Magnetodielectric Studies Research Articles

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.

Magnetic and magnetodielectric behavior of the Haldane spin-chain system, Ho2BaNiO5

The Haldane spin-chain compound, Ho2BaNiO5, crystallizing in an orthorhombic space group (Immm) and known to order antiferromagnetically below (TN=) 53 K, is investigated by magnetization, heat-capacity, dielectric, magnetodielectric, and pyrocurrent studies as well as neutron diffraction experiments down to 2 K. We report here another magnetic feature in the close vicinity of 12 K (as a consequence of the saturation of the magnetic moment of Ho) as revealed by qualitative changes in the behavior of isothermal magnetization when the temperature is varied across 12–15 K. Heat-capacity and magnetic susceptibility data also are consistent with this conclusion. The compound can be classified as ‘metamagnetodielectric’, when comparing the plots of dielectric constant as a function of temperature and magnetic field with the corresponding plots of magnetization. We are not able to find any evidence for ferroelectricity. This is notable considering that all the heavy rare-earth members of this family with the same crystal structure were known to be multiferroics. Pyrocurrent data however reveal a feature around 55 K, which is attributable to ‘thermally stimulated depolarization current’.

Experimental and theoretical interpretation of magnetic ground state of FeMnO3

Magnetic structure of FeMnO3 has been studied using temperature dependent neutron diffraction measurements. The compound exhibits ferrimagnetic structure at room temperature and undergoes another long range antiferromagnetic order below 36 K. The evolution of the magnetic structure is tracked as a function of temperature and compared with the physical property measurements such as magnetization and dielectric constants. Mössbauer spectroscopy measurements at selected temperatures and different magnetic fields exhibit strong influence of the magnetic field on the hyperfine splitting in the magnetically ordered state. Neutron diffraction experiments used for studying the magnetic structure also reveals the presence of a strong magnetically disordered phase in addition to the long range ferrimagnetic phase, leading to magnetic anisotropy in the system. Theoretical calculations were carried out to study ordered and disordered FeMnO3 models by means of Density Functional Theory using PBE0 hybrid functional. The magnetic structure resolved from the refinement of neutron diffraction data is in good agreement with the theoretical model for FeMnO3 model, and the values obtained from the magnetization measurements are in good agreement with the theoretically calculated and experimentally refined magnetic structural model. Dielectric measurements on FeMnO3 exhibits phase transition at two temperatures, firstly around 36 K coinciding with the magnetic ordering temperature, and another around 100 K. The influence of magnetic field does not change these temperatures much, but there is slight change in the dielectric response under the applied magnetic field, which can be attributed to the change in magnetodielectric effect from positive to negative with increase in the temperature. The magnetodielectric study shows the presence of positive and negative magnetodielectric values in this system where the cross over takes placed around the magnetic ordering temperature. From a combined look at the theoretical and experimental work, it can be said that FeMnO3 exhibits complex magnetism with interesting, but tunable physical properties.

Ferroelectric and magneto-dielectric properties of yttrium doped BaTiO3–CoFe2O4 multiferroic composite

Multiferroic composites of ferroelectric and ferrite phases having general formula xCoY0.1Fe1.9O4—(1 − x) Ba0.95Y0.05TiO3 (where x = 0.05, 0.1 and 0.15) were prepared using the conventional solid-state reaction method. X-ray diffraction studies were done to confirm the presence of constituent phases. The microstructural analysis revealed an increase in density with the increase of ferrite content in the ferroelectric matrix. Dielectric studies of the composites, in the temperature range 100–550 K revealed two ferroelectric phase transitions. Variation of dielectric constant and dielectric loss with frequency in the range of 20–3 MHz was carried out at room temperature. The low-temperature dc conductivity behaviour follows Motts law, confirming the variable range hopping mechanism in all the composites. All the composites showed P–E and M–H hysteresis loops; which confirm the ferroelectric and ferrimagnetic nature of the composites. At temperatures below 173 K, an increase in coercivity and saturation magnetization is observed due to frozen spins. The coupling between ferroelectric and ferromagnetic ordering was confirmed by room temperature magneto-dielectric studies. The decrease in real part of dielectric constant and dielectric loss was observed with an increase in the applied magnetic field. An appreciable increase in percentage magneto-capacitance was observed at lower frequencies and with the increase of ferrite content in the composites. The magneto-electric coupling coefficient was calculated by using the expansion of the thermodynamic potential φ (for x = 0.15) and was found to be 3.397 × 10−2 (emu/g)−2.

Novel multiferroicity in orthorhombic SmCrO3

This paper reports the ferroelectricity in antiferromagnetic SmCrO3 with a reasonably large value of spontaneous electric polarization (70 μC/cm2 mC cm−2 at 50 K) based on the measurement of pyroelectric current. The experimental results showed that the temperature of the onset of polar order was evidently higher (about 220 K) than the Neel temperature (TN =197 K) in the magnetodielectric studies. The calculated magnetodielectric effect (MDE) value was explicitly negative with maximum | MDE | values of about 3.6% around 230 K. The absence of a convincing signature in MDE at antiferromagnetic ordering temperature TN indicated the absence of magnetoelectric coupling around this temperature regime. The results further confirmed that the onset of polar order can be attributed to the structural distortion, rather than the canted noncollinear magnetic structure.

Magneto-dielectric studies on multiferroic composites of Pr doped CoFe2O4 and Yb doped PbZrTiO3

Multiferroic composites with remarkable magnetoelectric effect (ME) are unique candidates for multifunctional devices. This study focuses on the structural, dielectric, ferroelectric, magnetic and ME properties of ferromagnetic (Pr doped CoFe2O4) and ferroelectric (Yb doped PbZrTiO3) composites with the general formula x CoPr0.1Fe1.9O4- (1-x) Pb0.95Yb0.05Zr0.52Ti0.48O3 (x = 0.02, 0.05 and 0.1). The individual phases namely Pr doped CoFe2O4 (CFO) and Yb doped PbZrTiO3 (PZT) for the composites were synthesized by sol-gel auto combustion technique. The powder X-ray diffraction technique showed that the Yb doped PZT retains its tetragonal perovskite structure, while as Pr doped CFO possesses cubic spinel structure. The morphological studies showed a uniform distribution of grains of parent phases in the composites. The dielectric studies in the temperature range of 100–750 K at a selected frequency (100 kHz) revealed a diffused phase transition confirming a ferroelectric to paraelectric phase transition. The composites show well-saturated P-E loops. The room temperature magnetic studies exhibit saturated M-H loops, with dilution of magnetic parameters for the composite. A favorable decrease in magneto-crystalline anisotropy calculated by Law of Approach was observed with decreasing ferrite content in the composite. The magneto-dielectric studies at room temperature confirm a negative magnetoelectric coupling in the composite. The Landau free energy approach confirmed a bi-quadratic coupling in the composites.

Structurally tuned multifunctional characteristics of Nd-substituted lanthanum zirconates

Phase stable Nd-substituted lanthanum zirconates of the form La2−xNdxZr2O7 (x = 0.0, 0.2, 0.4, 0.6 and 2.0) with tunable structural parameters, were synthesized using a cost effective, wet chemical based sol–gel auto-combustion route. Rietveld’s refined X-ray diffraction patterns confirmed the formation of phase pure cubic pyrochlore structure with space group Fd3m. Data obtained from the refinement revealed that Nd-substitution at La-site reduced the lattice parameters from 10.782 to 10.637 A and thus caused the shrinkage of the unit cell volume from 1253.452 to 1203.820 A3. Field emission scanning electron microscopy operated in back scattered electron mode at 5.0 kV confirmed that Nd-substitution promoted the grain growth (77–430 nm) and densification of the prepared ceramics. Ferroelectric analysis showed that pure as well as Nd-substituted samples exhibited leaky ferroelectric loops at room temperature however, maximum value of polarization increased with substitution. Smaller values of coercivity and remanence obtained in the magnetic investigation suggested the superparamagnetic nature of the prepared samples. A profound effect of magnetic field on dielectric properties was also observed from the magnetodielectric study. The probable origin of magnetodielectric effect is discussed on the basis of dielectric study in the presence of magnetic field.

Enhanced antiferromagnetic transition and magnetodielectric study in cobalt and holmium co-substituted multiferroic Bi2Fe4O9

The search for new room temperature magnetoelectric materials for future multifunctional device application is the motivation behind the preparation of polycrystalline Bi2(1−x)Ho2xFe4(1−y)Co4yO9 (x = y = 0; x = 0, y = 0.02; x = 0.02, y = 0 and x = y = 0.02) ceramics. Along with substantial variation in antiferromagnetic transition (TN) in substituted samples (BFCO2, BHFO2), the co-substituted sample (BHFCO2) shows an enhanced TN at room temperature i.e. 288 K as compared to parent BFO (250 K). All the samples except BHFO2, depicts a clear irreversibility in ZFC-FCW for T < 360 K along with an opening in MH curves, thus indicating the onset of weak ferromagnetism associated with magnetic glassiness in the studied samples. The dielectric measurements reveal an unusual oppression/hump around TN, thereby providing evidence of magnetodielectric (MD) coupling, without any possibility of magnetoresistance. Moreover, a dielectric crossover at T ∼ 200 K is seen for BHFO2 and BHFCO2 while the mechanism is explained using a mean-field approximation model, thus validating the presence of isolated defects due to Ho3+. Lastly, all the samples are subjected to MD study, where the co-substituted sample exhibited ∼ −0.4% (∼7.5 times) of MD% effect at 288 K as compared to BFO (∼ −0.06%). Confirmation to the MD coupling is further established from MD% versus T plot and MD% versus H plot where later is found to obey behavior, with ‘m’ value being comparable to exponent (1-p) of thermoremanent magnetization.

Electric, Magnetic, and Magnetoelectric Properties of Yttrium-Containing BaY0.025Ti0.9625O3–SrFe12O19 Composite

The physical properties of BaY0.025Ti0.9625O3, SrFe12O19, and 0.90BaY0.025Ti0.9625O3–0.10 SrFe12O19 composite have been studied. The proposed composite was synthesized by solid-state reaction method from yttrium barium titanate processed by solid-state reaction and strontium hexaferrite obtained by a sol–gel process. Microstructural analysis revealed monophasic grains for yttrium barium titanate phase, while loosely packed biphasic structure was observed for the composite. Powder x-ray analysis showed that the individual phases retained their crystal structure in the composite, without formation of any new additional phase. Measurement of magnetic hysteresis loops at room temperature indicated that the magnetic parameters of the composite were diluted by the presence of the ferroelectric phase. The ferroelectric hysteresis of yttrium barium titanate confirmed the ferroelectric transition at 119°C. Meanwhile, the symmetrical ferroelectric loops observed at different fields established the ferroelectric nature of the composite. Improved dielectric properties and low dielectric losses were observed due to yttrium doping in the composite. The diffuseness of the ferroelectric transitions for the composite was confirmed by the Curie–Weiss law. Activation energy calculations revealed the charge-hopping conduction mechanism in the composite. Magnetodielectric studies confirmed that the overall magnetocapacitance in the composite exhibited combined effects of magnetoresistance and magnetoelectric coupling.

Synthesis and characterization of Ni-Cu-Zn ferrites by microwave reactive sintering technique

ABSTRACTNi0.3Cu0.08Zn0.62Fe2O4 (NCZF) ceramics were prepared by solid state reaction route and calcined at temperatures at (i) 600°C for 30 minutes and (ii) 900°C for 15 minutes in microwave furnace. Room temperature X-ray diffraction analysis of microwave sintered samples calcined at 900°C showed single phase cubic structure while, secondary peaks of iron oxide (Fe2O3) were found in sample calcined at 600°C. Single phase cubic structure of the sintered calcined at 600°C confirmed the microwave reactive sintering. Frequency and temperature dependent dielectric behavior and magneto-dielectric study showed comparable properties of both the microwave sintered samples.

Magnetodielectric coupling in multiferroic holmium iron garnets

Single phase magneto-electric multiferroics require a large magnetic or electric field for producing magneto-electric (ME) and magnetodielectric (MD) effects. For utilizing these effects in devices investigations on the room temperature and low field MD studies are necessary. Recently, efforts have been largely devoted to the investigation of rare earth iron garnets. In the physical method, the preparation of rare earth iron garnet requires high sintering temperature and processing time. To solve these problems, ball milling assisted microwave sintering technique is used to prepare nanocrystalline holmium iron garnets (Ho3Fe5O12). Magnetic and dielectric properties of the prepared sample are investigated. These properties get enhanced in nanocrystalline form when compared to the bulk. The MD coupling of the prepared sample is evident from the anomaly in the temperature dependent dielectric constant plot and the ME coupling susceptibility is derived from the room temperature MD measurements.

Room-temperature magneto-dielectric response in multiferroic ZnFe2O4/PMN-PT bilayer thin films

The magneto-dielectric response in multiferroic ZnFe2O4/PMN-PT bilayer thin films prepared on a glass substrate using RF magnetron sputtering has been investigated in this work. PMN-PT thin films (i.e. PMN-PT/LCMO/Pt/Ti/glass) deposited on glass were used as a substrate for deposition of ZnFe2O4 thin films. ZnFe2O4 thin films were annealed ex situ at different temperatures. Structural, magnetic, ferroelectric, dielectric and magneto-dielectric studies were carried out on these multiferroic bilayer thin films. Structural studies revealed the presence of each layer in its respective single phase. Magnetic and ferroelectric studies revealed the ferromagnetic and ferroelectric behaviors of these bilayers. To quantify the magnetoelectric coupling, the dielectric constant of the bilayer was measured at room temperature as a function of frequency with and without the applied magnetic field. The magneto-dielectric response MD(%) was calculated by finding the relative change in dielectric constant at 1 kHz as a percentage. The observed MD response was correlated with magnetization of the ferrite layer. An MD response of 2.60% was found for a bilayer film annealed at 350 °C. At this particular annealing temperature, the ZnFe2O4 layer also has the highest saturation magnetization of 1900 G.

Structural, magnetic, dielectric and magneto-dielectric coupling analysis of ferromagnetic-PbZr0.52Ti0.48O3 nanocomposites

Nanocomposites of La0.7Sr0.3MnO3–PbZr0.52Ti0.48O3 (LSMO–PZT) were synthesized by sol–gel route. The powder X-ray diffraction study confirmed the phase purity and the particle size (≈30–34 nm) of the nanocomposites. Magnetic, dielectric and magneto-dielectric studies of LSMO–PZT nanocomposites reflected a coupling between dielectric and magnetic parameters near the magnetic transition temperature (Tc ≈ 360 K). The quantitative magneto-dielectric coupling was found to be maximum value of 1.34 in magnitude at 1.5 T and decrease with increase in frequency. This provides the signature of magneto-dielectric coupling qualitatively as well as quantitatively in LSMO–PZT nanocomposites.

Dielectric, magnetic and enhanced magnetoelectric response in high energy ball milling assisted BST-NZF particulate composite

We report the magnetoelectric (ME) response in 0–3 particulate composites of Ba0.9Sr0.1TiO3–Ni0.9Zn0.1Fe1.98O4−δ assisted by high energy ball milling. Dielectric and magnetic characterizations are found to follow combination and dilution properties, respectively. ME voltage coefficient (αE) was found to increase with ferroelectric incorporation in composites and substantiated by the deviation of magnetic moment. The highest achieved αE (144 mV/cm.Oe) was obtained with 80 wt. % of ferroelectric incorporation in composite at room temperature, which is significantly higher than previous reported value (533 μV/cm.Oe) in BST-NF composite. The origin of enhanced response has been addressed with the help of Magnetodielectric (MD) study.

Interaction Between Optically-Generated Charge-Transfer States and Magnetized Charge-Transfer States toward Magneto-Electric Coupling.

This article reports the magneto-dielectric studies on the coupling between optically generated CT states and magnetized CT states based on thin-film devices with the architecture of ITO/TPD:BBOT/TPD/Co/Al. The magnetized CT states are generated at the Co/TPD interface, generating a magneto-dielectric response with a broad, non-Lorentzian line-shape. The optically generated CT states are formed at the TPD:BBOT interfaces in the heterojunction under photoexcitation, leading to a magneto-dielectric signal with a narrow, Lorentzian line-shape. We find that combining the optically generated CT states and magnetized CT states yields a new magneto-dielectric signal with distinctive line-shape and amplitude in the ITO/TPD:BBOT/TPD/Co/Al device. The magneto-dielectric analysis indicates that there exists a coupling between optically generated CT states and magnetized CT states through the interactions between the magnetic Co/TPD interface and the optically excited TPD:BBOT heterojunction. Furthermore, we show that the coupling between optically generated CT states and magnetized CT states experiences Coulomb interactions and spin-orbital interaction by changing (i) the density of optically generated CT states and (ii) the separation distance between optically generated CT states and magnetized CT states. Clearly, this coupling provides a new approach to mutually tune magnetic and electronic properties through thin-film engineering by combining magnetic and organic materials.

Giant dielectric response and magnetoelectric behavior of 95BiFeO3–5BaTiO3 (95BFO–5BT) ceramics

A detailed study of electrical behavior of 95BiFeO3–5BaTiO3 (95BFO–5BT) ceramics through comprehensive analysis of temperature and frequency dependent dielectric behavior, ac impedance and magnetodielectric (MD) properties is reported here. Addition of insulating BaTiO3 into BiFeO3 have exhibited enhanced dielectric and ferromagnetic responses. The remnant polarization (Pr) was found to be 6 µC/cm2, while the dielectric constant was found to be very high ( $$\varepsilon_{\text{r}}^{\prime }$$ ≥ 102, for T ≥ 140 °C). Along with strong conductivity contribution to $$\varepsilon_{\text{r}}^{\prime }$$ , spectroscopic plots of $$\varepsilon_{\text{r}}^{\prime }$$ revealed the Maxwell–Wagner type relaxation in the sample, which ultimately could have led to an apparent high $$\varepsilon_{\text{r}}^{\prime }$$ in 95BFO–5BT ceramics. Origin of such a high $$\varepsilon_{\text{r}}^{\prime }$$ in BFO–BT was further investigated using impedance spectroscopy. MD studies revealed the magneto-electric coupling in the ceramics. The MD studies along with anomaly in dielectric data near antiferromagnetic ordering temperature (TN = 170 °C) of BFO suggests the magnetic bearing on the electrical properties of 95BFO–5BT ceramics.

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.

Ferroelectric ordering and magnetoelectric effect of pristine and Ho-doped orthorhombic DyMnO3 by dielectric studies

In this paper, the magnetoelectric coupling and ferroelectric ordering of the orthorhombic Dy1-xHoxMnO3 (x = 0 and 0.1) are studied from the magnetodielectric response of the polycrystalline samples. The dielectric study on the DyMnO3 reveals ferroelectric transition at 18 K along with an addition transition at 12 K. We suggest that the transition at 12 K could have originated from the polarization flop rather than being the rare earth magnetic ordering. The magnetodielectric study reveals a magnetoelectric coupling strength of 10%, which is stronger by two orders of magnitude in comparison to the hexagonal manganites. Surprisingly, the Ho3+ substitution in DyMnO3 suppresses the magnetoelectric coupling strength via the suppression of the spiral magnetic ordering. In addition, it also reduces the antiferromagnetic ordering and ferroelectric ordering temperatures. Overall, the studies show that the rare earth plays an important role in the magnetoelectric coupling strength through the modulation of spiral magnetic structure.

Magnetic, magnetoelectric, magnetodielectric and magnetoresistance studies on CuO doped Sr2Bi4Ti5O18 lead free ferroelectric ceramics

As an effort to develop new multiferroic materials at room temperature, we have interstitially doped (x mol %) low dimensional CuO with strong superexchange interactions into lead-free ferroelectric Sr2Bi4Ti5O18 (SBT) ceramics. Upon increase of CuO doping concentration (x = 0, 0.25, 0.5, 0.75 & 1) in SBT + x mol % compound a small and noticeable, but not appreciable, SrTiO3 secondary phase segregates in the SBT. Thus, CuO doping induces SrTiO3 phase and induced volume increase with “x”. The undoped SBT shows weak magnetization (μemu/g) at room temperature and magnetization enhances three-fold (memu/g) with CuO doping in SBT. The thermomagnetic studies of all the samples under zero field cooled, and field cooled modes show typical paramagnetic nature below room temperature and no spin glass-like phases. The Curie–Weiss law analysis suggest that antiferromagnetic interactions are predominant at low temperature in all the CuO doped samples except x = 0.75, which show ferromagnetic interactions. In the direct test for magnetoelectric properties, all the samples show similar magnetoelectric coupling coefficient (α). Among all, the x = 1 sample shows relatively low “α” indicating the magnetoelectric coupling weakens at high CuO concentration. Further, in an indirect test (magnetocapacitance) for magnetoelectric coupling shows similar to magnetoelectric behavior. As reported, the magnetocapacitance may originate from magnetoresistance (MR) and Maxwell–Wagner polarization effect. Our low field MR study on all samples suggests that large magnetocapacitance in x = 1 sample is not due to the extrinsic phenomenon. However further studies are essential to confirm our present inference.

Dielectric modulus and magnetocapacitance behavior of Bi3.7Sm0.3Ti2.7Fe0.3O12 multiferroic

This communication presents the dielectric and magnetodielectric studies of Bi3.7Sm0.3Ti2.7Fe0.3O12 ceramic samples synthesized by a conventional solid state reaction method. X-ray diffraction (XRD) patterns reveal the formation of single phase orthorhombic structure. Dielectric measurements have shown anomalous behavior around 150°C. Dielectric modulus analysis confirms the non-Debye type relaxation behavior at higher temperature. Room temperature weak ferromagnetism has been observed for Sm/Fe modified bismuth titanate ceramic with 2Mr value of 5.4×10−3emu/g. Magnetodielectric measurement shows the variation in dielectric constant value with applied magnetic field of 5T, which confirms the magnetodielectric coupling in the system. The present results suggest a new material for room temperature multiferroic applications.