Apparent Coercive Field Research Articles

Optical, ferroelectric, and piezoresponse force microscopy studies of pulsed laser deposited Aurivillius Bi5FeTi3O15 thin films

Bi5FeTi3O15 (BFTO) based Aurivillius ferroelectric thin films were fabricated on strontium ruthanate coated amorphous fused silica substrates using pulsed laser deposition technique. Optical, ferroelectric, and piezoresponse properties of these thin films were investigated. The estimated refractive index (n) and extinction coefficient (k) for these films were in the range from 2.40 to 2.59 and 0.012 to 0.19, respectively. The bandgap of the BFTO thin layers was estimated to be 2.88 eV. Domain switching and hysteresis loops of BFTO films were studied utilizing piezoresponse force microscopy (PFM). The measured apparent polarization (Pr) and coercive field (Ec) for the samples were 20 μC/cm2 and 250 kV/cm, respectively. The amplitude and phase hysteresis curves obtained from PFM characterization reveal that these films can be switched below 5 V. These results suggest that BFTO in thin film form is a promising material for photo ferroelectric and optoelectronic devices applications.

Effect of magnetic field on ferroelectric properties of BiFeO3/La5/8Ca3/8MnO3epitaxial heterostructures

The temperature and magnetic field dependencies of ferroelectric properties for Au/BiFeO${}_{3}$/La${}_{5/8}$Ca${}_{3/8}$MnO${}_{3}$/SrTiO${}_{3}$ thin films were investigated systematically. It was found that the apparent coercive electric field of BiFeO${}_{3}$/La${}_{5/8}$Ca${}_{3/8}$MnO${}_{3}$ (BFO/LCMO) decreases obviously with increasing magnetic fields around the Curie temperature of LCMO (\ensuremath{\sim}220 K). The remanent polarization ${P}_{r}$ increases with increasing temperatures from 50 to 300 K but changes little with magnetic fields at a frequency of 2 kHz. The variations of ferroelectricity are mainly related to the voltage drops across the LCMO layer and the magnetoelectric-coupled interface between BFO and LCMO, which can be influenced by magnetic fields due to the phase separation in LCMO. Furthermore, the decrease of ${P}_{r}$ with decreasing temperature is probably due to the temperature dependencies of the lattice mismatch between the substrate and the BFO films. These findings are important for designing multifunctional devices in spintronics.

Nanosecond-range imprint and retention characterized from polarization-voltage hysteresis loops in insulating or leaky ferroelectric thin films

We transferred ferroelectric domain switching currents under pulses into polarization-voltage (P-V) hysteresis loops. With this transformation, it is possible to derive the remanent polarization and coercive voltage from domain switching currents after the shortest imprint and retention time of 35 ns. After the separation of film leakage current from domain switching current, we measured the P-V hysteresis loop in a semiconducting BiFeO3 leaky thin film, where the apparent coercive field highly reaches 320 kV/cm2, suggestive of a different domain switching mechanism from other insulators. This technique facilitates nanosecond-range measurements of both ferroelectric capacitive and resistive memories.

Study of helical induced anisotropy in amorphous ribbons for sensor applications

AbstractAmorphous samples with helical induced anisotropy show magnetization processes that can be controlled by applying a longitudinal magnetic field simultaneously with an alternating current flowing through the sample. By varying the current amplitude and the phase difference between current and applied field, a wide range of coercivity and susceptibility values can be achieved.This work shows that the apparent coercive field and the susceptibility can be controlled in amorphous ribbons with helical anisotropy. These characteristics make these samples very suitable for their application as sensor cores, magnetic amplifiers, variable reluctance transformer cores, etc (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of temperature on polarization reversal of strontium-doped lead zirconate titanate (PSZT) ceramics

The effect of temperature on polarization reversal of strontium-doped lead zirconate titanate ceramics was studied. The piezoelectric properties viz. dielectric constant and piezoelectric coupling coefficient, were used for polarization reversal characteristic. These properties and apparent coercive field were measured during polarization reversal at different temperatures. Results indicated that at higher temperature apparent coercive field decreased. Polarization reversal and further polarization reversal was quite asymmetric. After polarization reversal, dielectric constant was found to increase at all temperatures while piezoelectric coupling coefficient increased above the temperature of polarization. The trend shown by dielectric constant indicates that at 25°C, 1·5 kV/mm field can be applied safely to this material without much compromising the properties. D.c. field of 3·0 kV/mm and 100°C temperature can be predicted as poling parameters from their effect on \({\bf \textit{k}}_{{\bf p}}\). Apparent coercive field has shown non-linear relationship with temperature. It was of exponential decay type.

Measurement of Nonlinear Current–Voltage Dependence of Interfacial Passive Layers for Ferroelectric Thin Films

Theoretically, interfacial passive layers identified in most ferroelectric thin films are assumed to be highly insulating to tilt polarization–voltage (P–V) hysteresis loops and to reduce the apparent coercive field. Practically, the layers would be leaky under an extremely high field, where the P–V loop remains squared rather than tilted. In this work, we develop a technique to measure the nonlinear current–voltage dependence across the interfacial layers during domain switching. With the aid of this technique, we simulate the interfacial current–voltage relationship by using conventional conduction models. After elimination of the interfacial-layer effect on the coercive-voltage estimation with different film thicknesses, we extract domain switching current dependence of the intrinsic coercive field, irrespective of the film thickness. The thermal activation field derived from domain switching model of the Merz's law is around 1.4 kV/cm, unexpectedly smaller than those in bulk ceramics.

One-step synthesis of barium hexaferrite nano-powders via microwave-assisted sol–gel auto-combustion

Single phase barium hexaferrite nano-powders have been innovatively one-step synthesized via a microwave-assisted sol–gel auto-combustion in a specially designed quartz vessel using citric acid and ethylene diamine tetraacetic acid as composite chelating agents and freeze-drying technique to remove sols’ moisture. The auto-combustion product powder is characterized by fluffy particle aggregates with the crystallites ranging from 50 to 100 nm in diameter and containing single magnetic domains for each with a low apparent coercive field of 260 Oe and a high saturation magnetization of 64.1 emu/g. The direct formation of barium hexaferrite is believed to result from the effective improvement in the spatial distribution homogeneity of metal ions and oxidant in the gels. Moreover, the quartz vessel with barium hexaferrite ceramic pad on sample's support and film-strips on its interior wall can effectively build up a favorable temperature environment to promote the direct formation of barium hexaferrite with microwave assistance during the gel's uniform auto-combustion.

Spin filtering inLa0.7Sr0.3MnO3∕CoFe2O4∕Nb(0.5%):SrTiO3heterostructures

The magnetotransport properties of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}∕\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}∕\mathrm{Nb}(0.5%):\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ junctions were investigated. At low temperatures, these junctions show a magnetoresistance strongly dependent on current bias, temperature, and magnetic field. This is attributed to spin filtering in the $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ layer. Moreover, the apparent coercive field of the junctions depends on the applied bias current due to the interplay of magnetic switching and the bias dependent transport in the $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ layer.

Diversity of the magnetic coupling behaviors in the CoFe2∕CoFe2O4 system

The CoFe2∕CoFe2O4 system has been considered for use as hard electrodes in spin valve devices. In this letter, we focus on the nature and intensity of the magnetic exchange coupling between the two phases. CoFe2∕CoFe2O4 and CoFe2O4∕CoFe2 bilayers were fabricated by pulsed laser deposition from a metallic CoFe2 alloy target in a vacuum and in an O2:N2 oxidizing atmosphere for the metallic and oxide layers, respectively. Depending upon the elaboration conditions, three different coupling behaviors have been observed: Ferromagnetic, biquadratic, and antiferromagnetic. The minor loop shift and the apparent coercive field of the metal observed in the first two cases are of several hundreds of Oe for a metal thickness of 10nm. Those very large loop shifts and coercive fields confirm the potential usefulness of this system in terms of applications.

Ferroelectricity of single-crystalline, monodisperse lead zirconate titanate nanoparticles of 9 nm in diameter

Ferroelectric hysteresis at approximately 20 °C is measured in a lead zirconate titanate film prepared by stacking monodisperse and perovskite particles 9 nm in diameter. The particle-stacked film possesses a remanent polarization of 4.5μC∕cm2 and an apparent coercive field of 250 kV/cm. This result demonstrates that particulate lead zirconate titanate of 9 nm in diameter retains ferroelectricity even at room temperature.

Coercive field of ultrathin Pb(Zr0.52Ti0.48)O3 epitaxial films

The polarization reversal in single-crystalline ferroelectric films has been investigated experimentally and theoretically. The hysteresis loops were measured for Pb(Zr0.52Ti0.48)O3 films with thicknesses ranging from 8 to 250 nm. These films were grown epitaxially on SrRuO3 bottom electrodes deposited on SrTiO3 substrates. The measurements using Pt top electrodes showed that the coercive field Ec increases drastically as the film becomes thinner, reaching values as high as Ec≈1200 kV/cm. To understand this observation, we calculated the thermodynamic coercive field Eth of a ferroelectric film as a function of the misfit strain Sm in an epitaxial system and showed that Eth strongly depends on Sm. However, the coercive field of ultrathin films, when measured at high frequencies, exceeds the calculated thermodynamic limit. Since this is impossible for an intrinsic coercive field Ec, we conclude that measurements give an apparent Ec rather than the intrinsic one. An enormous increase of apparent coercive field in ultrathin films may be explained by the presence of a conductive nonferroelectric interface layer.

Preparation and properties of Bi4Ti3O12 thin films grown at low substrate temperatures

Bismuth titanate (Bi4Ti3O12) thin films were deposited on a (100) silicon substrate at a substrate temperature of 500°C, using rf magnetron sputtering. Ferroelectric Bi4Ti3O12 thin films were deposited on the pyrochlore phase buffer layer. The pyrochlore phase buffer layer was effective for lowering substrate temperature and suppressed the growth of the plate-like grains. The apparent remanent polarization and the apparent coercive field of this film at the measurement frequency of 50 Hz were estimated to be 2.4 μC cm−2 and 2.3 kV cm−1, respectively. The apparent remanent polarization was degraded to approximately 40% of the initial value by 1×106 switching cycles. The capacitance–voltage hysteresis loop of Bi4Ti3O12 thin film on silicon substrate is observed to correspond to ferroelectricity.

Effect of Grain Size on Bi4Ti3O12 Thin Film Properties

Bismuth titanate (Bi4Ti3O12) thin films with a c-axis orientation were prepared on (100) silicon wafers by rf magnetron sputtering. The average grain size and the c-axis lattice constant depend on the substrate temperature. The c-axis lattice constant was decreased with increasing grain size. The leakage current characteristics of the Bi4Ti3O12 thin film consisting of small grains with a closely packed structure, are superior to those of others. The apparent remanent polarization and the apparent coercive field of the film with small grain sizes were estimated to be 2.4 µC·cm-2 and 2.3 kV·cm-1, respectively. The refractive index dispersion characteristics for the film show that the large grains increase the distortion of the oxygen-octahedra. Consequently, this result indicates that the small grain size of Bi4Ti3O12 thin films can improve the ferroelectric properties.

Properties of Bi 4Ti 3O 12 Thin Films Grown at Low Temperatures

Bismuth titanate (Bi4Ti3O12) thin films have been deposited on a (100) silicon substrate with a buffer layer, using a rf planer magnetron sputtering technique. The buffer layer was formed by the heat treatment of an amorphous layer at 550°C, and exhibits a pyrochlore phase with a smooth surface. The amorphous layer consists of Bi, Ti and O atoms, which are the elements of the ferroelectric Bi4Ti3O12 thin film. Ferroelectric Bi4Ti3O12 thin films were deposited on the buffer layer at a low substrate temperature of 500°C. The Bi4Ti3O12 thin films have a dense and small grain structure. A D–E hysteresis characteristic has been observed at room temperature. The apparent remanent polarization and the apparent coercive field of this film at the measurement frequency of 50 Hz were estimated to be 2.4 µC/cm2 and 2.3 kV/cm, respectively.

A new type of fluxgate magnetometer for low magnetic fields

A new variant of a fluxgate magnetometer is introduced in this paper which uses the measurement of the change of the apparent coercive field strength derived from the primary current. The external magnetic field is measured periodically with a frequency which is the same as that of the magnetizing current of the sensor. The magnetometer shows a non-linearity of less than 0.02%, a resolution of less than 1 nT, and operates within a frequency range of the external field from 0 to 10 kHz.

A new type of fluxgate magnetometer using apparent coercive field strength measurement

A novel fluxgate magnetometer is introduced which employs the measurement of the change of the apparent coercive field strength derived from the magnetizing current. The external magnetic field is periodically measured at a frequency which is the same as that of the magnetizing current of the sensor. The magnetometer shows a standard deviation of the relative nonlinearity of less than 0.01% and a resolution of 0.1 nT, and it can measure transient external magnetic fields with a 100-kHz sampling rate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A new current sensor based on the measurement of the apparent coercive field strength

A current sensor that uses the change of the apparent coercive field strength caused by the current to be measured is introduced. The apparent coercive field strength is derived from the magnetizing current by detecting the zero crossing points of the average magnetic flux of a core pair. The current is periodically measured at a frequency which is the same as that of the magnetizing current of the sensor. The current sensor shows a standard deviation of the nonlinearity of less than 0.002%, and can measure transient current signals with an 80-kHz sampling rate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Coercivity of a macroscopic ferrimagnet near a compensation point

A ferrimagnet that does not have the opposing magnetic components in a unit cell of atomic dimensions is termed a macroscopic ferrimagnet. It is shown that the temperature dependence of the coercive field of such a magnet seems very different from that of a homogeneous ferrimagnet in that it decreases as T approaches the compensation temperature. This is understood to be the result of a weak magnetic coupling at the interface between the opposing components. This leads to an enhanced high-field susceptibility near the compensation temperature, which eventually controls the apparent coercive field. These suggestions are born out by a simple molecular field calculation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electrical Properties of Niobium-Doped Ferroelectric Pb(Zr, Sn, Ti)O3 Ceramics

Hysteresis loop, pulse switching, and aging measurements have been made as a function of composition, sample preparation and thickness, electrode material and size, on 24 rhombohedral ferroelectrics from the Pb0.99[(ZrxSny)1−z Tiz]0.98Nb0.02O3 ceramic family to determine their properties and usefulness for device applications. Results show that many of these ceramics have extremely high remanent polarization, saturation polarization, loop squareness, tolerance-to-matrix-type disturb pulses, switching speed, and minimal excitation aging. The polarizations, squareness, apparent coercive field, disturb-pulse tolerance, and aging of the apparent coerive field all increase with increasing x/y. The definite dependence of the properties on z is, in general, random. Samples prepared with smooth surfaces, such as by polishing or doctor-blading, had properties approaching those of a single crystal. The observed electrode-edge effects can be explained in ceramic terms. These materials, while retaining all the conventional advantages of ceramics, have electrical properties equal to, and in some cases, such as polarizations and disturb-pulse tolerance, superior to single-crystal ferroelectrics. These ceramics are thus very suitable for ferroelectric memory and display devices.