Showing papers on "Magnetocapacitance published in 2020"

Investigation of ferrimagnetism and ferroelectricity in AlxFe2−xO3 thin films

Abstract: AlxFe2−xO3 (x-AFO) thin films, belonging to the κ-Al2O3 family, are interesting because they show room temperature ferrimagnetism and have a polar crystal structure. These types of materials are being studied to observe simultaneous ferrimagnetism and ferroelectricity, and the possibility of coupling between the two. However, it is difficult to realise ferroelectric properties at room temperature, due to the low resistivity of the films. In this work, we have optimized the synthesis conditions to obtain x-AFO (0.5 ≤ x ≤ 1) thin films with high resistance. While magnetic measurements confirmed room temperature ferrimagnetism of the films, the maximum magnetization was observed for the composition x = 0.8. In addition, the Curie temperature was found to be influenced by oxygen pressure during deposition. Ferroelectric measurements on the films showed small remnant polarization (∼0.5–2 μC cm−2). In contrast, the predicted polarization from first principles calculations was calculated to be between 21 and 26 μC cm−2. The analysis also suggested that ferroelectric domain-switching occurs through shearing of in-plane oxygen layers. The presence of multiple in-plane domains, which oppose polarization switching of adjacent domains, is suggested to be the cause of the small observed polarization. The magnetocapacitance measurements showed weak magnetic coupling with the capacitance.

Magnetic and magnetoelectric response of Gd doped nickel ferrite and barium titanate nanocomposites

Abstract: Composites of NiFe2O4 (NFO)–BaTiO3 (BTO) and NiGd0.01Fe1.99O4 (G0.01)–BTO were investigated by x-ray diffraction, magnetization, transmission electron microscopy, magnetocapacitance, and ferroelectric studies. NFO and G0.01 nanoparticles were synthesized by the sol-gel method. The crystallite size of the nanoparticles estimated from the x-ray diffraction patterns is 20–22 nm. The average crystallite sizes of NFO and G0.01 nanoparticles were estimated from the transmission electron micrographs as 26 (1) nm and 22.3 (0.3) nm, respectively. These nanoparticles were encapsulated in a BTO shell, resulting in the formation of nanocomposites. Room temperature magnetization (at 60 kOe) of G0.01 nanoparticles was found to be slightly higher than that of NFO nanoparticles, due to the larger moment of Gd3+ than that of Fe3+. Also, the magnetization of G0.01–BTO is more than that of NFO–BTO nanocomposites. The magnetoelectric effect was observed with a magnetocapacitance value of approximately −10% at 10 kHz in both the composites.

Magnetoimpedance spectroscopy of phase-separated La0.5Ca0.5MnO3 polycrystalline manganites.

Abstract: Magnetoimpedance spectroscopy was carried out on phase-separated La0.5Ca0.5MnO3 polycrystalline manganites. The La0.5Ca0.5MnO3 powder was synthesized following an adapted sol–gel route. Structural and magnetic data showed the signs of phase coexistence of ferromagnetic (FM) Pnma and charge-ordered antiferromagnetic (CO-AFM) P21/m phases. Magnetization vs. temperature (M vs. T) measurements revealed several magnetic transitions from the high temperature paramagnetic (PM) to an FM phase upon cooling (PM–FM) at ≈240 K, FM–AFM (≈170 K) and AFM–FM (≈100 K). Magnetic field (H)-dependent impedance spectroscopy data were collected from sintered pellets and fitted with an equivalent circuit model to separately analyze the different dielectric contributions from the grain boundary (GB) and the grain interior bulk areas. This allowed separating the GB and bulk magnetoresistance (MR), which was shown to amount to a maximum of ≈80% for both GB and bulk at H = 10 T near the metal–insulator transition (MIT) at ≈100 K. The GB resistance was found to be larger than the bulk resistance by a factor of ≈3, which implies that the direct current (DC) resistance and DC MR are dominated by contributions from the GBs. The magnetocapacitance (MC) effects detected were all found to be small below ≈3%, including in the presence of a CO phase.

Synthesis, sintering by Cool-SPS and characterization of A2Cu(CO3)2 (A = K, Na): evidence for multiferroic and magnetoelectric cupricarbonates

Abstract: We report here the synthesis and densification of two magnetic materials in the system A2Cu(CO3)2 (A = Na, K). Based on literature data, the synthesis route has been modified to offer the possibility of gram-scale production with high powder purity. The cool spark plasma sintering (Cool-SPS) method has been used to obtain highly dense samples (>95% of theoretical density). Not only did the sintering method prove its efficiency for giving dense ceramics, but it also extended the stability domain of both compounds to higher temperatures. The case of Na2Cu(CO3)2 is highly instructive of the possibilities offered by the Cool-SPS technique, since the pure phase can be obtained during the sintering process, while it is impossible through conventional heat treatments in a furnace. Obtaining dense ceramics allowed the exploration of dielectric properties, leading to the observation of a step-like anomaly in K2Cu(CO3)2 similar to some multiferroic systems, while Na2Cu(CO3)2 exhibits an interesting magnetocapacitance evolution, probably linked to a magnetoelectric coupling.

Room temperature multiferroism in BaCoF4 films prepared by pulsed laser deposition

Abstract: The much lower magnetic ordering temperature of multiferroic fluorides strongly limits their practical applications [J. F. Scott and R. Blinc, J. Phys. 23, 113202 (2011)]. In this paper, (010) oriented BaCoF4 films have been prepared on (0001) Al2O3 substrates by pulsed laser deposition. The clear observation of amplitude and phase hysteresis loops by piezoresponse force microscopy confirms the ferroelectricity at room temperature. In contrast to the antiferromagnetism in bulk BaCoF4 with a much lower Neel temperature of 69.6 K [Eibschutz et al., Phys. Rev. B 6, 2677 (1972)], clear magnetic hysteresis loops are observed in BaCoF4 films, indicating the weak room temperature ferromagnetism. The nearly unchanged saturated ferromagnetic magnetization of about 30 emu/cm3 between 5 K and 300 K suggests that the Curie temperature of BaCoF4 films is much higher than room temperature. Exchange bias is clearly observed, with a blocking temperature of 250 K. Magnetoelectric coupling is demonstrated by the observed magnetocapacitance effect at room temperature.

Improved room-temperature multiferroicity in Co-doped Aurivillius Sr 0.5 Bi 5.5 Fe 1.5 Ti 3.5 O 18 ceramics

Abstract: Multiferroic Sr0.5Bi5.5Fe1.5−xCoxTi3.5O18 (0 ≤ x ≤ 0.5, SBFCT-x) ceramics with layered perovskite structure were successfully prepared by the sol–gel auto-combustion method. The coexistence of ferroelectricity and ferromagnetism was observed at room temperature (RT) for all Co-doped samples. Co substitution can not only remarkably improve ferroelectricity but also enhance the ferromagnetism obviously. In particular, the SBFCT-0.4 sample shows the highest remnant magnetization (2Mr) ~ 1.82 emu/g and the largest remnant polarization (2Pr) ~ 24.4 μC/cm2. Furthermore, dielectric anomalies (x = 0.2–0.5) have been found, which can be ascribed to the long-range migration of oxygen vacancies. The effects of Co doping on ferroelectric, magnetic, and dielectric properties were discussed. The SBFCT-0.4 sample was also found to exhibit the magnetoelectric (ME) effect detectable under a low response magnetic field at RT. And the obvious magnetocapacitance (MC) performance was also observed at and above RT (373 K), which is important to potential applications in sensor technology and memory devices.

Magnetocapacitance of FC-ATiO3 (A = Ba, Ca, Sr) for supercapacitor electrode

Abstract: The effort of exploring a new stable oxide electrode for a supercapacitor has been extensively performed. For the conventional capacitor, the capacitive storage could strongly depend on high dielectric materials. Besides the high capacity, the release rate is one of the crucial issues for supercapacitors. For the latest case, we should control the optimum electrical conductivity of electrode materials, because it has an important factor to influence the high-performance supercapacitor. For those purposes, a composite combination of high ferroelectric materials of ATiO3 perovskite by introducing FC (Functionalized Carbon) with FC stands for enhancing the conductivity. The new supercapacitor materials made of FC (graphene, graphite, acetylene carbon black) and ATiO3 (A = Ba, Ca, Sr) could be a good candidate for high energy density supercapacitor. A series of ATiO3 has been synthesized using conventional solid-state reaction with intermediate calcination. The composite of FC-ATiO3 (A = Ba, Ca, Sr) with ethanol adhesive has been fabricated using conventional die pressed. The capacitance under the influence of the magnetic field was characterized using DC capacitance meter with various magnetic fields generated by a double coil magnetic generator.

Study of Structural, Magnetic, Dielectric Properties and Estimation of Magnetoeletric Coupling of La, Mn co-doped Bi 1−x La x Fe 0.97 Mn 0.03 O 3 Ceramics

Abstract: 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.

Capacitive detection of magnetic field induced quantum phase transitions in an imbalanced bilayer electron system

Abstract: Ground states that appear in a quantizing magnetic field in an imbalanced bilayer electron system hosted by a dual-gated, wide GaAs quantum well are explored with a magnetocapacitance technique that enables detection of the compressibility of each layer separately, the characterization of the charge distribution, as well as the distinction of single- or double-layer-like behavior. Magnetic field induced reentrant quantum phase transitions are observed between a compressible double-layer ground state and a single-layer-like incompressible phase for both total fillings 1 and 2. The transitions are accompanied by a charge redistribution across the well. Our observations indicate for both incompressible states easy-plane pseudospin ferromagnetism as the origin.

Tunnel magnetocapacitance in Fe/MgF2 single nanogranular layered films

Abstract: The tunnel magnetocapacitance (TMC) effect in two-dimensional single nanogranular layered Fe / MgF 2 films is investigated both experimentally and theoretically. We measured the frequency dependence of TMC ratios in a frequency range of 20 Hz–1 MHz and discovered that TMC ratios strongly depend on the frequency, reaching a peak value at a specific frequency. We observe that the largest TMC ratios occur at lower frequencies and that TMC values steadily reduce with increasing frequency. Notably, we achieved a maximum TMC ratio of 1.45%, which is the largest low-field TMC ever reported for granular films. A combination of the Debye–Frohlich (DF) model and the Julliere formula is used to fit the experimental data to theory, and an excellent agreement between the calculated values and the experimental data is obtained. To perfectly fit the experimental data, the conventional DF model is extended to a composite model in which three capacitors (with three different relaxation times) are introduced. Our findings will give further insights into the exact mechanism of the TMC effect in nanogranular films and will open broader opportunities for device applications, such as magnetic sensors and impedance tunable devices.

Dielectric relaxation and magnetodielectric response of mesoporous terbium manganate nanopaticles

Abstract: We report here the investigation on dielectric response, charge transport mechanism and magnetocapacitance of mesoporous perovskite Terbium Manganate (TMO) nanoparticles. TMO NPs possess relatively high specific surface area (20.9 m2 g−1) than that of general perovskite-type metal oxides. To study the dielectric/magnetodielectric response in TMO material, a capacitive system is fabricated. Impedence spectroscopy shows electrically heterogeneous nature of the material. Due to the existence of inhomogeneity at electrode-TMO interface, ideality factor is larger than unity. Various Schottky diode electrical parameters are obtained here from temperature dependent capacitance–voltage (C–V) measurements. The function of Au/TMO/Au Schottky diode system is well explained from current–voltage (I–V) characteristics study. Ferroelectric-like hysteretic behaviour is observed at and above room temperature from non-ferroelectric TMO. Presence of defect states at TMO interface layer dominates the charge conduction phenomenon. Negative magnetodielectric effect arises in heterogeneous TMO sample due to the effect of magnetic field on Maxwell–Wagner space charge at the interface. The thermionic current–voltage (I–V) characteristics of the Au/TMO/Au Schottky diode have been measured within the voltage range ± 40 V. Ferroelectric type hysteretic behaviour is observed in non-ferroelectric TMO sample.

Influence of Pr 3+ and Lu 3+ co-doping on the magnetoelectric coupling response in BiFeO 3 mutiferroic ceramics

Abstract: Perovskite-type bismuth ferrite (BiFeO3) multiferroic material is being considered as a potential candidate for magnetoelectric coupling and field-controlled multi-state memory applications. We report the Pr3+ and Lu3+ co-doping effect on the magnetoelectric coupling in BiFeO3 ceramics. In this study, Bi0.94Pr0.06FeO3 (BPO), Bi0.94Lu0.06FeO3 (BLO), and Bi0.94Pr0.03Lu0.03FeO3 (BPLO) ceramics were synthesized by conventional solid-state ceramic synthesis technique. The BPO sample displayed higher saturation magnetization as compared to BLO and BPLO samples which may be due to canting of spin cycloid of BFO and also due to the discrepancy between the antiparallel sublattice of Fe3+/Fe2+, the value of ferroelectric polarization of the BPO sample was found to be better than BLO and BPLO samples. However, the magnetocapacitance (MC) of the BPLO sample is found to be MC ~ 2.3% which is higher than that of BLO (MC ~ 1.4%) and BPO (MC ~ 1.7%) samples. In addition, the enhanced value of magnetoelectric coupling coefficient (αE) has been obtained for the BPLO sample. Overall, our results indicate that BPLO sample displays interesting magnetoelectric response for device applications.

Magnetocapacity of manganese sulphides substituted by thulium ions

Abstract: The capacity and the dielectric loss tangent of a TmxMn1-xS (x=0.05, x=0.1) solid solution in the frequency range (1–300) kHz without field H=0 and in a magnetic field of H=8 kOe in the temperature range (80–500) K were measured. The increase of dielectric permittivity and the maximum of dielectric losses at high temperatures were found. The shift of the maximum of the imaginary part of the permittivity towards high temperatures with increasing concentration is found. The magnetocapacitance effect for two compositions and change the sign of magnetocapacity with increasing concentration were revealed. The magnetocapacity is explained in terms of the model with orbital electron ordering.