Decrease Of Coercive Field Research Articles

Structural, magneto-optical properties and cation distribution of SrBixLaxYxFe12−3xO19 (0.0 ≤ x ≤ 0.33) hexaferrites

SrBixLaxYxFe12−3xO19 (0.0≤x≤0.33) hexaferrites were produced via sol-gel auto combustion. XRD patterns show that all the samples are single-phase M-type strontium hexaferrite (SrM). The magnetic hysteresis (σ-H) loops revealed the ferromagnetic nature of nanoparticles (NPs). The coercive field decreases from 4740Oe to 2720Oe with increasing ion content. In particular, SrBixLaxYxFe12−3xO19 NPs with x=0.0, 0.1, 0.2 have suitable magnetic characteristics (σs=62.03–64.72emu/g and Hc=3105–4740Oe) for magnetic recording. The intrinsic coercivity (Hci) above 15000Oe reveals that all samples are magnetically hard materials. Tauc plots were used to specify the direct optical energy band gap (Eg) of NPs. The Eg values are between 1.76eV and 1.85eV. 57Fe Mössbauer spectroscopy data, the variation in line width, isomer shift, quadrupole splitting, relative area and hyperfine magnetic field values on Bi3+ La3+ and Y3+ substitutions have been determined.

Exchange bias effect in CuCr2O4/Cr2O3 nanogranular systems

We have successfully synthesized the CuCr2O4/Cr2O3 nanogranular system through a high-temperature phase separation route from Cu doped Cr2O3 nanoparticles, which are confirmed by X-ray diffraction measurements. Magnetic susceptibility shows two magnetic transitions at 130 K and 310 K corresponding to the ferrimagnetic and antiferromagnetic phases of CuCr2O4 and Cr2O3, respectively. Significant exchange bias behavior could be observed in the field cooling magnetic hysteresis loop at low temperatures. Both exchange bias field (HEB) and coercive field (HC) decrease with increasing temperature and disappear near 200 K. This exchange bias behavior is discussed in terms of the existence of exchange coupling between the ferrimagnetic CuCr2O4 and antiferromagnetic Cr2O3. The uncompensated spins in antiferromagnetic phase frozen along field cooling direction provide the pinning force on the ferrimagnetic spins leading to the exchange bias effect.

Energy storage properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 lead-free ceramics modified by La and Zr co-doping

Lead-free [(Bi0.5Na0.5)0.93Ba0.07]1-xLaxTi1-yZryO3 (BNBLTZ) ceramics were investigated for energy storage applications. In order to adjust its energy storage properties, the La and Zr co-doping contents varied at 0.01 ≤ x, y ≤ 0.04. BNBLTZ ceramics show a single phase perovskite structure without phase transition after La and Zr co-doping. The compact and uniform microstructure with similar grain morphology and different grain sizes is obtained. The remnant polarization and coercive field decrease with the increase of La and Zr additions, and the energy storage density increases drastically. The maximum energy storage density of 1.21 J/cm3 is obtained when x = 0.04 and y = 0.01. There are two dielectric anomalies at Tp and Tm due to the phase transformation. The results suggest that lead-free BNBLTZ ceramics should be good candidates for energy storage applications.

Change in dielectric properties of triglycine sulfate in a constant magnetic field

The effect of a constant magnetic field on the dielectric properties of triglycine sulfate crystals has been investigated. It has been shown that, after the magnetic treatment of the crystal (2 T, 20 min), the hysteresis loop becomes narrower; i.e., the coercive field decreases, and the dielectric permittivity changes in the region of the phase transition. It has been found that the observed effect is anisotropic with respect to the orientation of the crystal in a magnetic field and occurs when the vector of magnetic induction is perpendicular to the polar axis of the crystal. The relative orientation of the magnetic field and the domain structure determines its sign. The doping of the crystal with chromium makes the effect more pronounced and leads to a change in the kinetics of the magnetically stimulated increase in the dielectric permittivity.

Effects of 60Co γ-ray irradiation on microstructure and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films

Bi3.25La0.75Ti3O12 (BLT) thin films were deposited on Pt/Ti/SiO2/Si substrates through sol–gel method. Films underwent 60Co γ-ray irradiation with different doses; 0, 50, 100 and 150kGy, respectively. Impacts of γ-ray on the microstructure, ferroelectric properties, leakage current density and fatigue characteristic were studied in detail. The results of SEM images show that grain patterns become irregular. Remnant polarization (2Pr) and coercive field (2Ec) decrease with irradiation dose increase. C–V curves reveal obvious asymmetry along y-axis. The irradiated thin films display lower leakage current density and fatigue endurance up to more than 1010 switching cycles. These results suggest that radiation can improve the film performance in some areas.

SrTiO3 and Pr Effects on Structural, Dielectric and Ferroelectric Properties of Na0.5Bi0.5TiO3 Ceramic

[(Na0.5Bi0.5TiO3)0.7Sr0.3]1-xPrxTiO3(NBTS30P) (0 ≤ x ≤ 0.025) ceramics were prepared by solid-state synthesis process. Their crystal structure, microstructure dielectric and ferroelectric properties were studied. XRD data show that all of the ceramics possess a single perovskite phase. SEM results confirm that the ceramics are well sintered and exhibit relative densities higher than 97%. The results also reveal that the microstructure of the different compositions do not show a significant variation. The praseodymium (Pr) substitution to NBTS30 results in a change of crystal structure from cubic to tetragonal at x = 0.025. At the same time, electric permittivity decreases, the maximum is broadened and slightly shifted to lower temperature, polarization and coercive field decreases. It is also found that the frequency dependence of Tm(the temperature where electric permittivity is maximum) increases with increasing Pr. A possible origin for these observed effects is discussed. The results show that the investigated ceramics are promising lead-free materials for electronic applications.

Dielectric and ferroelectric behavior of cerium modified SrBi2Ta2O9 ceramic

Cerium modified bismuth layered ferroelectric SrBi2Ta2O9 (SBT) with general formula SrBi2−xCe3x/4Ta2O9 (x=0, 0.025, 0.05, 0.075 and 0.1) is prepared by solid state reaction route. Structural and electrical properties of SrBi2Ta2O9 ceramic are studied as a function of cerium content. The X-ray diffraction pattern indicates the formation of the single phase material for all compositions. The morphological study is done by Scanning electron microscopy, which shows plate like structure. The temperature dependent dielectric study shows a diffuse phase transition with a linear decrease in transition temperature and dielectric constant with an increase in Ce content. An increase in the dielectric diffuseness is obtained by an increase in Ce content. The ferroelectric property is studied by P–E hysteresis loop which shows a decrease in coercive field and increment in remanent polarization with an increase in Ce content.

Consequences of electronic excitations in CoFe1.90Dy0.10O4

Present work reports the irradiation induced effects in Dy3+ doped cobalt ferrite nanoparticles in the regime of dominant electronic excitation processes induced by 100 MeV O7+ ion irradiation. Irradiation leads to the deterioration of crystalline phase as envisaged by X-ray diffraction. Crystallite size decreases with the increase of irradiation fluence. Disappearance of certain bands in Raman spectra at higher fluence of irradiation confirms the crystalline disorder induced by electronic excitations. Fourier transform infrared spectra show onset of cation migration from tetrahedral site to octahedral site and vice versa. X-ray absorption fine structure measurements depict the preservation of valence state of metal ions after irradiation. These measurements further infer bond breaking process in irradiated materials. Magnetic measurements carried out on these materials indicate slight increase of saturation magnetization at room temperature followed by the decrease of coercive field. Obtained results are discussed on the basis of appropriate mechanism.

Effect of Ba addition on the structural, dielectric and ferroelectric properties of Na0.5Bi0.5TiO3 ceramics

Abstract Lead-free (Na0.5Bi0.5)1-xBaxTiO3 (x = 0, 0.04 and 0.06) ceramics were fabricated by conventional solid phase sintering process. X-ray diffraction analysis shows that obtained specimens possess the perovskite structure. The microstructure study shows a dense structure, in good agreement with the relative density determined by the Archimedes method (above 95 %). Electric permittivity anomaly is shifted to low temperature after Ba doping of NBT. The pyroelectric and hysteresis loops measurements show that polarization and coercive field increases and decreases, respectively, after Ba doping of NBT. The obtained results are discussed in terms of ions/lattice imperfections, which create local electromechanical fields. The investigated ceramics are considered to be promising candidates for lead-free electronic materials.

Effect of grain size on dielectric and ferroelectric properties of nanostructured Ba0.8Sr0.2TiO3 ceramics

Barium strontium titanate (Ba0.8Sr0.2TiO3, BST) nanocrystalline ceramics have been synthesized by high energy ball milling. As the sintering temperature increases from 1200 °C to 1350 °C, the average grain size of BST ceramics increases from 86 nm to 123 nm. The X-ray diffraction (XRD) studies show that these ceramics are tetragonal. The phase and grain size of the sintered pellets have been estimated from the XRD patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The effect of grain size on dielectric and ferroelectric properties is studied. The dielectric and piezoelectric parameters are greatly improved at room temperature with increase in grain size. The Curie transition temperature is found to shift slightly towards higher temperatures as the grain increases from 86 nm to 123 nm. The coercive field decreases and the remnant polarization and spontaneous polarization increase as the grain size of BST nano ceramics increases. These ceramics are promising materials for tunable capacitor device applications.

The role of nanoporosity on the local piezo and ferroelectric properties of lead titanate thin films

Nanoporous PbTiO3 films present enhanced tetragonality at lower temperatures than respective dense films. Moreover, the porosity present in the nanoporous films allows an increase of the local piezoelectric response and a decrease of the local coercive field. As a result, these nanoporous films might be used to improve the switching behaviour of ferroelectric thin films.

Dielectric and ferroelectric properties of xBaZr0.52Ti0.48O3–(1−x)BiFeO3 solid solution ceramics

xBaZr0.52Ti0.48O3–(1−x)BiFeO3 (short for xBZT–(1−x)BFO) solid solution ceramics were synthesized by conventional solid state reaction method. The microstructure, dielectric and ferroelectric properties of xBZT–(1−x)BFO (x = 0.2–0.5) solid solution ceramics have been investigated systematically. The XRD results show that the crystal structure of xBZT–(1−x)BFO solid solution ceramics evolves gradually from rhombohedral to tetragonal phase and the lattice parameters increase as BZT content increases. The grain size of xBZT–(1−x)BFO ceramics increases initially to the maximum (x = 0.3) and then decreases with the increase of BZT content. As BZT content increases, the temperature at which the maximum in the dielectric constant appears and the corresponding maximum dielectric constant increase first and then decrease. There is obvious frequency dispersion and diffuse phase transition in xBZT–(1−x)BFO solid solution ceramics, and the dielectric diffuseness is enhanced by a certain amount of BZT. As BZT content increases, the leakage current density of xBZT–(1−x)BFO ceramics increases first and then decreases, while the remnant polarization and coercive field decrease initially by a large margin and then increase. Moreover, the remnant polarization and coercive field decrease as frequency increases.

Magnetic Properties of Fe49Co33Ni18Nanowire ArraysStudied by First-Order Reversal Curve Diagrams

Fe49Co33Ni18 nanowire arrays (175 nm in diameter and lengths ranging from 5 to 32μm) were fabricated into nanopores of hard-anodized aluminum oxide templates using pulsed ac electrodeposition technique. Hysteresis loop measurements indicated that increasing the length decreases coercivity and squareness values (from 274 Oe and 0.58 to200 Oe and 0.105, respectively), indicating the increase in magnetostatic interactions between the nanowires (NWs).On the other hand, first-order reversal curve measurements showed a linear correlation between the magnetostatic interactions and length of NWs. It was also found that with increasing length, the domain structure of NWs changed from single-domain to pseudo single-domain state. A multidomain-like behavior is also seen for the longest NWs length.Increasing the length of NWs resulted in an increase inthe interaction and decrease in the array coercive field(H ୡ ) as beingsmaller than that of individual NWs (H ୡ ).The observed CFD component in the FORC diagrams of FeCoNi NWs with shorter lengths was a consequence ofnon-uniform length distributions.

Solving the electrical control of magnetic coercive field paradox

The ability to tune magnetic properties of solids via electric voltages instead of external magnetic fields is a physics curiosity of great scientific and technological importance. Today, there is strong published experimental evidence of electrical control of magnetic coercive fields in composite multiferroic solids. Unfortunately, the literature indicates highly contradictory results. In some studies, an applied voltage increases the magnetic coercive field and in other studies the applied voltage decreases the coercive field of composite multiferroics. Here, we provide an elegant explanation to this paradox and we demonstrate why all reported results are in fact correct. It is shown that for a given polarity of the applied voltage, the magnetic coercive field depends on the sign of two tensor components of the multiferroic solid: magnetostrictive and piezoelectric coefficient. For a negative applied voltage, the magnetic coercive field decreases when the two material parameters have the same sign and increases when they have opposite signs, respectively. The effect of the material parameters is reversed when the same multiferroic solid is subjected to a positive applied voltage.

The field and temperature dependence of hysteresis loops in P(VDF–TrFE) copolymer films

The ferroelectric hysteresis loops of poly(vinylidene fluoride/trifluoethylene) [P(VDF–TrFE)] copolymer films are investigated as a function of external field and temperature. Starting from a plateau at the low frequency side the coercive field increases with increasing frequency. It exhibits a maximum in the kHz-range and decreases then. The remanent polarization is almost constant at low frequencies and decreases above the kHz-range. For a constant frequency, the coercive field increases with increasing the amplitude of the external field. Furthermore, the hysteresis loops at different temperatures at a given frequency and amplitude exhibit a linear decrease of coercive field with increasing temperature. A double hysteresis loop is observed close to the Curie point and a sharp jump of the remanent polarization is obtained in samples as thick as 600nm indicating a first order phase transition. For samples as thin as 90nm, the double hysteresis loop of the polarization is absent and the change of remanent polarization dependent on temperature is smoother. The above experimental results can be explained and simulated in the Weiss mean field model. The simulation results are compared with the experiments and show a good consistency.

Structure and electrical properties of Na1/2Bi1/2TiO3–xK1/2Bi1/2TiO3 lead-free ferroelectric single crystals

Dielectric, ferroelectric and piezoelectric properties of lead-free Na1/2Bi1/2TiO3–xK1/2Bi1/2TiO3 (NBT–xKBT) ferroelectric single crystals were characterized systematically. With increasing KBT concentration, the remanent polarization and coercive electric field decrease, while the piezoelectric constant and the electromechanical coupling coefficient increase. The evolution of average and local structures as a function of temperature and composition was investigated using in-situ X-ray diffraction and Raman spectroscopy. The structure/property relationship of NBT–xKBT single crystals was discussed in detail.

Electromechanical and microstructural study of (1-x) Bi0.5 (Na0.40 K0.10) TiO3-x (Ba0.70 Sr0.30) TiO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1-x) BNKT-xBST (x = 0 ~ 0.150) were studied, and the effects of BST content on the electromechanical properties and crystal structure were examined. X-ray diffraction patterns confirmed a single perovskite phase, and the tetragonality was found to increase with increasing BST ratio. The ceramics exhibited good performance with the following properties: piezoelectric coefficient d33 = 223 pC/N, coupling factor kp = 38 %, electromechanical quality factor Qm = 120, and tanδ = 3 % at 1 kHz. Significantly enhanced temperature stability of the ferroelectric properties was found in the range from room temperature to 125 °C. Further, the ceramics displayed a high piezoelectric constant d*33 calculated as Smax/Emax = 300 pm/V. Remnant polarization (Pr) was found to increase with the increasing temperature up to 125 °C; however, a significant increase was observed at 50 °C. Moreover, the Pr at room temperature was increased with increasing BST content up to (x = 0.075) along with a significant decrease in coercive field.

The erbium׳s amphoteric behavior effects on sodium bismuth titanate properties

The effects of erbium doping on lead-free ferroelectric Na0.5Bi0.5TiO3 ceramics are investigated. The scanning electron microscopy shows a highly dense grain structure for all specimens. A pure perovskite structure was observed by X-ray diffraction for all compositions. However, Raman spectroscopy investigations suggest that Er3+ doping is associated with local structural disorder. Doping with Er3+ ions also leads to photoluminescence effect. The depolarization temperature in the dielectric measurement vanishes gradually as the Er3+ amount increases. The ferroelectric properties display a slight decrease in coercive field for the x=0.01 composition.

Role of point defects in bipolar fatigue behavior of Bi(Mg1/2Ti1/2)O3 modified (Bi1/2K1/2)TiO3-(Bi1/2Na1/2)TiO3 relaxor ceramics

Lead-free Bi(Mg1/2Ti1/2)O3-(Bi1/2K1/2)TiO3-(Bi1/2Na1/2)TiO3 (BMT-BKT-BNT) ceramics have been shown to exhibit large electromechanical strains under high electric fields along with negligible fatigue under strong electric fields. To investigate the role of point defects on the fatigue characteristics, the composition 5BMT-40BKT-55BNT was doped to incorporate acceptor and donor defects on the A and B sites by adjusting the Bi/Na and Ti/Mg stoichiometries. All samples had pseudo-cubic symmetries based on x-ray diffraction, typical of relaxors. Dielectric measurements showed that the high and low temperature phase transitions were largely unaffected by doping. Acceptor doping resulted in the observation of a typical ferroelectric-like polarization with a remnant polarization and strain hysteresis loops with significant negative strain. Donor-doped compositions exhibited characteristics that were indicative of an ergodic relaxor phase. Fatigue measurements were carried out on all of the compositions. While the A-site acceptor-doped composition showed a small degradation in maximum strain after 106 cycles, the other compositions were essentially fatigue free. Impedance measurements were used to identify the important conduction mechanisms in these compositions. As expected, the presence of defects did not strongly influence the fatigue behavior in donor-doped compositions owing to the nature of their reversible field-induced phase transformation. Even for the acceptor-doped compositions, which had stable domains in the absence of an electric field at room temperature, there was negligible degradation in the maximum strain due to fatigue. This suggests that either the defects introduced through stoichiometric variations do not play a prominent role in fatigue in these systems or it is compensated by factors like decrease in coercive field, an increase in ergodicity, symmetry change, or other factors.

Improved ferroelectricity of (1−x)Na0.5Bi0.5TiO3–xBaTiO3 ceramics rapidly sintered at low temperature

High insulating (1−x)Na0.5Bi0.5TiO3–xBaTiO3 (x=0–0.08) ceramics were rapidly sintered at low temperature and then enhanced polarizations were observed. (1−x)Na0.5Bi0.5TiO3–xBaTiO3 nano-powders were prepared by the sol–gel method and then their ceramics were rapidly sintered at 920–1150°C. According to the saturated ferroelectric hysteresis loops, the acquired saturated and remnant polarizations are ~48μC/cm2 and ~40μC/cm2 at x=0.03–0.08, and the coercive electric field decreases from ~70kV/cm of pure Na0.5Bi0.5TiO3 to ~30kV/cm at x=0.06–0.08. Furthermore, the piezoelectric d33 is 166.8pC/N at x=0.06, where the morphotropic phase boundaries occur. These improved properties are achieved because the leakage current is reduced through compressing the loss of Na and Bi components during ceramics׳ preparation process.