Magnetodielectric Research Articles

Size dependent dielectric, magnetic, transport and magnetodielectric properties of BiFe0.98Co0.02O3 nanoparticles

Nanostructured polycrystalline samples of BiFe0.98Co0.02O3 (BFCO) with various sizes were prepared using acetate precursor based modified sol–gel method. Rhombohedral phase of nanoparticles has been confirmed by X–ray diffraction (XRD) measurement. The electrical and magnetic properties of samples were investigated for different sized nanoparticles (47.25–93.30 nm). The effect of sintering temperature on the structural, microstructural and dielectric properties has been discussed in the context of crystallite size and oxygen vacancy concentration. The temperature dependent impedance study confirms that the temperature dependent relaxation process is non Debye type and dominated by the short–range movement of oxygen vacancies. The value of magnetization is found to increase with decreasing particle size. Large magnetodielectric (MD) effect has been observed for BFCO nanoparticles with size less than ∼ 62 nm. Complex impedance spectroscopy analysis has been used to ensure intrinsic MD effect.

Dielectric, magnetic and magneto-dielectric properties of (La, Co) co-doped BiFeO3

BLFCO [Bi1−xLaxFe1−xCoxO3 (x = 0.00, 0.01, 0.03, 0.05, 0.10)] ceramics were synthesized using sol-gel technique. The effect of Lanthanum (La) and Cobalt (Co) co-doping in BiFeO3 (BFO) on structural, electrical, magnetic and magneto-dielectric (MD) properties are systematically investigated. The phase purity and crystal structures were studied using x-ray diffraction technique. The dielectric properties [dielectric constant (ε) and tangent loss (tanδ)] were studied as a function of frequency and temperature for pure and co-doped BFO samples. The magnetization study reveals that the ferromagnetic (FM) properties in La and Co co-doped samples enhance with an increasing dopant concentration. The coercive field is found to increase with the doping concentration suggest a competition between the antiferromagnetic and FM ordering. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements of these samples also confirm the change in the magnetic behaviour of BLFCO as doping concentration increases. ZFC and FC magnetization curves split at low temperature with sharp cusps suggest a spin-glass like behaviour of BLFCO. The pure and co-doped samples exhibit a MD coupling effect. Furthermore, the anomaly near the magnetic phase transition (around the Neel temperature of BFO) implies the MD coupling in BLFCO samples.

Miniaturized Wearable Antennas with Improved Radiation Efficiency Using Magneto-dielectric Composites

In this paper, two miniaturized flexible wearable antennas based on a planar dipole with a reflector are proposed and investigated. To realize miniaturization, the concept of magneto-dielectric (MD) composite materials was chosen. Numerical and experimental analyses were carried out to investigate the antenna performance (radiation efficiency, gain, and bandwidth) in the free space and on a tissue-equivalent phantom. Results demonstrated that MD composites are beneficial for wearable antenna miniaturization (up to 36%) and radiation efficiency improvement (up to 65% in free space and up to 23% on-body). Moreover, the antennas exhibit a low specific absorption rate.

Giant magnetodielectric effect in composites of nanodimensional spin glass of system CoO-SiO2 and mesoporous silica SBA-15

Abstract Nanodimensional silica-based spin glasses of compositions xCoO-(100-x)SiO2 with x having values 20 and 30 were synthesized by sol-gel method within mesoporous silica SBA-15 (Santa Barbara Amorphous-15) having a pore diameter ~5.5 nm. X-ray photoelectron spectroscopy (XPS) analysis showed the presence of both Co2+ and Co3+ species within the glasses. This caused electronic conduction in this amorphous system. The nanoglasses exhibited resistivity values at room temperature which were about three orders of magnitude lower than those of the corresponding bulk glasses. The resistivity data for the nanodimensional glasses on analysis confirmed the conduction to arise due to small polaron hopping between the localized states represented by these ions with an activation energy in the range 0.08–0.07 eV. The inter-site separation values extracted from the analysis of the resistivity data were found to lie in the range 6.7–6.6 A. The values of magnetodielectric (MD) parameter for the different nanocomposites were rather large with the highest value found to be in the range 523–49% for the frequencies 1 kHz and 1 MHz in case of nanocomposite with a glass composition 30CoO-70SiO2. The dissipation factor for the samples was, however, in the range 0.3–1.6. By using suitable measuring frequency the dissipation factor could be brought down to a value less than 1.0 with the M.D. parameter exhibiting values in the range 10–49%. The results were fitted to Catalan’s model based on two dielectrics with different resistivity values connected in series. The satisfactory fit of the experimental data to the theoretical model based on a negative magnetoresistance of the nanoglasses leads to the conclusion that an enhancement of spin polarized electron hopping is the reason behind this effect. The results obtained should be further explored to find applications of these materials as magnetic sensors as well as magnetically controlled supercapacitors.

Destruction of multiferroicity in Tb2BaNiO5 by Sr-doping and its implication to magnetodielectric coupling

The Haldane spin-chain compound, Tb2BaNiO5, with two antiferromagnetic transitions, one at T1 = 63 K and the other at T2 = 25 K, has been recently shown to be an exotic multiferroic below T2. Here, we report the results of our investigation of Sr doping at the Ba site by magnetization, heat-capacity, magnetodielectric (MDE) and pyrocurrent measurements. An intriguing finding, which we stress, is that the ferroelectricity is lost even for a doping level of ten atomic percent, though magnetic ordering prevails. The doped specimens however retain significant MDE behaviour, but with reduced magnitudes and qualitative changes with respect to the behaviour of the parent compound. This implies that ferroelectric order is also crucial for the anomalously large MDE in the parent compound, in addition to the role of 4f single-ion anisotropy.

Solar panel integrated compact magneto-dielectric loaded sequentially rotated navigation antenna

In this paper we present a novel solar panel integrated navigation antenna for solar powered vehicles and devices. The radiator is a compact sequentially rotated patch array loaded with a magneto-dielectric (MD) layer to achieve miniaturization for ease of integration with solar panels. The magneto-dielectric substrate of the proposed antenna consists of MD unit cells created by vertical split ring resonators (SRR). The unit cells are arranged in 11 × 1 fashion beneath each radiator to reduce the dimension of the same to 70% with equivalent performance compared with the respective original patch radiators. An array of such radiators is formed and fed with a sequential feed network comprising T-junction power dividers and delay lines. The proposed antenna is then integrated and tuned with a solar panel and measured. The measured results show that the antenna operates in the L1 GPS band with an impedance bandwidth of 20 MHz centered at 1.575 GHz. The antenna is Right Handed Circularly Polarized (RHCP) with axial ratio bandwidth (ARBW) of 100% due to the sequential rotation. The peak Axial Ratio (AR) values are <3 dB with a peak gain of 7.89 dBic. The proposed solar panel integrated navigation antenna is developed with the view of using it as a multipurpose candidate in a compact shared dimension in solar powered vehicles and devices to provide power to the systems as well as global positioning and navigation of the same.

Spin phonon coupling and magneto-dielectric coupling in BiFeO3–TbMnO3 composite

Magneto-structural and magneto-electric coupling in multiferroic composite BiFeO3–TbMnO3 have been investigated. Anomalous softening of the phonon modes was observed ∼210 K (close to the spin reorientation transition temperature T*) in the temperature dependent Raman spectra which might be a result of spin phonon coupling in the lattice. Relaxor type ferroelectric behaviour was observed in the temperature dependent dielectric measurements where the relaxation was found near T*. The frequency dependent dielectric study and the impedance spectroscopy revealed that the dielectric properties are a mixture of intrinsic Debye relaxation and Maxwell-Wagner relaxation arising from the space charge polarization at the interface of the composite. A large positive magneto-dielectric (MD) coupling was found near the relaxation temperature which is understood to be originated as a result of the magneto-structural coupling in the system.

Magnetic and anomalous dielectric behavior of Mn modified Ba2Mg2Fe12O22 hexaferrite

We have investigated structural, dielectric, magnetic and room temperature magnetodielectric properties of Mn-doped Y-type hexaferrite Ba2Mg2(Fe1-xMnx)12O22 (0 ≤ x ≤ 0.08). Rietveld refined of X-ray pattern of all samples confirms the phase purity with the rhombohedral crystal structure (space group R-3 m). Magnetization measurements suggest that ferrimagnetic to paramagnetic transition temperature (Tc) decreased from 647 K (x = 0.0, BMF) to 621 K (x = 0.08, BMFM8) for Mn-doped sample. Our results confirm the decrease in magnetocrystalline anisotropy constant (K) by ∼49% and ∼117% in BMFM4 (x = 0.04) and BMFM8 samples respectively in comparison to BMF due to substantial increase in superexchange angle of Fe/Mg ions in octahedral and tetrahedral sites. Substantial decrease in dielectric constant and inverse distorted butterfly loop like behavior in magnetodielectric (MD) effect is observed at room temperature.

Persistence of large magnetodielectric coupling anomalies and multiferroicity for significant dilution of Tb sublattice by Y in Tb2BaNiO5

We report temperature dependence of dc magnetization, heat capacity, dielectric, magnetodielectric (MDE) coupling, and pyrocurrent on polycrystalline specimens resulting from a gradual dilution of Tb sublattice by Y in an exotic multiferroic Haldane spin-chain system, Tb2BaNiO5, viz., for the series Tb2−xYxBaNiO5 (x = 0.1, 0.2, 0.5, 1, and 1.5). Among various observations, the ones to be stressed are as follows: (i) The Néel temperature (TN1 = 63 K) and the second magnetic feature occurring at (TN2) 25 K get reduced to lower temperatures with increasing x in a linear fashion, with a concomitant lowering of multiferroicity onset temperature below TN2, scaling essentially with x. This finding emphasizes that local effects due to Tb play a major role on multiferroicity. Clearly, 4f dominates to determine this phenomenon—a rare finding in this field. (ii) The magnitude of the change (Δɛ′) in the dielectric constant surprisingly gets maximized for x = 0.1 with respect to that of the parent compound, attaining a relatively large value of about 22% at 15 K for a field of 100 kOe, thereby revealing that a delicate balance of nonmagnetic doping or defects in favorable cases can enhance MDE coupling. (iii) Nonmonotonic changes in Δɛ′ with a magnetic field, also a changing sign for some compositions, reveal interesting metamagnetodielectric behavior. (iv) The magnetoelectric behavior of x = 1.5 composition is found to be somewhat peculiar with respect to that of other compositions. Thus, this solid solution reveals exotic magneto(di)electric coupling anomalies.

Dielectric, multiferroic and magnetodielectric properties of Co/Fe co-doped Bi4Ti3O12 ceramics

Polycrystalline Bi4CoxFe2xTi3−3xO12 (BC2FT) ceramics (0 ≤ x ≤ 0.4) were synthesized by conventional solid state reaction and their dielectric, multiferroic and magnetodielectric (MD) properties have been investigated. The XRD data results reveals that all samples possess a layered perovskite structure belonging to the n = 3 members of the Aurivillius family and a secondary phase of Bi5Ti3FeO15 could be observed at higher concentrations. The average grain size is found to decrease with the increase of x. Permittivity values decrease with the content of Co and Fe increasing (0.1 ≤ x ≤ 0.4). BC2FT ceramics exhibit ferroelectric and weak ferromagnetic properties at room temperature. Room-temperature magnetodielectric (MD) and magnetodielectricloss effect were observed in all samples, resulting from the contributions of the magnetostriction effect. Furthermore, with the dopant content of Co and Fe increasing, the negative peaks shift to a lower frequency and the positive peak get the largest room-temperature MD response of 0.649% at 97.88 Hz with x = 0.4.

Emergent Vibronic Excitations in the Magnetodielectric Regime of Ce_{2}O_{3}.

The strong coupling between spin, lattice, and electronic degrees of freedom in magnetic materials can produce interesting phenomena, including multiferroic and magnetodielectric (MD) behavior, and exotic coupled excitations, such as electromagnons. We present a temperature- and magnetic field-dependent inelastic light (Raman) scattering study that reveals the emergence of vibronic modes, i.e., coupled vibrational and crystal-electric-field (CEF) electronic excitations, in the unconventional rare-earth MD material Ce_{2}O_{3}. The energies and intensities of these emergent vibronic modes are indicative of enhanced vibronic coupling and increased modulation of the dielectric susceptibility in the Néel state (T_{N}≈6.2 K). The field dependences of the energies and intensities of these vibronic modes are consistent with a decrease of both the vibronic coupling and the dielectric fluctuations associated with these modes below T_{N}. These results suggest a distinctive mechanism for MD behavior in Ce_{2}O_{3} that is associated with a field-tunable coupling between CEF and phonon states.

Lead-free (K, Na, Li) NbO3/NiFe2O4 thin films by pulsed laser deposition: Structure, dielectric, magnetic and magnetodielectric behavior

In this paper we propose the fabrication of eco-friendly (1-x) (K, Na, Li) NbO3-xNiFe2O4(KNNL-NFO) thin films on Pt/Si substrate with (x = 0.01, 0.02, 0.03, and 0.04) as target source by Pulsed Laser Deposition (PLD) technique. Structural analysis carried out by X-ray diffraction and Raman spectroscopy method confirmed the perovskite phase of KNNL and ferrite phase for NFO of the composite films. Chemical state evaluated by X-ray photoelectron spectroscopy confirmed the presence of Fe3+ along with minor Fe2+ ions. These films exhibit a clear magnetic hysteresis behavior and with increasing ferrite composition the saturation magnetization increases and sufficiently large room temperature magnetodielectric (MD) constant with low losses are obtained. Exchange between thermally activated charge carriers created by hole-electron pair in d-orbital satisfying Maxwell-Wagner polarization was observed during room temperature dielectric permittivity measurements. All the composite films exhibit poor saturation polarization and leakage current was found to be decrease with increasing ferrite composition. Ac conductivity plot infers hopping mechanism to be responsible for the conduction process obeying Jonscher's power law.

Low-temperature Raman, high magnetic field Fe57 Mössbauer, and x-ray diffraction study of magnetodielectric coupling in polycrystalline GdFeO3

The present work reports the magnetodielectric (MD), ferroelectric, magnetoelectric (ME), low-temperature (LT) Raman, low-temperature high magnetic field (LTHM) $^{57}\mathrm{Fe}$ M\ossbauer and x-ray diffraction (XRD) studies of polycrystalline multiferroic ${\mathrm{GdFeO}}_{3}$. Ferroelectric ordering as a result of magnetic ordering of ${\mathrm{Gd}}^{3+}$ ions (${T}_{N,\mathrm{Gd}}$) is confirmed from the pyrocurrent measurements carried out in conventional and bias-electric field protocols. Coupling between magnetic and electric orderings, viz., magneto-electric (ME) coupling, is confirmed by measuring the dielectric constant and pyrocurrent in the presence of magnetic field. A huge magnetodielectric (MD) coefficient ($\ensuremath{\approx}\ensuremath{-}12%$) is observed below the ${\mathrm{Gd}}^{3+}$ ordering temperature, reaching a saturation value with a field of about 3 Tesla. From LT Raman data across ${T}_{N,\mathrm{Gd}}$, the presence of spin-phonon coupling and the development of new modes indicating the symmetry reduction are observed. The average displacement of cations obtained from LTHM XRD measurement corroborates the ferroelectric polarization measurements. Signatures of a transferred hyperfine field are observed from LTHM $^{57}\mathrm{Fe}$ M\ossbauer data. The variation of the effective internal magnetic field obtained from in-field M\ossbauer data gives evidence for the predicted spin structures of ${\mathrm{GdFeO}}_{3}$ with magnetic fields below ${T}_{N,\mathrm{Gd}}$. The observed anomalous variation of the center shift and resonance area from $^{57}\mathrm{Fe}$ M\ossbauer data across ${T}_{N,\mathrm{Gd}}$ ordering is explained in terms of phonon mode softening. Observed MD and ME effects are explained in terms of spin-phonon coupling as evidenced from LTHM $^{57}\mathrm{Fe}$ M\ossbauer and LT Raman data.

Sintering effect and magnetodielectric studies on nanostructured BiFe0.95Co0.05O3

Abstract Multiferroic nanoparticles of BiFe0.95Co0.05O3 (BFCO) with different sizes were prepared by low temperature acetate precursor based modified sol–gel method. X–ray diffraction (XRD) measurement revealed rhombohedral phase of nanoparticles. Crystallite size increases from ∼41.05 nm (700 °C) to ∼ 48.42 nm (800 °C). The dielectric constant and loss tangent were studied as a function of temperature at different frequencies. The appearance of dielectric anomalies around antiferromagnetic transition temperature prove the existence of possible magnetoelectric (ME) coupling. Possible present ME coupling has been studied by performing magnetodielectric (MD) measurements at room temperature. The strong variation in dielectric constant and loss has been found under the influence of magnetic field at room temperature. The magnetoimpedance (MI) measurement was studied to separate out intrinsic and extrinsic MD effect. Spin spiral suppression at nanoscale with crystallite size less than ∼62 nm and lattice strain, arises due to Cobalt doping, play an important role in controlling multiferroic properties.

Extraordinary Ferromagnetic Coupling and Magnetodielectric Phenomena in NiO Nanoparticles

We intend to understand the ferromagnetism and magnetodielectric (MD) phenomenon in NiO nanoparticles (NPs) with average diameter ~23 nm synthesized by a chemical route. Magnetic studies revealed that NiO NPs are weakly ferromagnetic (FM). Notably, temperature-dependent magnetization studies show a bifurcation of the FC-ZFC curves, suggesting a competition between FM and antiferromagnetic (AFM) interactions with the blocking temperature (TB) at ~210 K. In addition, we observed field-dependent exchange bias effect in the NPs. Furthermore, MD studies revealed that the changes in the dielectric constant and loss induced by the magnetic field are strongly frequency dependent, which originates from the combined effect of extrinsic Maxwell-Wagner polarization along with magnetoresistance. It was found that the magnetoimpedance (MI) changes sign from negative at low frequencies to positive at higher frequencies of an excitation signal. Cole-Cole studies showed that the negative MI at low frequencies arises from FM surface/grain boundary, and positive MI at higher frequencies originates from AFM core of NiO NPs.

Magnetodielectric effect in a metamaterial consisting of xerogel with embedded ε-Fe2O3 iron oxide nanoparticles

The ε-Fe2O3 iron oxide is a fairly rare polymorphic modification, which only exists in the form of nanoparticles embedded, as a rule, into a silica gel matrix. This magnetically ordered iron oxide, which exhibits a significant room-temperature coercivity, is a ferroelectric; therefore, the magnetoelectric and magnetodielectric properties of this material evoke keen interest. In this work, we investigate the magnetodielectric (MD) effect in a metamaterial consisting of xerogel SiO2 with embedded ε-Fe2O3 nanoparticles 9 nm in size on average in a concentration of 20 mass.%. This bulk material exhibits the MD effect in a wide temperature range. The temperature behavior of the permittivity is related to the magnetic state of the ε-Fe2O3 oxide, which undergoes the magnetic transition from the magnetically hard to magnetically soft phase in the temperature range of 80–150 K, indicating the interplay of the ε-Fe2O3 magnetic and charge subsystems.

The origin of enhanced magnetodielectric effect in Y3-xYbxFe5O12 ceramics

In this work, dielectric and magnetic behaviors, as well as magnetodielectric (MD) effect of Yb-substituted yttrium iron garnet (Y3-xYbxFe5O12) ceramics, have been studied systematically. Three dielectric relaxations are detected from the temperature spectrums, which can be attributed to the charge carries hopping between Fe2+ and Fe3+, inhomogeneous structure, and conduction, respectively. All the samples exhibit ferrimagnetic behaviors at room temperature and the saturation magnetization (Ms) increases first and then decreases with increasing Yb-substitution. The variation of Ms can be viewed as a comprehensive effect of superexchange interaction of Fe1-O-Fe2 and double exchange interaction between Fe2+ and Fe3+ ions. The room-temperature MD effect of Y3-xYbxFe5O12 ceramics measured at 15 kOe and 1 MHz are −1.11%, −1.33%, −2.04%, −1.48%, and −1.57%, for x = 0, 0.5, 1, 2, and 3, respectively. With the analysis of XPS spectra, the origin of enhanced MD effect has been investigated, and it is mainly caused by the response of local dipoles (Fe2+-Fe3+) under applied magnetic field.

Structural, magnetic and magnetodielectric study in the layered oxide BiPbSr2MnO6

Layered oxide BiPbSr2MnO6 (BPSMO) is investigated from structural, magnetic and magnetodielectric perspective. Room temperature Rietveld refinement of neutron powder diffraction pattern shows that the compound crystallizes in orthorhombic ‘Amaa’ structure. Along with an antiferromagnetic ordering (TN) at 190 K, a weak ferromagnetic component is substantiated from magnetization isotherm measured at 175 K as well as temperature dependent neutron powder diffraction. Magnetic reflections were indexed in the nuclear lattice with propagation vector k = (0, 0, 0). Rietveld refinement of powder pattern at 2.8 K demonstrates that magnetic moments are almost normal to the a–b plane in an antiferromagnetic arrangement, but are canted slightly towards a–b plane. The Jahn–Teller distortion shows a substantial increase in the ordered state with decrease in temperature while the variation is trifling above the ordering temperature. The dielectric measurement shows a contrasting nature of the dielectric data for temperatures below and above TN, thus hinting a plausible magnetodielectric (MD) coupling. A confirmation to this coupling is seen in the temperature and field dependent magnetodielectric (MD) results. Substantial MD effect of ~ − 2% is observed at 175 K, near to the vicinity of magnetic transition.

Dielectric anomalies and robust magnetodielectricity in Y-type Ba2Mg2Fe12O22 hexaferrite

We have investigated dielectric, magnetic and magnetodielectric (MD) properties of polycrystalline Y-type hexaferrite Ba2Mg2Fe12O22 (BMFO). Rietveld refinement of x-ray diffraction pattern and hexagonal plate like Field Emission Scanning Electron Microscope (FESEM) micrograph confirms the phase purity with rhombohedral crystal structure (R-3m space group). Magnetization measurement suggests irreversibility in zero field cooled and field cooled data below 270 K whereas opening of hysteresis in isothermal magnetization (M-H plot) indicates weak ferromagnetism. Surprisingly, two distinct dielectric anomalies, near 25 K (TI) and 270 K (TII) are observed in the vicinity of magnetic spin phase transition, suggesting a strong coupling between dielectric and magnetic properties. A confirmation of this magnetodielectric coupling is seen in both temperature as well as magnetic field dependent MD results. In addition, a butterfly loop like switchable MD effect is observed below 200 K which owes to enhanced magnetic ordering and reduced leakage current. Finally, we confirm magnetic field induced MD effect below 200 K may be due to intrinsic coupling between electric and magnetic order parameter whereas above 200 K this effect is due to Maxwell-Wagner space charge polarization. The existence of substantial MD coupling (∼30%) at 15 K is obtained which is exciting and pave the way for further scientific research.

Fabrication and Characterization of Flexible Polymer Iron Oxide Composite Substrate for the Imaging Antennas of Wearable Head Imaging Systems

Given the increased interest in wearable electromagnetic imaging systems, developing a low-cost, lightweight, flexible, and conformal customized substrate to accommodate the imaging antenna array is essential. The characterization and assessment of a custom-made composite substrate using a flexible polymer poly-di-methyl-siloxane (PDMS) and magnetite iron oxide ( ${\text{FeO.Fe}}_{2}{\text{O}}_{3}$ ) for wearable head imaging systems is presented. Micro-scale ${\text{FeO.Fe}}_{2}{\text{O}}_{3}$ particles are homogeneously combined with PDMS in different ratios to build the flexible engineered magneto-dielectric (MD) composite substrate. Besides the low cost, fabrication simplicity, and durability, the magnetite ${\text{FeO.Fe}}_{2}{\text{O}}_{3}$ particles can be used to control the relative permittivity and permeability over a wide range of values to suit the proposed application. The permittivity, permeability, and losses of the developed substrate are extracted using a custom-made two-port multilayer microstrip transmission line test fixture with the help of conformal mapping algorithms. The characterization is performed across the microwave frequency range 1.2–4 GHz, which is widely adopted for head imaging. The extracted permittivity is successfully verified by using a Keysight 85070E dielectric slim probe kit.