Negative Magnetocapacitance Research Articles

Magnetic properties and linear negative magnetocapacitance in rutile like LiFe2SbO6

Abstract We report the high-temperature magnetic ordering and the observation of linear negative magnetocapacitance mediated by a low-temperature magnetic phase crossover in the compound LiFe2SbO6. The combined observations of the absence of a second harmonic generation (SHG) signal, and Rietveld refinements of X-ray and neutron powder diffraction (NPD) patterns of the sample confirm that LiFe2SbO6 crystalizes in a centrosymmetric Pnnm structure. The DC magnetization measurements reveal that an antiferromagnetic ordering sets in at high temperature (TN= 850). The magnetic anomaly in the DC magnetization at a lower temperature T = 13 K, is corroborated by AC susceptibility and the specific heat measurements. We observe a linear negative magnetocapacitance below T = 13 K, demonstrating an indirect magneto-dielectric coupling. Further, the neutron powder diffraction study reveals a collinear antiferromagnetic ordering with a wavevector k = (0, 0, 0).

Detailed investigation on structural, optical, dielectric and magneto-dielectric properties with enhanced magneto-impedance characteristic of NdFeO3 nanoparticles

NdFeO3 (NFO) was synthesized using sol-gel citrate method and subsequent calcination at 1023 K. Utilizing Rietveld refinement analysis of the XRD pattern, it has been verified that NFO exhibits an orthorhombic crystal structure, with space group Pbnm. NFO grains were seen to be strongly agglomerated in a SEM image, despite their irregular shapes. The analysis of optical properties revealed a transmittance range of ∼20%–∼50% and an optical band gap of 2.15 eV for the NFO material. The dielectric properties of NFO were analyzed with respect to their dependence on frequency and temperature, while the ac electrical conductivity was studied in terms of its frequency variation. The dielectric dispersion behavior of NFO is explained by Maxwell Wagner-type polarization. A negative magneto-dielectric (MD) coupling of −19% and −35% was measured for ε and tanδ at 1 kHz for NFO. Magneto-impedance measurements on NFO showed positive magneto-resistance and both positive and negative magneto-capacitance, suggesting that the grain effect had a substantial role in the material's magnetic properties. These results suggest that NFO is an excellent choice for memory-based electronic devices with an appropriate band gap for optoelectronic applications having a large MD effect.

Fe+3- Mn+4 and Ti+4- Mn+3 interactions effect on the structural, electronic, magnetic and magnetoelectronic properties of perovskite manganites

In this study, we report room temperature positive magneto capacitance due to Ti +4 and Mn +3 valence states interactions in La 0.7 Pb 0.3 Mn 0.4 Ti 0.6 O 3+δ (δ = 0.15) i.e. La 0.66 Pb 0.3 Mn 0.4 Ti 0.56 O 3 and negative magneto capacitance due to Fe +3 and Mn +4 valance states interactions in Fe doped composition La 0.7 Pb 0.3 Mn 0.4 Fe 0.6 O 3. Rietveld analysis shows similar structural properties of Fe and Ti doped compositions. Synchrotron X-ray absorption spectroscopy (XAS) studies show (i) the presence of Ti +4 , Mn +3 and Mn +4 in La 0.66 Pb 0.3 Mn 0.4 Ti 0.56 O 3 and (ii) Fe +3 , Mn +3 and Mn +4 in La 0.7 Pb 0.3 Mn 0.4 Fe 0.6 O 3 . At room temperature, Fe doped composition shows nearly three times more magnetic moment in comparison to the magnetic moment of Ti doped composition. La 0.7 Pb 0.3 Mn 0.4 Fe 0.6 O 3 composition shows maximum ∼64% negative magneto capacitance while La 0.66 Pb 0.3 Mn 0.4 Ti 0.56 O 3 shows maximum ∼ 12% positive magneto capacitance. Size, quantity, number of d electrons and type of interactions among the valence states Fe +3 (0.645 Å), Ti +4 (0.605 Å), Mn +3 (0.645 Å), Mn +4 (0.53 Å) affect the electronic, magnetic and magnetoelectric properties of La 0.66 Pb 0.3 Mn 0.4 Ti 0.56 O 3 and La 0.7 Pb 0.3 Mn 0.4 Fe 0.6 O 3 manganites.

Multiferroic, optical and magneto-dielectric properties with enhanced magneto-impedance characteristic of KBiFe2O5

KBiFe2O5 (KBFO) is an interesting multiferroic compound with the coexistence of ferroelectric and ferromagnetic phases with a narrow band gap. In this work, KBiFe2O5 (KBFO) is synthesized using a sol-gel technique, followed by calcination at 973 K. Rietveld refinement analysis of XRD data confirms the presence of monoclinic crystal structure with space group P2/c. The SEM micrograph of KBFO reflects the irregular shape of grains with slight agglomeration. The optical band gap measured by UV visible spectroscopy of KBFO is found to be 1.56 eV. The multiferroic behavior of KBFO has been confirmed by the electrical (dielectric) and magnetic (M-H loop) measurements. A weak ferromagnetic nature is observed due to canting of the magnetic moment of G-type anti-ferromagnetic ordering. The dielectric studies reveal the Maxwell-Wagner type dispersion and an anomaly near magnetic phase transition temperature. KBFO shows negative magneto-dielectric (MD) coupling as both ε and tan δ decreases with the increasing magnetic field. Impedance spectroscopy is performed to analyze the intrinsic and extrinsic effects in MD coupling. The Nyquist plot shows positive magneto-resistance and negative magneto-capacitance which represent strong MD coupling in KBFO. It also reveals that the bulk property (intrinsic effect) dominates over the extrinsic effect in the sample. These results affirm that KBFO is a new candidate for room-temperature multiferroicity with a suitable bandgap and strong MD coupling for photovoltaic applications.

Dielectric relaxation and current conduction mechanism of Tb and Mn codoped bismuth ferrite grafted poly (vinyl alcohol) nanocomposite film

Abstract Investigation on current conduction mechanism through Tb and Mn codoped Bismuth Ferrite grafted polyvinyl alcohol (BTFMO-PVA) nanocomposite film above room temperature (300 K – 415 K) is reported here in detail. A detailed study on dielectric properties of the sample is done over a wide temperature range in a frequency range of 20 Hz - 2MHz. The conduction is attributed to correlated barrier hopping model. Bipolaron hopping dominates over single-polaron hopping in this system. Complex electric modulus spectra and complex modulus spectra are well explained by suitable models to understand the effective dielectric response. The sample responds to the externally applied magnetic field exhibiting negative magnetocapacitance at room temperature.

Study of Structural, Magnetic, Dielectric Properties and Estimation of Magnetoeletric Coupling of La, Mn co-doped Bi1−xLaxFe0.97Mn0.03O3 Ceramics

La and Mn co-doped Bi1−xLaxFe0.97Mn0.03O3 (BLFMOx, x = 0.05, 0.1, 0.2) ceramics were prepared by solid-state reaction method, and their structural, magnetic, dielectric and magnetocapacitance properties were studied. It was discovered that the co-substitution of La & Mn at sites of Bi & Fe suppressed impure phases which normally occur in BiFeO3 synthesis. BLFMOx samples were calcinated, and well crystalline phases were acquired at a sintering temperature of 950 °C. X-rays diffraction patterns of the samples were recorded and investigated for the affirmation of crystal structure and determination of the lattice parameters. The normal grain size of the samples was observed to be between 1 and 2 μm. M–H graphs of BiFeO3 and BLFMO0.05 ceramics consist of straight-line, confirming antiferromagnetic nature of samples. Dielectric constant was diminished with increase in frequency for each composition. BLFMOX ceramics showed negative magnetocapacitance and decrease in dielectric constant with magnetic field. Relative difference in dielectric constant initiated by external magnetic field might be approximated by ∆e/e = kM2 for BLFMOX, and here, magnetoelectric interaction (k) is negative.

Fabrication of 1D-Nanofiber/Fe₂O₃ Composites with Tailored Magnetic Properties.

A novel flexible electrospun nanofiber/γ-Fe₂O₃ composite has been obtained from suspension of γ-Fe₂O₃ nanoparticles in polyvinylpyrolidone solution in dimethylformamide. The impedance spectroscopy of the synthesized nanofiber/γ-Fe₂O₃ composite was carried out. Negative magnetoresistance and giant magnetocapacitance effects, as well as phenomenon of a "negative capacitance" at room temperature were observed in magnetic field (2.75 κOe) in infra-low frequency range. The polarization properties and volt-ampere characteristics of the nanocomposite in the applied magnetic field indicate the increase in the dielectric permittivity ɛ and the emergence of spin electromotive force, which enables us to accumulate of electric energy at quantum level. A quantum-mechanical model, which explained the non-monotonous behaviour of the volt-ampere characteristic of the novel nanofiber based composite, has been suggested.

Extremely Large Magnetic-Field-Effects on the Impedance Response of TiO2 Quantum Dots

Here, we report large magnetoresistance and magnetocapacitance response of undoped TiO2 quantum dots weighting the contribution of both grain and grain boundaries by means of impedance spectroscopy. We also performed a complete characterization of the TiO2 quantum dots (~5 nm) prepared by sol-gel via water vapor diffusion method, using X-ray diffraction, small angle X-ray scattering, transmission electron microscopy and Raman spectroscopy. In addition, we showed a complete theoretical study on the electronic properties of TiO2 surface and subsurface oxygen and titanium vacancies to shed some light in their electronic and magnetic properties. Based in our study, we can conclude that the presence of defects, mainly at the grain boundary of these undoped TiO2 quantum dots, could be responsible for the large positive magnetoresistance (+1200%) and negative magnetocapacitance (−115%) responses at low applied magnetic fields (1.8 kOe) and room temperature.

Complex magnetic structure and magnetocapacitance response in a non-oxide NiF2 system

We report here on the complex magnetic structure and magnetocapacitance in NiF2, a non-oxide multifunctional system. It undergoes an anti-ferromagnetic transition near 68.5 K, superimposed with canted Ni spin driven weak ferromagnetic ordering, followed by a metastable ferromagnetic phase at or below 10 K. Our density functional calculations account for the complex magnetic structure of NiF2 deduced from the temperature and the field dependent measurements. Near room temperature, NiF2 exhibits a relatively large dielectric response reaching >103 with a low dielectric loss of <0.5 at frequencies >20 Hz. This is attributed to the intrinsic grain contribution in contrast to the grain boundary contribution in most of the known dielectric materials. The response time is 10 μs or more at 280 K. The activation energy for such temperature dependent relaxation is ~500 meV and is the main source for grain contribution. Further, a large negative magneto capacitance >90% is noticed in 1 T magnetic field. We propose that our findings provide a new non-oxide multifunctional NiF2, useful for dielectric applications.

Magnetocapacitance and magnetoelectric coupling effect of Ni0.5Cu0.5Fe2O4/BaTiO3 mixed multiferroic fluids

Ni0.5Cu0.5Fe2O4/BaTiO3 (NCFO/BTO) mixed multiferroic fluids were prepared by distributing surfactant treated NCFO and BTO particles in to a highly-insulating base fluid, the magnetocapacitance and magnetoelectric coupling effect were investigated. The results indicate that the sample shows strong magnetoelectric coupling effect, in which both the coercive field Ec and remanent polarization Pr enhance obviously under the action of magnetic field, while the dielectric constant decreases with increasing the magnetic field, revealing negative magnetocapacitance phenomenon. Large direct magnetolectric coupling coefficient was estimated to be α ∼ 76 V/(cm−1.Oe), which is several orders of magnitude larger than that of conventional magnetoelectric composite ceramics. Further analysis indicates that the possibility of movement, aggregation and chain-like structure of the particles in the presence of external field is believed to be responsible for the observed negative magnetocapacitance and strong magnetoelectric coupling effect. The results show that the strong room temperature magnetoelectric effect of the mixed magnetoelectric fluids is the product of the deformation of the particle distribution and the utilization of clamping force under external field. This magnetoelectric coupling effect is different from that of conventional composite materials which is directly coupled via the interface interaction, which will be advantageous for practical applications.

Sign reversal of magneto-capacitance in an organic heterojunction based opto-spintronic system

Positive and negative magneto-capacitance can be achieved in an organic opto-spintronic device comprising an NPB/C60 heterojunction. The effect can be ascribed to the competition between the spin-dependent dissociation and bi-molecular recombination mechanisms.

Room temperature large positive and negative magnetocapacitance in CaMn0.95Fe0.05O3−δ

We report large positive and negative magnetocapacitance (MC) at room temperature in non-magnetic CaMn0.95Fe0.05O3−δ (CMFO) for low magnetic field (<1T). Large MC value (~13.65% at 7.8kG magnetic field) and negative MC value (~−2% at 1kG) shows strong magnetoelectric effect in CMFO due to the presence of Fe+3 and local magnetoresistance in oxygen deficient Mn+4–O–Mn+4 lattice. Magnetic studies show the signature of paramagnetic property for CMFO while CaMnO3−δ (CMO) shows the paramagnetic to antiferromagnetic transition ~125K. Transport studies show the insulator type of the behavior for CMFO and conductivity value has been observed more than two orders of magnitude lesser for CMFO in comparison to the conductivity of CMO.

Exchange bias effect and magnetoelectric coupling behaviors in multiferroic Co/Co3O4/PZT composite thin films

The multiferroic Co/Co3O4/PZT composite films are prepared on Pt/Ti/SiO2/Si wafers by sol-gel process combined with pulsed laser deposition method. The phase structures, microstructural topographies and element valence states of the composite films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectrum (XPS). The ferroelectric, electrical and magnetic properties as well as the magnetoelectric coupling behaviors are measured, and the exchange bias effect and its influence on the magnetoelectric coupling behavior of the composite film are studied systematically. #br#The results show the composite films have well-defined ferroelectric hysteresis loops with a remanent polarization value of ～17 μ C/cm2. The composite film exhibits evidently an exchange bias effect. Typically, a exchange bias field of ～80 Oe is observed at 77 K. Both the exchange bias field and magnetic coercive field increase with reducing the temperature. The exchange bias field increases to 160 Oe when the temperature decreases to 10 K. The XPS results confirm that an about 5 nm-thick CoO layer appears at the Co/Co3O4 interface due to the oxygen diffusion during the preparation, indicating that the exchange bias effect at 77 K is caused by the pinning effect of the antiferromagnetic CoO layer while the exchange bias effect at 10 K originates from the combining effect of antiferromagnetic CoO and Co3O4 layers. #br#The measureflent results of magnetocapacitance versus magnetic field curves at different temperatures show that the composite films have remarkable magnetoelectric coupling properties. The response of capacitance to temperature changes with the variation of external magnetic field. Further investigations show that the composite film possesses distinct anisotropic magnetocapacitance effect. When the direction of the magnetic field changes, the magnetocapacitance of the composite film changes from positive value to negative value. Moreover, the magnetocapacitance value changes with the variations of temperature and magnetic field magnitude. Typically, at 300 K a maximum value of positive magnetocapacitance (5.49%) and a minimum value of negative magnetocapacitance of (1.85%) are obtained at -4000 and 4000 kOe, respectively. When the temperature is reduced to 10 K, the positive magnetocapacitance decreases to a minimum value (0.64%) while the negative magnetocapacitance increases to a maximum value (5.4%). We perform a detailed analysis on such a magnetoelectric coupling behavior, and elucidate its origin, which should be attributed to the exchange bias effect and interface-mediated magnetism-stress-electricity coupling process.

Room temperature multiferroicity in Bi4.2K0.8Fe2O9+δ

Magnetoelectric multiferroics are materials that have coupled magnetic and electric dipole orders, which can bring novel physical phenomena and offer possibilities for new device functions. In this report, single-crystalline Bi4.2K0.8Fe2O9+δ nanobelts which are isostructural with the high-temperature superconductor Bi2Sr2CaCu2O8+δ are successfully grown by a hydrothermal method. The regular stacking of the rock salt slabs and the BiFeO3-like perovskite blocks along the c axis of the crystal makes the Bi4.2K0.8Fe2O9+δ nanobelts have a natural magnetoelectric–dielectric superlattice structure. The most striking result is that the bulk material made of the Bi4.2K0.8Fe2O9+δ nanobelts is of multiferroicity near room temperature accompanied with a structure anomaly. When an external magnetic field is applied, the electric polarization is greatly suppressed, and correspondingly, a large negative magnetocapacitance coefficient is observed around 270 K possibly due to the magnetoelectric coupling effect. Our result provides contributions to the development of single phase multiferroics.

Magnetocapacitance effect of magnetoelectric laminated composite at resonant frequency

A sandwich-like laminated composite of TbxDy1-xFe2-y/Pb(Zr, Ti)O3/TbxDy1-xFe2-y is prepared with a bonding method. The experimental study shows that the capacitance of the sample has several resonant peaks in the range of the frequency manipulated, and the resonant points shifte with the increase of applied magnetic field. The impedance of the sample also varies from capacitive to inductive ones at about the resonant point by changing the magnetic field. Giant positive and negative magnetocapacitance effects are observed simultaneously near the resonant frequency. From the constitutive equations of magnet and piezoelectrics involved, the capacitances as functions of frequency and magnetic field were theoretically modeled respectively. The results show that the experimental results are in good agreement with the theoretical ones, suggesting that the magnetocapacitance effect of the layered composite of magnetostriction/ piezoelectric originates from the magnetic field-controlled compliance coefficient of the ferromagnetic phase in the sample.

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

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.

Magnetoelectric properties of (Ca1−xSrx)2CoSi2O7 Crystals

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 5.6 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 ≥ 0.5. Namely, the compound of x ≥ 0.5 does not show the magnetic-field-induced electric polarization. On the other hand, the negative magnetocapacitance is enhanced by Sr substitution.

Magnetodielectric coupling in nonmagnetic Au/GaAs:Si Schottky barriers

We report on a heretofore unnoted giant negative magnetocapacitance (>20%) in non-magnetic Au/GaAs:Si Schottky barriers that we attribute to a magnetic field in-duced increase in the binding energy of the shallow donor Si impurity atoms. Depletion capacitance (Cdep) dispersion identifies the impurity ionization and capture processes that give rise to a magnetic field dependent density of ionized impurities. Internal photoemission experiments confirm that the large field-induced shifts in the built-in potential, inferred from 1/Cdep^2 vs voltage measurements, are not due to a field-dependent Schottky barrier height, thus requiring a modification of the abrupt junction approximation that accounts for the observed magnetodielectric coupling.

Coupled negative magnetocapacitance and magnetic susceptibility in a Kagomé staircase-like compoundCo3V2O8

The dielectric constant of the Kagomé staircase-likeCo3V2O8 polycrystalline compound has been measured as a function of temperature and magneticfield up to 14 T. It is found that the application of an external magnetic field suppresses theanomaly for the dielectric constant beyond 6.1 K. Furthermore, its magnetic field dependencereveals a negative magnetocapacitance which is proportional to the magnetic susceptibility,suggesting a common magnetostrictive origin for the magnetic field dependence of the twoquantities. This result is very different from that obtained from the isostructural compoundNi3V2O8 that presents a peak in the dielectric constant at the incommensurate magnetic phasetransition coupled to a sign change of the magnetocapacitance.

Positive and negative magnetocapacitance in magnetic nanoparticle systems

The dielectric properties of MnFe2O4 and γ-Fe2O3 magnetic nanoparticles embedded in insulating matrices were investigated. The samples showed frequency dependent dielectric anomalies coincident with the magnetic blocking temperature and significant magnetocapacitance above this blocking temperature, as large as 0.4% at H=10kOe. For both samples the magnetic field induced change in dielectric constant at high temperatures was proportional to the square of the sample magnetization. These measurements suggest that the dielectric properties of magnetic nanoparticles are closely related to the disposition of magnetic moments in the system. This magnetodielectric coupling is believed to arise from extrinsic effects, which are discussed in light of recent work relating magnetoresistive and magnetocapacitive behaviors.