Dielectrics and Waves.

A La2NiMnO6 polycrystalline sample prepared by the sol–gel method showed monoclinic crystal structure with the P21/n space group and a saturation magnetization of 4.63 μB/f.u. at 5 K. Impedance spectroscopy results in the temperature range of 10 K < T < 300 K have revealed a distinct conduction process at grains and grain boundaries, where the grains followed the variable range hopping mechanism and the grain boundaries obeyed Arrhenius thermal activation. A negative magnetoresistance of 2.5% was observed at the paramagnetic to ferromagnetic transition, and this became temperature independent below the magnetic ordering. A marginal positive magnetodielectric (MD) effect that followed the dielectric relaxation was observed and its magnitude was found to decrease with increase of the frequency. A systematic study on the magnetic field induced dielectric properties, dc transport and dc bias effect on the dielectric permittivity has revealed the extrinsic origin of the MD effect in the bulk sample of La2NiMnO6.

https://iopscience.iop.org/article/10.1088/0953-8984/24/49/495901/pdf

The extrinsic origin of the magnetodielectric effect in the double perovskite La2NiMnO6.

We show that the first order magneto-structural phase transitions observed in various classes of magnetic solids are often accompanied by useful multi-functional properties, namely giant magneto-resistance, magneto-caloric effect and magneto-striction. We highlight various characteristic features associated with a disorder influenced first order phase transition namely supercooling, superheating, phase-coexistence and metastability, in several magnetic materials and discuss how a proper understanding of the transition process can help in fine tuning of the accompanied functional properties. Magneto-elastic coupling is a key element in this first order phase transition, and methods need to be explored for maximizing the contributions from both the lattice and the magnetic degree of freedom while simultaneously minimizing the thermomagnetic hysteresis loss. An analogy is also drawn with the first order phase transition observed in dielectric materials and vortex matter of type-II superconductors.

https://iopscience.iop.org/article/10.1088/0953-8984/25/18/183201/pdf

First order magneto-structural phase transition and associated multi-functional properties in magnetic solids

We report magnetization, dielectric and dc transport properties of La2NiMnO6 nanoparticles. Both dc and ac magnetization measurements indicated a metastable magnetic behaviour with random ferromagnetic and antiferromagnetic interactions below 110 K; critical slow-down, memory and rejuvenation properties signify the spin glass nature. The dc resistivity shows a semiconducting nature but the temperature dependent magnetoresistance (MR) shows a peak at the spin glass transition. The colossal dielectric property and its frequency dependence were interpreted using the Maxwell–Wagner (MW) interfacial polarization model. Impedance analysis along with magnetodielectric (MD) and magnetoresistance (MR) indicates that the observed MD originates from the combined effect of MR and MW interfacial polarization.

https://iopscience.iop.org/article/10.1088/0953-8984/24/37/376003/pdf

Spin glass behaviour and extrinsic origin of magnetodielectric effect in non-multiferroic La 2 NiMnO 6 nanoparticles

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl3 and FAPbBr3, compared to those of MAPbCl3 and previously reported MAPbBr3, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX3 system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results.

Critical Comparison of FAPbX3 and MAPbX3 (X = Br and Cl): How Do They Differ?

We report magnetic, dielectric, and magnetodielectric responses of the pure monoclinic bulk phase of partially disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this compound is an intrinsically multiglass system with a large magnetodielectric coupling (8%-20%) over a wide range of temperatures (150-300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative-spin orientations between neighboring magnetic ions in a transition-metal oxide system. We discuss the role of antisite (Ni-Mn) disorder in emergence of these unusual properties.

S. Rajan

Papers

Near-room-temperature colossal magnetodielectricity and multiglass properties in partially disordered La2NiMnO6