P. D. Babu

Dielectric relaxations in La2NiMnO6 with signatures of Griffiths phase

Polycrystalline La2NiMnO6 prepared by sol–gel auto combustion technique is found to possess dual crystallographic phases R-3c (71 vol. %) and Pbnm (29 vol. %), with respective ferromagnetic transitions at 277 K and 160 K, respectively. The impedance data are analysed via Havriliak-Negami equivalent for impedance (HNEI). Using HNEI equation, the resistance “R,” relaxation time “τ,” and symmetric “α” and asymmetric “β” broadening parameters, for the two crystallographic phases are obtained. The dielectric relaxations for the two crystallographic phases are found to be independent of each other. The activation energy of relaxation for the R-3c and Pbnm phases is found to be 147 meV and 88 meV, respectively. Strong correlations of dielectric relaxations with the magnetic phases are seen. Before the onset of ferromagnetic transition, i.e., above 160 K for Pbnm and 277 K for R-3c phase, relaxations are dominantly non-Debye type. Once the ferromagnetic state is achieved, relaxations attain Debye nature. Hopping ...

Magnetoelectric coupling in Ca3CoMnO6

Magnetic structure and dielectric response of Ca3CoMnO6 have been studied as a function of temperature and applied magnetic field. Neutron diffraction experiments at low temperatures reveal changes in the magnetic structure in the temperature range 2–10 K. The pristine magnetic structure undergoes significant change with the application of magnetic field at these temperatures. Dielectric measurements reveal the presence of magnetoelectric coupling below magnetic ordering temperature (TN=15 K). The present work brings out the existence of a correlation between magnetic structure and dielectric properties.

Study of magnetization and magnetoelectricity in CoFe2O4/BiFeO3 core-shell composites

CoFe2O4 (core)/BiFeO3 (shell) nanoparticles are prepared by varying the relative molar concentration of core and shell materials (40%CoFe2O4-60%BiFeO3, 50%CoFe2O4-50%BiFeO3, and 60%CoFe2O4-40%BiFeO3). The core-shell nature is confirmed from transmission electron microscopy on these samples. A plot of ΔM (=MFC-MZFC) vs temperature suggests the presence of two types of spin dynamics: (a) particle size dependent spin blocking and (b) spin-disorder. These two spin dynamic processes are found to contribute independently to the generation of magnetoelectric voltage. Very clear first order and second order magnetoelectric voltages are recorded. The resemblance of the first order magnetoelectric coefficient vs temperature plot to that of building up of order parameters in the mean field theory suggests that spin disorder can act like one of the essential ingredients in building the magnetoelectric coupling. The best result is obtained for the 50-50 composition sample, which may be due to better coupling of magnet...

Effect of Cu substitution on the magnetic and dielectric properties of La2NiMnO6

In the continuation of our previous work [Biswal et al., J. Appl. Phys. 115, 194106 (2014)] of biphasic La2NiMnO6 (LNM) prepared via sol-gel technique, here we are reporting the effect of Cu substitution at Ni site of LNM. The powder x-ray diffraction results of La2Ni1−xCuxMnO6 (x = 0, 0.05, 0.1) confirm the biphasic nature (Pbnm + R-3c) for all samples along with increasing R-3c phase with Cu content in the sample. The temperature variation of magnetization (M) plot shows sharp ferromagnetic transitions of R-3c phase at 285 K, 278 K, and 270 K (obtained from dM/dT plot) and that of Pbnm phase at 165 K, 145 K, and 145 K for x = 0, 0.05, and 0.1, respectively. Weak antiferromagnetic type feature at 25 K (x = 0), is getting pronounced with Cu content in the sample and is now seen at 32 K & 266 K (for x = 0.05) and 32K & 257 K (for x = 0.1). Fitting of Curie-Weiss law in the paramagnetic region of χ−1 vs T plot results slightly lower values of corresponding Tcs of R-3 c phase (277 K, 270 K, and 260 K), thus...

Interrupted Magnetic First Order Transitions and Kinetic Arrest probed with In-field Neutron Diffraction

In-field neutron diffraction studies were carried out on two compounds that exhibit magnetic first order phase transitions (FOPT). It is shown that the FOPT can be interrupted by an external magnetic field, resulting in a coexistence of kinetically arrested metastable states and equilibrium phases. Use of a novel protocol CHUF (Cooling and Heating under Unequal Fields) helps to determine the coexisting phase fractions and also to observe the devitrification of the kinetically arrested phase into the equilibrium phase, in a manner similar to that found in structural glassy systems.

Tuning of magnetic structure and its effect on magnetic properties in Co(Cr1–xMnx)2O4 (x = 0–0.3)

We report here the magnetic properties of Co(Cr1–xMnx)2O4 (x = 0.00–0.30) through bulk-magnetization measurements and temperature variable neutron powder diffraction experiments. We have noticed the compensation temperature (Tcomp) and then the onset of negative magnetization at T = 60 K and 83 K only for Co(Cr0.73Mn0.27)2O4, i.e., x = 0.27, and Co(Cr0.70Mn0.30)2O4, i.e., x = 0.30 samples. Low temperature neutron diffraction experiments on these polycrystalline samples (i.e., x = 0.27 and 0.30) are performed to explore the understanding of observed negative magnetization, complex magnetic properties, and correlation between chemical and magnetic structures. A broad hump has been observed around the (111) Bragg reflection which gives the signature of diffuse scattering from short-range magnetic interactions coexisting with long range magnetic order below ordering temperature. The variation of bond lengths, bond angles, and magnetic moments with temperature is correlated, which clearly indicates magneto-elastic coupling around compensation temperature in these compounds. The presence of this magneto-elastic coupling results in the sign change of the magneto-caloric effect at compensation temperature.We report here the magnetic properties of Co(Cr1–xMnx)2O4 (x = 0.00–0.30) through bulk-magnetization measurements and temperature variable neutron powder diffraction experiments. We have noticed the compensation temperature (Tcomp) and then the onset of negative magnetization at T = 60 K and 83 K only for Co(Cr0.73Mn0.27)2O4, i.e., x = 0.27, and Co(Cr0.70Mn0.30)2O4, i.e., x = 0.30 samples. Low temperature neutron diffraction experiments on these polycrystalline samples (i.e., x = 0.27 and 0.30) are performed to explore the understanding of observed negative magnetization, complex magnetic properties, and correlation between chemical and magnetic structures. A broad hump has been observed around the (111) Bragg reflection which gives the signature of diffuse scattering from short-range magnetic interactions coexisting with long range magnetic order below ordering temperature. The variation of bond lengths, bond angles, and magnetic moments with temperature is correlated, which clearly indicates magneto-ela...

Grain boundary-dominated electrical conduction and anomalous optical-phonon behaviour near the Neel temperature in YFeO3 ceramics

We investigated the anomalous behaviour in the dielectric properties, occurring nearly at room temperature and at elevated temperatures (near the Neel temperature TN) of the polycrystalline samples of YFeO3 (YFO) ceramics. On the prepared YFO ceramics, the magnetic measurements showed the Neel temperature of YFO to be 650 K, below which the compound exhibited the weak ferromagnetic behaviour. X-ray photoelectron spectroscopy (XPS) shows the presence of Fe ions (Fe2+ and Fe3+ states) and also revealed the formation of the oxygen vacancies. The frequency dependence of the complex dielectric constant within the frequency domain of 100 Hz–1 MHz shows the presence of grain dominated dielectric relaxation over the thermal window of 300–373 K. The activation energy Eact.e=0.611eV extracted from the imaginary permittivity spectrum indicates the involvement of oxygen vacancies in the relaxation process. Above 493 K, the ac conductivity, complex impedance, and modulus studies revealed appreciable conduction and rel...

Magnetoelectricity in La2NiMnO6 and its PVDF impregnated derivative

La2NiMnO6 is prepared for two different sintering time durations: 4 hour (4H) and 24 hour (24H). The magnetization data suggest a double ferromagnetic (FM) transition for 24H, whereas a single FM transition for 4H sample. The 4H sample was found to possess several microscopic pores compared with the 24H sample, and thus, these pores are vacuum impregnated with polyvinylidene fluoride (PVDF). The PVDF in the impregnated sample is found to crystallize in beta phase. The first and second order magnetoelectric coefficients are extracted from the measured data and plotted as a function of temperature. The room temperature region is found to be dominated by the magnetoconductivity/magnetoloss and thus resulting low ME voltage. With lowering of temperature, the situation improves, and for T < 200 K, the loss component becomes negligible so that the true magnetoelectricity is observed. A comparative study on all the three samples (4H, 24H, and 4H + PVDF) shows a significant enhancement in the magnetoelectricity in the 4H + PVDF sample. The highlight of this report is the unique way to arrest the porosity by PVDF impregnation into the pores of La2NiMnO6 so that leakage is minimized.La2NiMnO6 is prepared for two different sintering time durations: 4 hour (4H) and 24 hour (24H). The magnetization data suggest a double ferromagnetic (FM) transition for 24H, whereas a single FM transition for 4H sample. The 4H sample was found to possess several microscopic pores compared with the 24H sample, and thus, these pores are vacuum impregnated with polyvinylidene fluoride (PVDF). The PVDF in the impregnated sample is found to crystallize in beta phase. The first and second order magnetoelectric coefficients are extracted from the measured data and plotted as a function of temperature. The room temperature region is found to be dominated by the magnetoconductivity/magnetoloss and thus resulting low ME voltage. With lowering of temperature, the situation improves, and for T < 200 K, the loss component becomes negligible so that the true magnetoelectricity is observed. A comparative study on all the three samples (4H, 24H, and 4H + PVDF) shows a significant enhancement in the magnetoelectricity i...

Effect of Yttrium Substitution on the Structural and Magnetic Properties of SmFeO3

Polycrystalline samples of SmFeO3 (SFO) and Y0.5Sm0.5FeO3 (YSFO 50/50) are prepared by conventional solid state reaction method. Crystal structure refinement shows the decrease in the lattice parameter ‘a’of the orthorhombic cell in SFO relative to that of (YSFO 50/50), which has been ascribed to the increasing FeO6 octahedral distortions and vast deviation from their non-rigidity. The Magnetic measurements on both the samples in the temperature range of 2-350K shows thermomagnetic irreversibility between the ZFC-FC curves for both the samples, which can be ascribed to the formation of the nearly collinear magnetic state in the parent and the doped samples. These results indicate the pronounced effects of Sm3+ ions in determining the structural and magnetic properties of these materials.Polycrystalline samples of SmFeO3 (SFO) and Y0.5Sm0.5FeO3 (YSFO 50/50) are prepared by conventional solid state reaction method. Crystal structure refinement shows the decrease in the lattice parameter ‘a’of the orthorhombic cell in SFO relative to that of (YSFO 50/50), which has been ascribed to the increasing FeO6 octahedral distortions and vast deviation from their non-rigidity. The Magnetic measurements on both the samples in the temperature range of 2-350K shows thermomagnetic irreversibility between the ZFC-FC curves for both the samples, which can be ascribed to the formation of the nearly collinear magnetic state in the parent and the doped samples. These results indicate the pronounced effects of Sm3+ ions in determining the structural and magnetic properties of these materials.

Low Temperature Neutron Diffraction Studies on Ca3CoMnO6

The magnetic structure of the quasi‐1D compound Ca3CoMnO6 (CCMO) has been determined by carrying out Rietveld analysis of low temperature neutron diffraction (ND) patterns. The magnetic measurements on CCMO have established that the compound exhibits a long range antiferromagnetic ordering (TN) around 15 K, and interesting magnetic properties below TN. In order to understand the correlation of the magnetic behavior with the magnetic structure, we have recorded ND patterns at temperatures across TN, from 2 K to 30 K and have also studied the influence of externally applied magnetic field on the magnetic ordering in CCMO. The magnetic field induces drastic changes in the pristine magnetic structure at temperatures below 15 K. We discuss here the parameters associated with magnetic structure of CCMO at low temperatures and the influence of external field on them.