Low Field Ac Research Articles

Anomalous circular ac susceptibility and magnetoimpedance for nearly nonmagnetostrictive amorphous wire

The frequency dependent low-field circular ac susceptibility χ=χ′−jχ″ of a nearly nonmagnetostrictive Co68.2Fe4.3Si12.5B15 amorphous wire annealed at 470 °C is determined by impedance Z measurements under stepwise decreased longitudinal dc field Hdc from 216 to −216 A/m after premagnetization at 16 kA/m. It is found that compared with the results of the classical eddy-current model, the measured low-frequency loss factor is larger by an anomaly factor η between 1.5 and 3, depending on the value of Hdc. However, the χ(f) curve at each value of Hdc may be well fitted by an eddy-current model of bamboo-domain wall displacements if the material conductivity is replaced by a greater effective one and a proper value of effective domain size is chosen. The physical mechanism of such a fitting is discussed based on previous studies on anomalous dc circular susceptibility. This anomalous χ(f,Hdc) leads to an anomalous magnetoimpedance Z(Hdc) with a two-peak feature when frequency is high.

Vortex melting line and anisotropy of aBa2Ca3Cu4O8(O1−yFy)2 multilayered superconductor

We have measured the vortex melting lines of high-pressure-synthesizedBa2Ca3Cu4O8(O1−yFy)2 (nominalcomposition, 2y = 1.3, 1.6and 2.0) F(2y)-0234multilayered high-Tc superconductor using fundamental and third harmonic susceptibility responsescarried out on preferentially oriented crystallites with very low ac field amplitude(5 µT) and in applied dc fields up to 6 T. The vortex melting lines of all three F-substitutedsamples show interesting doping dependence and are very well described by the commonlyaccepted theory of melting lines. Interestingly, we discovered that the models describing thetemperature dependence of the in-plane London penetration depth also dependon the doping level: the 3D XY model for the nearly-optimum-doped sample[F(1.3)], the mean-field theory for the under-doped sample [F(1.6)] and the two-fluidmodel for the heavily-under-doped sample [F(2.0)]. We found that the vortexmelting lines in F-0234 are determined by the interplay in the coupling of pancakevortices through the charge reservoir layer (CRL), and through the block of innerCuO2 planes (IPs), respectively. The anisotropy values (47 for the near-optimally-doped sample)derived from experimental vortex melting lines are consistent with the values obtained byfirst-principles electronic-band-structure calculations.

Multiple superconducting transitions in theSr3Ru2O7region ofSr3Ru2O7-Sr2RuO4eutectic crystals

We report the superconducting properties of ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}{\text{-Sr}}_{2}{\text{RuO}}_{4}$ eutectic crystals, which consist of the spin-triplet superconductor ${\text{Sr}}_{2}{\text{RuO}}_{4}$ with a monolayer stacking of ${\text{RuO}}_{2}$ planes and the metamagnetic normal metal ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ with a bilayer stacking. Although ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ so far has not been reported to exhibit superconductivity, our ac susceptibility measurements revealed multiple superconducting transitions that occur in the ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ region of the eutectic crystals. The diamagnetic shielding essentially reached the full fraction at low ac fields parallel to the $c$ axis. However, both the shielding fraction and the onset temperature are easily suppressed by ac fields larger than 0.1 mT rms and no anomaly was observed in the specific heat. Moreover, the critical field curves of these transitions have a positive curvature near zero fields, which is different from the upper critical field curve of the bulk ${\text{Sr}}_{2}{\text{RuO}}_{4}$. These facts suggest that the superconductivity observed in the ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ region is not a bulk property. To explain these experimental results, we propose the scenario that stacking ${\text{RuO}}_{2}$ planes, which are the building blocks of superconducting ${\text{Sr}}_{2}{\text{RuO}}_{4}$, are contained in the ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ region as stacking faults.

Study of magnetic ordering in MnAl xCr xFe 2−2xO 4

The spinel solid solution series MnAl x Cr x Fe 2−2 x O 4 with x=0.0–0.9 in steps of 0.1 was studied using powder X-ray diffraction profile refinement, magnetization measurement at 80 K using an applied magnetic field 3.18×10 5 A-turn/m, Mössbauer spectroscopy at 300 K and low field AC susceptibility measurements. We have seen that the increase of Cr 3+ ions and Al 3+ ions in B sites at the cost of Fe 3+ ions leads the present system towards the Néel type of collinear ordering of spins. The negative quadruple shifting seen in Mössbauer results and high value of oxygen positional ‘ u’ parameter indicate oblate type of deformation of 3d 5 shell. The results indirectly support the theory of “exchange disorder” of Fe 3+ ions in B sites as the cause of the reduction in magnetic moment for MnFe 2O 4 from Néel type of ordering. There is the emergence of ‘spin clusters’ in the long-range network of spins as indicated in the ac susceptibility and Mössbauer results at room temperature in x>0.7 compounds.

Low-temperature scaling of the susceptibility of Ni films

Measurement of low field ac susceptibility of Ni thin films over the temperature range of $5--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reveals a surprising power law scaling. The temperature-dependent part of the normalized susceptibility, ${\ensuremath{\chi}}_{\ensuremath{\parallel}}∕{M}_{S}\ensuremath{-}{\ensuremath{\chi}}_{\mathrm{rot}}∕{M}_{S}$, where ${\ensuremath{\chi}}_{\ensuremath{\parallel}}$ is the initial susceptibility for in-plane magnetization, ${\ensuremath{\chi}}_{\mathrm{rot}}$ is the domain rotation contribution, and ${M}_{S}$ is the saturation magnetization, scales with the nonlinear reduced temperature as ${t}^{\ensuremath{-}2}$ over the entire temperature range, where $t=(T\ensuremath{-}{T}_{C})∕(T+{T}_{C})$ and ${T}_{C}$ is the Curie temperature. Thickness and reduced temperature dependences are completely decoupled. This result implies that domain wall motion does not contribute to the low field susceptibility.

Vortex melting line and dimensional crossover in Ba2Can−1CunO2n(O1−y,Fy)2 cuprate superconductors

Ba2Can−1CunO2n(O1−y,Fy)2[F(2y)-02(n−1)n], homologous series with F-substitution at apical oxygen site possess a thinner, 7.4Å, charge reservoir layer (CRL) and a strong interlayer vortex coupling was expected in these systems. We measured the vortex melting lines of high pressure synthesized Ba2Ca3Cu4O8(O1−yFy)2 (2y=2.0) multilayered system with 2 inner planes (IP) of CuO2 layers and Ba2Ca1Cu2O4(O1−yFy)2 (2y=1.4, 1.6 and 2.0) bilayer system with no IP; using fundamental and third harmonic susceptibility response carried out onto preferentially oriented crystallites at very low ac field amplitude and applied dc fields up to 12T. These experimental melting lines are described by the commonly accepted theory of two-fluid model. We observed 3D to 2D crossover in heavily underdoped F(2.0)-0212 sample, whereas such a vortex lattice crossover is not evident in F(2.0)-0234 sample; and explained on the basis of heavily underdoped and self doped states.

Griffiths phase and critical behavior in single-crystalLa0.7Ba0.3MnO3: Phase diagram forLa1−xBaxMnO3(x⩽0.33)

Analyses of temperature- and field-dependent ac susceptibility and magnetization data from a colossal magnetoresistive ${\mathrm{La}}_{0.7}{\mathrm{Ba}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ single crystal in terms of scaling behavior yield exponent values of $\ensuremath{\delta}=5.5\ifmmode\pm\else\textpm\fi{}0.3$ and $\ensuremath{\gamma}=1.41\ifmmode\pm\else\textpm\fi{}0.02$ (both slightly larger than Heisenberg model predictions), with $\ensuremath{\beta}=0.35\ifmmode\pm\else\textpm\fi{}0.04$ and a Curie temperature ${T}_{C}=310\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Detailed investigation of the low-field dc and ac susceptibilities reveals features consistent with the presence of a Griffiths phase (GP)---viz., an inverse susceptibility characterized by ${\ensuremath{\chi}}^{\ensuremath{-}1}\ensuremath{\propto}{(T\ensuremath{-}{T}_{C}^{R})}^{1\ensuremath{-}\ensuremath{\lambda}}$ with $\ensuremath{\lambda}=0.67\ifmmode\pm\else\textpm\fi{}0.05$. These, combined with previous results, enable a phase diagram summarizing the evolution of the GP with composition in this system to be constructed, and in this context, the possible importance of the variation of the acoustic spin-wave stiffness $D$ with composition is discussed.

Structural and magnetic properties of zinc- and aluminum-substituted cobalt ferrite prepared by co-precipitation method

Spinal ferrites having the general formula Co1 − xZnxFe2 − xAlxO4 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were prepared using the wet chemical co-operation technique. The samples were annealed at 800°C for 12 h and were studied by means of X-ray diffraction, magnetization and low field AC susceptibility measurements. The X-ray analysis showed that all the samples had single-phase cubic spinel structure. The variation of lattice constant with Zn and Al concentration deviates from Vegard’s law. The saturation magnetization σs and magneton number nB measured at 300 K using high field hysteresis loop technique decreases with increasing x, suggesting decrease in ferrimagnetic behaviour. Curie temperature TC deduced from AC susceptibility data decreases with x, suggesting a decrease in ferrimagnetic behaviour.

Electrical and magnetic characterization of nanocrystalline Ni–Zn ferrite synthesis by co-precipitation route

Nickel zinc ferrite nanoparticles Ni 1− x Zn x Fe 2O 4 ( x=0.25, 0.5, 0.75, 1) have been prepared by the chemical co-precipitation route. The samples were characterized by X-ray diffraction (XRD), DC electrical resistivity, dielectric constant and low field AC magnetic susceptibility. The powder XRD patterns confirm the single phase spinel structure for the synthesized materials. The crystallite size was calculated from the most intense peak (3 1 1) using the Scherrer formula. The crystallite size was found within the range 7–15 nm. The crystallite size decreases with increasing zinc concentration. DC electrical resistivity decreases as the temperature increases indicating that the samples have semiconductor like behavior. DC electrical resistivity of the samples at room temperature was found to vary from 1.67×10 9 to 4×10 9 Ω cm with zinc concentration. The activation energy and drift mobility were calculated from DC electrical resistivity measurements. The dielectric constant for all the compositions has been studied as a function of frequency in the range from 500 Hz to 1 MHz at room temperature. The dielectric constant follows the Maxwell–Wagner interfacial polarization. AC magnetic susceptibility measurements were carried out as a function of temperature to measure the transition temperature, which was found to decrease with zinc concentration. The decrease of transition temperature has been explained by the distribution of cations on A and B sites.

Tectonic implications of finite strain variations in Baraboo-interval quartzites (ca. 1700 Ma), Mazatzal orogen, Wisconsin and Minnesota, USA

Variably deformed Baraboo-interval quartzites (1750–1630 Ma) are now correlated by detrital zircon ages [Holm, D., Schneider, D., Coath, C.D., 1998. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: implications for extent of foreland deformation during final assembly of Laurentia. Geology 26, 907–910] and were analyzed using finite strain techniques which document a SE–NW tectonic shortening orientation from the Baraboo syncline (south) to the Flambeau syncline (north). Fabric interpretations using bedding and the principal strain axis orientations indicate there was a mix of layer-parallel and layer-normal (synfolding) shortening in the plane of tectonic transport (SE–NW) in the thrust belt. Finite strain magnitudes increase from the foreland Barron–Sioux quartzites south toward the Baraboo syncline. Electron backscatter device (EBSD) fabric diagrams (pole figures) of quartz optic axes also demonstrate an increasingly penetrative strain to the south. Anisotropy of [low-field ac] magnetic susceptibility (AMS), as a strain proxy, preserve K min measurements in the Baraboo syncline quartzites that are sub-horizontal, E–W and intersect bedding. AMS measurements in underlying rhyolite on opposite limbs of the Baraboo syncline are clean, appear to be primary (magmatic) as K max intersects bedding, but require a complex folding interpretation that is inconsistent with field kinematic indicators. The flat-lying foreland Barron and Sioux Quartzites preserve a mix of results: quartzites with no strain (finite strain and EBSD measurements) or small strains that preserve vertical shortening. AMS measurements in the foreland preserve K min axes that are sub-horizontal and parallel to the regional shortening orientation in the adjacent thrust belt. The Mazatzal orogen produced a thrust belt that is orders of magnitude wider (∼500 km) and of higher metamorphic grade, than most thrust belts, where both the thrust belt and foreland are quartzites.

Low field AC-susceptibility study on γ -irradiated Bi1.2Pb0.33Sr1.54Ca2.06Cu3O10+δ superconductor

We report here, the effect of γ-irradiation on low field AC susceptibility as a function of temperature on the grown polycrystalline Bi1.2Pb0.33Sr1.54Ca2.06Cu3O10+δ sample. The sample was irradiated with γ-ray dose of 100 MR and 300 MR. The temperature dependent AC susceptibility measurements have been carried out with varying AC field amplitude H ac (0.5 to 9.0 Oe) at frequency 131.1 Hz on both un-irradiated and irradiated samples. AC susceptibility measurements as a function of temperature at AC field amplitude H ac = 2.0 Oe have also been carried out with varying frequency (31.1 Hz to 1131.1 Hz). The temperature dependence of AC susceptibility with varying amplitude has been analyzed using well defined Bean's critical state model. The critical current density (J c) is calculated as a function of both AC field amplitude (H ac) and temperature. An increase in J c with the increase of γ-dose has been found.

Scaling Parameters in Frustrated Systems: Spin Glasses and Relaxor Ferroelectrics

Through analysis of low-field AC susceptibility data in relaxor ferroelectric and magnetic spin glass solids, the key parameters of activation energy, EA, and freezing temperature, Tf, were extracted from the Vögel–Fulcher dependence of the maxima. At higher temperatures the deviation temperature, TD, was obtained from departure of the high temperature Curie–Weiss behavior. Collectively, these parameters were found to scale across different compounds and solid solutions for both the magnetic spin glasses and relaxor ferroelectrics.

Investigation of Sm → Ca substitution in Bi(Pb)SrCaCuO high temperature superconductor by low field AC magnetic susceptibility

We have investigated the effect of Sm substitution in Bi(Pb)SrCaCuO system by performing AC susceptibility ( \(\chi = \chi ' + i\chi ''\)), XRD (X Ray Diffraction) and SEM (Scanning Electron Microscopy) measurements. The Sm → Ca substitution (Bi1.6Pb0.4Sr2Ca2−xSmxCu3Oy) was carried out by conventional solid-state reaction method. The susceptibility measurements were carried out at different values of the AC field amplitudes. The imaginary part of susceptibility is used to estimate the intergranular critical current density by means of the Bean’s model. The intergranular critical current density (Jc) of pure sample is found to be about 68 A/cm2 at 95 K. The intergranular Jc is seen to decrease with increasing Sm substitution. XRD pattern and SEM micrographs are given to provide information about Bi-2223 phase and grain size respectively.

Dynamic magnetic behavior ofBaCoO3quasi-one-dimensional perovskite

Polycrystalline $\mathrm{Ba}\mathrm{Co}{\mathrm{O}}_{3}$ was synthesized by a citrate technique using thermal treatments at high oxygen pressure. Magnetic susceptibility measurements were carried out at very low ac and dc fields on the compound. The magnetic properties of the material between 14 and $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ were identified with the appearance of nanoscale ferromagnetic regions. The observed magnetic irreversibilities can be understood in the framework of a superparamagnetic model. The real and imaginary parts of the complex susceptibility are consistent with an Arrhenius-like thermal relaxation law with an energy barrier of about $0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Through this model, the mean size of the ferromagnetic clusters could be determined; it is about $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter, if spherical geometry is assumed for them.

Low-field AC susceptibility in Pr 1− xHo xMn 2Ge 2

We report the structure and magnetic properties of Pr 1− x Ho x Mn 2Ge 2 ( 0.0 ≤ x ≤ 1.0 ) germanides by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr 2Si 2-type structure with the space group I 4 / m m m . Substitution of Ho for Pr leads to a linear decrease in the lattice constants and the unit cell volume. The samples with x = 0 and x = 0.8 have spin reorientation temperature. The results are collected in a phase diagram.

AC susceptibility of Sr 2RuO 4–Sr 3Ru 2O 7 eutectics: Dependence on AC field strength and frequency

The low-field AC susceptibility of a eutectic crystal of the Sr 2RuO 4–Sr 3Ru 2O 7 system exhibits superconducting transition features at three temperatures. We investigate the dependence on AC field strength from 0.021 to 2.1 Oe-rms and frequency from 17 to 3 kHz. The two lower-temperature peaks in the imaginary part originate in the Sr 3Ru 2O 7 part. The lowest-temperature peak is exceedingly sensitive to the field amplitude and shifts to lower temperatures with increasing amplitude. By contrast, it slightly shifts to higher temperatures with increasing frequency. These results support that the sensitive features involve weak links forming the screening paths.

Combined effect of Pr and Mn on the resistivity and low-field AC susceptibility of Y0.95Pr0.05Ba2(Cu1−xMnx)3O7−δ superconductors for x ⩽ 0.02

Resistivity and low-field (40A/m) susceptibility measurements are performed on the samples of Y0.95Pr0.05Ba2(Cu1−xMnx)3O7−δ (x⩽0.02). It has been observed that above x=0.005 resistivity decreases drastically in the presence of Pr atoms. Analyzing this feature it has been argued that potential scattering alone is inadequate for explaining the resistivity behaviour. Prominent role of the electronic effects is expected for increasing Mn content. Analyzing the imaginary part of ac susceptibility at the peak temperatures it has been found that the superconducting volume fraction fg is 0.12 for x=0, but reduces drastically to 0.03 for x=0.005. For other values of x, fg is higher than 0.12.

Short-range anisotropic ferromagnetic correlations in the paramagnetic and antiferromagnetic phases ofGd5Ge4

Signatures of short range anisotropic ferromagnetic correlations and ferromagnetic clustering, manifested as unusually large hysteresis and other anomalies of the low magnetic field dc magnetization and ac magnetic susceptibility, have been observed in both the antiferromagnetic and paramagnetic states of single crystal ${\mathrm{Gd}}_{5}{\mathrm{Ge}}_{4}$. Ferromagnetic correlations, which are most pronounced in a weak magnetic field applied along the $b$ axis, are readily suppressed by fields exceeding $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ and are believed to be related to a Griffiths-like phase that develops in ${\mathrm{Gd}}_{5}{\mathrm{Ge}}_{4}$ below ${T}_{\mathrm{G}}\ensuremath{\cong}240\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Structural, magnetic and electrical properties of Co 1−xZn xFe 2O 4 synthesized by co-precipitation method

Nanoparticles of Co 1− x Zn x Fe 2O 4 with stoichiometric proportion ( x) varying from 0.0 to 0.6 were prepared by the chemical co-precipitation method. The samples were sintered at 600 °C for 2 h and were characterized by X-ray diffraction (XRD), low field AC magnetic susceptibility, DC electrical resistivity and dielectric constant measurements. From the analysis of XRD patterns, the nanocrystalline ferrite had been obtained at pH=12.5–13 and reaction time of 45 min. The particle size was calculated from the most intense peak (3 1 1) using the Scherrer formula. The size of precipitated particles lies within the range 12–16 nm, obtained at reaction temperature of 70 °C. The Curie temperature was obtained from AC magnetic susceptibility measurements in the range 77–850 K. It is observed that Curie temperature decreases with the increase of Zn concentration. DC electrical resistivity measurements were carried out by two-probe method from 370 to 580 K. Temperature-dependent DC electrical resistivity decreases with increase in temperature ensuring the semiconductor nature of the samples. DC electrical resistivity results are discussed in terms of polaron hopping model. Activation energy calculated from the DC electrical resistivity versus temperature for all the samples ranges from 0.658 to 0.849 eV. The drift mobility increases by increasing temperature due to decrease in DC electrical resisitivity. The dielectric constants are studied as a function of frequency in the range 100 Hz–1 MHz at room temperature. The dielectric constant decreases with increasing frequency for all the samples and follow the Maxwell–Wagner's interfacial polarization.

Interplay between spin-glass and non-Fermi-liquid behavior inY1−xUxPd3

We report the results of a study of the ac and dc magnetic susceptibilities of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{U}}_{x}{\mathrm{Pd}}_{3}$. The magnetic properties of the system gradually evolve from the spin-glass (SG) behavior observed for $x=0.4$ to the non-Fermi-liquid (NFL) ground state that develops at $x=0.2$. For $x=0.4$, the SG properties are remarkably similar to those found in canonical spin glasses. We have been able to analyze our data in terms of critical slowing down when $T$ approaches the critical temperature associated with a spin-glass freezing phase transition. The values obtained for the dynamic and static critical exponents [$z\ensuremath{\nu}=10\ifmmode\pm\else\textpm\fi{}1$, $\ensuremath{\beta}=0.9(1)$, $\ensuremath{\gamma}=1.8(2)$] support the existence of a true spin-glass phase transition in ${\mathrm{Y}}_{0.6}{\mathrm{U}}_{0.4}{\mathrm{Pd}}_{3}$. On the other hand, the evolution of magnetic properties with a decreasing U concentration suggests an interplay between the SG and NFL ground states. The ground state that finally appears for $x=0.2$ displays a temperature dependence of its low-field ac susceptibility consistent with the prediction of the Griffith's phase model of NFL behavior. The role played by compositional disorder in the development of the NFL state is discussed.