Low Field Dc Magnetization Research Articles

DC magnetic field dependent microwave absorption in CMR material, La 0.7Sr 0.3MnO 3

The effect of temperature and DC magnetic field on the microwave absorption at 9.2 GHz has been measured in as synthesized pellet samples of La 0.7Sro 0.3MnO 3 and compared with previous measurements of the temperature and DC magnetic field dependence of the bulk resistance and susceptibility in a sample of the same composition. The onset of the ferromagnetic transition at 374 K is marked by a small decrease in the surface resistance followed by a large increase below the Curie temperature in contrast to the bulk resistance which shows only a decrease. The onset of the ferromagnetic transition is also indicated by the appearance of a derivative of the absorption versus DC magnetic field centered at zero magnetic field recorded using an electron paramagnetic resonance spectrometer. The surface resistance at room temperature is much more strongly dependent on DC magnetic field strength at low DC magnetic fields decreasing by a factor of 50% for a field of 600 G. compared to only a 0.5% decrease in the resistance for the same DC magnetic field. The difference is due to the strong negative dependence of the sample permeability on DC magnetic field at room temperature. The large DC magnetic field dependence of the microwave absorption at room temperature in as synthesized samples may have potential for device applications.

Surface resistance of YBa 2Cu 3O thin films at low dc magnetic fields

The surface resistance R s of YBa 2Cu 3O 7 thin films has been measured as functions of temperature and magnetic field through a resonant cavity technique. Temperature dependence of R s was found to follow a BCS-like variation at higher temperature and deviate from the theoretical prediction at lower temperature. As the field was increased, R s increased linearly at low fields and nonlinearly at higher fields, which can be well accounted for in terms of flux motion. The anisotropy of R s is observed for H//c and H//ab. The result can be understood in terms of the intrinsic flux pinning and the relative orientation between rf current and dc field.

Transport AC losses in multifilamentary Ag/Bi-2223 tapes in low external DC magnetic fields

Experimental results of measurements of the AC transport current losses on silver sheathed (Bi,Pb) 2Sr 2Ca 2Cu 3O x multifilamentary tapes are presented. In the frequency range 33–180 Hz the self-field losses are hysteretic. Transport loss changes due to low DC, applied transverse magnetic fields are also studied. External transverse DC magnetic field causes an increase in the transport losses due to lowering of the critical current I c of the superconductor. Critical current dependence on the external magnetic field extracted from loss data is compared with that measured directly by standard DC four-probe measurements.

Low-field microwave magnetoabsorption in manganites

We report the magnetic field dependence of microwave absorption at low fields (200 Oe), in thin film and bulk samples of La1−xBaxMnO3−z. We demonstrate, using a very simple cavity configuration, that the magnetic field sensitivity of the microwave response of the giant magnetoresistance materials offers possibilities of using such materials in devices operating at low fields. Low-field dc magnetization and ac resistivity measurements are also presented.

Anisotropic magnetic properties of TbNi2B2C single crystals.

Magnetic and transport properties of single crystals of ${\mathrm{TbNi}}_{2}$${\mathrm{B}}_{2}$C have been investigated by ac susceptibility, dc magnetization, specific heat, and resistance measurements. The compound shows highly anisotropic magnetic properties which come about as a result of the Tb magnetic moments lying predominantly in the ab plane. The ac susceptibility and low-field dc magnetization measurements indicate the presence of two magnetic transitions; one due to the antiferromagnetic (AFM) ordering of the Tb moments around 15 K, and another at 5 K, which we believe is due to a spin reorientation of the Tb moments. The resistivity (${\mathrm{\ensuremath{\rho}}}_{\mathit{ab}}$) measurements show a sharp decrease in the resistance at 15 K but contain no observable features corresponding to the lower-temperature transition. The compound does not show superconductivity above 300 mK. Specific heat at low temperatures shows a large \ensuremath{\lambda}-type anomaly at 13.8 K in addition to the anomalies associated with the AFM ordering of the Tb moments at 15 K (${\mathit{T}}_{\mathrm{AF}}$) and the spin reorientation at 5 K. Magnetization (M) and ${\mathrm{\ensuremath{\rho}}}_{\mathit{ab}}$(T) measurements indicate a decrease in ${\mathit{T}}_{\mathrm{AF}}$ as a function of applied field (H) for H\ensuremath{\parallel}ab, whereas for H\ensuremath{\parallel}c, ${\mathit{T}}_{\mathrm{AF}}$ is independent of field. The M-H isotherms at 2 K show that for H\ensuremath{\parallel}ab the compound goes through a series of temperature-dependent metamagnetic states before finally saturating into a ferromagnetic state for H\ensuremath{\gtrsim}21 kOe. For H\ensuremath{\parallel}c the compound shows a linear M-H behavior expected for a normal AFM compound. The ${\mathrm{\ensuremath{\rho}}}_{\mathit{ab}}$(H) measurements show anomalies due to the metamagnetic transitions and a large negative magnetoresistance above 21 kOe for H\ensuremath{\parallel}ab. \textcopyright{} 1996 The American Physical Society.

Spin-glass-like behavior in the diluted antiferromagnet FexMg1−xTiO3 near the percolation concentration

Low-field dc magnetization has been measured in the diluted Ising antiferromagnet FexMg1−xTiO3. The sample with 0.13 ≤ x ≤ 0.25 exhibited various properties which are characteristic of spin glass (SG), including the long-time relaxation phenomena of remanent magnetizations. In contrast, samples with x = 0.3 and 0.35 have shown typical reentrant SG-like behavior. By combining the Mössbauer data reported previously, the origin of these kinds of behavior is attributed to the freezing of antiferromagnetic clusters.

Concentration—temperature phase diagram of the diluted antiferromagnet MnXMg1-XTiO3

The temperature variation of low-field dc magnetization of the diluted antiferromagnet MnXMg1-XTiO3 has been measured. It has been found that the crossover phenomenon from random-field to spin-glass behavior occurs near the percolation concentration XP = 0.57, which is experimentally estimated. The origin of the appearance of the spin-glass-like behavior is discussed in relation to spin structures near the percolation concentration.

Temperature and magnetic field dependence of microwave absorption in Rb 3C 60 superconductor

The temperature dependence of the surface resistance of Rb 3C 60 at 9.2 GHz has been measured from 370 to 5 K. As well as the expected drop at the superconducting transition temperature, anomalous changes are observed at 370 and 190 K and attributed to phase transitions in the material. The dc and ac magnetic field dependence of the surface resistance in the superconducting state is measured. Both low dc and ac magnetic field weak link induced absorption are observed as well as high dc magnetic field vortex induced dissipation.

First flux-penetration fields in L2-xCexCuO4-y single crystals.

High-resolution measurements of low-field dc magnetization are used to obtain flux-penetration fields ${\mathit{H}}_{\mathit{p}}$(T) along the two main crystallographic directions of several ${\mathit{L}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ (L=Pr,Nd,Sm) single crystals. The departure from the linearity of the virgin M(H) curve and the appearance of remanence and magnetic relaxation are used to determine ${\mathit{H}}_{\mathit{p}}$. It is shown that ${\mathit{H}}_{\mathit{p}}$(T) may not be safely identified with ${\mathit{H}}_{\mathit{c}1}$(T) when H\ensuremath{\parallel}c; the implications of these results are discussed in order to estimate the values of the fundamental superconducting quantities as the lower critical field, the penetration depth, the Ginzburg-Landau parameter, and the anisotropy in these electron-doped copper oxide superconductors.

Magnetic Property and Phase Diagram of a Frustrated System with Competing Exchange Interactions, FexMn1-xTiO3

Systematic studies of a random mixture Fe x Mn 1- x TiO 3 with competing exchange interactions have been made by low-field dc-magnetization measurements. Typical behavior characterizing a spin-glass transition is observed in the samples with intermediate Fe concentrations. The samples with Fe- and Mn-rich concentrations undergo the transition to the antiferromagnetic phase, and the samples with certain concentrations reenter into the reentrant spin-glass phase at low temperatures. From these results, the concentration versus temperature phase diagram is determined. In Fe x Mn 1- x TiO 3 the characteristic properties of the spin-glass and reentrant spin-glass appear strongly, that is because Fe x Mn 1- x TiO 3 is a magnetically condensed system with strong frustration. The magnetic properties observed in the antiferromagnetic phase of this system have clarified that the spin configuration is quite different from those in ordinary antiferromagnets.

Theory for the hysteretic properties of the low-field dc magnetization in type-II superconductors.

We examine theoretically the following types of magnetization measurements of type-II superconductors: (1) zero-field cooled (ZFC), (2) field cooled with data collected on cooling (FCC), (3) field cooled with data collected on warming (FCW), and (4) remanent. In analyzing the irreversible behavior of the low-field dc susceptibilities, the important parameters are the critical current density, sample dimensions, and lower critical field. We point out that the irreversibility temperature is determined by the merging point of the FCC and ZFC, but not the FCW and ZFC, magnetization curves.

Energy dissipation and flux trapping in a high- Tc superconductor in low AC and DC magnetic fields

The results of measurements of low frequency AC losses in a cylindrical YBa 2Cu 3O 7− x sample subjected to a sinusoidally varying magnetic field, with a coaxially superimposed DC-bias field, are presented. It is found that magnetic history effects are not observed if only the AC field is applied. A quite different behavior is observed if a DC magnetic field is trapped in the sample. In such a case, a significant increase, above one order of magnitude, of the intergranular loss is observed, whereas the dissipation inside the granules is practically unchanged. Moreover, there exists a threshold value of the applied DC magnetizing field, below which changes of the AC losses are not observed. Its value is approximately equal to the first penetration field for the grains.

Effects of weak low frequency sinusoidal and dc magnetic fields on myosin phosphorylation in a cell-free preparation

The effects of weak, environmental range, sinusoidal (ac), and dc magnetic fields on myosin light chain phosphorylation in a cell-free system were studied. This enzyme system is Ca 2+calmodulin dependent, providing a well established ion binding mechanism for this investigation. The exposure system applied dc (0–200 μT, vertically and/or horizontally controlled) and/or ac (16 Hz, 20.9 μT) magnetic fields. The results appear to suggest that all of the ac and dc, as well as combinations of ac/dc magnetic fields, significantly (30–100%, p< 0.01) influenced myosin phosphorylation. The most surprising and important finding was that variations of the dc magnetic field (in the absence of ac components) were not only sufficient to alter the rate of phosphorylation, but also gave the maximum effect. Experiments with Mumetal shielding, which decreased the ambient dc and ac magnetic fields to ±0.1 μT, also resulted in a decrease of phosphorylation, to 35% of the control value. Study of phosphorylation kinetics showed a typical “S” shaped enzyme-substrate dependence, which again was modified by the dc field.

Magnetic fields and intracellular calcium: effects on lymphocytes exposed to conditions for 'cyclotron resonance'

There are a number of claims in the literature that specific combinations of low level DC and AC magnetic fields can cause biologically significant effects. The combinations of fields required to elicit these responses fulfil the theoretical conditions for classical cyclotron resonance of the selected ion. Because of the biological importance of calcium ions any effects on them are of particular interest, for instance the claimed increase in calcium uptake by electromagnetically exposed lymphocytes. The authors have measured the intracellular calcium concentration, by means of a sensitive fluorescent probe, during a 60 min exposure of mouse lymphocytes to 'cyclotron resonance' conditions for calcium ions. 'Resonance' conditions at two frequencies (16 Hz and 50 Hz) were tested, with a range of DC field amplitudes used to shift the frequency up to 25% either side of the calculated optimum. No change in intracellular calcium concentration could be detected when lymphocytes were exposed to 'cyclotron resonance' conditions or to the other magnetic field combinations used.

A SQUID magnetometer for low field DC magnetization and AC susceptibility measurements

The design and performance of an inexpensive high sensitivity SQUID magnetometer for measurements at low fields over a wide temperature range is described. A top-loading facility enables rapid measurements to be made on small single crystals, thin film and powder HTC samples. Operation is possible either in DC mode with sensitivity of 2*10-12 A m2 and drift of approximately 10-11 A m2 h-1 or in AC mode with sensitivity 10-12 A m2 and negligible drift.

Response of microwave surface impedance in EuBa 2Cu 3O 7− x film to a low magnetic field

The response of microwave impedance in a highly c -axis oriented EuBa 2 Cu 3 O 7− x film to a low DC magnetic field ( H 2 dependence, as predicted by the G–L model for pair-breaking in the Meissner state. However, clear saturation was observed in the surface impedance response with the field normal to the surface. Therefore, these responses were thought to arise from the properties of the weak links remaining in the film.

Anomalous Low-Field Magnetic Behaviour of High-TC Superconductors

ABSTRACTWe report on the studies of low-field magnetic properties in the well-characteristic superconducting LaCaBaCu3Oy, YBa2Cu3Oy, YaBa4Cu8Oy, (La1–85Sr0–15)CuO4-y, (Tl0–5Pb0–5)Ca2Sr2Cu3Oy and (Bi2−xPbx)Ca2Sr2Cu3Oy oxides. The studies of magnetic properties include ac zero-field cooled (ZFC), field-cooled (FC), and ac susceptibilities, and their dependences on the applied magnetic field and temperature. As the applied dc magnetic field is low enough, the FC susceptibility varies flat with the increasing of temperature from low temperature, but the diamagnetic signal becomes increasing at some temperature and reaches to the maximum values, and then continously decreases with temperature, at last the diamagnetism rapidly transfers to paramagnetism with temperature going to above Tc. The magnetization reduction at low temperature significantly appears at the low dc magnetic field and exists within a lot of single-phase high-Tc superconductors. However, this phenomenon is not observed in the measurements of low-frequency ac complex susceptibility for the ac applied fields from 0.001 G to 10 G and disappears for higher dc applied field. But, there exists a two-step drop in ac and ZFC susceptibilities curve and the drop correlates with the de FC magnetization dip. The magnetic field range of the dip occurrence is dependent on specimens. The strange magnetic property may be regard with superconducting coupling link in grain boundaries and interpreted by the grain-cluster model associated with Josephson effect.

Susceptibility and magnetization characterization of bulk high Tc superconductors: scaling length and critical current densities

Low field DC and AC susceptibility (swept temperature) and magnetization (swept field) are routinely used to characterize bulk samples of high Tc superconducting materials. More information can be obtained by extending these techniques, for example, by adding a DC offset field in the AC susceptibility measurements and by making magnetization measurements for several different cycles of the applied field. The relationships between the critical current and its scaling length are developed with respect to these experimental techniques.

The crystal structure and low-magnetic-field properties of the Pb- and Ba-doped Bi-Sr-Ca-Cu-O system

The XRD analyses for the Bi-Pb-Sr-Ca-Ba-Cu-O system show that the addition of Ba ions is favourable to the formation of the higher-Tc phase (Tc>100 K), and TEM observations show that the partial substitution of Ba ions for Ca ions induces a change in the crystal microstructure of the higher-Tc phase (Tc>100 K); a new modulation mode with a wavelength of 43 AA appears in the electron diffraction pattern of the higher-Tc phase. The EDS results confirm that Ba ions really intercalate into the crystal of the 2:2:2:3 phase. The DC magnetization measurements show that the sample displays a much larger hysteresis than that of a general Pb-doped sample. In addition, a study on the influence of low DC magnetic fields on the real component chi ' and imaginary component chi " of the susceptibility for the Ba-doped sample shows that Tp is related to the applied field HDC and their relation can be fitted by (1-Tp/Tc) varies as H0.23 when HDC<140 G, and (1-Tp/Tc) varies as H0.65 when HDC>140 G. These data can be excellently interpreted in terms of Bean's critical-state model.

Noncritical behavior and remanent magnetization in magnetically frustrated FeSbO4.

Low-field dc-magnetization and thermoremanent-magnetization measurements have been recorded from the rutile-related iron antimonate of composition ${\mathrm{FeSbO}}_{4}$. The zero-field-cooled susceptibility curve at low magnetic fields (H=20 Oe) shows a small peak at \ensuremath{\sim}72 K and a much broader peak at lower temperatures of \ensuremath{\sim}25 K. The temperature dependence of the remanent magnetization below \ensuremath{\sim}72 K indicates the existence of two different relaxation regimes which depend on the strength of the applied field in which the sample was cooled. The thermoremanent-magnetization measurements as a function of time show aging effects and the characteristic parameters are quite similar to those found in canonical spin glasses. The temperature dependence of the first nonlinear coefficient ${\mathit{b}}_{3}$ in the expansion of the magnetization in odd powers of (${\mathit{a}}_{1}$H) shows no divergence around the peak at 72 K, which is indicative of the absence of a true spin-glass transition. The moderate degree of frustation associated with an inhibited antiferromagnetic transition at \ensuremath{\sim}72 K, is nevertheless insufficient to produce a true spin-glass transition where the magnetic correlation length is even much shorter.