Ac Resistivity Research Articles

Probing quantum spin liquids in equilibrium using the inverse spin Hall effect

We propose an experimental method utilizing a strongly spin-orbit coupled metal to quantum magnet bilayer that will probe quantum magnets lacking long range magnetic order, e.g., quantum spin liquids, via examination of the voltage noise spectrum in the metal layer. The bilayer is held in thermal and chemical equilibrium, and spin fluctuations arising across the single interface are converted into voltage fluctuations in the metal as a result of the inverse spin Hall effect. We elucidate the theoretical workings of the proposed bilayer system, and provide precise predictions for the frequency characteristics of the enhancement to the ac electrical resistance measured in the metal layer for three candidate quantum spin liquid models. Application to the Heisenberg spin-$1/2$ kagom{\'e} lattice model should allow for the extraction of any spinon gap present. A quantum spin liquid consisting of fermionic spinons coupled to a $U(1)$ gauge field should cause subdominant $\W^{4/3}$ scaling of the resistance of the coupled metal. Finally, if the magnet is well-captured by the Kitaev model in the gapless spin liquid phase, then the proposed bilayer can extract the two-flux gap which arises in spite of the gapless spectrum of the fermions. We therefore show that spectral analysis of the ac resistance in the metal in a single interface, equilibrium bilayer can test the relevance of a quantum spin liquid model to a given candidate material.

Structural, Magnetic and DC Electrical Resistivity Studies of Ni–Zn–Cr Ferrites Prepared by the Citrate-Gel Auto-Combustion Method

Nanocrystalline chromium substituted Ni–Zn nanoferrites with composition Ni0.5Zn0.5CrxFe2−xO4 (0 ≤ x ≤ 0.25, in steps of 0.05) have been synthesized using citrate-gel auto-combustion method. The as synthesized powder of the starting composition has been characterized by using TG–DTA in air atmosphere for the temperature up to 800 °C and X-ray diffraction for its structure. Upon confirming the spinel phase for the starting composition, the chromium substituted Ni–Zn ferrite nano powders were synthesized by the similar procedure. The as synthesized powders were pressed into pellets and toroids under uniaxial pressure followed with sintering in air atmosphere for 4 h at 1100 °C for electrical and magnetic characterizations. The X-ray diffraction studies revealed a decrease of the lattice constant and considerable modification in the structure of Ni–Zn ferrite with the substitution of Cr3+ for Fe3+ ions. The room temperature FT-IR spectra displayed three significant IR absorption bands in the wave number range of 350–600 cm−1 which is a characteristic of the spinel structure. The band positions and intensities were found to vary with chromium substitution in Ni–Zn ferrite. The room temperature DC electrical resistivity and activation energy for conduction in the Ni–Zn–Cr ferrite samples were measured by using the two-probe method while the AC resistivity studies were carried out in the frequency range of 1 kHz to 13 MHz and the all the observations have been discussed in the light of existing understanding.

Synthesis, structural, dielectric and magnetic properties of hexagonal ferrites

Neodymium-substituted strontium hexa-ferrites Sr0.5-xNdxCo0.5Al0.5Fe11.5O19 (x = 0.0, 0.01, 0.03, 0.05) were synthesized by applying the Sol–gel scheme. The samples were characterized via X-ray diffraction, vibrating sample magnetometer and scanning electron microscopy. The XRD results exposed that the structure was a hexagonal type with some extra peaks and c/a ratio was less than 3.98 which confirmed the M-type hexagonal structure. From VSM results, it was perceived that saturation magnetization enlarged up to 0.01 then dropped with Nd concentration while the coercivity decreased up to 0.01 and then high by the substitution of Nd. SEM showed that grains were platelet-like and grain growth decreased with Nd concentration. Electrical properties were measured and it was observed that AC resistivity dwindled with rising temperature which showed that the material was like a semiconductor. These ferrites may be suitable for perpendicular recording media.

Investigation of relaxation phenomenon in lanthanum orthoferrite extracted through complex impedance and electric modulus spectroscopy

Impedance and electric modulus spectroscopy is exploited over a broad frequency and temperature range to find the relaxation phenomenon in LaFeO3 (LFO), which otherwise was concealed by the dc conductivity in dielectric ɛ*(ω) representation. The impedance measurements and the ac resistivity determined from Z′(ω) indicate that LFO is an insulator at room temperature and divulges the negative temperature coefficient of resistance. At higher temperatures, capacitive behavior flips to inductive behavior. The ac resistivity is exploited to determine the activation energy using the Arrhenius model. The relaxation peaks appear in the imaginary parts of electric modulus [M*(ω)] and impedance [Z(ω)], which have been exploited to determine the activation energy. The single distorted semicircle in the Nyquist and complex plots of electric modulus is evidence of the contribution of grains in the conduction process. At higher temperatures, data corresponding to the grain interior transform from an arc to a line with an intercept on the Z′(ω) axis and is parallel to the imaginary axis Z″(ω). Relaxation times calculated from the imaginary parts of impedance and electric modulus fit well in accordance with the Arrhenius law. Electron hopping, hole hopping, and oxygen vacancies play an important role in the dielectric response of grains. The relaxation frequencies of Z″(ω) and M″(ω) follow the sequence of scaling of magnitude of relaxation frequencies, i.e., fz′′≤fM′′. The separation of relaxation peaks of M″(ω) and Z″(ω) are indicative of a localized conduction process. The Giuntini law is applied to determine the hopping energies of charge carriers.

Ferromagnetic, electric, and ferroelectric properties of samarium and cobalt co-doped bismuth ferrite nanoparticles

Pure and co-doped multiferroic bismuth ferrite (BFO) nanoparticles were synthesized by the sol-gel method. Samarium and cobalt were used as co-dopants in BFO. The co-doped composites had the formula Bi1-xSmxFe1-yCoyO3, where x = 0.1 and y = 0.05, 0.1, or 0.15. Rietveld refinement of X-ray diffraction patterns showed a well-arranged crystalline rhombohedral structure with space group R3c. The rhombohedral structure transformed to an orthorhombic structure with space group Pbam in co-doped BFO nanoparticles. The ferromagnetic properties of synthesized nanoparticles were investigated in detail. Bi0.9Sm0.1Fe0.85Co0.15O3 showed a large increase in saturation magnetization and remanent magnetization to 3.2 and 1.5 emu/g, respectively. The large increase in magnetization with a reasonable squareness ratio and coercive field of co-doped BFO nanoparticles means they have potential uses in memory and spintronic devices. Electric properties such as resistance, reactance, AC resistivity, and AC conductivity were analyzed as a function of frequency. The AC resistivity decreased and the AC conductivity increased greatly with increasing co-dopant concentration. The ferroelectric behavior of pure and co-doped samples was also investigated. The maximum polarization and remanent polarization greatly increased to 12.5 and 7.5 μC/cm2, respectively, in Bi0.9Sm0.1Fe0.85Co0.15O3. The increased conductivity and reduced resistivity of co-doped samples may also be useful in efficient photovoltaic solar cells.

Direct estimation of the activation energy and relaxation times from the anomalies observed in the dielectric, AC and DC resistivity data using modified Lorentz equation

In the present study, we have investigated the dielectric, AC and DC resistivity properties of [Ba(Nd0.1Ti0.8Nb0.1)O3](1−y)[Na0.5Bi0.5TiO3]y ceramics with y = 0.60, 0.65 and 0.70. These ceramic materials were prepared through a solid-state double sintering route. It is shown in this paper, the other possible way to estimate the activation energy and relaxation times using modified Lorentz equations as proposed by us instead of well-known Arrhenius, modified Arrhenius and Vogel–Fulcher laws.

Crystal structure and electrical transport of nano-crystalline strontium-doped neodymium ortho-ferrites

Strontium-doped neodymium orthoferrites belong to the perovskite type of materials of a wide range of interests and applications. This material has an orthorhombic perovskite crystal system of space group (Pbnm) no. 62. Nd0.6Sr0.4FeO3 is synthesized using the co-precipitation method, and the resulting precipitated powder was fired at different temperatures to control the nano-crystalline size. The effect of the nano-crystalline size on the structure and the electrical properties were studied. The EDS spectra confirmed that the compound was formed according to the suggested calculations. SEM micrographs showed the homogeneity of the nano grain size in the compound. The nano-crystalline size of the prepared samples, fired at 705 °C and at 850 °C were 40 nm and 90 nm, respectively. Nd0.6Sr0.4FeO3 showed a semiconducting behavior according to the exponential dependence of resistivity on temperature in the range from 350 K up to 473 K. The temperature independence of impedance is found at high frequencies starting from 0.5 MHz. The frequency dependence–activation energy is found in the temperature range from 350 K up to 473 K. The found decrease in AC resistivity with frequency increase for Nd0.6Sr0.4FeO3 showed that the hopping mechanism occurred in the charge carrier transfer.

Dielectric, Magnetic and Spectral studies of gel grown Mg2+ Doped Copper Cadmium Oxalate Single Crystals

Mg2+ doped Copper Cadmium oxalate (MCuCO) single crystals were grown by a single diffusion method in silica hydrogel at room temperature. Powder X-ray diffraction (XRD) studies show the triclinic nature of the crystals. The dielectric constant, ac resistivity and ac conductivity have been measured as a function of frequency. The dielectric constant of MCuCO crystal decreases as the frequency of applied field increases; at the end it gives the diminishing value of dielectric constant. AC conductivity increases with the increase of frequency. Absorbance, transmittance, reflectance, refractive index, and energy gap of MCuCO crystal were determined by UV-Visible spectroscopy. A high bandgap corresponds to the dielectric behavior of the crystal.

Increased flux pinning force and critical current density in MgB2 by nano-La2O3 doping

MgB2 superconducting wires and bulks with nano-La2O3 addition have been studied. A series of MgB2 superconducting bulk samples with nano-La2O3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc2 and unchanged Birr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB6 in the nano-La2O3 doped MgB2 bulk samples and decreased MgB2 grain size in nano-La2O3 doped bulks. Monocore powder-in-tube (PIT) MgB2 wires without and with 5 wt% nano-La2O3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La2O3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M–H), transport critical current density (Jc) was measured at 4.2 K for the nano-La2O3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density Jc (B) of P-05 at 4.2 K and 8 T (6.0 ×104 A/cm2) was twice that of the control wire. The critical magnetic fields (Bc2 and Birr ) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05’s transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc2 or Birr.

Superconducting-like behavior of Bi/Bi2O3 interface

Metastable superconducting-like behavior of rhombohedral Bi samples exposed to surface oxidation under special conditions has been observed below the critical temperature range of 15–35 K. The ac magnetic susceptibility and electrical resistance measurements imply that the superconductivity appears in the Bi/Bi2O3 interfacial layer formed at the surface of the samples.

Structural, magnetic, and insulator-to-metal transitions under pressure in the GaV4S8 Mott insulator: A rich phase diagram up to 14.7 GPa

In addition to its promising potential for applications, GaV4S8 shows very interesting physical properties with temperature and magnetic field. These properties can be tuned by applying hydrostatic pressure in order to reveal and understand the physics of these materials. Not only pressure induces an insulator-to-metal transition in GaV4S8 but it also has an interesting effect on the structural and magnetic transitions. Using a combination of AC calorimetry, capacitance, and resistivity measurements under pressure, we determine the evolution of the structural and magnetic transitions with pressure and thus establish the T-P phase diagram of GaV4S8. To detect the insulator-to-metal transition, we use optical conductivity and DC resistivity measurements and we follow the evolution of the Mott gap under pressure. The structural transition temperature increases with pressure and a second transition appears above 6 GPa indicating a possible new phase with a very small gap. Pressure has surprisingly a very weak effect on the ferromagnetic transition that persists even very close to the IMT that occurs at around 14 GPa, implying that the metallic state may also be magnetic.

Dielectric, ferroelectric and ferromagnetic properties of samarium doped multiferroic bismuth ferrite

In this research, un-doped and samarium doped functional bismuth ferrite nanoparticles were synthesized by sol-gel method. The samples were annealed at 600 °C to obtain crystalline phase. The concentration of samarium was varied in bismuth ferrite as Bi1−xSmxFeO3 (where x = 0.05, 0.10 and 0.15). Rietveld refinement was conducted and crystal parameters were extracted. Crystallite size for doped nanoparticles was found to vary from 66 to 44 nm. Doping of 5 and 10% Sm in BFO, the impurity phases were found to suppress totally. The rhombohedral structure of bismuth ferrite with R3c space group has found to transform to orthorhombic structure with Pbam space group in doped samples. The frequency dependence dielectric properties namely dielectric constant, dielectric loss, resistance, reactance, AC resistivity, AC conductivity and modulus of electricity were measured on nanocrystalline solids. The influence in ferroelectric property, P-E hysteresis loop of un-doped and doped samples was also investigated on nanocrystalline solids. The maximum polarization was found to be 0.084 μC cm−2 in Bi0.85Sm0.15FeO3. The coercive field was decreased in doped samples due to suppression of impurity phases and a minimum value was found in Bi0.9Sm0.1FeO3. The M–H hysteresis loop of synthesized nanoparticles has also been determined. The magnetization of doped samples showed an enhanced saturation magnetization and remnant magnetization of 1.87 and 0.57 emu gm−1 respectively in Bi0.85Sm0.15FeO3 nanoparticles.

Dielectric properties of superparamagnetic titanium doped nanophased Mn–Zn ferrites for high frequency applications

Effect of Ti4+ ions on Structural, Dielectric and Magnetic properties of nanophasedMn0.5Zn0.5TixFe2–4x/3(x = 0.0, 0.01, 0.02,0.03, 0.04 and 0.05) ferrites synthesized by hydrothermal method is studied. XRD peaks reveal pure spinel phase without extra peaks. Lattice parameter (a) is found to vary non-linearly with dopant concentration (x). An overall decrease in Crystallite Size (D) (varying from 78 nm–41 nm) with x is witnessed. Values of dielectric constant () and loss factor (tan δ) of Ti4+ doped Mn–Zn ferrites are lower than that of the undoped sample. Increase of AC resistivity (ρ) by an order of 10 in Ti4+ doped Mn–Zn ferrites is ensued due to locking of Ti4+–Fe2+ pairs. Lowered values of Ms is attributed to spin canting due to growth of nanosized grains, weakening of exchange interactions by non-magnetic Ti4+ doping and lower values of x-ray density. Transition from single to multi- domain region of Mn–Zn–Ti ferrites is clearly evinced from the plot of Coercivity (Hc) with D. Reduced value of coercivity to zero upto a critical size of ∼49 nm indicates the existence of superparamagnetism in these ferrites. Superparamagnetismis first ever reported in the present caseof Ti4+ doped Mn–Zn ferritessynthesized by hydrothermal method. Relatively lowered values of (29–18), tan δ (of the order of 10–2–10–3), higher values of ρ (106 Ω—cm) and lowered values of Hc obtained with Ti4+ doping improve the eddy current losses and direct these materials for high frequency applications.

Tailoring the properties of Ni-Zn-Co ferrites by Gd3+ substitution

Structural, magnetic and electrical properties of Ni0.7Zn0.2Co0.1Fe2−xGdxO4 (x = 0.00, 0.02, 0.05, 0.07, 0.1 and 0.12) ferrites, prepared by the standard solid-state reaction technique have been studied. The single phase of cubic spinel structure was confirmed by X-ray diffraction. The lattice constant, bulk density and porosity were calculated. The FE-SEM examined the surface morphology of the prepared samples. Real and imaginary parts of the complex permeability and magnetic properties of the samples were measured using Impedance Analyzer and VSM respectively. Initial permeability decreases with increasing of Gd content up to x = 0.05 and then increases with Gd. The dielectric constant and dielectric loss tangent of the studied samples decreased with the increase of Gd concentration. The eddy current loss of the prepared samples is expected to decrease with the increase of Gd content due to increase in ac resistivity. Complex impedance plots were studied to understand the contribution of grains and grain boundary resistances.

Synthesis and Elucidation of Structural, Morphological, Magnetic, and Electrical Properties of Al3+ Substituted Ba0.7Sr0.3Ti1 − xAlxO3 Perovskites

In this paper, the structural, morphological, magnetic, and electrical properties of Al3+ substituted Ba0.7Sr0.3Ti1 − xAlxO3 (where x = 0, 0.05, 0.10, 0.15, 0.20) perovskites were properly elucidated which were successfully synthesized by the standard solid-state reaction method. All the compositions were calcined at 700 °C for 4 h and sintered at 1150 °C for 4 h. The XRD patterns of all the prepared compositions were of single phase and showed good crystallinity. The crystalline phase was indexed as a simple cubic structure preferentially oriented along (110) plane. The lattice constant increased for the sample with Al content; x = 0.05 then decreased with further increase of Al content. FESEM images confirmed good microstructural and morphological properties with a homogeneous distribution of grains in the sample which was surrounded by a visible grain boundary. The result observed from VSM showed that all the compositions exhibited weak ferromagnetic properties at room temperature. The real part of complex permeability showed that the resonance frequency shifted towards higher frequency because Al3+ substitution and the quality factor increased at high frequency which indicated that the prepared perovskites had high-frequency magnetic applications. A high-quality factor was observed at a higher frequency which confirmed high-frequency dielectric applications of the prepared compositions. AC resistivity increased with the increase in Al content which indicated that the prepared perovskites had low eddy current loss characteristics.

Study of ac and dc electrical resistivity variation with high pressure of as-grown pure, indium and antimony doped SnS single crystals

The direct vapour transport technique grown single crystals of pure SnS, indium (5 and 15 at%) doped SnS and antimony (5 and 15 at%) doped SnS are used in the present study. The as-grown single crystals are completely characterized. The single crystal growth and comprehensive characterization results are presented in another research paper of the authors. All the as-grown single crystals are p-type semiconductors. The ac and dc electrical resistivity variation with high pressure of the as-grown single crystals up to 5 GPa are measured. The obtained ac and dc electrical resistivity variation with high pressure results are deliberated in this paper.

Study of physical properties towards optimizing sintering temperature of Y-substituted Mg-Zn ferrites

Optimum sintering temperature (Ts) plays an important role in controlling densification and growth of grains which greatly affect the magnetic and electrical properties of polycrystalline materials. Y-substituted Mg-Zn [Mg0.5Zn0.5YxFe2−xO4 (0 ≤ x ≤ 0.05)] ferrites have been prepared by using conventional standard ceramic technique and were sintered between 1100 and 1300 °C in the steps of 50 °C. Characterizations of the samples have been done by X-ray diffraction (XRD) technique, field emission scanning electron microscopy (FESEM), dielectric and permeability measurements to find the suitable Ts. XRD data have been analyzed and the results confirmed the same information regarding phase analysis at different Ts. Further characterization of the samples, sintered at 1100 and 1150 °C, were not continued due to their low density and high porosity. FESEM images indicate the change in microstructure with Ts. The decrease (increase) of ac electrical resistivity with Ts (Y content) has been observed while the dielectric constant behaves in opposite manner. Impedance spectroscopy also exhibited similar trend as ac electrical resistivity. The initial permeability revealed the wide stability zone of frequency and different optimum Ts for different compositions. The ac resistivity values for the compositions have been found higher at 1200 °C, but the compositions have highest value of bulk density and permeability at other Ts. The effect of Ts on the physical properties (bulk density, porosity and permeability) of Y-substituted Mg0.5Zn0.5YxFe2−xO4 (0 ≤ x ≤ 0.05) ferrites elucidate that the optimum Ts should be of 1250 °C for x = 0.01, 0.02 and 0.03 while the Ts should be of 1300 °C for x = 0.04 and 0.05.

Frequency and temperature dependent intrinsic electric properties of manganese doped cobalt ferrite nanoparticles

This paper reports a series of investigations on the frequency and temperature dependent intrinsic electric properties of manganese doped cobalt ferrite nanoparticles. The investigated samples were prepared via solid state reaction route using the planetary ball milling technique. The XRD patterns confirm their crystallinity and single phase spinel structure. The Field Emission Electron Scanning Microscopic (FE-SEM) micrographs show the agglomerated particle with a nearly spherical shape. The estimated lattice constant is found to decrease but the crystallites size to increase with the Mn content. The ac resistivity is found to decrease exponentially with the increase of frequency due to the effect of multilayer capacitance. Conversely, the ac conductivity is observed to increase exponentially with the frequency due to increased hopping between metallic cationsFe2+and Fe3+. The temperature dependent ac resistivity exhibits the semiconducting behavior of the materials above room temperature. The variation in both the resistivity and conductivity is assumed to follow the Arrhenius equation at a selected frequency of 10 kHz. The estimated activation energy using the Arrhenius equation of ac conductivity in the ferro region is found to increase with the Mn content. This increasing trend in activation energy signifies ferrimagnetic-to-paramagnetic phase transition. Besides, the electric modulus shows the contributions of both the grain and grain boundaries to the conduction mechanism in the materials. This nature of temperature dependent resistivity is as usual for the majority of nanoferrite materials.

Metal-organic frameworks filled epoxy composites: exploring new chances for low-κ polymer dielectrics

ABSTRACTHerein, a simple strategy was proposed to explore new chances for low-κ epoxy composites by adding ZIF-8 as additive. Compared with that of pure epoxy sample, the dielectric constant of the epoxy/ZIF-8 composite (0.3 wt%) was decreased to 3.45 from 4.12, the dielectric loss was decreased, the AC electrical resistivity was increased, and the water absorption was decreased from 0.41% to 0.18%. Moreover, the epoxy/ZIF-8 composites possessed improved thermal stability under air atmosphere. Motivated by these results, the ZIF-8 was identified as a promising candidate for preparing low-κ epoxy composite, which maybe get further development in the future.

Effect of Hydrothermal Reaction Time on Electrical, Structural and Magnetic Properties of Cobalt Ferrite

Abstract Cobalt ferrite was synthesized by hydrothermal route in order to investigate the effect of hydrothermal reaction time on structural, magnetic and dielectric properties. The synthesized cobalt ferrite was characterized by X-ray diffraction, Fourier transform infrared and Vibrating-Sample Magnetometer (VMS). XRD data analysis confirmed the formation of cubic inverse spinel ferrite for complete time series as the high intensity peak corresponds to cubic normal spinel structure. The ionic radii, cation distribution among tetrahedral and octahedral sites, lattice parameters, X-ray density, bond lengths were also investigated cobalt ferrite prepared at different hydrothermal reaction time. The crystallite size was found to be in the range of 11.79–32.78 nm. Tolerance factor was near unity that also confirms the formation of cubic ferrites. VSM studies revealed the magnetic nature of cobalt ferrite. The coercivity (1076.3Oe) was observed for a sample treated for 11 h. The squareness ratio was 0.56 that is close to 0.5 which shows uniaxial anisotropy in cobalt ferrite. Frequency dependent dielectric properties i.e. dielectric constant, AC conductivity, tangent loss and AC resistivity are calculated with the help of Impedance Analyzer. Intrinsic cation vibration of cubic spinel ferrites are confirmed from FTIR analysis in the range of 400–4000 cm−1. In view of enhanced properties, this technique could possibly be used for the synthesis of cobalt ferrite for different applications.