R. Ranjith

Periodicity dependence of the ferroelectric properties in BiFeO3∕SrTiO3 multiferroic superlattices

Artificial superlattices of (BiFeO3)m(SrTiO3)m (m=1–10 unit cells) consisting of multiferroic BiFeO3 and insulating SrTiO3 layers were fabricated on (100)-oriented SrTiO3 substrates by pulsed laser ablation. The remnant polarization and leakage current behavior of these films were studied by varying the periodicity (8–80A) of the superlattice. Compared to single layer BiFeO3 thin films, the leakage current was reduced by a few orders of magnitude on increasing the periodicity. Reduced leakage and intrinsic polarization hysteresis were observed and confirmed by pulsed polarization analysis for all periodicities (∼20–60A). The leakage current was dominated by space charge limited conduction.

Dc leakage behavior and conduction mechanism in (BiFeO3)m(SrTiO3)m superlattices

Leakage current behavior of (BiFeO3)m(SrTiO3)m superlattice structures was studied and analyzed at different temperatures (303–473K) in the light of various models. While bulk limited Poole–Frenkel emission was observed to dominate the leakage current in the temperature range of 303–383K, the space charge limited conduction was observed up to 473K. With a Poole–Frenkel emission type of conduction, the activation energy range of ∼0.06–0.25eV was calculated. The physical parameters, calculated from the analysis, correlate with the intrinsic properties. Such analysis of leakage current facilitates interface engineering of heterostructures for device applications.

A multiferroic ceramic with perovskite structure: (La0.5Bi0.5)(Mn0.5Fe0.5)O3.09

An ABO3 perovskite multiferroic, where the B-site cation is responsible for the magnetic properties and the A-site cation with lone pair electron is responsible for the ferroelectric properties, was synthesized with the composition (La0.5Bi0.5)(Mn0.5Fe0.5)O3.09 at normal conditions. This oxide exhibits a ferromagnetic transition around 240 K with a well defined hysteresis loop, and a significant reversible remnant polarization below 67 K similar to ferroelectric behavior. The magnetic interaction is interpreted by the ferromagnetic Fe3+–O–Mn3+ and antiferromagnetic Fe3+(Mn3+)–O–Fe3+(Mn3+) interactions competed with each other, whereas the ferroelectricity is predominantly due to the polar nature introduced by the 6 s2 lone pair of Bi3+ cations.

Ferromagnetism and magnetodielectric effect in insulating LaBiMn4∕3Co2∕3O6 thin films

High quality epitaxial thin films of LaBiMn4∕3Co2∕3O6 perovskite were fabricated on (001)-oriented SrTiO3 and LaAlO3 substrates by the pulsed laser deposition technique. Magnetization measurements reveal a strong magnetic anisotropy and a ferromagnetic behavior that is in agreement with a superexchange interaction between Mn4+ and Co2+ ions, which are randomly distributed in the B site. A distinct anomaly is observed in the dielectric measurements at 130K corresponding to the onset of the magnetic ordering, suggesting a coupling. Above this temperature, the extrinsic Maxwell–Wagner effect is dominating. These results are explained using the Raman spectroscopic studies indicating a weak spin-lattice interaction around this magnetic transition.

Interfacial contribution to the dielectric response in semiconducting LaBiMn4∕3Co2∕3O6

Impedance measurements have been performed on a sintered polycrystalline sample of the perovskite LaBiMn4∕3Co2∕3O6. Colossal dielectric permittivity is often measured in this class of semiconducting materials as a result of extrinsic factors. Our results show that a large offset in the capacitance, measured on a series of samples with different thickness, is due to the interfacial polarization. This contribution can then be removed from the data, creating a general procedure for dielectric measurements in semiconducting samples.

Enhanced low field magnetoresistance in nanocrystalline La0.7Sr0.3MnO3 synthesized on MgO nanowires

We report on the structure and transport properties of nanocrystalline manganite La0.7Sr0.3MnO3 (LSMO) synthesized on nanowires-engineered MgO substrates by pulsed laser deposition, which is compared with reference samples deposited directly on flat MgO substrates. Such LSMO/MgO nanocomposites show enhanced low field magnetoresistance, especially at low temperature, due to the dominant spin-polarized intergrain tunneling. This work suggests that growing on nanoengineered substrates is a viable route to achieve nanostructured materials with desired crystalline structure and physical properties.

Constrained ferroelectric domain orientation in (BiFeO3)m(SrTiO3)n superlattice

Ferroelectric domains were investigated using piezoresponse force microscopy in superlattices composed of multiferroic BiFeO3 and SrTiO3 layers. Compared to single BiFeO3 thin films, a reduction in the domains size and a suppression of the in-plane orientation of domains are observed in a superlattice of (BiFeO3)4(SrTiO3)8, suggesting a constrained ferroelectric domain orientation along the out-of-plane ⟨001⟩ direction. Such modification of domain size and orientation in BiFeO3-based heterostructures could play a vital role on engineering the domains and domain wall mediated functional properties necessary for device applications.

Magneto-transport and magneto-dielectric effects in Bi-based perovskite manganites

The study of the substitution of manganese by cobalt in the perovskite (LaBi)MnO3 in view of discovering magneto-dielectric properties in manganites, has allowed the perovskite La1.2Bi0.8Mn1.2Co0.8O6.02 to be selected. This phase is indeed ferromagnetic, like the parent phase La1.2Bi0.8Mn2O6.20, yet with a much higher TC (181 K instead of 103 K). Moreover, it is a better insulator than the other members with an activation energy of 86 meV. Importantly, it exhibits a significant magneto-dielectric effect of around 0.25% at 80 K. Finally, it is also shown to exhibit magnetoresistance (MR) properties at low temperature, with MR values up to −19% at 125 K. These properties are explained on the basis of Mn4+/Co2+ ferromagnetic interactions, electronic phase separation and spin–lattice interactions.

Effect of the external fields on the polar and dielectric properties of Eu0.8Y0.2MnO3

Eu0.8Y0.2MnO3 has been widely studied due to its very distinctive phase diagram, where it is still poorly understood the actual ferroelectric character of the low temperature magnetic phases. In order to figure out what is the origin of the microscopic mechanisms that drive its behavior, we carried out a detailed study of the displacement currents for both different starting conditions and polarizing electric fields, and of the field dependent magnetodielectric effect in Eu0.8Y0.2MnO3 ceramics. The experimental results provide clear evidence for the existence of two dipolar systems, one stemming from an electric field-induced process, likely associated with the isovalent substitution of Eu3+ by the smaller off-center Y3+ ions at A-lattice sites, which is independent of any cooperative phenomena occurring in the system. The other dipolar system, strongly dependent on the existence of the first one, drives the polar behavior of the nonmodulated magnetic phase AFM-2, stable in the temperature range of 23–30 ...

Insight on the ferroelectric properties in a (BiFeO3)2(SrTiO3)4 superlattice from experiment and ab initio calculations

Ferroelectric domain properties of a (BiFeO3)2(SrTiO3)4 superlattice were studied by means of piezoresponse force microscopy and density functional theory calculations. A combination of out-of-plane and in-plane piezoresponse force imaging confirms that the ferroelectric domains are oriented along the out-of-plane [001] direction of the film. Density functional theory calculations evidence that this orientation is due to the tetragonal-like structure adopted by the BiFeO3 units inside the superlattice in response to the interfacial strains. In addition, antiferrodistortive rotations of the BO2 planes within both types of ABO3 blocks (i.e., SrTiO3 as well as BiFeO3 units) are highlighted. Besides, a much lower coercive voltage is measured on superlattices compared to BiFeO3 single layers, suggesting a more reliable switching capability. The results are expected to enable the design of promising multifunctional oxide superlattices.