Multiferroic La0 Research Articles

Study of spin-phonon interaction in multiferroic La0.9Bi0.1CrO3 by Raman spectroscopy

Spin-phonon interaction is an interesting phenomenon that may find applications in spintronics, quantum information processing, etc. In multiferroic materials, spin-phonon interaction, if exists, may provide additional degrees of freedom that could lead to the development of novel technologies. Thus, investigating and understanding this interaction in a multiferroic material is of both fundamental and applied interests. Perovskite La1-xBixCrO3 is a mutiferroic material which turns from antiferromagnetic/antiferroelectric to ferromagnetic/ferroelectric with 10 % substitution of Bi3+ for La3+. Raman spectrum feature changes dramatically with 10 % substitution of Bi3+ for La3+ also, indicating a spin-phonon interaction. Furthermore, in La0.9Bi0.1CrO3, the ∼700 cm−1 mode is split into two Raman sub-modes. The intensity of the lower frequency (680 cm−1) sub-mode starts increasing sharply around the Néel temperature TN = 269 K upon cooling. Below TN, the monotonic change in intensity reflects the magnetic exchange modulation, resulted from the interacting with the chromium atoms oscillation at the original Cr4+ position in LaCrO3, related to the competing ferromagnetic and antiferromagnetic interactions, with the former prevailing at low temperatures. The interplay of magnetic exchange modulation interactions and chromium atoms oscillation (decreasing of the lower frequency sub-mode of the Bi3+ doping related John-Teller-like 700 cm−1 mode) coincides with the corresponding magnetic phase transition (TN), demonstrating a spin-phonon coupling in La0.9Bi0.1CrO3 which may find applications in magnonic devices.

Full Antiferroelectric Performance and GMR Effect in Multiferroic La0.75Ba0.25Fe12O19 Ceramic

The potential application of multiferroic materials in new electronic devices attracts more and more attention from people either in an academic field or industry. This paper reports that M-type lanthanum-doped barium ferrite (La0.75Ba0.25Fe12O19) demonstrates full antiferroelectric (AFE) and excellent magnetoelectric coupling effects at room temperature, while its AFE phase displays a zero macroscopic net polarization. The dramatic change in the dielectric constant near the Curie temperature far below room temperature represents the transition from ferroelectrics (FE) to antiferroelectrics. The fully separated double electric polarization hysteresis (P–E) loops confirmed its AFE performance. Its EF and EA are located at 1100 kV/cm and 850 kV/cm, respectively. The large M–H loop showed a strong magnetic property simultaneously. The UV-Vis-NIR optical spectrum revealed that La0.75Ba0.25Fe12O19 is also a semiconductor, whose direct bandgap energy (Eg) was determined to be 1.753 eV. Meanwhile, La0.75Ba0.25Fe12O19 showed strong ME coupling and a GMR effect. A 1.1 T magnetic field reduced its resistance by 110% at 30 kHz. The multiple functions combined in one phase would create new options for high energy storage capacitors, microactuators, pyroelectric safety sensors, cooling devices, and pulsed power generators and so on, as well as great opportunities for generating new electronic devices with active magnetoelectric coupling effects.

Tuning electronic transport through magnetic field and magnetoelectric coupling of multiferroic nanocomposites

Detailed analysis on structural, dielectric and ac electrical transport with dc magnetic field variation and magnetoelectric response are studied on semiconductor based multiferroic La0.7Sr0.3MnO3 - ZnO nanocomposites. The impedance data is observed to gradually enhance with magnetic field due to the strain developed in the sample, hampering the charge flow in the system. The nanocomposites display prominent magneto-impedance (∼87%) and magnetodielectric (∼35%) at room temperature. Nyquist plots are fitted utilizing two resistance-capacitor parallel circuits at varying magnetic fields demonstrating the governing role of La0.7Sr0.3MnO3 and ZnO grain boundaries. The grain boundary resistance of La0.7Sr0.3MnO3 decreases with field due to the depinning of domain walls from the grain boundary pinning centers and therefore improving the spin dependent transport mechanism while the grain boundary resistance of ZnO increases with field due to the scattering of the charge carrier. The magnetoelectric coupling investigated at a frequency of 1 kHz is accomplished in both longitudinal and transverse mode. Further, the magnetoelectric voltage induced by ac magnetic field executes a linear magnetoelectric response. The viewed magnetoelectric coupling might be in consequence of the field influenced magnetostriction of the piezomagnetic element of the composites. Such multiferroic material with tuning ac electrical property with magnetic field alongside magnetoelectric response can be pertinent in future solid-state microelectronic devices and sensor applications.

Effect of Bi-substitution into the A-site of multiferroic La0.8Ca0.2FeO3 on structural, electrical and dielectric properties.

(La0.8Ca0.2)1−xBixFeO3 (x = 0.00, 0.05, 0.10, 0.15 and 0.20) (LCBFO) multiferroic compounds have been prepared by the sol–gel method and calcined at 800 °C. X-ray diffraction results have shown that all samples crystallise in the orthorhombic structure with the Pnma space group. Electrical and dielectric characterizations of the synthesized materials have been performed using complex impedance spectroscopy techniques in the frequency range from 100 Hz to 1 MHz and in a temperature range from 170 to 300 K. The ac-conductivity spectra have been analysed using Jonscher's power law σ(ω) = σdc + Aωs, where the power law exponent (s) increases with the temperature. The imaginary part of the complex impedance (Z′′) was found to be frequency dependent and shows relaxation peaks that move towards higher frequencies with the increase of the temperature. The relaxation activation energy deduced from the Z′′ vs. frequency plots was similar to the conduction activation energy obtained from the conductivity. Hence, the relaxation process and the conduction mechanism may be attributed to the same type of charge carriers. The Nyquist plots (Z′′ vs. Z′) at different temperatures revealed the appearance of two semi-circular arcs corresponding to grain and grain boundary contributions.

Anomalous freezing of dielectric polarons near magnetic ordering in multiferroic La0.5Bi0.5FeO3

A single-phase multiferroic (MF) La0.5Bi0.5FeO3 (LBFO) ceramic sample was prepared by the conventional solid-state reaction method. The crystallographic phase formation and morphological studies were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The temperature-dependent magnetization exhibits weak ferromagnetic ordering near TC = 220 K. Complex dielectric and impedance properties of LBFO as a function of temperature and frequency were performed to understand the correlation between magnetic and dielectric properties. The analysis of electrical modulus spectrum reveals that the magnetic ordering imposes a change in the activation energy of dielectric polarons. The thermal fluctuations of magnetic moments activate the freezing of dielectric polarons with enhancement in temperature.

Thin film growth of multiferroic La0.8Bi0.2Fe0.7Mn0.3O3 on LaNiO3 using RF sputtering and its characterization

Single phase multiferroic La0.8Bi0.2Fe0.7Mn0.3O3 thin film is grown successfully on LaNiO3 buffer with LaAlO3 as substrate using RF sputtering. Structural studies show the presence of strain in the thin film with slight expansion in unit cell compared to bulk form. Root mean square (rms) value of surface roughness is ~1.703nm with grain size of ~20nm. Dielectric studies reveal the dispersion behaviour. Frequency and temperature dependent decrease of dielectric loss is conducive for the device applications. Capacitance vs voltage (0±5V) hysteresis at 350K, 100kHz confirms the ferroelectric nature. Magnetization measurement confirms the ferrimagnetism in thin film. Presence of ferroelectricity and ferrimagnetism authenticate the multiferroic nature of thin film.

Effect of La and Sr co-doping on electric and magnetic properties of multiferroic La0.1Bi0.9−xSrxFeO(3−x/2) ceramics

[Formula: see text] ceramics were synthesized by solid-state reaction method, and their magnetic properties and conductive characteristics were investigated and discussed. It is found that La-doped [Formula: see text] ceramic without [Formula: see text] doping is a rhombohedrally distorted perovskite structure, and with the increase of the [Formula: see text] concentration, the phase transits gradually from rhombohedral to pseudo cubic symmetry. Electrical and magnetic properties show strong dependence on [Formula: see text] dopant level. The measurement demonstrates that [Formula: see text] with 30% [Formula: see text] dopant exhibits the lowest values of the leakage current, dielectric constant [Formula: see text] and dielectric loss [Formula: see text]. And the highest value of saturation magnetization of about 4.5 emu/g is observed in 60% Sr-doped [Formula: see text]. An abnormal enhancement of the conductivity was observed in [Formula: see text] ceramics with [Formula: see text] = 0.60 and 0.70, and their magnitude of conductivity is nearly six orders larger than that of pure [Formula: see text].

Magneto-electric coupling study in multiferroic La0.7Ba0.3MnO3–BaTiO3 composite ceramic at room temperature

A multiferroic La0.7Ba0.3MnO3–BaTiO3 composite sample was prepared by the solid state method. The composite exhibits a ferroelectric feature (2Pr=10.52µC/cm2, 2Ec=32.2kV/cm) and weak ferromagnetic feature (2Mr=13.6memu/g, 2Hc=60.5Oe). More importantly magneto-electric coupling, as well as magneto-capacitance and magneto-impedance effects, was also observed. The average changes of the 2Pr and 2Ec were −21.1% and −9.2%, respectively, when a relatively low external magnetic field (H=200Oe) was applied. The MC value was ~−1.9% at 1MHz after the sample was magnetized. These ME effects were mainly originated from the coexistence of magnetostriction and magneto-resistance in LBMO.

Direct observation of temperature dependent magnetic domain structure of the multiferroic La0.66Sr0.34MnO3/BiFeO3 bilayer system by x-ray linear dichroism- and x-ray magnetic circular dichroism-photoemission electron microscopy

Low-thickness La0.66Sr0.34MnO3 (LSMO)/BiFeO3 (BFO) thin film samples deposited on SrTiO3 were imaged by high resolution x-ray microscopy at different temperatures. The ultra-thin thickness of the top layer allows to image both the ferromagnetic domain structure of LSMO and the multiferroic domain structure of the buried BFO layer, opening a path to a direct observation of coupling at the interface on a microscopic level. By comparing the domain size and structure of the BFO and LSMO, we observed that, in contrast to LSMO single layers, LSMO/BFO multilayers show a strong temperature dependence of the ferromagnetic domain structure of the LSMO. Particularly, at 40 K, a similar domain size for BFO and LSMO is observed. This indicates a persistence of exchange coupling on the microscopic scale at a temperature, where the exchange bias as determined by magnetometer measurements is vanishing.

Heat capacity, thermopower and magnetoresistance effects in multiferroic La0.5Bi0.5Mn0.5Fe0.5O3

We have carried out systematic investigations in perovskite multiferroic La0.5Bi0.5Mn0.5Fe0.5O3 by means of X-ray diffraction, magnetisation, electrical resistivity, thermoelectric and heat capacity measurements. The magnetic behaviour of this composition is rather complex, though the magnetisation curve seems to be like a weak ferromagnetic material. However, there is no clear evidence of λ-anomaly in the heat capacity data down to 2 K, yet this behaviour corroborate the trends of semiconducting silicon below room temperature. The sensitivity of magnetic behaviour to the iron-manganese ratio is also demonstrated. In presence of an external field of 7 T, it exhibits a magnetoresistance of −5 % at 130 K. The thermoelectric value increases linearly with decreasing temperature, and at room temperature the value is +85 μV/K, which is associated with the p-type polaronic conductivity.

A comparative study of magnetic and dielectric behaviors for La1−xBixMn1−yFeyO3 series (with x = 0.5, 0.7 and y = 0.3, 0.7)

We have carried out an extensive investigation into the effect of doping on both the A- and B-sites for the multiferroic La0.5Bi0.5Mn0.5Fe0.5O3 in relation to its physical properties. The temperature dependent magnetization and dielectric response are determined for different percentages of Bi- and Fe-substitutions. For La0.5Bi0.5Mn0.7Fe0.3O3, there is a prominent ferromagnetic transition TC around 110 K, whereas the other La0.5Bi0.5Mn0.3Fe0.7O3 and La0.3Bi0.7Mn0.3Fe0.7O3 phases fail to exhibit any clear transition. On the other hand, for the Fe-rich phases, the coercive field increases to 2450 Oe compared to 1720 Oe (for the Mn-rich phase). All the compositions exhibit coexistence of ferromagnetic and antiferromagnetic phases at low temperatures. The temperature dependent dielectric constant of the investigated samples varies from 32 000 to 500 at room temperature and the data has been analyzed using the universal dielectric response model.

Electrical reliability and leakage mechanisms in highly resistive multiferroic La0.1Bi0.9FeO3 ceramics

Multiferroic La0.1Bi0.9FeO3 (LBFO) ceramics with high resistivity were synthesized by using a modified rapid thermal process. The LBFO ceramics show very low leakage and low dielectric loss. Well saturated ferroelectric hysteresis loops and polarization switching currents have been observed. For a maximum applied electric field of 145 kV/cm, the remanent polarization is 17.8 μC/cm2 and the coercive filed is 75 kV/cm. The dominant conduction mechanism in the LBFO ceramics has been found to be the space-charge-limited current mechanism rather than the thermal excitation mechanism. Electrical reliability related to the fatigue and polarization retention of the LBFO ceramics has also been discussed with the leakage mechanisms.

Structural, dielectric, and magnetic properties of La0.8Bi0.2Fe1−xMnxO3 (0.0≤x≤0.4) multiferroics

Polycrystalline multiferroic La0.8Bi0.2Fe1−xMnxO3 (0.0≤x≤0.4) samples were synthesized by the conventional solid state reaction method. Reitveld refinement of the x-ray diffraction patterns confirms the single phase character of all the compositions with orthorhombic structure having space group Pnma (No. 62). Dielectric properties of the samples at temperatures 200–475 K and frequencies 500 kHz–1 MHz authenticate the stabilization of ferroelectric phase with Mn substitution. Dielectric responses of these multiferroics have been analyzed carefully, in the light of “universal dielectric response” model. While cooling from room temperature to 20 K, systematic shifts in magnetization hysteresis loops indicate the presence of exchange bias (EB) phenomenon in the system. Magnetic behavior of these samples has been briefly discussed on the basis of “EB” model for granular systems. Temperature and magnetic field dependent magnetization data demonstrate enhanced magnetization due to the Mn substitution. Magnetocapacitance measurement reveals the magnetoelectric coupling for wide range of temperature (180–280 K) and decrease in dielectric loss at high magnetic field (3 T).

Correlation between structure and properties in multiferroic La0.7Ca0.3MnO3∕BaTiO3 superlattices

Superlattices composed of ferromagnetics, namely La0.7Ca0.3MnO3 (LCMO), and ferroelectrics, namely, BaTiO3 (BTO), were grown on SrTiO3 at 720 °C by a pulsed laser deposition process. While the out-of-plane lattice parameters of the superlattices, as extracted from the x-ray diffraction studies, were found to be dependent on the BTO layer thickness, the in-plane lattice parameter is almost constant. The evolution of the strains, their nature, and their distribution in the samples were examined by the conventional sin2ψ method. The effects of structural variation on the physical properties, as well as the possible role of the strain on inducing the multiferroism in the superlattices, have also been discussed.