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Multiferroic materials having good magneto-electric coupling are of great interest due to their enormous variety of applications in the field of spintronic devices. Despite having good magneto-electric coupling and multiferroic properties, gallium ferrite (GaFeO3) is plagued for having a very high leakage current compared to other multiferroic materials, which shadows the possibility of its application in device fabrication. The conduction mechanism behind this exceptionally high leakage current in GaFeO3 is still not identified properly. Here, we have carefully investigated the variation of leakage conduction with the change in morphologies, especially at the grain boundaries due to heat treatment temperature during sample preparation. A connected conductive network is detected along the fused grain boundaries, which provides the pathway for the leakage electrons to propagate through the material. Interestingly, the conductive atomic force microscopy image shows that oxygen vacancies at the near grain boundaries produce poor conductive regions. Small polaron hopping could be an effective transport mechanism present in the conductive ridges at the grain boundaries. Therefore, modification of grain boundaries could produce more effective results for the reduction in leakage current of GaFeO3.

Magnetic Barkhausen noise and hysteresis loops have been measured in the presence of various tensile stresses from three wire-shaped amorphous magnetic materials having different domain structures. Long duration large Barkhausen pulse indicates the presence of an inner core (IC) domain in positive magnetostrictive glass-coated Co83.2Mn7.6Si5.8B3.3 and Co58.59Fe4.41Ni10Si11B16 microwires where magnetization takes place by ~180° rotation of domain magnetization. This long duration Barkhausen pulse is missing in the case of low negative magnetostrictive Co68.1Fe4.4Si12.5B15 wire indicating a negligible volume of IC domain in it. Short duration pulses observed in all three samples suggest the presence of a multi-domain outer shell where small angle rotation and domain wall displacement are responsible for the magnetization process. The shapes of the hysteresis loops support the Barkhausen noise studies.

The size reduction to very small nanoparticles has earlier revealed the transformation of NiFe2O4 from its known inverse spinel structure to a normal phase, preceded and probably forced by an abrupt lattice contraction. Since such lattice contractions are also likely to occur due to lowering of temperature, the defects-related properties of NiFe2O4 nanoparticles at low temperatures (300?20?K) were studied using positron lifetime and Doppler broadened line shape parameter measurements. A change in the predominant positron trapping sites from inherent structural vacancies within the spinel structure to the large microvoid-like cavities at the intersection of the grain interfaces during pelletization of the powdered samples has been identified from the values of the measured positron lifetimes. Significant, and at the same time anomalous, changes have been observed to take place around 90?K owing to an inversion of the normal spinel structure. At lower temperatures, cationic redistribution will take place due to the strong preference of Ni2+ ions towards the octahedral sites and of Fe3+ ions towards the tetrahedral sites, thereby attaining a state of lower energy. The cationic redistribution due to the change in temperature is verified through M?ssbauer spectroscopic studies as well. The results are encouraging from the point of investigating identical structural transformations brought in by grain size reduction and/or lowering of temperatures in similar materials.

Magnetocaloric effect (MCE) in low-cost iron-based binary alloys R2Fe17 (R = Y, Pr) has been investigated by measuring their magnetic properties. The Curie temperature of these alloys is found to be close to room temperature. The specific heat measurement of these materials indicates a second order ferromagnetic/paramagnetic phase transition. The MCE properties in these alloys are comparable to that of gadolinium, which is very expensive for domestic use. Therefore these binary alloys are suitable as room temperature MCE materials. The effect of pulsed magnetic field on the MCE properties has also been studied up to 25 T.

Magnetic-flux-leakage (MFL) measurements are used for corrosion inspection of buried oil and gas pipelines. MFL signals are sensitive to the depth of defects, such as corrosion pits, in the pipe wall and to external stresses generated mainly by the line pressure. MFL signal variations have been studied from the inside wall in the presence of various circumferential bending stresses and magnetic flux densities for three outside wall pits of depths 30%, 50% and 80% of the wall thickness. Magnetic Barkhausen noise (MBN) measurements have been used to study the stress-induced changes in the direction of the bulk magnetic easy axis and directional anisotropies of the pipe wall. These help one to understand the stress-dependent MFL results.

Magnetic flux leakage (MFL) is used for in-line inspection of underground oil and gas pipelines. MFL signals are sensitive to the line pressure which should be taken into account when estimating the sizes of defects. MFL signals have been studied from electrochemically milled pits with 50% penetration in pipeline steel under the influence of different axial or circumferential stresses and a range of magnetic flux densities. Magnetic Barkhausen noise (MBN) measurements have been used to study the changes in the direction of the magnetic easy axis of the pipe wall. These help one to understand the stress-dependent MFL results. MBN measurements have also been used to estimate the directional magnetic anisotropies of the steels.

Herein, we investigate the origin of enhanced absorption and complex conductivity of magnetite (Fe3O4) nano-hollow spheres (NHSs) in contrast to its nanoparticles (NPs) configuration in the frequency range 0.4–2.0 THz. The maximum absorption for NHSs and NPs of the same average diameter (~100 nm) are found to be 246.27 and 48.35 cm−1 at 1.8 THz, respectively. A detailed study suggests that the multiple resonance peaks in the absorption spectra are due to low frequency acoustic vibrational phonon modes of Fe3O4 nanostructures. Moreover, we demonstrate that the magnitude of total absorption can be tailored by varying the shell thickness of NHSs. It is found to increase with increasing shell thickness, and attain a maximum value of 498.5 cm−1 for the NHSs of average diameter 350 nm at 1.8 THz. The invariance of frequency dependent magnetic permeability points out that the absorption is basically due to dielectric loss instead of magnetic loss. The enhanced THz conductivity of Fe3O4 NHSs, as compared to NPs is described in light of thermally activated polaronic hopping which is found to increase with increasing THz absorption. Finally, the size dependent THz conductivity of NHSs confirms its sole dependence on the magnitude of THz absorptivity.

Structural and magnetic phase transitions of Mn-rich Mn50.5−xNi41Sn8.5+x (x = 0, 1 and 2) alloys were studied by magnetic and electrical measurements. The shift in martensitic transition temperature was less sensitive to the Mn/Sn ratio as compared to the Ni/Sn ratio. A magnetic entropy change (ΔSM) ∼ 11.85 J kg−1 K−1 was obtained for x = 0 at 270 K due to a change in magnetic field ΔH of only 1.5 T. Incorporating hysteresis losses, the net refrigerant capacity was estimated to be 44.82 J kg−1 for the same sample, which was found to be larger than that of other reported Ni-rich Ni–Mn–Sn alloys. Nearly 33% magnetoresistance was achieved for these alloys in presence of 8 T field differences.

Enhanced magnetoelectric coupling is observed in bismuth ferrite samples, co-doped with non-magnetic Ba and magnetic Gd ions replacing Bi and Fe, respectively. Distortion in Fe–O octahedra has a significant effect on the magnetic properties of the samples. Ferromagnetic signature is found to increase significantly in the co-doped samples with respect to the only-Gd-doped sample both at 80 and 300 K. The co-doped samples show enhanced electric polarization as well as the highest resistivity at room temperature, which might be due to the reduction in the leakage current and oxygen vacancy in the compositions. An interesting correlation between the antiferromagnetic Néel temperature (T N) of bismuth ferrite and the temperature-dependent dielectric constant is observed in all samples. Bi0.9Ba0.1Fe0.95Gd0.05O3 ceramic possesses maximum coupling between electric dipole and magnetic dipole with an estimated magnetodielectric effect MD ([ε r(H) − ε r (0)]/ε r (0)) ∼ 380 at an applied field of 6 kOe. Impedance spectroscopy in the frequency range 40–107 Hz and temperature within 30–300 °C suggests that grain relaxation is dominant in the samples. Electrical parameters (such as capacitance and resistance) of the grains are determined using the real and imaginary parts of impedance (Z′ and Z″) and the electrical modulus (M′ and M″) plot. The results of electrical conductivity indicate a correlated barrier hopping conduction mechanism in the samples.