Showing papers on "Magnetocapacitance published in 2010"

Multiferroic and magnetoelectric properties of core-shell CoFe2O4@BaTiO3 nanocomposites

Abstract: Core-shell CoFe2O4@BaTiO3 nanoparticles and nanotubes have been prepared using a combination of solution processing and high temperature calcination. Both the core-shell nanostructures exhibit magnetic and dielectric hysteresis at room temperature and magnetoelectric effect. The dielectric constant of both the nanocomposites decreases upon application of magnetic field. The core-shell nanoparticles exhibit 1.7% change in magnetocapacitance around 134 K at 1 T, while the core-shell nanotubes show a remarkable 4.5% change in magnetocapacitance around 310 K at 2 T.

Excess-electron induced polarization and magnetoelectric effect in yttrium iron garnet

Abstract: Magnetoelectric (ME) properties in yttrium iron garnet $(\text{YIG}:{\text{Y}}_{3}{\text{Fe}}_{5}{\text{O}}_{12})$, including both the first-order and second-order effects, have long been under dispute. In particular, the conflict between observations of the first-order ME effect and the centrosymmetric lattice structure has remained as a puzzling issue. As a key to solve the problem, we found that YIG shows quantum ME relaxation; the dielectric relaxation process is correlated closely with the magnetic one and has characteristic features of quantum tunneling. An application of magnetic field enhances the dielectric relaxation strength (by 300% at 10 K with 0.5 T), which gives rise to the large second-order ME (magnetocapacitance) effect critically dependent on the magnetization direction. The temperature and magnetic-field dependence of dielectric relaxation strength is well described by the noninteracting transverse-field Ising model for the excess-electron or ${\text{Fe}}^{2+}$ center with the quantum tunneling and spin-orbit coupling effects. We could also spectroscopically identify such a ME ${\text{Fe}}^{2+}$ center in terms of linear dichroism under a magnetic field along the specific direction. On this basis, the fictitious first-order ME effect---the magnetic-field induced electric polarization without the presence of external electric field---as observed for the electric-field cooled sample is ascribed to the combined effect of the above large second-order ME effect and the poling induced charge accumulation. The correlation between the ME effect and the thermally stimulated depolarization current indicates that hopping electrons freeze below around 125 K and the frozen-in dipoles generate an internal electric field (i.e., an electret-like effect). Investigation of electron-compensating doping effect on dielectric relaxation phenomena gives compelling evidences that excess electrons forming ${\text{Fe}}^{2+}$ ions play a critical role in the charge accumulation as well as in the ME effect in YIG.

Structural, dielectric, and magnetic properties of La0.8Bi0.2Fe1−xMnxO3 (0.0≤x≤0.4) multiferroics

Abstract: Polycrystalline multiferroic La0.8Bi0.2Fe1−xMnxO3 (0.0≤x≤0.4) samples were synthesized by the conventional solid state reaction method. Reitveld refinement of the x-ray diffraction patterns confirms the single phase character of all the compositions with orthorhombic structure having space group Pnma (No. 62). Dielectric properties of the samples at temperatures 200–475 K and frequencies 500 kHz–1 MHz authenticate the stabilization of ferroelectric phase with Mn substitution. Dielectric responses of these multiferroics have been analyzed carefully, in the light of “universal dielectric response” model. While cooling from room temperature to 20 K, systematic shifts in magnetization hysteresis loops indicate the presence of exchange bias (EB) phenomenon in the system. Magnetic behavior of these samples has been briefly discussed on the basis of “EB” model for granular systems. Temperature and magnetic field dependent magnetization data demonstrate enhanced magnetization due to the Mn substitution. Magnetocapacitance measurement reveals the magnetoelectric coupling for wide range of temperature (180–280 K) and decrease in dielectric loss at high magnetic field (3 T).

Magnetocapacitive effects in the Néel N -type ferrimagnet SmMnO 3

Abstract: Magnetocapacitive effects were investigated for N\'eel's $N$-type ferrimagnetic ${\text{SmMnO}}_{3}$, which shows temperature-induced magnetization reversal at the compensation temperature $({T}_{\text{comp}}=9.4\text{ }\text{K})$ in a low magnetic field $B$ $(l\ensuremath{\sim}1\text{ }\text{T})$. When higher $B$ is applied, remarkable anomalies were observed in the temperature profiles of magnetization, dielectric constant, and dielectric relaxation time around ${T}_{\text{comp}}$. These coupled field-induced anomalies give rise to striking magnetocapacitance and could be attributed to simultaneous reversals of ferrimagnetically coupled Sm and Mn moments. In addition, peculiar cooling-field dependent asymmetric magnetocapacitance was observed at ${T}_{\text{comp}}$.

The effect of Ni substitution on magnetic, dielectric and magnetoelectric properties in BiFe1−xNixO3 system

Abstract: Multiferroic nanoceramics BiFe 1− x Ni x O 3 (where x =0, 0.15, 0.20, and 0.25) were prepared by the sol–gel method. XRD analysis of samples calcined at a low temperature of 400 °C for 2 h shows the formation of a single phase perovskite rhombohedral structure. Average particle size was observed to be ∼50 nm by TEM measurement. Magnetization was found to increase with increase in the concentration of Ni. Dielectric constant and dielectric loss were measured up to 1 MHz frequency. Variation of dielectric constant with temperature shows a peak at ∼395 °C (Neel temperature) for x =0.15 and the peak shifts towards lower temperature at a higher concentration of Ni with diffused type of phase transition. Magnetocapacitance was found to decrease with magnetic field. For BiFe 1− x Ni x O 3 (with x =0.15, 0.20, 0.25) nanoceramics, the fractional change of the magnetic field induced a change in the dielectric constant, which may well be approximated by Δ e / e = γM 2 , (where γ (magnetoelectric interaction ) is small and negative). A linear fit gave the value of γ of ∼−2.14×10 −3 , −2.11×10 −3 , and −2.09×10 −3 for BiFe 1− x Ni x O 3 nanoceramics, with x =0.15, 0.20, and 0.25, respectively.

Impedance spectroscopy of micron sized magnetic tunnel junctions with MgO tunnel barrier

Abstract: We have studied the magnetoimpedance of micron sized magnetic tunnel junction sensors with 1.7 nm MgO tunnel barrier. We performed ac impedance spectroscopy in the frequency range between 100 Hz–40 MHz as a function of applied magnetic field in the sensing direction. We model our devices with a simple RLC circuit. Fitting the model to our data results in frequency independent R, L, and C, and our low frequency results are in agreement with dc measurements. Despite excellent agreement with published result on interface capacitance for MgO barrier magnetic tunnel junctions similar to ours we do not observe any magnetocapacitance in our devices.

Structural, magnetic and dielectric properties of xCrFe2O4–(1−x)BiFeO3 multiferroic nanocomposites

Abstract: Spinel-perovskite nanocomposites of xCrFe2O4–(1−x)BiFeO3 with x=0.0, 0.1, 0.2, 0.3, 0.4 were prepared by sol gel method. The XRD showed the phase formation of nanocomposites at 700 °C. Particle size was observed to be ∼100 nm by TEM. The variation of dielectric constant and dielectric loss with frequency showed dispersion in the low frequency range. Magnetization was found to increase with increasing concentration of ferrite content. Dielectric analysis showed the conducting behaviour at higher temperature. Magnetocapacitance was also observed in the prepared nanocomposites which may be the sign of magnetoelectric coupling in the synthesized nanocomposites at room temperature.

Extraction of the tunnel magnetocapacitance with two-terminal measurements

Abstract: The tunnel magnetocapacitance (TMC) of the magnetic tunnel junction has been investigated with a series of complex impedance spectra measured at varying magnetic field. To avoid the circuit complication in the four-terminal measurement with high frequency operation, two-terminal approach was developed by elimination of spin independent contribution apart from the junction area. A subsequent fitting process based on the difference spectra analysis gave the TMC ratio of −0.43% with the opposite dependence on the field as compared to the tunnel magnetoresistance (TMR) of 30.67%. This technique would be applied in the further development and integration of spintronics devices.

Enhanced magnetic moment in ErMnO3 by copper doping and negative magnetocapacitance effect

Abstract: Single phase ErCuxMn1?xO3(0 ? x ? 0.1) compounds were prepared by the standard solid-state reaction. The doping effects on the crystal structure, magnetic properties and dielectric properties were systematically studied. X-ray diffraction patterns show that all samples are single phase and crystallized in a hexagonal structure belonging to the P63cm space group. Rietveld refinement indicates that the a lattice parameter increases and the c lattice parameter decreases with increasing Cu concentration. The magnetic moments of the doped samples are enhanced, which is due to modification of the frustrated spin arrangement by the superexchange interaction between Cu2+ ions and Mn3+ ions. The specific heat capacity data show a peak at the antiferromagnetic transition temperature, which decreases from 77?K for x = 0 to 61?K for x = 0.1. The samples become more conductive upon doping, which is responsible for the increase in the dielectric constant. A large negative magnetocapacitance effect was observed in paramagnetic-state ErCu0.05Mn0.95O3 at 300?K.

Suppressed magnetoelectric effect in epitaxially grown multiferroic Pb(Zr 0.57 Ti 0.43 )O 3 -Pb(Fe 2/3 W 1/3 )O 3 solid-solution thin films

Abstract: We report on the epitaxial growth of single-phase [Pb(Zr0.57Ti0.43)O3]0.8[Pb(Fe2/3W1/3)O3]0.2 (PZT–PFW) solid-solution thin films using pulsed laser deposition. X-ray diffraction measurements reveal that the films have a tetragonal structure. The films exhibit ferroelectric properties and weak ferromagnetic responses at room temperature. The magnetoelectric effects were investigated; the nonlinear magnetocapacitance coefficient, β33, was measured and found to be comparable to those of multiferroic hexagonal manganites, but at least two orders of magnitude smaller than that for polycrystalline PZT–PFW films.

Charge-order driven multiferroic and magneto-dielectric properties of rare earth manganates

Abstract: Charge-order driven magnetic ferroelectricity is shown to occur in several rare earth manganates of the general formula, Ln1−x A x MnO3 (Ln = rare earth, A = alkaline earth). Charge-ordered manganates exhibit dielectric constant anomalies around the charge-ordering or the antiferromagnetic transition temperature. Magnetic fields have a marked effect on the dielectric properties of these compounds, indicating the presence of coupling between the magnetic and electrical order parameters. Magneto-dielectric properties are retained in small particles of the manganates. The observation of magneto-ferroelectricity in these manganates is in accordance with theoretical predictions.

Large magnetodielectric effect in (1-X) La2NiMnO6 - (X) La2/3Sr1/3MnO3 composites

Abstract: A large magnetodielectric effect in (1-x) La(2)NiMnO(6)-(x) La(2/3)Sr(1/3)MnO(3) composites over wide frequency and temperature ranges is reported. The results indicate that variation of dielectric constants with magnetic fields depends non-monotonously on the molar ratio of two compounds and reaches a maximum up to 20% for x = 0.4 at the frequency of 1 MHz and field of 1 kOe at room temperature. The further analyses suggest that this effect is closely related to the magnetoresistance. These findings provide an alternative strategy to achieve a room temperature magnetodielectric response in multiferroic composites, indicating that it is promising for potential applications.

Magnetoelectric properties of (Ca1−xSrx)2CoSi2O7 Crystals

Abstract: We have investigated the magnetoelectric properties of (Ca1−xSrx)2CoSi2O7 (0 ≤ x ≤ 1) crystals with a quasi-two-dimensional structure In Ca2CoSi2O7 (x = 0), a canted antiferromagnetic transition occurs at 56 K The transition temperature TN is increasing with increasing Sr concentration, and the rises of the magnetization and dielectric constant become larger Since the dielectric constant shows large change at TN and the magnetocapacitance effect is observed below TN, a coupling between the magnetism and dielectricity is strong in (Ca1−xSrx)2CoSi2O7 The positive magnetocapacitance is reduced by Sr substitution, and is not observed in x ≥ 05 Namely, the compound of x ≥ 05 does not show the magnetic-field-induced electric polarization On the other hand, the negative magnetocapacitance is enhanced by Sr substitution

Magnetoelektrische Eigenschaften von Manganat-Titanat Übergittern

Abstract: Magnetoelectric multiferroics are materials in which the order phenomena ferromagnetism and ferroelectricity coexist. Due to pronounced magnetic as well as electric susceptibility, this class of material is especially suitable for the study of magnetoelectric coupling phenomena. However, the coexistence of the ferroic orders has rarely been observed in single phase materials. Composites give the possibility to further study magnetoelectric effects Within this work, the combination of ferromagnetic La2/3Ca1/3MnO3 (LCMO) and ferroelectricBaTiO3 (BTO) constitutes a multiferroic composite, in which a magnetoelectric coupling can occur via the interfaces. LCMO-BTO superlattices have been deposited on MgO substrates with the metallorganic aerosol deposition technique. Using coplanar electrodes on the top of the films, dielectric properties have been measured by a capacitance bridge. The electric capacitance and the dielectric loss as a function of sample temperature and external magnetic field shall give information about potential magnetoelectric couplings. Comparison of the experimental results with model calculations of the dielectric characteristics of a Maxwell-Wagner-circuit shall clarify, to what extent the observed magnetocapacitance can be attributed to the colossal magnetoresistance (CMR), a well-known property of LCMO. To further study the influence of the superlattice interfaces on the magnetoelectric properties, an interface-modified superlattice has been fabricated, in which thin layers of LaMnO3 have been deposited at every LCMO-BTO interface. The modified sample, in comparison to a conventional one, exhibiting the same thicknesses of LCMO- and BTO-layers, first of all shows enhanced magnetic and transport properties. The effect of the modification and thus the influence of the interfaces is discussed within LCMO as a material possessing strong electronic as well as polaronic correlations.

Surface Superconductivity Controlled by Electric Field

Abstract: We discuss an effect of the electrostatic field on superconductivity near the surface. First, we use the microscopic theory of de Gennes to show that the electric field changes the boundary condition for the Ginzburg–Landau function. Second, the effect of the electric field is evaluated in the vicinity of H c3, where the boundary condition plays a crucial role. We predict that the field effect on the surface superconductivity leads to a discontinuity of the magnetocapacitance. We estimate that the predicted discontinuity is accessible for experimental tools and materials nowadays. It is shown that the magnitude of this discontinuity can be used to predict the dependence of the critical temperature on the charge carrier density which can be tailored by doping.

Doping effect on the magnetocapacitance in Cd1−xFexCr2S4

Abstract: The influence of cation substitution on the Curie temperature and the magnetocapacitance in the doped magnetic relaxor ferroelectrics CdCr2S4 is investigated within the framework of the spherical random-bond-random-field model for the relaxor ferroelectric sublattice and the Heisenberg model for the magnetic sublattice. The site-dilution model is also introduced to describe the doping effect. A-site substitution of the Cd by Fe in Cd1−xFexCr2S4 is found to increase significantly the value of the Curie temperature, which well agrees with the experimental results. Additionally, the doping Fe ions play a crucial role in the fluctuation of spin-pair correlation and magnetocapacitive response.

Temperature Dependence of Magnetocapacitance in n-AlGaAs/GaAs Selectively Doped Heterojunction with InGaAs Quantum Dots

Abstract: The magnetocapacitance between a two-dimensional electron gas (2DEG) and a gate electrode has been studied in a selectively doped n-AlGaAs/GaAs heterojunction in which InGaAs quantum dots (QDs) are embedded in the vicinity of a GaAs channel. By comparing the experimental results with a resistive plate model, we have estimated the frequency f dependence of the bulk conductance σxx of the 2DEG in a quantum Hall regime and found that σxx is well fitted by the form σdc + AfS. We have also examined σdc, A, and S as functions of temperature T. It was found that the dc part σdc is well described as σ0exp [-(T0/T)2] in the conduction of variable range hopping. It was also found that A is proportional to Tn (n ~6.4), while S behaves as 1 - CT (C ~1.35), which can be explained by the pair approximation.