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La 0.67 Ca 0.2 Ba 0.13 Fe 1- x Ti x O 3 samples ( x = 0 and 0.03) were synthesized by the auto-combustion method. Analysis of XRD diffractograms revealed that these compounds crystallize in the cubic system with the space group Pm 3̄ m . The dielectric properties have been studied in the 10 2 -10 6 frequency range and the 120-280 K temperature range. Analysis of AC conductivity shows that the conduction mechanisms are of polaronic origin and that they are co-dominated by the NSPT and OLPT models. The monotonic increase in conductivity with increasing temperature results from the reduction of defect centers and the increase in charge carrier mobility. Such variation is consistent with impedance variation at different frequencies and temperatures indicating semiconductor behavior. Nyquist diagrams are characterized by the appearance of semi-circular arcs. These spectra are modeled in terms of equivalent electrical circuits confirming the contribution of grains ( R g //CPE g ) and grain boundaries ( R gb //CPE gb ). The dielectric analysis showed an evolution of the dielectric constant in accordance with Koop's theory and the phenomenological model of Maxwell-Wagner. The low conductivity and the high values of the real permittivity at low frequency make our compounds potential candidates for energy storage and applications for electronic devices and microwaves., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (This journal is © The Royal Society of Chemistry.)

Ferrite compounds have recently attracted significant interest because of their multifunctional properties. This work investigates the optical, magnetic, and dielectric properties of a LiMn 0.5 Fe 2 O 4 ferrite prepared by a solid-state reaction. Raman spectroscopy analysis substantiated the presence of the 5 active modes representing the vibration of the oxygen anion at both tetrahedral and octahedral sites. The direct optical band gap was estimated to be 3.51 eV, which indicates the semiconductor behavior of the compound. A theoretical modulation of the hysteresis loop was done to confirm the dominance of the ferromagnetic contribution over the antiferromagnetic one. Furthermore, the dielectric permittivity result indicated a colossal value of the order of 10 3 . The dielectric losses are characterized by the Giuntini law to extract the relaxation process, which is hidden by the DC-conductivity process. Our results indicate the potential of LiMn 0.5 Fe 2 O 4 for applications in multifunctional devices., Competing Interests: The authors declare that there are no competing interests regarding the publication of this paper., (This journal is © The Royal Society of Chemistry.)

In this work we synthesized the multifunctional (La 0.8 Ca 0.2 ) 0.4 Bi 0.6 FeO 3 material using a sol-gel process. Structural and morphologic investigations reveal a Pnma perovskite structure at room temperature with spherical and polygonal nanoparticles. A detailed study of the temperature dependence of the dielectric and electrical properties of the studied material proves a typical FE-PE transition with a colossal value of real permittivity at 350 K that allows the use of this material in energy storage devices. Thus, the investigation of the frequency dependence of the ac conductivity proves a correlated barrier hopping (CBH) conduction mechanism to be dominant in the temperature ranges of 150-170 K; the two observed Jonscher's power law exponents, s 1 and s 2 between 180 K and 270 K correspond to the observed dispersions in the ac conductivity spectra in this temperature region, unlike in the temperature range of 250-320 K, the small polaron tunnel (NSPT) was considered the appropriate conduction model., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

La 1-2 x Ba x Bi x FeO 3 (0.00 ≤ x ≤ 0.20) nanoparticles were prepared by the auto-combustion method using glycine as a combustion fuel. X-ray diffractometry (XRD) measurements confirmed the orthorhombic structure of the synthesized compounds with the Pnma space group as a principal majority phase and showed the presence of a very minor secondary phase when x > 0.1. The nanosize criterion of the prepared compounds was confirmed from the crystallite size values calculated using the Williamson-Hall formalism. The relaxation process has been studied by the frequency dependence of the imaginary parts of impedance and modulus ( Z '' and M '') which satisfied the Arrhenius law. Nyquist plots allowed us to obtain an adequate equivalent circuit involving the grains and grain boundary contributions. The activation energies calculated from Z '', M '' and the resistance of both contributions deduced from the Nyquist plots are found to be very similar. The conduction mechanism has been analyzed using the temperature dependence of the exponent Jonscher's power law parameter which confirms the NSPT conduction mechanism type for all compounds with an enhancement of the binding energy of the charge carrier ( W H ) with the substitution., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

This work involves the synthesis and study of physical properties of the La 0.8 Ca 0.1 Pb 0.1 Fe 0.975 Mg 0.025 O 3 compound, which has been characterized by various experimental techniques, such as X-ray diffraction, SEM and complex impedance spectroscopy. The structural study showed that the La 0.8 Ca 0.1 Pb 0.1 Fe 0.975 Mg 0.025 O 3 compound crystallized in the orthorhombic structure with the Pnma space group. The particle size and the surface morphology of this compound have been analysed using SEM. The particle size was found to be around 120 nm and we confirmed that one particle contains more than one crystallite. Importantly, the studied compound presented a giant dielectric permittivity ( ε ' of around 9 × 10 4 at high temperature and low frequencies). An equivalent electric circuit has been deduced from the Nyquist plots of the complex impedance parts ( Z '' vs. Z ') to correctly describe the electrical behavior of the La 0.8 Ca 0.1 Pb 0.1 Fe 0.975 Mg 0.025 O 3 compound. The chosen circuit consists of two cells mounted in series corresponding to the grain and grain boundary contributions. The electrode contribution has been detected from the frequency dependence of the imaginary part of modulus where the activation energy of each constitution has been calculated. The relaxation process and the electrical conductivity are attributed to the same type of charge carriers characterized by similar values of the activation energy determined from loss factor tangent (tg( δ )), the imaginary part of the permittivity and the modulus spectrum., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The hexaferrite Ba 1- x Sr x Fe 12 O 19 compounds with x = 0, 0.5 and 1 were synthesized by the autocombustion method. X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) were used for structural and morphological studies., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

(La 0.8 Ca 0.2 ) 1- x Bi x FeO 3 ( 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 ) = σ dc + Aω s , 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., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this study, our central focus is to investigate the magnetocaloric characteristics of a La 1- x □ x MnO 3 ( x = 0.1, 0.2 and 0.3) series prepared by a sol-gel technique published in Prog. Mater. Sci. , 93 , 2018, 112-232. The crystallographic study revealed that our compounds crystallize in a rhombohedral structure with R 3̄ c . Ferromagnetic (FM) and paramagnetic (PM) characters were detected from the variation in magnetization as a function of magnetic fields at different temperatures. The second order transition was verified from the Arrott plots ( M 2 vs. ( μ 0 H / M )), where the slopes have a positive value. In order to verify the second order, we traced the variation of magnetization vs. temperature at different magnetic fields for x = 0.2. This revealed a ferromagnetic (FM)-paramagnetic (PM) transition when temperature increases. Relying on the indirect method while using the Maxwell formula, we determined the variation in the entropy (-Δ S M ) as a function of temperature for different magnetic fields for the three samples. We note that all the studied systems stand as good candidates for magnetic refrigeration with relative cooling power (RCP) values of around 131.4, 83.38 and 57.26 J kg -1 with magnetic fields below 2 T, respectively. Subsequently, the magnetocaloric effect was investigated by a phenomenological model for x = 0.2. The extracted data confirm that this phenomenological model is appropriate for the prediction of magnetocaloric properties. The study also demonstrated that this La 0.8 □ 0.2 MnO 3 system exhibits a universal behaviour., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this study, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80-330 nm thick) via pulsed-laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirmed the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phases. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with the coexistence of four-layer stacking. Such stacking defects are found to be pertinent to the high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of the orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr of 35 μC cm-2) with a fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization as opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1-5 nm), distributed across the film, with Bi and Fe-rich compositions and oxygen vacancies that contribute to the weak ferromagnetic behavior mediated by the Dzyaloshinskii-Moriya interactions. Subtle changes in the structural strain and lattice distortions of thin films with varied thicknesses led to distinct ferroic properties. Stronger ferroelectric polarization of 80 nm and 120 nm films compared to that of thicker ones can be due to structural strain and the possible rearrangement of BO6 octahedra. The observation of the improved magnetoelectric coefficient of 50 mV cm-1 Oe-1 for 120 nm film, as compared to that of several Aurivillius oxides, indicates that the structural strain modification in SBLFT is beneficial for the fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions was found to be responsible for the improved multiferroic behaviour of the SBLFT films.

Magnetite (Fe3O4) offers unique physical and chemical properties, being an important material for many industrial applications. Certain limitations on the application conditions are, however, imposed by the redox stability issue. Fine control of the iron oxidation states represents a challenge for materials engineering. The present work explores relevant redox processes in iron oxides, processed under highly non-equilibrium laser floating zone (LFZ) conditions under atmospheres with different oxygen activities. The as-grown fibres showed a structure composed of the Fe3O4 core and the Fe2O3 shell. This study uncovers unexpectedly lower hematite content and shell thickness for the fibres processed under more oxidizing conditions. Combined structural and microstructural studies, supported by the analysis of the existing literature data, strongly suggest that the redox processes during the LFZ process can be rather determined by kinetics of melt crystallization, nuclei formation and heat transfer than by the oxygen content in the gas phase. The proposed mechanisms are further confirmed by electrical and magnetic studies of the composite fibres.

Lead-free particulate multiferroic composites of [0.94(Na0.5Bi0.5)TiO3-0.06 BaTiO3]:(Co0.6Zn0.4)(Fe1.7Mn0.3)O4 were synthesized and magnetoelectric (ME) properties were studied. X-ray diffraction and microstructural studies indicated the formation of a two-phase composite system without any impurities. The shift of Raman modes corresponding to ferroelectric and ferrite phases was assigned to the induced strain amid the formation of a two-phase system, in relation to the fraction of each phase in the samples. A strong local piezoresponse and hysteresis loops observed for composites established the ferroelectric properties at a nanoscale. Magnetostriction measurements revealed values of λ11 = -10.4 and λ12 = 5.3 ppm and piezomagnetic coefficient dλ11/dH = -0.0087 ppm Oe(-1) at 0.45 kOe for a composite with a ferrite concentration of 35 mol%. A maximum change of 18.5% in magnetization after electrical poling indicates a strong magnetoelectric response of the present composites followed by a direct ME coefficient of 8.2 mV cm(-1) Oe(-1). Our studies point to the fact that the present multiferroic composites having strong ME coupling are useful for lead-free electronic applications.