Magnetocapacitance Research Articles

Energetics of the complex phase diagram of a tunable bilayer graphene probed by quantum capacitance

Bilayer graphene provides a unique platform to explore the rich physics in quantum Hall effect. The unusual combination of spin, valley and orbital degeneracy leads to interesting symmetry broken states with electric and magnetic field. Conventional transport measurements like resistance measurements have been performed to probe the different ordered states in bilayer graphene. However, not much work has been done to directly map the energetics of those states in bilayer graphene. Here, we have carried out the magneto capacitance measurements with electric and magnetic field in a hexagonal boron nitride encapsulated dual gated bilayer graphene device. At zero magnetic field, using the quantum capacitance technique we measure the gap around the charge neutrality point as a function of perpendicular electric field and the obtained value of the gap matches well with the theory. In presence of perpendicular magnetic field, we observe Landau level crossing in our magneto-capacitance measurements with electric field. The gap closing and reopening of the lowest Landau level with electric and magnetic field shows the transition from one ordered state to another one. Further more we observe the collapsing of the Landau levels near the band edge at higher electric field ($\bar D > 0.5$ V/nm), which was predicted theoretically. The complete energetics of the Landau levels of bilayer graphene with electric and magnetic field in our experiment paves the way to unravel the nature of ground states of the system.

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.

Spin glass freezing, magnetocapacitance and dielectric anomalies in 0.3NiFe2O4-0.7BiFeO3 nanocomposite

Abstract We report on magnetocapacitance, spin dynamics through AC succeptability measurements and dielectric anomalies in 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite. Structural studies (purity and phase) and surface morphology of the 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite were examined by X-ray diffraction (XRD), Raman Spectra, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM and SEM images indicate the uniform distribution of nanoparticles in parent pahase as well as in the composite. Energy dispersive X-ray (EDX) analysis indicates the presence of Ni, Fe, O and Bi elements in 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite. Magnetization measurements were performed at 300 K, 50 K and at 2 K and the Field Cooled (FC) and Zero Field Cooled (ZFC) measurements were carried out from 350 K to 2 K. ZFC and FC magnetization curves exhibit a hump around 30 K indicating the existence of competing magnetic interactions and possible spin galss behaviour. AC succeptibility measurements at various frequencies confirm the spin glass freezing behaviour of 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite around 25 K with the relaxation time and activation energy in the order of 10−7 s and 0.938 meV respectively. The dielectric constant and dielectric loss at various frequencies and temperatures indicate the dielectric relaxation behaviour of 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite. A rapid increase in the dielectric constant near the antiferromagnetic transition temperature (TN ∼ 643 K) of BiFeO3 indicates the existence of magneto-electric coupling in this composite. Magnetocapacitance (MC) of 0.3NiFe2O4 – 0.7BiFeO3 composite was measured at room temperature and the MC values of 13.5% and 7.8% (at 5000 Oe) are observed when the pellet is perpendicular and parallel to the magneticfield direction respectively. The presented magnetic and dielectric studies reveal the existence of magnetoelectric coupling and spin glass frezing of 0.3NiFe2O4 – 0.7BiFeO3 nanocomposite.

Nature of magnetoelectric coupling in corundum antiferromagnet Co4Ta2O9

Abstract We study magnetocapacitance (MC) effect and magnetoelectric (ME) coupling in spin-flop driven antiferromagnet Co4Ta2O9. Powder neutron diffraction data reveal that the magnetic structure corresponds to a non-collinear arrangement along with a non-centrosymmetric crystal structure below Neel Temperature. Electric polarization is achieved below Neel temperature only when the sample is cooled in the presence of external magnetic field. The magnetocapacitance data at high magnetic fields are analyzed by phenomenological Ginzburg-landau theory of ferro-electromagnets and it is found that change in dielectric constant is proportional to the square of magnetization. The saturation polarization and magnetoelectric coupling are estimated to be 52 µC/m2 and γ = 1.4 × 10−3 (emu/g)−2 respectively at 6 Tesla. Strong magnetoelectric coupling and ferroelectric phase in the anti-ferromagnetic Co4Ta2O9 are correlated to magnetic structure as derived from neutron diffraction data.

Large Landau-level splitting in a tunable one-dimensional graphene superlattice probed by magnetocapacitance measurements

The unique zero energy Landau Level of graphene has a particle-hole symmetry in the bulk, which is lifted at the boundary leading to a splitting into two chiral edge modes. It has long been theoretically predicted that the splitting of the zero-energy Landau level inside the {\it bulk} can lead to many interesting physics, such as quantum spin Hall effect, Dirac like singular points of the chiral edge modes, and others. However, so far the obtained splitting with high-magnetic field even on a hBN substrate are not amenable to experimental detection, and functionality. Guided by theoretical calculations, here we produce a large gap zero-energy Landau level splitting ($\sim$ 150 meV) with the usage of a one-dimensional (1D) superlattice potential. We have created tunable 1D superlattice in a hBN encapsulated graphene device using an array of metal gates with a period of $\sim$ 100 nm. The Landau level spectrum is visualized by measuring magneto capacitance spectroscopy. We monitor the splitting of the zeroth Landau level as a function of superlattice potential. The observed splitting energy is an order higher in magnitude compared to the previous studies of splitting due to the symmetry breaking in pristine graphene. The origin of such large Landau level spitting in 1D potential is explained with a degenerate perturbation theory. We find that owing to the periodic potential, the Landau level becomes dispersive, and acquires sharp peaks at the tunable band edges. Our study will pave the way to create the tunable 1D periodic structure for multi-functionalization and device application like graphene electronic circuits from appropriately engineered periodic patterns in near future.

Investigations on magnetodielectric and magnetoelectric properties of CNFO-BST composite

The paper reports the synthesis of Ba1-xSrxTiO3 (BSTx) for x = 0.20 and 0.30 via hydrothermal route followed by microwave sintering to result into the BST nano particles of size nearly 60 nm. The Co0.9Ni0.1Fe2O4 (CNFO) is also synthesized using hydrothermal route to achieve Nano-crystalline particles. The composites yCNFO-(1-y)BSTx with y = 0.2,0.3 & 0.4 and x = 0.2 and 0.3 are investigated for the variation of dielectric constant as a function of applied magnetic field (H) for the frequency between 100 Hz and 1 MHz. The maximum value of Magnetocapacitance (MC) is −6% for the 0.20CNFO-0.8BST0.20 while the maximum value of Magnetoelectric Coefficient (α) is 5 mV/Oe.cm for the 0.20CNFO-0.8BST0.30.

Improved magneto-electric properties of LaFeO3 in La0.8Gd0.2Fe0.97Nb0.03O3

Samples of LaFeO3 (LFO) and La0.8Gd0.2Fe0.97Nb0.03O3 (LGFNO) are prepared by solid state reaction method. The combination of these ions (3d and 4d) is chosen to enhance the magnetic and ferroelectric property of the multiferroic LFO. Crystallographic phase analyses of LFO and LGFNO are carried out by Rietveld method using MAUD program which confirms the successful substitution of Gd3+ and Nb5+ in the lattice of LFO. Zero field cooled (ZFC) and field cooled (FC) magnetizations of LGFNO are measured in the range of 300–5K in vibrating sample magnetometer (VSM) which shows an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition below ~ 85K. Magnetic hysteresis loops recorded at 300 and 5K exhibits the enhancement of magnetization of the co-doped sample (LGFNO) at 300 (2.02 times) and 5K (31.7 times) with reference to LFO. Dielectric properties such as dielectric constant (ε′) and loss tangent (tanδ) of LFO and LGFNO are measured with the variation of temperature from 300K up to 900K and also with the variation of frequency from 42Hz to 5MHz. The thermal variation of dielectric constant (ε′) shows ferroelectric to paraelectric phase transition of LGFNO at ~ 780K. Polarization vs. electric field (P-E) loops of these samples recorded at 300K indicate the onset of ferroelectric ordering with enhanced polarization of LGFNO compared to that of LFO. The dielectric constant of LGFNO measured in presence and absence of magnetic field confirms that the samples are type-II multiferroic with positive magneto-electric coupling and also the value of magneto-capacitance (MC) of LGFNO is enhanced compared to that of LFO which suggests that the co-doping with Gd3+ and Nb5+ ions in place of La3+ and Fe3+ ions also enhance the magneto-electric coupling.

Inverse Tunnel Magnetocapacitance in Fe/Al-oxide/Fe3O4

Magnetocapacitance (MC) effect, observed in a wide range of materials and devices, such as multiferroic materials and spintronic devices, has received considerable attention due to its interesting physical properties and practical applications. A normal MC effect exhibits a higher capacitance when spins in the electrodes are parallel to each other and a lower capacitance when spins are antiparallel. Here we report an inverse tunnel magnetocapacitance (TMC) effect for the first time in Fe/AlOx/Fe3O4 magnetic tunnel junctions (MTJs). The inverse TMC reaches up to 11.4% at room temperature and the robustness of spin polarization is revealed in the bias dependence of the inverse TMC. Excellent agreement between theory and experiment is achieved for the entire applied frequency range and the wide bipolar bias regions using Debye-Fröhlich model (combined with the Zhang formula and parabolic barrier approximation) and spin-dependent drift-diffusion model. Furthermore, our theoretical calculations predict that the inverse TMC effect could potentially reach 150% in MTJs with a positive and negative spin polarization of 65% and −42%, respectively. These theoretical and experimental findings provide a new insight into both static and dynamic spin-dependent transports. They will open up broader opportunities for device applications, such as magnetic logic circuits and multi-valued memory devices.

Room-temperature multiferroic and magnetocapacitance effects in M-type hexaferrite BaFe10.2Sc1.8O19

The room-temperature multiferroic and magnetocapacitance (MC) effects of polycrystalline M-type hexaferrite BaFe10.2Sc1.8O19 have been investigated. The results show that the magnetic moments of insulating BaFe10.2Sc1.8O19 can be manipulated by the electric field at room temperature, indicating the existence of magnetoelectric coupling. Moreover, large MC effects are also observed around the room temperature. A frequency dependence analysis shows that the Maxwell-Wagner type magnetoresistance effect is the dominant mechanism for MC effects at low frequencies. Both the magnetoelectric-type and non-magnetoelectric-type spin-phonon couplings contribute to the MC effects at high frequencies with the former being the dominant mechanism. The above results show that the hexaferrite BaFe10.2Sc1.8O19 is a room-temperature multiferroic material that can be potentially used in magnetoelectric devices.

A nonlinear model for magnetocapacitance effect in PZT-ring/Terfenol-D-strip magnetoelectric composites

In previous works, most of them employ a linear constitutive model to describe magnetocapacitance (MC) effect in magnetoelectric (ME) composites, which lead to deficiency in their theoretical results. In view of this, based on a nonlinear magnetostrictive constitutive relation and a linear piezoelectric constitutive relation, we establish a nonlinear model for MC effect in PZT-ring/Terfenol-D-strip ME composites. The numerical results in this paper coincide better with experimental data than that of a linear model, thus, it’s essential to utilize a nonlinear constitutive model for predicting MC effect in ME composites. Then the influences of external magnetic fields, pre-stresses, frequencies, and geometric sizes on the MC effect are discussed, respectively. The results show that the external magnetic field is responsible for the resonance frequency shift. And the resonance frequency is sensitive to the ratio of outer and inner radius of the PZT ring. Moreover, some other piezoelectric materials are employed in this model and the corresponding MC effects are calculated, and we find that different type of piezoelectric materials affect the MC effect obviously. The proposed model is more accurate for multifunction devices designing.

Magnetically controlled space charge capacitance at La1−xSrxMnO3/SrxLa1−xTiO3 interfaces

This work reports on magnetocapacitance (MC) effects in epitaxial heterostructures of nominally 15 unit cells (u.c.) LaMnO3 (LMO) and 2 u.c. SrTiO3 (STO) with an alternating layer‐repetition rate of 8: (LMO15/STO2)8. Epitaxial multilayer growth at high temperatures (900 °C) activates a selective inter‐diffusion of La3+ and Sr2+ cations across the interfaces, which gives rise to Sr p‐doping of the LMO and La n‐doping of the STO layers. MC effects at the buried La1–xSrxMnO3/SrxLa1–xTiO3 (LSMO/SLTO) interfaces are probed by frequency, temperature and magnetic field dependent AC impedance spectroscopy. The technique is shown to be appropriate to account for the separate analysis of different resistance and capacitance contributions at the buried interfaces. As a result of the La/Sr inter‐diffusion process, Schottky barriers are formed at the LSMO/SLTO interfaces, which give rise to massive MC of up to ≈ −200% in the out‐of‐plane film direction. The capacitance of the manganite‐titanate LSMO/SLTO interfaces may be coupled indirectly to the resistance of the LSMO layers, because the Schottky space‐charge layers and their capacitance can be modulated by varying the concentration of highly mobile charge carriers in the LSMO with a magnetic field.

Room temperature magnetodielectric studies on Mn-doped LaGaO3

Polycrystalline samples of LaGa1−xMnxO3 (0 ≤ x ≤ 0.3) were prepared by the solid-state reaction route. The phase purity of these samples was confirmed by powder x-ray diffraction experiments carried out on BL-12 at the Indus-2 synchrotron radiation source. The sample with x = 0.2 shows significant change in the value of capacitance with the application of a magnetic field. The observed results were understood by systematically analyzing magnetocapacitance (MC), magnetoresistance (MR), and dielectric loss as a function of frequency. Our results and analysis suggest that the observed magnetodielectric (MD) coupling may be due to the MR effect of the Maxwell–Wagner type and/or field-induced dipolar relaxation. Further, it is observed that oxygen stoichiometry plays a very crucial role in the observed MD coupling.

Enhanced magneto-electric property and exchange bias effect of Zn substituted LaFeO3 (La0.50Zn0.50FeO3)

Nanoparticles of Zn substituted lanthanum orthoferrite, La0.50Zn0.50FeO3 (LZFO) are prepared by co-precipitation method and the crystallographic phase is confirmed by analyzing the X-ray diffractograms using FULLPROF program. Magnetic and ferroelectric orderings of LZFO are observed at room temperature (RT). An exchange bias effect is also observed at RT. Temperature dependent dielectric constant of LZFO exhibits a ferroelectric to paraelectric transition at ~776K. Magneto-capacitance (MC) co-efficient is estimated from the dielectric constant of LZFO measured in presence and absence of magnetic field. Interestingly, the values of magnetization, electric polarization and MC of LZFO are substantially enhanced compared to that of LaFeO3 (LFO) and thus the doping of Zn would enhance the quality of applications of LFO.

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.

Modulated magneto-dielectric property and exchange bias effect of BiFeO3 incorporated in (BiFeO3)0.50 (Li0.30Zn0.35 Fe2.35O4)0.50 nanocomposite

Nanoparticles of BiFeO3 (BFO) are prepared by the sol–gel method. These prepared nanoparticles are incorporated in the nanocomposite of (BiFeO3)0.50 (Li0.30Zn0.35 Fe2.35O4)0.50 (BFO-LZFO). The formation of pure crystallographic phase of each component (BFO and LZFO) in the nanocomposite of BFO-LZFO has been confirmed by Rietveld analysis of the X-ray diffractograms using FULLPROF program. Morphology, average particle size and its distribution, crystallographic phase etc. are determined from the high-resolution transmission electron microscopy of BFO-LZFO. Magnetic property of BFO is drastically enhanced in the nanocomposite. An exchange bias effect is also observed in the field cooled and zero field cooled hysteresis loops, which suggest that antiferromagnetic nanoparticles of BFO are encapsulated by ferrimagnetic nanoparticles of LZFO in BFO-LZFO. Magneto-capacitance (MC) co-efficient has been estimated from the values of dielectric constants, measured in presence and absence of magnetic field. Results of magnetic, dielectric and MC co-efficient indicate a strong modulation of magneto-dielectric property of BFO in BFO-LZFO and this would be quite interesting for applications of BFO, a lead free multiferroic.

Magnetocapacitance and structure properties of Bi0.85−xLa0.15PbxFeO3 ceramics

To study the effects of La and Pb co-doping on the magnetocapacitance (MC) and structure properties of BiFeO3, single-phase Bi0.85–xLa0.15PbxFeO3 (BLPF, x=0–0.10) multiferroic ceramics were prepared by a solid-state reaction. In the Rietveld refinement, good agreement between the observed and calculated pattern was observed. The dielectric response of these samples was analyzed in the frequency range of 100Hz–10MHz, and it was found that Pb-doping drastically improved the dielectric properties. Moreover, the MC and magnetoelectric effect were enhanced, show a large MC effect (~18.8%) and magnetoelectric voltage coefficient αE (~90.6mV/cm/Oe) for BLPFx=0.05 ceramic.

Magnetodielectric behaviour in La0.53Ca0.47MnO3

We explore magneto-dielectricity in La0.53Ca0.47MnO3 across its paramagnetic (PMI) to ferromagnetic (FMM) isostructural transition at TC ∼ 253 K, by magnetic (M), caloric (W), dielectric (ε′), magnetoresistive (MR), and magnetocapacitance (MC) investigations. A skew-broadened first-order transition character is confirmed via heating/cooling hystereses in M(T) and W(T), with a superheating temperature T** next to TC and supercooling temperature T* exhibiting kinetics. Above TC, linearly related MC and MR reflect purely a magnetoresistance effect. Near TC, the high-frequency MC (5 Tesla (T)), far exceeds the magneto-losses, and is uncorrelated with dc MR (5 T) in the FM-ordered state. The intrinsic magneto-dielectricity manifest below TC and above ∼kHz is traced to an intra-granular Maxwell–Wagner-type effect at the interface region of PMI–FMM phase coexistence.

Multiferroicity in La1/2Nd1/2FeO3 nanoparticles

Nano-sized La1/2Nd1/2FeO3 (LNF) powder is synthesized by the sol–gel citrate method. The Rietveld refinement of the X-ray diffraction profile of the sample at room temperature (303 K) shows the orthorhombic phase with Pbnm symmetry. The particle size is obtained by transmission electron microscope. The antiferromagnetic nature of the sample is explained using zero field cooled and field cooled magnetisation and the corresponding hysteresis loop. A signature of weak ferromagnetic phase is observed in LNF at low temperature which is explained on the basis of spin glass like behaviour of surface spins. The dielectric relaxation of the sample has been investigated using impedance spectroscopy in the frequency range from 42 Hz to 1 MHz and in the temperature range from 303 K to 513 K. The Cole–Cole model is used to analyse the dielectric relaxation of LNF. The frequency dependent conductivity spectra follow the power law. The magneto capacitance measurement of the sample confirms its multiferroic behaviour.

Enhanced electric property and magneto-capacitance co-efficient co-related with modulated Raman spectroscopy of GaFeO3 in (GaFeO3)0.50(Ni0.40Zn0.40Cu0.20Fe2O4)0.50

Multiferroic (MF) nanoparticles of GaFeO3 (GFO) were prepared by sol–gel method. To modulate the magneto-dielectric property, nanoparticles of GFO are incorporated in Ni0.40Zn0.40Cu0.20Fe2O4 (NZCF). Crystallographic phases of all the samples are confirmed by analyzing the X-ray diffractograms using FULLPROF program. The hysteretic behavior observed in the polarization vs. electric field loops recorded at room temperature (RT) confirmed the ferroelectric ordering of GFO and (GaFeO3)0.50(Ni0.40Zn0.40Cu0.20Fe2O4)0.50 (GFO-NZCF). Temperature dependent dielectric constant of GFO and GFO-NZCF shows the ferroelectric to paraelectric transition at 625 and 680 K, respectively. The dielectric constant measured in presence and absence of magnetic field confirmed that the samples are MFs with positive magneto-capacitance (MC) co-efficient. Interestingly, electric polarization and MC co-efficient of GFO are increased in GFO-NZCF. Room temperature Raman spectra of GFO and NZCF have been compared with that of GFO-NZCF and the observed changes have been co-related with the enhanced polarization of GFO-NZCF.

HIGH-FREQUENCY MAGNETOCAPACITANCE EFFECT IN ORGANIC SPIN VALVE WITH A 3,4,9,10-PERYLENE-TERACARBOXYLIC-DIANHYDRIDE SPACER

The frequency-dependent impedance of a series of ferromagnet (FM)/organic semiconductor (OSC)/FM tri-layered organic spin valves (OSV) is investigated in the frequency range of 10 Hz–1 MHz. An equivalent resistor–capacitor (RC) parallel network model is employed to analyze the magnetoresistance (MR) and magnetocapacitance (MC) effects. Fitting with the model yields field-dependent parameters and the resistive parameters agree with the experimental results. The analysis of the impedance spectra indicates an effective magnetotransport mechanism dominated by the charge accumulation at the organic–FM interfaces.