Hamed Sharifi Dehsari

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, H_ac cos(ωt), where H_ac 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 H_ac, ω, and H_acω. 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 H_ac, a saturating behavior with ω, and H_acω. Furthermore, a measurement protocol for reliable reporting of magnetoelectric coupling coefficient has been provided.

We have developed a method to solution process strongly hydrogen-bonded odd nylons into ferroelectric thin films.

Polymeric nanocomposite thin-films of magnetic nanoparticles blended with the ferroelectric polymer poly(vinylidene-fluoride-co-triflouroethylene) (P(VDF-TrFE)) are promising candidates for multi-ferroic applications. To date, only thick-film multi-ferroic nanocomposites have been reported. Fabrication of nanocomposite thin-films along with the study of the ferroic properties with magnetic nanoparticle loading is crucial for the realization of functional devices. However, systematic studies and, in particular the dynamic of ferroelectric polarization switching and a solid understanding of the microstructure formation in thin-films, are still missing. Here, we present solution-processed P(VDF-TrFE):magnetic nanoparticle thin-films for multi-ferroic applications, wherein the ferroic properties, polarization switching dynamic, and the microstructure formation are studied as a function of nanoparticle loading. Our results demonstrate that, as the nanoparticle loading increases, the ferroelectric polarization of the nanocomposite decreases and the saturation magnetization increases. Moreover, presence of the nanoparticles substantially increases the polarization switching time and shifts the switching mechanism to one-dimensional growth. The P(VDF-TrFE):magnetic nanoparticle solution phase separates upon film casting. The crystalline regions of P(VDF-TrFE) are pure. The amorphous regions accommodate the nanoparticles. The phase separation leads to agglomerated nanoparticles at higher loadings, and eventually stratified vertical phases occur. The insight gained from the study of thin-film microstructure would help to optimize the performance of the nanocomposite for multi-ferroic applications, and can be used for better understanding of the polymer:nanoparticle nanocomposites for energy storage and memory applications.

Solution-processed memory diodes based on phase separated blends of ferroelectric and semiconducting polymers in the low resistance on-state operate similar to a vertical field-effect transistor at the pinch-off. Numerical simulations have shown that the performance of the diode is dominated by the conduction of charge carriers at the interface between the semiconductor and ferroelectric phases. Here, we present an unambiguous experimental demonstration of the charge injection process in the diodes. We employ a modified diode structure, wherein the electrode in contact with the semiconductor phase has been intentionally removed. Even in the absence of an electrical contact with the semiconductor phase, the diode still shows resistance switching. We provide numerical simulations that reproduce the experimentally measured I-V characteristics and therefore confirm interfacial conduction in the diodes. Furthermore, we discuss the implications of the proposed memory structure particularly in the performance of light-emitting diodes with built-in memory functionality, i.e., MEMOLEDs.

Thermal decomposition is a promising route for the synthesis of magnetic nanoparticles. The simplicity of the synthesis method is counterbalanced by the complex chemistry of the system such as precursor decomposition and surfactant–reducing agent interactions. Control over nanoparticle size is achieved by adjusting the reaction parameters, namely, the precursor concentration. The results, however, are conflicting as both an increase and a decrease in nanoparticle size, as a function of increasing concentration, have been reported. Here, we address the issue of size-controlled synthesis via the precursor concentration. We synthesized iron oxide nanoparticles with sizes from 6 nm to 24 nm with narrow size distributions. We show that the size does not monotonically increase with increasing precursor concentration. After an initial increase, the size reaches a maximum and then shows a decrease with increasing precursor concentration. We argue that the observation of two different size regimes is closely related to the critical role of the amount of surfactant. We confirm the effect of surfactant amount on nucleation and growth and explain the observed trend. Furthermore, we show that the nanoparticles show size-dependent but superior superparamagnetic properties at room temperature.

We demonstrate that ferroelectric memory diodes can be utilized as switching type positive temperature coefficient (PTC) thermistors. The diode consists of a phase separated blend of a ferroelectric and a semiconducting polymer stacked between two electrodes. The current through the semiconducting polymer depends on the ferroelectric polarization. At the Curie temperature the ferroelectric polymer depolarizes and consequently the current density through the semiconductor decreases by orders of magnitude. The diode therefore acts as switching type PTC thermistor. Unlike their inorganic counterparts, the PTC thermistors presented here are thin film devices. The switching temperature can be tuned by varying the Curie temperature of the ferroelectric polymer.