Creation and erasure of polar bubble domains in PbTiO_(3)films by mechanical stress and light illuminations

The controllable manipulation of polar topological structures(e.g.skyrmion bubble)in ferroelectric materials have been considered as a cornerstone for future programmable nano-electronics.Here,we present the effective creation and erasure of polar bubble states PbTiO_(3)(PTO)multilayers trigged by mechanical stress and light illumination,respectively.It was found that applying atomic force microscope(AFM)tip force can induced formation of nanoscale bubble domains from the initial monodomain state.Moreover,the created bubble domain can be eliminated by exposure to ultraviolet or infrared light illumination.The above results can be understood by modulation of depolarization screening charges and bias fields,as reflected by scanning Kelvin potential microscopic(SKPM)observations,whereby the flexoelectric effect from the tip force tends to remove the screening charges on top surface and modulate the bias field that favors the formation of bubble state while light illumination tends to recover the screen charges and favor the monodomain state.The results provide a good example for multi-field manipulation of polar topologies,which might create a new avenue towards the immerging new concept electronic devices.

Bubble domain evolution in well-ordered BiFeO_(3) nanocapacitors

Ferroelectric nanocapacitors have attracted intensive research interest due to their novel functionalities and potential application in nanodevices.However,due to the lack of knowledge of domain evolution in isolated nanocapacitors,precise manipulation of topological domain switching in the nanocapacitor is still a challenge.Here,we report unique bubble and cylindrical domains in the well-ordered BiFeO_(3) nanocapacitor array.The transformation of bubble,cylindrical and mono domains in isolated ferroelectric nanocapacitor has been demonstrated via scanning probe microscopy(SPM).The bubble domain can be erased to mono domain or written to cylindrical domain and mono domain by positive and negative voltage,respectively.Additionally,the domain evolution rules,which are mainly affected by the depolarization field,have been observed in the nanocapacitors with different domain structures.This work will be helpful in understanding the domain evolution in ferroelectric nanocapacitors and providing guidance on the manipulation of nanoscale topological domains.

Patterning of large area nanoscale domains in as-grown epitaxial ferroelectric PbTiO_(3)films

Effective tuning of nanoscale domain structures provides fundamental basis for controlling and engineering of various functionalities in ferroelectric materials.In this work,we demonstrate the precise patterning of nanoscopic domain structures in as-grown epitaxial PbTiO_(3)(PTO)films by merely introducing an ultrathin pre-patterned doping layer(e.g.,Fe-doped PTO).The doping layer can effectively reverse the interfacial built-in bias,consequent to a reversed initial polarization reorientation in the as-grown film,which makes it possible to transfer the nano-patterns in the doping layer into the domain structure of ferroelectric films.For instance,we have successfully fabricated large area ordered array of nanoscale cylindrical domains(downward polarization)embedded in the matrix domain with opposite polarization(upward polarization)in PTO film.These nanoscale cylinder domains also allow deterministic and reversible erasure and creation induced by biased tip scanning.The results provide an effective pathway for on-demand patterning of large area nanoscale domains in the as-grown films,which may find applications in a wide range of nanoelectronic devices.

Magnetic skyrmions:materials,manipulation,detection,and applications in spintronic devices

Magnetic skyrmions are vortex-like spin configurations that possess nanometric dimensions,topological stability,and high controllability through various external stimuli.Since their first experimental observation in helimagnet MnSi in 2009,magnetic skyrmions have emerged as a highly promising candidate for carrying information in future high-performance,low-energy-consumption,non-volatile information storage,and logical calculation.In this article,we provide a comprehensive review of the progress made in the field of magnetic skyrmions,specifically in materials,manipulation,detection,and application in spintronic devices.Firstly,we introduce several representative skyrmion material systems,including chiral magnets,magnetic thin films,centrosymmetric materials,and Van der Waals materials.We then discuss various methods for manipulating magnetic skyrmions,such as electric current and electric field,as well as detecting them,mainly through electrical means such as the magnetoresistance effect.Furthermore,we explore device applications based on magnetic skyrmions,such as track memory,logic computing,and neuromorphic devices.Finally,we summarize the challenges faced in skyrmion research and provide future perspectives.

Emergent strain engineering of multiferroic BiFeO_(3) thin films

BiFeO_(3),a single-phase multiferroic material,possesses several polymorphs and exhibits a strong sensitivity to strain.Recently,emergent strain engineering in BiFeO_(3) thin films has attracted intense interest,which can overcome the confines of traditional strain engineering introduced through the mismatch between the film and substrate.In this review,we discuss emerging non-traditional strain engineering approaches to create new ground states and manipulate novel functionalities in multiferroic BiFeO_(3) thin films.Through fabricating freestanding thin films,inserting an interface layer or utilizing thermal expansion mismatch,continuously tunable strain can be imposed beyond substrate limitations.Nanostructured evolution and defect introduction are discussed as efficient routes to introduce strain,promising for the development of new nanodevices.Ultrafast optical excitation,growth conditions and chemical doping driven strain are summarized as well.We hope this review will arouse the readers’interest in this fascinating field.

Ferroelectricity in dopant-free HfO_(2) thin films prepared by pulsed laser deposition

As a high-k material,hafnium oxide(HfO_(2))has been used in gate dielectrics for decades.Since the discovery of polar phase in Si-doped HfO_(2) films,chemical doping has been widely demonstrated as an effective approach to stabilize the ferroelectric phase in HfO_(2) based thin films.However,the extra capping layer deposition,post-growth annealing and wake-up effect are usually required to arouse the ferroelectricity in HfO_(2) based thin films,resulting in the increase of complexity for sample synthesis and the impediment of device application.In this study,the ferroelectricity is observed in non-capped dopant-free HfO_(2) thin films prepared by pulsed laser deposition(PLD)without post-growth annealing.By adjusting the deposited temperature,oxygen pressure and thickness,the maximum polarization up to 14.7 m C/cm^(2) was obtained in 7.4 nm-thick film.The fraction of orthorhombic phase,concentrations of defects and size effects are considered as possible mechanisms for the influences of ferroelectric prop-erties.This study indicates that PLD is an effective technique to fabricate high-quality ferroelectric HfO_(2) thin films in the absence of chemical doping,capping layer deposition and post-growth annealing,which may boost the process of nonvolatile memory device application.

Controlled manipulation of conductive ferroelectric domain walls and nanoscale domains in BiFeO_(3) thin films

Recently,there is a surge of research interest in configurable ferroelectric conductive domain walls which have been considered as possible fundamental building blocks for future electronic devices.In this work,by using piezoresponse force microscopy and conductive atomic force microscopy,we demonstrated the controlled manipulation of various conductive domain walls in epitaxial BiFeO_(3) thin films,e.g.neutral domain walls(NDW)and charged domain walls(CDWs).More interestingly,a specific type of nanoscale domains was also identified,which are surrounded by highly conductive circular CWDs.Similar nano-scale domains can also be controlled created and erasured by applying local field via conductive probe,which allow nondestructive current readout of different domain states with a large on/off resistance ratio up to 102.The results indicate the potential to design and develop high-density non-volatile ferroelectric memories by utilizing these programable conductive nanoscale domain walls.

Versatile SrFeO_(x) for memristive neurons and synapses

Spiking neural network(SNN)consisting of memristor-based artificial neurons and synapses has emerged as a compact and energy-efficient hardware solution for spatiotemporal information processing.However,it is challenging to develop memristive neurons and synapses based on the same material system because the required resistive switching(RS)characteristics are different.Here,it is shown that SrFeO_(x)(SFO),an intriguing material system exhibiting topotactic phase transformation between insulating brownmillerite(BM)SrFeO_(2).5 phase and conductive perovskite(PV)SrFeO_(3) phase,can be engineered into both neuronal and synaptic devices.Using a BM-SFO single layer as the RS medium,the Au/BM-SFO/SrRuO_(3)(SRO)memristor exhibits nonvolatile RS behavior originating from the formation/rupture of PV-SFO filaments in the BM-SFO matrix.By contrast,using a PV-SFO(matrix)/BM-SFO(interfacial layer)bilayer as the RS medium,the Au/PV-SFO/BM-SFO/SRO memristor exhibits volatile RS behavior originating from the interfacial BM-PV phase transformation.Synaptic and neuronal characteristics are further demonstrated in the Au/BM-SFO/SRO and Au/PV-SFO/BM-SFO/SRO memristors,respectively.Using the SFO-based synapses and neurons,fully memristive SNNs are constructed by simulation,which show good performance on unsupervised image recognition.Our study suggests that SFO is a versatile material platform on which both neuronal and synaptic devices can be developed for constructing fully memristive SNNs.

Structural, magnetic behaviors and temperature-dependent Raman scattering spectra of Y and Zr codoped BiFeO_(3) ceramics

Multiferroic BiFeO_(3)(BFO)and Y,Zr codoped BFO(Bi_(1-x)Y_(x)Fe_(0.95)Zr_(0.05)O_(3)ceramics were prepared and the influence of codoping on the crystal structure and magnetic properties were investigated in this work.Confirmed by the evolution of X-ray diffraction and Raman modes,the codoping has changed the crystal structure from rhombohedral to tetragonal in bulk BFO ceramics.The enhancement of magnetic behaviors is demonstrated by the damage of space-modulated spiral spin structure,and it can be attributed to the crystal structure change and size effects.Meanwhile,Raman spectra from 300 to 800K demonstrates that lower frequency phonon modes show rapid softening near the Neel temperature.

Experimental search for high-performance ferroelectric tunnel junctions guided by machine learning

Ferroelectric tunnel junction(FTJ)has attracted considerable attention for its potential applications in nonvolatile memory and neuromorphic computing.However,the experimental exploration of FTJs with high ON/OFF ratios is a challenging task due to the vast search space comprising of ferroelectric and electrode materials,fabrication methods and conditions and so on.Here,machine learning(ML)is demonstrated to be an effective tool to guide the experimental search of FTJs with high ON/OFF ratios.A dataset consisting of 152 FTJ samples with nine features and one target attribute(i.e.,ON/OFF ratio)is established for ML modeling.Among various ML models,the gradient boosting classification model achieves the highest prediction accuracy.Combining the feature importance analysis based on this model with the association rule mining,it is extracted that the utilizations of{graphene/graphite(Gra)(top),LaNiO_(3)(LNO)(bottom)}and{Gra(top),Ca_(0.96)Ce_(0.04)MnO_(3)(CCMO)(bottom)}electrode pairs are likely to result in high ON/OFF ratios in FTJs.Moreover,two previously unexplored FTJs:Gra/BaTiO_(3)(BTO)/LNO and Gra/BTO/CCMO,are predicted to achieve ON/OFF ratios higher than 1000.Guided by the ML predictions,the Gra/BTO/LNO and Gra/BTO/CCMO FTJs are experimentally fabricated,which unsurprisingly exhibit≥1000 ON/OFF ratios(~8540 and~7890,respectively).This study demonstrates a new paradigm of developing high-performance FTJs by using ML.