Magnetoelectric coupling coefficient in multiferroic capacitors:Fact vs Artifacts

Multiferroic materials are characterized by their magnetoelectric coupling coefficient, which can be obtained using a lock-in amplifier by measuring the voltage developed across a multiferroic capacitor in a time-variable magnetic field, Hac cos(ωt), where Hac and ω are the amplitude and frequency of the applied magnetic field. The measurement method, despite its simplicity, is subject to various parasitic effects, such as magnetic induction, which leads to significant over-estimation of the actual magnetoelectric response. This article outlines the measurement theory for a multiferroic capacitor using the lock-in technique. It is demonstrated that the inductive contribution has linear proportionality with Hac, ω, and Hacω. It is shown that the true magnetoelectric coupling response is retrieved from the real component of the lock-in signal. Using a polymer-nanoparticle multiferroic composite, the internal consistency of the proposed measurement method is experimentally demonstrated, and it is shown that the actual multiferroic signal can be retrieved using the lock-in technique by removing the magnetic induction contribution from the signal. It is observed that the magnetoelectric voltage shows only a linear dependence with Hac, a saturating behavior with ω, and Hacω. Furthermore, a measurement protocol for reliable reporting of magnetoelectric coupling coefficient has been provided.</p

Solution-processed multiferroic thin-films with large magnetoelectric coupling at room-temperature

Experimental realization of thin films with a significant room-temperature magnetoelectric coupling coefficient, αME, in the absence of an external DC magnetic field, has been thus far elusive. Here, a large coupling coefficient of 750 ± 30 mV Oe-1 cm-1 is reported for multiferroic polymer nanocomposites (MPCs) thin-films in the absence of an external DC magnetic field. The MPCs are based on PMMA-grafted cobalt-ferrite nanoparticles uniformly dispersed in the piezoelectric polymer poly(vinylidene fluoride-co-trifluoroethylene, P(VDF-TrFE). It is shown that nanoparticle agglomeration plays a detrimental role and significantly reduces αME. Surface functionalization of the nanoparticles by grafting a layer of poly(methyl methacrylate) (PMMA) via atom transfer radical polymerization (ATRP) renders the nanoparticle miscible with P(VDF-TRFE) matrix, thus enabling their uniform dispersion in the matrix even in submicrometer thin films. Uniform dispersion yields maximized interfacial interactions between the ferromagnetic nanoparticles and the piezoelectric polymer matrix leading to the experimental demonstration of large αME values in solution-processed thin films, which can be exploited in flexible and printable multiferroic electronic devices for sensing and memory applications.</p

Solution-processed multiferroic thin-films with large magnetoelectric coupling at room-temperature

Experimental realization of thin films with a significant room-temperature magnetoelectric coupling coefficient, αME, in the absence of an external DC magnetic field, has been thus far elusive. Here, a large coupling coefficient of 750 ± 30 mV Oe-1 cm-1 is reported for multiferroic polymer nanocomposites (MPCs) thin-films in the absence of an external DC magnetic field. The MPCs are based on PMMA-grafted cobalt-ferrite nanoparticles uniformly dispersed in the piezoelectric polymer poly(vinylidene fluoride-co-trifluoroethylene, P(VDF-TrFE). It is shown that nanoparticle agglomeration plays a detrimental role and significantly reduces αME. Surface functionalization of the nanoparticles by grafting a layer of poly(methyl methacrylate) (PMMA) via atom transfer radical polymerization (ATRP) renders the nanoparticle miscible with P(VDF-TRFE) matrix, thus enabling their uniform dispersion in the matrix even in submicrometer thin films. Uniform dispersion yields maximized interfacial interactions between the ferromagnetic nanoparticles and the piezoelectric polymer matrix leading to the experimental demonstration of large αME values in solution-processed thin films, which can be exploited in flexible and printable multiferroic electronic devices for sensing and memory applications.</p

Graphene memristors based on humidity-mediated reduction of graphene oxide

Memristors have emerged as promising devices for neuromorphic applications, particularly as synaptic weight. Graphene oxide, a partially oxidised and electrically insulating form of graphene, has been employed in metal/insulator/metal devices, where resistance switching based on the filamentary growth of the contacting metals has been demonstrated. Here we demonstrate an alternative highly reproducible resistance switching mechanism based on solid-state reduction of GO thin-films mediated by adsorbed water. It is shown that distinguishable and highly stable resistance states can be controllably realised in graphene oxide metal/insulator/metal devices. We have unravelled the growth mechanism and determined the growth kinetic of reduced graphene oxide, which enables a deterministic way to tune the resistance in GO devices. The demonstration of highly reproducible memristors based on graphene oxide crossbar devices is very promising for the realisation of low-cost and environmentally benign solution-processable neuromorphic synaptic weight

Graphene memristors based on humidity-mediated reduction of graphene oxide

Memristors have emerged as promising devices for neuromorphic applications, particularly as synaptic weight. Graphene oxide, a partially oxidised and electrically insulating form of graphene, has been employed in metal/insulator/metal devices, where resistance switching based on the filamentary growth of the contacting metals has been demonstrated. Here we demonstrate an alternative highly reproducible resistance switching mechanism based on solid-state reduction of GO thin-films mediated by adsorbed water. It is shown that distinguishable and highly stable resistance states can be controllably realised in graphene oxide metal/insulator/metal devices. We have unravelled the growth mechanism and determined the growth kinetic of reduced graphene oxide, which enables a deterministic way to tune the resistance in GO devices. The demonstration of highly reproducible memristors based on graphene oxide crossbar devices is very promising for the realisation of low-cost and environmentally benign solution-processable neuromorphic synaptic weight

Organic non-volatile memories from ferroelectric phase-separated blends

New non-volatile memories are being investigated to keep up with the organic-electronics road map(1). Ferroelectric polarization is an attractive physical property as the mechanism for non-volatile switching, because the two polarizations can be used as two binary levels(2). However, in ferroelectric capacitors the read-out of the polarization charge is destructive(3). The functionality of the targeted memory should be based on resistive switching. In inorganic ferroelectrics conductivity and ferroelectricity cannot be tuned independently. The challenge is to develop a storage medium in which the favourable properties of ferroelectrics such as bistability and non-volatility can be combined with the beneficial properties provided by semiconductors such as conductivity and rectification. Here we present an integrated solution by blending semiconducting and ferroelectric polymers into phase-separated networks. The polarization field of the ferroelectric modulates the injection barrier at the semiconductor-metal contact. The combination of ferroelectric bistability with (semi) conductivity and rectification allows for solution-processed non-volatile memory arrays with a simple cross-bar architecture that can be read out nondestructively. The concept of an electrically tunable injection barrier as presented here is general and can be applied to other electronic devices such as light-emitting diodes with an integrated on/off switch.</p

Mechanically stable solution-processed transparent conductive electrodes for optoelectronic applications

The bilayer structure of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coating on silver nanowires (AgNWs) film is a promising structure for replacing indium tin oxide (ITO) as a flexible transparent conductive electrode. Pristine PEDOT:PSS film due to its hydrophilicity and high permeability cannot fully protect AgNWs from mechanical stress and oxidation. Here, we present a composite approach that improves mechanical properties and lifespan of the AgNWs/PEDOT:PSS electrode by adding polyvinyl alcohol (PVA) as a polymer-surfactant. It is shown that addition of PVA improves the conductivity as well as the stability of hybrid electrode under demanding mechanical stress conditions. The drop in conductivity of the hybrid electrode is only 17% after 2000 repeated bending cycles whereas the reference electrode has shown a dramatic drop of 180% in the conductivity. We speculate that generation of hydrogen bonds between PEDOT:PSS and PVA increases adhesivity and cohesivity of the conductive polymer film to the sublayer. So PEDOT:PSS-PVA film not only fixes the arrangement of AgNWs but also improves the welding on cross junction points. By addition of PVA, optoelectronic performance (Figure-of-merit (ΦTC)) of the electrode is improved from ΦTC = 2.646 × 10-3 Ω-1 for AgNWs/PEDOT:PSS to ΦTC = 3.819 × 10-3 Ω-1 for AgNWs/PEDOT:PSS-PVA electrode and power conversion efficiency (PCE) of the polymer solar cell (PSC) is increased by over 17%.</p