One of the new materials that have recently attracted wide attention of researchers are magnetoelectric (ME) composites. Great interest in these materials is due to their properties associated with ...the transformation of electric polarization/magnetization under the influence of external magnetic/electric fields and the possibility of their use to create new devices. In the proposed review, ME magnetic field sensors based on the widely used structures Terfenol—PZT/PMN-PT, Metglas—PZT/PMN-PT, and Metglas—Lithium niobate, among others, are considered as the first applications of the ME effect in technology. Estimates of the parameters of ME sensors are given, and comparative characteristics of magnetic field sensors are presented. Taking into account the high sensitivity of ME magnetic field sensors, comparable to superconducting quantum interference devices (SQUIDs), we discuss the areas of their application.
•The relaxation of magnetization in thin FeRh films has been studied during phase transition.•A phenomenological model that describes the evolution of the phase transition in thin-film FeRh systems ...has been constructed.•Phase transition dependence on the initial state of the system is demonstrated.•A new approach to the interpretation of TFORC diagrams is proposed.
The process of the nucleation and evolution of the ferromagnetic phase in the thin Fe49Rh51 film on a MgO substrate near the phase transition from the antiferromagnetic (AFM) to the ferromagnetic (FM) state was comprehensively characterized by means of measurements of the temperature, time and magnetic field dependences of the magnetization. The observed relaxation processes of magnetization indicate the nucleation of the FM phase on the film surface and its further growth towards the film-substrate interface upon heating or applying the external magnetic field. The confirmation and verification of the FM phase growth evolution were carried out by the methods of vibrational magnetometry and magnetic force microscopy. A phenomenological model described the evolution of the FM phase growth in thin films based on the Bean-Rodbell and Kolmogorov-Johnson-Mehl-Avrami (KJMA) models is proposed. The article presents a new approach for interpreting Temperature First Order Reversal Curve (TFORC) diagrams based on comparative analysis of the coercivity and squareness coefficient temperature dependences and the peak position on the diagram.
This paper presents data on the macroscopic polarization of copolymer films of vinylidene fluoride with tetrafluoroethylene obtained with a modified apparatus assembled according to the Sawyer-Tower ...Circuit. The kinetics of the polarization process were analyzed taking into consideration the contributions of both bound and quasi-free (impurity) charges. It was shown that an "abnormal" decrease in conductivity was observed in fields near the coercive fields. This could be associated with the appearance of deep traps of the impurity charge carriers formed by the polar planes of β-phase crystals. The conductivity data obtained from the charge and current responses differed. It was concluded that chain segments contributing to polarization with sufficiently low fields were present in the amorphous phase. A comparison showed that the average size of β-phase crystals (crystals of X-ray diffraction reflection width) was almost one order of magnitude lower than the domain size obtained using piezoresponse force microscopy (PFM). The analysis of the fast-stage dielectric response before and after polarization indicated that as the external polarizing field increased in the ferroelectric polymer chains, conformational transitions occurred according to the T
GT
G
→ (-TT-)
и TGTG → (-TT-)
types. This was accompanied by an increase in the effective dipole moment in the amorphous phase chains. The analysis of the IR spectroscopy data obtained in transmission and ATR modes revealed a difference in the conformational states of the chains in the core and surface parts of the film.
General problems of structural changes which occur at storage of two‐phase crystalline polymers with a metastable structure at room conditions are considered on the example of a vinylidene fluoride ‐ ...hexafluoropropylene copolymer. The physical aging occurs in flexible‐chain crystalline polymers (polyethylene, fluoropolymers under consideration, etc.), where, due to low glass transition temperatures, the liquid‐like dynamics of amorphous phase chains is realized at room temperature through cooperative micro‐Brownian motions with short relaxation times. Taking into account that covalent‐bound sections of the chains of the amorphous phase can enter the crystallites, the noted mobility may initiate changes in the size of the latter. Such a possibility is proved by the example of the noted copolymer. At low‐temperature crystallization of a vinylidene fluoride ‐ hexafluoropropylene copolymer from a solution the formed α‐phase crystals have little perfection. The size of the crystals increases when the films are stored in room conditions. Because of the crystal polymorphism, at the same time a certain fraction of γ‐phase crystals which are present in initial films undergo a polymorphic transformation γ → α. These processes lead to an increase in crystallinity. Moreover, during such processes additional structuring is observed, which is reduced to the displacement of various kinds of intra‐chain defects into the amorphous phase (and especially into the surface). Since the copolymers under study are ferroelectrics, they were studied by piezo force microscopy. It was found that despite the crystallization predominantly in nonpolar α‐phase, piezo force microscopy revealed a domain structure, which formation mechanism is discussed. The structural changes at physical aging of the films affect the character of the noted domain structure. Thus, it is suggested that the mechanism of the described structural changes is realized through the developed cooperative mobility of the chains in the amorphous phase, which characterizes the processes of rotational diffusion.
The use of the ferroelectric materials in hybrid nanogenerators based on triboelectricity requires the information on their surface potential. The latter was studied by the Kelvin‐mode method on VDF‐HFP copolymer films obtained by crystallization from solution in ethyl acetate. Freshly prepared films have a positive surface potential. Exposure of such a film at room temperature for 4 years is accompanied by structural changes: increase in the volume degree of crystallinity, but its strong decrease in the surface. The observed decrease occurs due to displacement of the amorphous phase chains, which contains sections of chains with monomer units attached in the "wrong" position, into the surface. Such structural changes are accompanied by significant changes in the surface potential, which changes its sign.
With the recent thriving of low-power electronic microdevices and sensors, the development of components capable of scavenging environmental energy has become imperative. In this article, we studied ...bidomain congruent LiNbO 3 (LN) single crystals combined with magnetic materials for dual, mechanical, and magnetic energy harvesting applications. A simple magneto-mechano-electric composite cantilever, with a trilayered long-bar bidomain LN/spring-steel/metglas structure and a large tip proof permanent magnet, was fabricated. Its vibration and magnetic energy harvesting capabilities were tested while trying to optimize its resonant characteristics, load impedance, and tip proof mass. The vibration measurements yielded a peak open-circuit voltage of 2.42 kV/g, a short-circuit current of <inline-formula> <tex-math notation="LaTeX">60.1~\mu \text{A} </tex-math></inline-formula>/g, and an average power of up to 35.6 mW/g 2 , corresponding to a power density of 6.9 mW/(cm<inline-formula> <tex-math notation="LaTeX">^{{3}}\cdot \text {g}^{{2}} </tex-math></inline-formula>), at a low resonance frequency of 29.22 Hz and with an optimal load of 40 <inline-formula> <tex-math notation="LaTeX">\text{M}\Omega </tex-math></inline-formula>. The magnetic response revealed a resonant peak open-circuit voltage of 90.9 V/Oe and an average power of up to <inline-formula> <tex-math notation="LaTeX">49.9~\mu \text{W} </tex-math></inline-formula>/Oe 2 , corresponding to a relatively large magnetoelectric coefficient of 1.82 kV/(cm<inline-formula> <tex-math notation="LaTeX">\cdot </tex-math></inline-formula>Oe) and a power density of <inline-formula> <tex-math notation="LaTeX">9.7~\mu \text{W} </tex-math></inline-formula>/(cm<inline-formula> <tex-math notation="LaTeX">^{{3}}\cdot \text {Oe}^{{2}} </tex-math></inline-formula>). We thus developed a system that is, in principle, able to scavenge electrical power simultaneously from low-level ambient mechanical and magnetic sources to feed low-power electronic devices.
Polymer-based magnetoelectric composite materials have attracted a lot of attention due to their high potential in various types of applications as magnetic field sensors, energy harvesting, and ...biomedical devices. Current researches are focused on the increase in the efficiency of magnetoelectric transformation. In this work, a new strategy of arrangement of clusters of magnetic nanoparticles by an external magnetic field in PVDF and PFVD-TrFE matrixes is proposed to increase the voltage coefficient (α
) of the magnetoelectric effect. Another strategy is the use of 3-component composites through the inclusion of piezoelectric BaTiO
particles. Developed strategies allow us to increase the α
value from ~5 mV/cm·Oe for the composite of randomly distributed CoFe
O
nanoparticles in PVDF matrix to ~18.5 mV/cm·Oe for a composite of magnetic particles in PVDF-TrFE matrix with 5%wt of piezoelectric particles. The applicability of such materials as bioactive surface is demonstrated on neural crest stem cell cultures.
We present a low-frequency sensor for the detection of vibrations, with a sub-nm amplitude, based on a cantilever made of a single-crystalline lithium niobate (LiNbO₃) plate, with a bidomain ...ferroelectric structure. The sensitivity of the sensor-to-sinusoidal vibrational excitations was measured in terms of displacement as well as of acceleration amplitude. We show a linear behavior of the response, with the vibrational displacement amplitude in the entire studied frequency range up to 150 Hz. The sensitivity of the developed sensor varies from minimum values of 20 μV/nm and 7 V/
(where
= 9.81 m/s² is the gravitational acceleration), at a frequency of 23 Hz, to peak values of 92.5 mV/nm and 2443 V/
, at the mechanical resonance of the cantilever at 97.25 Hz. The smallest detectable vibration depended on the excitation frequency and varied from 100 nm, at 7 Hz, to 0.1 nm, at frequencies above 38 Hz. Sensors using bidomain lithium niobate single crystals, as sensitive elements, are promising for the detection of ultra-weak low-frequency vibrations in a wide temperature range and in harsh environments.
The values of the surface potentials of two sides of films of polyvinylidene fluoride, and its copolymers with tetrafluoroethylene and hexafluoropropylene, were measured by the Kelvin probe method. ...The microstructures of the chains in the surfaces on these sides were evaluated by ATR IR spectroscopy. It was found that the observed surface potentials differed in the studied films. Simultaneously, it was observed from the IR spectroscopy data that the microstructures of the chains on both sides of the films also differed. It is concluded that the formation of the surface potential in (self-polarized) ferroelectric polymers is controlled by the microstructure of the surface layer. The reasons for the formation of a different microstructure on both sides of the films are suggested on the basis of the general regularities of structure formation in flexible-chain crystallizing polymers.
Relaxor ferroelectrics are a prototypical example of ferroic systems in which interplay between atomic disorder and order parameters gives rise to emergence of unusual properties, including ...non‐exponential relaxations, memory effects, polarization rotations, and broad spectrum of bias‐ and temperature‐induced phase transitions. Despite more than 40 years of extensive research following the original discovery of ferroelectric relaxors by the Smolensky group, the most basic aspect of these materials – the existence and nature of order parameter – has not been understood thoroughly. Using extensive imaging and spectroscopic studies by variable‐temperature and time resolved piezoresponse force microscopy, we find that the observed mesoscopic behavior is consistent with the presence of two effective order parameters describing dynamic and static parts of polarization, respectively. The static component gives rise to rich spatially ordered systems on the ∼100 nm length scales, and are only weakly responsive to electric field. The surface of relaxors undergoes a mesoscopic symmetry breaking leading to the freezing of polarization fluctuations and shift of corresponding transition temperature.
Relaxor ferroelectrics are known as an enigma of solid‐state physics. Using extensive imaging and spectroscopic studies by variable‐temperature and depth‐resolved piezoresponse force microscopy, we propose a model explaining their behaviour by introducing two order parameters describing dynamic and static polarization. The surface of relaxors undergoes a symmetry breaking leading to the freezing of polarization fluctuations and shift of transition temperature.
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