.

Electric field control of deterministic current-induced magnetization switching in a hybrid ferromagnetic/ferroelectric structure

Subject Code:F04 With the support by the National Natural Science Foundation of China,a study by the research group led by Prof.Wang Kaiyou(王开友)from the Institute of Semiconductors,Chinese Academy of Sciences demonstrates all-electric and programmable manipulations of ferromagnetic bits without external

C-Type Antiferromagnetic Structure of Topological Semimetal CaMnSb_(2)

Determination of the magnetic structure and confirmation of the presence or absence of inversion(P)and time reversal(Τ)symmetry is imperative for correctly understanding the topological magnetic materials.Here highquality single crystals of the layered manganese pnictide CaMnSb_(2)are synthesized using the self-flux method.

Robust and Tunable Ferroelectricity in Ba/Co Codoped (K_(0.5)Na_(0.5))NbO_(3) Ceramics

The 0.98(K_(0.5)Na_(0.5))NbO_(3)-0.02Ba(Nb_(0.5)Co_(0.5))O_(3-δ) ceramics with doped Ba^(2+) and Co^(2+) ions are fabricated,and the impacts of the thermal process are studied.Compared with the rapidly cooled (RC) sample,the slowly cooled (SC) sample possesses superior dielectric and ferroelectric properties,and an 11 K higher ferroelectricparaelectric phase transition temperature,which can be attributed to the structural characteristics such as the grain size and the degree of anisotropy.Heat treatment can reversibly modulate the content of the oxygen vacancies,and in turn the ferroelectric hysteresis loops of the samples.Finally,robust and tunable ferroelectric property is achieved in SC samples with good structural integrity.

Hole-Doped Nonvolatile and Electrically Controllable Magnetism in van der Waals Ferroelectric Heterostructures

Electrical control of magnetism in van der Waals semiconductors is a promising step towards development of two-dimensional spintronic devices with ultralow power consumption for processing and storing information.Here, we propose a design for two-dimensional van der Waals heterostructures(vdWHs) that can host ferroelectricity and ferromagnetism simultaneously under hole doping. By contacting an In Se monolayer and forming an InSe/In_(2)Se_(3) vd WH, the switchable built-in electric field from the reversible out-of-plane polarization enables robust control of the band alignment. Furthermore, switching between the two ferroelectric states(P_↑ and P_↓)of hole-doped In_(2)Se_(3) with an external electric field can interchange the ON and OFF states of the nonvolatile magnetism. More interestingly, doping concentration and strain can effectively tune the magnetic moment and polarization energy. Therefore, this provides a platform for realizing multiferroics in ferroelectric heterostructures,showing great potential for use in nonvolatile memories and ferroelectric field-effect transistors.

Structural and Ferroelectric Transition in Few-Layer HfO_(2) Films by First Principles Calculations

The discovery of ferroelectricity in HfO_(2)-based materials with high dielectric constant has inspired tremendous research interest for next-generation electronic devices.Importantly,films structure and strain are key factors in exploration of ferroelectricity in fluorite-type oxide HfO_(2) films.Here we investigate the structures and straininduced ferroelectric transition in different phases of few-layer HfO_(2) films(layer number𝑁=1–5).It is found that HfO_(2) films for all phases are more stable with increasing films thickness.Among them,the Pmn2_(1)(110)-oriented film is most stable,and the films of𝑁=4,5 occur with a𝑃21 ferroelectric transition under tensile strain,resulting in polarization about 11.8μC/cm^(2) along in-plane𝑎-axis.The ferroelectric transition is caused by the strain,which induces the displacement of Hf and O atoms on the surface to non-centrosymmetric positions away from the original paraelectric positions,accompanied by the change of surface Hf–O bond lengths.More importantly,three new stable HfO_(2)2D structures are discovered,together with analyses of computed electronic structures,mechanical,and dielectric properties.This work provides guidance for theoretical and experimental study of the new structures and strain-tuned ferroelectricity in freestanding HfO_(2) films.

BaTiO_(3)/p-GaN/Au self-driven UV photodetector with bipolar photocurrent controlled by ferroelectric polarization

Ferroelectric materials are promising candidates for ultraviolet photodetectors due to their ferroelectric effect.In this work,a BaTiO_(3)/p-GaN/Au hybrid heterojunction-Schottky self-driven ultraviolet photodetector was fabricated with excellent bipolar photoresponse property.At 0 V bias,the direction of the photocurrent can be switched by flipping the depolarization field of BaTiO_(3),which allows the performance of photodetectors to be controlled by the ferroelectric effect.Meanwhile,a relatively large responsivity and a fast response speed can be also observed.In particular,when the depolarization field of BaTiO_(3) is in the same direction of the built-in electric field of the Au/p-GaN Schottky junction(up polarized state),the photodetector exhibits a high responsivity of 18 mA/W at 360 nm,and a fast response speed of<40 ms at 0 V.These findings pave a new way for the preparation of high-performance photodetectors with bipolar photocurrents.

New‑Generation Ferroelectric AlScN Materials

Ferroelectrics have great potential in the field of nonvolatile memory due to programmable polarization states by external electric field in nonvolatile manner.However,complementary metal oxide semiconductor compatibility and uniformity of ferroelectric performance after size scaling have always been two thorny issues hindering practical application of ferroelectric memory devices.The emerging ferroelectricity of wurtzite structure nitride offers opportunities to circumvent the dilemma.This review covers the mechanism of ferroelectricity and domain dynamics in ferroelectric AlScN films.The performance optimization of AlScN films grown by different techniques is summarized and their applications for memories and emerging in-memory computing are illustrated.Finally,the challenges and perspectives regarding the commercial avenue of ferroelectric AlScN are discussed.

Large and nonlinear electric field response in a two-dimensional ferroelectric Rashba material

The achievement of electrical spin control is highly desirable.One promising strategy involves electrically mod-ulating the Rashba spin orbital coupling effect in materials.A semiconductor with high sensitivity in its Rashba constant to external electric fields holds great potential for short channel lengths in spin field-effect transistors,which is crucial for preserving spin coherence and enhancing integration density.Hence,two-dimensional(2D)Rashba semiconductors with large Rashba constants and significant electric field responses are highly desirable.Herein,by employing first-principles calculations,we design a thermodynamically stable 2D Rashba semiconductor,YSbTe_(3),which possesses an indirect band gap of 1.04 eV,a large Rashba constant of 1.54 eV·Åand a strong electric field response of up to 4.80 e·Å^(2).In particular,the Rashba constant dependence on the electric field shows an unusual nonlinear relationship.At the same time,YSbTe_(3)has been identified as a 2D ferroelectric material with a moderate polarization switching energy barrier(~0.33 eV per formula).By changing the electric polarization direction,the Rashba spin texture of YSbTe_(3)can be reversed.These out-standing properties make the ferroelectric Rashba semiconductor YSbTe_(3)quite promising for spintronic applications.

Semiclassical approach to spin dynamics of a ferromagnetic S = 1 chain

Motivated by recent experimental progress on the quasi-one-dimensional quantum magnet Ni Nb2O6, we study the spin dynamics of an S = 1 ferromagnetic Heisenberg chain with single-ion anisotropy by using a semiclassical molecular dynamics approach. This system undergoes a quantum phase transition from a ferromagnetic to a paramagnetic state under a transverse magnetic field, and the magnetic response reflecting this transition is well described by our semiclassical method.We show that at low temperature the transverse component of the dynamical structure factor depicts clearly the magnon dispersion, and the longitudinal component exhibits two continua associated with single-and two-magnon excitations,respectively. These spin excitation spectra show interesting temperature dependence as effects of magnon interactions. Our findings shed light on the experimental detection of spin excitations in a large class of quasi-one-dimensional magnets.

Ultra‑Stable Sodium‑Ion Battery Enabled by All‑Solid‑State Ferroelectric‑Engineered Composite Electrolytes

Symmetric Na-ion cells using the NASICON-structured electrodes could simplify the manufacturing process,reduce the cost,facilitate the recycling post-process,and thus attractive in the field of large-scale stationary energy storage.However,the long-term cycling performance of such batteries is usually poor.This investigation reveals the unavoidable side reactions between the NASICON-type Na_(3)V_(2)(PO_(4))_(3)(NVP)anode and the commercial liquid electrolyte,leading to serious capacity fading in the symmetric NVP//NVP cells.To resolve this issue,an all-solid-state composite electrolyte is used to replace the liquid electrolyte so that to overcome the side reaction and achieve high anode/electrolyte interfacial stability.The ferroelectric engineering could further improve the interfacial ion conduction,effectively reducing the electrode/electrolyte interfacial resistances.The NVP//NVP cell using the ferroelectric-engineered composite electrolyte can achieve a capacity retention of 86.4%after 650 cycles.Furthermore,the electrolyte can also be used to match the Prussianblue cathode NaxFeyFe(CN)_(6−z)·nH_(2)O(NFFCN).Outstanding long-term cycling stability has been obtained in the all-solid-state NVP//NFFCN cell over 9000 cycles at a current density of 500 mA g^(-1),with a fading rate as low as 0.005%per cycle.

Interfacial stress engineering toward enhancement of ferroelectricity in Al doped HfO_(2)thin films

Ferroelectric HfO_(2)has attracted much attention owing to its superior ferroelectricity at an ultra-thin thickness and good compatibility with Si-based complementary metal-oxide-semiconductor(CMOS)technology.However,the crystallization of polar orthorhombic phase(o-phase)HfO_(2)is less competitive,which greatly limits the ferroelectricity of the as-obtained ferroelectric HfO_(2)thin films.Fortunately,the crystallization of o-phase HfO_(2)can be thermodynamically modulated via interfacial stress engineering.In this paper,the growth of improved ferroelectric Al doped HfO_(2)(HfO_(2):Al)thin films on(111)-oriented Si substrate has been reported.Structural analysis has suggested that nonpolar monoclinic HfO_(2):Al grown on(111)-oriented Si substrate suffered from a strong compressive strain,which promoted the crystallization of(111)-oriented o-phase HfO_(2)in the as-grown HfO_(2):Al thin films.In addition,the in-plane lattice of(111)-oriented Si substrate matches well with that of(111)-oriented o-phase HfO_(2),which further thermally stabilizes the o-phase HfO_(2).Accordingly,an improved ferroelectricity with a remnant polarization(2P_(r))of 26.7C/cm^(2) has been obtained.The results shown in this work provide a simple way toward the preparation of improved ferroelectric HfO_(2)thin films.

Evolution of anomalous Hall effect in ferromagnetic Weyl semimetal Nb_(x)Zr_(1-x)Co_(2)Sn

Magnetic topological semimetal can host various topological non-trivial states leading to exotic novel transport properties.Here we report the systematic magneto-transport studies on the Heusler alloy Nb_(x)Zr_(1-x)Co_(2)Sn considered as a ferromagnetic(FM)Weyl semimetal.The cusp anomaly of temperature-dependent resistivity and large isotropic negative magneto-resistivity(MR)emerge around the FM transition consistent with the theoretical half-metallic predictions.The prominent anomalous Hall effect(AHE)has the same behavior with the applied field along various crystal directions.The Nb doping introduces more disorder resulting in the enhancement of the upturn for the temperature-dependent resistivity in low temperatures.With Nb doping,the AHE exhibits systemic evolution with the Fermi level lifted.At the doping level of x=0.25,the AHE mainly originates from the intrinsic contribution related to non-trivial topological Weyl states.

Ferroelectricity Induced by Oxygen Vacancies in Rhombohedral ZrO_(2) Thin Films

Rhombohedral phase Hf_(x)Zr_(1.x)O_(2)(HZO,x from 0 to 1)films are promising for achieving robust ferroelectric polarization without the need for an initial wake-up pre-cycling,as is normally the case for the more commonly studied orthorhombic phase.However,a large spontaneous polarization observed in rhombohedral films is not fully understood,and there are also large discrepancies between experimental and theoretical predictions.In this work,in rhombohedral ZrO_(2)thin films,we show that oxygen vacancies are not only a key factor for stabilizing the phase,but they are also a source of ferroelectric polarization in the films.This is shown experimentally through the investigation of the structural properties,chemical composition and the ferroelectric properties of the films before and after an annealing at moderate temperature(400℃)in an oxygen environment to reduce the V_(o)concentration compared.The experimental work is supported by density functional theory(DFT)calculations which show that the rhombohedral phase is the most stable one in highly oxygen defective ZrO_(2)films.The DFT calculations also show that V_(o)contribute to the ferroelectric polarization.Our findings reveal the importance of V_(o)for stabilizing rhombohedral ZrO_(2)thin films with superior ferroelectric properties.

Half-metallic ferromagneticWeyl fermions related to dynamic correlations in the zinc-blende compound VAs

The realization of 100%polarized topologicalWeyl fermions in half-metallic ferromagnets is of particular importance for fundamental research and spintronic applications.Here,we theoretically investigate the electronic and topological properties of the zinc-blende compound VAs,which was deemed as a half-metallic ferromagnet related to dynamic correlations.Based on the combination of density functional theory and dynamical mean field theory,we uncover that the half-metallic ferromagnet VAs exhibits attractive Weyl semimetallic behaviors which are very close to the Fermi level in the DFT+U regime with effect U values ranging from 1.5 eV to 2.5 eV.Meanwhile,we also investigate the magnetization-dependent topological properties;the results show that the change of magnetization directions only slightly affects the positions of Weyl points,which is attributed to the weak spin–orbital coupling effects.The topological surface states of VAs projected on semi-infinite(001)and(111)surfaces are investigated.The Fermi arcs of all Weyl points are clearly visible on the projected Fermi surfaces.Our findings suggest that VAs is a fully spin-polarized Weyl semimetal with many-body correlated effects in the effective U values range from 1.5 eV to 2.5 eV.

Enhanced ferroelectric and improved leakage of BFO-based thin films through increasing entropy strategy

BiFeO_(3)(BFO)has received considerable attention as a lead-free ferroelectric film due to its large theoretical remnant polariza-tion.However,BFO suffers from a large leakage current,resulting in poor ferroelectric properties.Herein,the sol-gel method was used to deposit a series of BFO-based thin films on fluorine-doped tin oxide substrates,and the effects of the substitution of the elements Co,Cu,Mn(B-site)and Sm,Eu,La(A-site)on the crystal structure,ferroelectricity,and leakage current of the BFO-based thin films were invest-igated.Results confirmed that lattice distortion by X-ray diffraction can be attributed to the substitution of individual elements in the BFO-based films.Sm and Eu substitutions contribute to the lattice distortion in a pseudo-cubic structure,while La is biased toward pseudo-tet-ragonal.Piezoelectric force microscopy confirmed that reversible switching of ferroelectric domains by nearly 180°can be realized through the prepared films.The ferroelectric hysteresis loops showed that the order for the polarization contribution is as follows:Cu>Co>Mn(B-site),Sm>La>Eu(A-site).The current density voltage curves indicated that the order for leakage contribution is as follows:Mn<Cu<Co(B-site),La<Eu<Sm(A-site).Scanning electron microscopy showed that the introduction of Cu elements facilitates the formation of dense grains,and the grain size distribution statistics proved that La element promotes the reduction of grain size,leading to the increase of grain boundaries and the reduction of leakage.Finally,a Bi_(0.985)Sm_(0.045)La_(0.03)Fe_(0.96)Co_(0.02)Cu_(0.02)O_(3)(SmLa-CoCu)thin film with a qualitative leap in the remnant polarization from 25.5(Bi_(0.985)Sm_(0.075)FeO_(3))to 98.8µC/cm^(2)(SmLa-CoCu)was prepared through the syner-gistic action of Sm,La,Co,and Cu elements.The leakage current is also drastically reduced from 160 to 8.4 mA/cm^(2)at a field strength of 150 kV/cm.Thus,based on the increasing entropy strategy of chemical engineering,this study focuses on enhancing ferroelectricity and decreasing leakage current,providing a promising path for the advancement of ferroelectric devices.

Influence of exchange bias on spin torque ferromagnetic resonance for quantification of spin–orbit torque efficiency

Antiferromagnet(AFM)/ferromagnet(FM)heterostructure is a popular system for studying the spin–orbit torque(SOT)of AFMs.However,the interfacial exchange bias field induces that the magnetization in FM layer is noncollinear to the external magnetic field,namely the magnetic moment drag effect,which further influences the characteristic of SOT efficiency.In this work,we study the SOT efficiencies of IrMn/NiFe bilayers with strong interfacial exchange bias by using spin-torque ferromagnetic resonance(ST-FMR)method.A full analysis on the AFM/FM systems with exchange bias is performed,and the angular dependence of magnetization on external magnetic field is determined through the minimum rule of free energy.The ST-FMR results can be well fitted by this model.We obtained the relative accurate SOT efficiencyξ_(DL)=0.058 for the IrMn film.This work provides a useful method to analyze the angular dependence of ST-FMR results and facilitates the accurate measurement of SOT efficiency for the AFM/FM heterostructures with strong exchange bias.

Reliable ferroelectricity down to cryogenic temperature in wakeup free Hf_(0.5)Zr_(0.5)O_(2)thin films by thermal atomic layer deposition

The performance and reliability of ferroelectric thin films at temperatures around a few Kelvin are critical for their application in cryo-electronics.In this work,TiN/Hf_(0.5)Zr_(0.5)O_(2)/TiN capacitors that are free from the wake-up effect are investigated systematically from room temperature(300 K)to cryogenic temperature(30 K).We observe a consistent decrease in permittivity(εr)and a progressive increase in coercive electric field(Ec)as temperatures decrease.Our investigation reveals exceptional stability in the double remnant polarization(2P_(r))of our ferroelectric thin films across a wide temperature range.Specifically,at 30 K,a 2P_(r)of 36μC/cm^(2)under an applied electric field of 3.0 MV/cm is achieved.Moreover,we observed a reduced fatigue effect at 30 K in comparison to 300 K.The stable ferroelectric properties and endurance characteristics demonstrate the feasibility of utilizing HfO_(2)based ferroelectric thin films for cryo-electronics applications.

Enhancing BiVO_(4)photoanode performance by insertion of an epitaxial BiFeO_(3)ferroelectric layer

BiVO_(4)(BVO)is a promising material as the photoanode for use in photoelectrochemical applications.However,the high charge recombination and slow charge transfer of the BVO have been obstacles to achieving satisfactory photoelectrochemical performance.To address this,various modifications have been attempted,including the use of ferroelectric materials.Ferroelectric materials can form a permanent polarization within the layer,enhancing the separation and transport of photo-excited electron-hole pairs.In this study,we propose a novel approach by depositing an epitaxial BiFeO_(3)(BFO)thin film underneath the BVO thin film(BVO/BFO)to harness the ferroelectric property of BFO.The self-polarization of the inserted BFO thin film simultaneously functions as a buffer layer to enhance charge transport and a hole-blocking layer to reduce charge recombination.As a result,the BVO/BFO photoanodes showed more than 3.5 times higher photocurrent density(0.65 mA cm^(-2))at 1.23 V_(RHE)under the illumination compared to the bare BVO photoanodes(0.18 m A cm^(-2)),which is consistent with the increase of the applied bias photon-to-current conversion efficiencies(ABPE)and the result of electrochemical impedance spectroscopy(EIS)analysis.These results can be attributed to the self-polarization exhibited by the inserted BFO thin film,which promoted the charge separation and transfer efficiency of the BVO photoanodes.

Stacking-dependent exchange bias in two-dimensional ferromagnetic/antiferromagnetic bilayers

A clear microscopic understanding of exchange bias is crucial for its application in magnetic recording, and further progress in this area is desired. Based on the results of our first-principles calculations and Monte Carlo simulations,we present a theoretical proposal for a stacking-dependent exchange bias in two-dimensional compensated van der Waals ferromagnetic/antiferromagnetic bilayer heterostructures. The exchange bias effect emerges in stacking registries that accommodate inhomogeneous interlayer magnetic interactions between the ferromagnetic layer and different spin sublattices of the antiferromagnetic layer. Moreover, the on/off switching and polarity reversal of the exchange bias can be achieved by interlayer sliding, and the strength can be modulated using an external electric field. Our findings push the limits of exchange bias systems to extreme bilayer thickness in two-dimensional van der Waals heterostructures, potentially stimulating new experimental investigations and applications.