P.R. Mandal

Evolution of Griffith phase in hole doped double perovskite La2−xSrxCoMnO6 (x = 0.0, 0.5, and 1.0)

We report on the magnetic properties of hole doped La2−xSrxCoMnO6 (x = 0.0, 0.5, and 1.0) double perovskite synthesized by the low-temperature sol-gel method. X-ray photoelectron spectroscopy (XPS) analysis reveals the increase of the number of Mn3+ and Co3+ ions with Sr content. The reduction in magnetic moment with the increase in Sr content is assigned to the increase of antiferromagnetic (AFM) phases. Detailed analysis of zero field cooled dc magnetic susceptibility as a function of temperature for the samples reveals the features of Griffiths-like singularity. Second harmonic nonlinear ac magnetic susceptibility (χ2) also demonstrates the aspects of Griffiths phase (GP) singularity above the ferromagnetic Curie temperature for all the samples. The value of the exponent of the GP (λ < 1) is found to decrease with Sr substitution in the La site, and it is not related to the structural disorder originating from the Jahn–Teller active Mn3+ions. Hence, it is concluded that with increasing Sr content, the nucleation of GP singularity is supressed due to the increase in the AFM phase volume fraction.

A comparative study of structural, magnetic, dielectric behaviors and impedance spectroscopy for bulk and nanometric double perovskite Sm2CoMnO6

A comparative study regarding the structural, magnetic, dielectric and conductivity behavior and impedance spectroscopy of double perovskite Sm2CoMnO6 (SCMO) with different grain sizes of nearly 35 nm (SCMO_N) and 1 μm (SCMO_B) synthesized by a chemical sol-gel route has been performed in detail. The high-resolution recorded XRD pattern confirms the crystalline phase with monoclinic symmetry (space group P21/n) in SCMO_B. The XRD pattern of SCMO_N discloses the coexistence of both monoclinic and orthorhombic (Pnma) phases in the system. The magnetic measurements reveal that, in SCMO_N, the increase of the disordered phase (orthorhombic phase) results in an increase in the Curie temperature and a decrease in the magnetic moment. SCMO_B shows ferromagnetic behavior with two magnetic transitions, whereas the absence of a high-temperature magnetic transition reveals the disorder nature of the SCMO_N sample. The value of effective paramagnetic moment peff (5.75 μB) of SCMO_B is comparable to the calculated value of the spin-only moment for intermediate-spin Co3+ and high-spin Mn3+, as well as high-spin Mn2+ and Co4+, whereas the observed reduced peff (5.5 μB) value is attributed to the spin-only moment due to the increased number of high-spin Mn3+ and low-spin Co3+ in SCMO_N. SCMO_B exhibits a high dielectric constant with prominent high-frequency dielectric dispersion as compared to SCMO_N. The impedance spectroscopy studies disclose different conduction processes at the grain and grain boundary where the grain boundary obeys Arrhenius behavior and the grains follow the variable range hopping (VRH) model. SCMO_N shows high grain and grain boundary resistance as compared to SCMO_B. The grain and grain boundary capacitance show an anomaly near the dielectric relaxation. The nature of the temperature-dependant exponent s in the ac conductivity predicts that the overlapping large polaron tunneling mechanism (QLPT) is the dominant conduction mechanism for both the SCMO_B and SCMO_N samples.

Temperature and frequency dependence of AC electrical properties of Zn and Ni doped CoFe2O4 nanocrystals

Abstract We have investigated the structure and the electrical properties of CoFe2O4, Ni0.5Co0.5Fe2O4 and Zn0.5Co0.5Fe2O4 nanoparticles, prepared through a chemical pyrophoric reaction technique. The study of the dielectric constant reveals evidence of Rabinkin and Novikova polarisation in the system. The increased value of the dielectric constant at low frequency is attributed to the presence of interfacial and dipolar polarisation in the system. The impedance for Zn0.5Co0.5Fe2O4 nanoparticles is found to decrease with increase in temperature, indicating the presence of a temperature-dependent electrical relaxation process in the system. Nyquist plots have been fitted using parallel combinations of grain boundaries resistance and grain boundaries capacitance. The activation energy is estimated from Nyquist plots, dc and ac conductivity data using the Arrhenius relation. This is indicating that the same type of charge carrier is responsible for the relaxation and the conduction processes in the system. Ac conductivity curves follow a double power law, as proposed by Jonscher. The conduction mechanism with temperature is mainly due to the large polaronic hopping in the system.

Investigation of Magnetic and Electrical Properties of Multiferroic CZFO‐PZT Nanocomposites

The room temperature multiferroic nanocomposite materials of ferrite‐ferroelectric ceramics viz. cobalt‐zinc ferrite (i.e. Co0.65Zn0.35Fe2O4) and lead zirconate titanate (i.e. PbZr0.52Ti0.48O3) sintered at three different temperatures were prepared by sol‐gel technique. Magnetization versus magnetic field measurements show typical hysteresis loops, attesting room temperature ferrimagnetic behavior of the composites. The ferrimagnetic Curie temperature is nearly 230 °C which is slightly decreased from pure Co‐Zn ferrite. The dielectric constant increases with increasing particle size. The ferroelectric Tc decreases gradually but the transition becomes increasingly diffuse with decrease in particle size. The polarization versus electric field (P‐E) measurement exhibits a clear saturating nature of hysteresis loop which proves the excellent ferroelectricity of the nanocomposite.