Dependent Ac Susceptibility Research Articles

Observation of spin-glass behavior in homogeneous (Ga,Mn)N layers grown by reactive molecular-beam epitaxy

We present a detailed study of the magnetic properties of (Ga,Mn)N layers grown directly on 4H-SiC substrates by reactive molecular-beam epitaxy. X-ray diffraction and transmission electron microscopy demonstrates that homogeneous (Ga,Mn)N alloys of high crystal quality can be synthesized by this growth method up to a Mn-content of 10--12 %. Using a variety of magnetization experiments (temperature-dependent dc magnetization, isothermal remanent magnetization, frequency and field dependent ac susceptibility), we demonstrate that insulating (Ga,Mn)N alloys represent a Heisenberg spin-glass with a spin-freezing temperature around 4.5 K. We discuss the origins of this spin-glass characteristics in terms of the deep-acceptor nature of Mn in GaN and the resulting insulating character of this compound.

AC loss and critical current density in Bi-2223 tapes with oxide additives and reinforced Ag sheaths

A series of measurements have been conducted on the field and temperature dependent AC susceptibility (ACS) of Bi-2223 based superconducting tapes with Ag sheath of different degrees of hardness and various oxide additives (SrCO 3 and BaZrO 3) in the core materials. The applied AC field was aligned perpendicular to the tape surface, i.e. along the c crystallographic axes. The analysis of these data based on the strip model shows that SrCO 3 additive does not give rise to meaningful changes in the AC loss density ( q) and current density ( J c) while BaZrO 3 additive appears to lower q and J c considerably, especially at lower temperature. A reversed trend of changes in q and J c is exhibited by reinforcement of the Ag sheath.

Crystal and magnetic structures of AFe IIFe III(C 2O 4) 3 (A=organic cation): two-dimensional honeycomb ferrimagnets

Ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFe IIFe III(C 2O 4) 3 (A=quaternary ammonium, phosphonium) all crystallise in two-dimensional hexagonal honeycomb lattices but while some show conventional Néel type Q magnetisation at low temperature others show negative Néel type N magnetisation below a compensation temperature T comp. The compounds having negative magnetisation show a discontinuity in the magnetisation at a temperature between T comp and T c suggesting a magneto-strictive transition. For the type Q compounds, hysteresis and frequency dependent AC susceptibility show that the low temperature magnetic state is glassy, either because of frustration of random anisotropy domains below a blocking temperature, or imbalance in the fractions of Fe II and Fe III.

Low-field magnetic properties of amorphous and nanocystalline FeCrCuNbSiB alloys

The AC susceptibility dependence on magnetic field, time and temperature of amorphous as well as nanocrystalline Fe 73.5− x Cr x Cu 1Nb 3Si 13.5B 9 ( x=0–4) alloys was studied. Micromagnetic model is used for calculating the activation energy spectra (AES) of the magnetic after-effect (MAE). It was observed that addition of Cr to the amorphous FeCrCuNbSiB alloys highly decreases the amplitude of the MAE so that no MAE is observed for Cr content higher than 2 at%. After annealing at 550°C, the initial susceptibility increases as a result of magnetic softening during nanocrystallization and the MAE vanishes. The nanocrystalline state was characterized by the high magnetic as well as structural stability. Moreover, addition of 1 at% Cr makes the initial susceptibility of the nanocrytalline sample higher than in the FINEMET alloy.

Scaling of vortex transport properties and nonlinear ac response of high-temperature superconductors

The macroscopic behavior of high-temperature superconductors is described by a nonlinear response function in combinations with Maxwell equations. This function is compatible with the suggested different model pinning barriers U(J). A comparison of this function to the scaling behavior of the isothermal current-voltage characteristics measured in twinned YBa2Cu3O (YBCO) samples shows fair agreement. We also compare the amplitude dependence of ac susceptibility derived from this function with several experimental results of high-temperature superconductors and find a general power law in the out-of-phase peak shift.

Effect of Organic Cation A on the Crystal Structure and Magnetisation of the Layer Molecular Ferrimagnets AFeIIFeIII(C2O4)3

We survey the crystal and magnetic structures and bulk magnetisation for representative examples of the series of ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFeII FeII(C2O4)3 (A = quaternary ammonium, phosphonium). The compounds all crystallise in two-dimensional hexagonal honeycomb lattices but while some show positive magnetisation at low temperature (Néel type Q) others show a large negative magnetisation below a compensation temperature Tcomp (Néel type N). In compounds that exhibit negative magnetisation a discontinuity in the magnetisation is observed at Tt (Tcomp < Tt < Tc) indicative of a magneto-strictive transition. In compounds that exhibit positive magnetisation, hysteresis and frequency dependent AC susceptibility indicate a glassy magnetic order at low temperatures, possibly arising from the frustration of random anisotropy domains below a blocking temperature, or imbalance and disorder of Fe(II) and Fe(III).

Double transition behavior induced by coherent anisotropy in random anisotropy magnets amorphous HoFe and ErFe

Magnetic phase transition in weak random anisotropy magnets amorphous a-HoFe and ErFe thin films with “in-plane” coherent anisotropy are studied by measurements of temperature (T) dependence of ac susceptibility. We observed distinct difference in the ac susceptibilities between samples with the coherent anisotropy and without the coherent anisotropy; double transition behavior from paramagnetic to ferrimagnetic (FWA) to correlated spin-glass-like is induced by the coherent “in-plane” anisotropy field larger than about 8 kOe even without H. Such a behavior is also observed in a-DyGdFe with “perpendicular” coherent anisotropy. But, larger coherent anisotropy field than about 20 kOe is needed to induce the FWA state, which may arise from the symmetry difference in the coherent anisotropy.

Flux creep determined ac magnetic response in type-II superconductors

The ac magnetic response in type-II superconductors was analyzed on the basis of flux creep theory. A model was proposed to describe the variation of current density in alternating field, which simultaneously took into account the sweeping applied field and the flux creep inside the sample. The frequency dependent ac susceptibility of a textured YBa2Cu3O7 bulk sample was measured and explained in this model.

Dynamic susceptibility of a strong random anisotropy magnet

Frequency dependent complex ac susceptibility measurements have been made on amorphous Dy{sub {ital x}}Y{sub 7{minus}{ital x}}Ni{sub 3}, a model strong random magnetic anisotropy (RMA) system. Analysis of these measurements in terms of coexisting free and correlated or clustered Dy spins has enabled the evolution of the distribution of spin relaxation times to be mapped as a function of temperature from the paramagnetic phase into the low-temperature speromagnetic phase. Qualitatively the results are similar to those observed close to {ital T}{sub {ital g}} for spin-glass systems, but with a major difference: for the strong RMA {ital a}-Dy{sub {ital x}}Y{sub 7{minus}{ital x}}Ni{sub 3} system the spin relaxation times, as {ital T}{sub {ital g}} is approached from above, are of the order of seconds. This is some four orders of magnitude longer than those observed for typical spin glasses. {copyright} {ital 1996 The American Physical Society.}

Frequency dependence of ac magnetic response in type-II superconductors

The ac magnetic response in type-II superconductors was analyzed on the basis of flux creep theory. A modified model was proposed to describe the variation of current density in a sample immersed in an alternating field, which simultaneously took into account the sweeping applied field and the flux creep inside the sample. The frequency dependent ac susceptibility of a textured YBa 2Cu 3O 7 bulk sample was measured and explained in this model. Also discussed was the relation between our model and other models that appeared in literature.

Magnetic properties of glassy Fe/sub 91-x/Zr/sub 7/B/sub 2/Ni/sub x/ (x=0, 5, 10, 15) alloys

We have studied the magnetization behaviour and the temperature dependence of AC-susceptibility at very low fields (5 mOe to 50 mOe) of glassy Fe/sub 91-x/Nr/sub 7/B/sub 2/Ni/sub x/ (x=0, 5, 10, 15) alloys. At zero Ni concentration our data suggests the existence of spin-glass behaviour at temperatures below 30 K. The addition of Ni drastically modifies the magnetic properties: the antiferromagnetic exchange coupling is reduced, the magnetic moment of Fe increases from 1.58 to 2.29 /spl mu//sub s/ and T/sub c/ increases from 230 to 478 K. Thus, substitution of Fe by Ni favours the increase of magnetic order in this system.

Intergrain magnetization in high temperature superconductors

The results of intergrain magnetization calculations in high-temperature superconductors in a critical state model with the generalized expression for the critical current density Jc= alpha 0(1-T/Tc)q/(B0+ mod B mod )beta are presented. The temperature and field dependent local fields, magnetic loops and ac susceptibility have been calculated. The procedure of determining parameters of pinning centres is also discussed. The experimental data of temperature dependences of fundamental ac susceptibility measured with various ac field amplitudes are compared with the calculated ones for a ceramic sample of YBa2Cu3O7- delta .

Frequency dependence of ac susceptibility in superconducting BiPbSrCaCuO

Abstract The frequency dependence of the ac susceptibility in polycrystalline samples of Bi 0.7 Pb 0.3 SrCaCu 1.8 O x has been studied in the range 1–20 kHz. The peak in the imaginary part of the susceptibility shifts slightly to higher temperature as the frequency increases. This behavior could be explained in terms of thermally activated flux creep. The activation energy for flux creep ranges from 2 eV at 0.06 Oe to 0.5 eV at 1.2 Oe.

Influence of exchange bond disorder on the magnetic properties of (Pd1−xFex)95Mn5 near T c

A summary of a detailed study of the field and temperature dependent ac susceptibility of a series of ternary (Pd1−xFex)95Mn5 alloys near the ferromagnetic ordering temperature Tc is presented. By following the behavior of the crossover line (a line of maxima in the field dependent susceptibility above Tc, the maxima moving upwards in temperature but decreasing in amplitude with increasing field) it is possible to catalog the influence of exchange bond disorder on aspects of the critical behavior. These ternary systems are not soft ferromagnets and thus asymptotic critical exponents are difficult to estimate using this technique. Nevertheless, effective exponents (i.e., obtained away from the critical point) clearly reflect the presence of significant variance in the distribution of exchange bonds.

The longitudinal and transverse magnetoresistivities and the AC susceptibility of Fe10Pt90

A summary of detailed measurements of field and temperature dependent magnetic and transport properties in F10Pt90 is presented, with particular emphasis on attempting to evaluate the influence of spin-orbit coupling. Analysis of the field dependent AC susceptibility near the Curie temperature Tc approximately=170 K enables effective exponent values to be estimated, although the presence of magnetic anisotropy arising from spin-orbit coupling prevents asymptotic exponent values from being found. The anisotropy of the magnetoresistance in this system also originates from spin-orbit coupling, and this behaviour is discussed in terms of both itinerant and localized models. A number of estimates for Tc are deduced from these different measurements and compared.

Complex ac susceptibility studies of the disordered molecular based magnets V(TCNE)x: Role of spinless solvent

The effect of different spinless solvents on the magnetism of V(TCNE)x⋅y(solvent) (TCNE=tetracyanoethylene) is studied by temperature (T) dependent ac susceptibility above 2.3 K. Though V(TCNE)x⋅y(CH2Cl2) has a critical temperature Tc ∼400 K, for V(TCNE)x⋅y(C4H8O), Tc ∼205 K and the coercive field is reduced, while for V(TCNE)x⋅y(CH3CN), Tc ∼140 K or below and a spin glass phase exists below a spin freezing temperature of Tf∼7–15 K. The variations with solvent are suggested to be due to spin dilution and random anisotropy introduced by the spinless solvents.

Measurements of the ac susceptibility and the spontaneous resistive anisotropy in dilute P_d_Fe

The results of detailed measurements of the field and temperature dependent ac susceptibility (χac) and the spontaneous resistive anisotropy (SRA) of P_d_Fe samples with between 0.35 and 2.4 at. % Fe are summarized. The χac data enable estimates for the critical exponents γ, β, and δ to be made, and in the 1.4 at. % sample these are close to Heisenberg model predictions. In the other samples evidence is found for exchange bond disorder. The SRA results are consistent with this transport coefficient exhibiting critical behavior governed by percolationlike behavior.

Effects of hydrogen on the magnetic properties of amorphous Fe 90Sc 10

Detailed measurements of the field and temperature dependent ac susceptibility of amorphous Fe 90Sc 10, with and without hydrogen loading, are reported.

Linear and non-linear magnetic response of potentially re-entrant Fe-Zr glasses

Detailed measurements are presented of the field and temperature dependent AC susceptibility of a number of FeZr metallic glasses. By following the behaviour of the 'cross-over' line, the presence of a paramagnetic to ferromagnetic transition below room temperature in this system is confirmed; the asymptotic exponent values characterizing this transition display near Heisenberg-like values (for the index delta , measurements extend to much lower fields than previous studies), although complications arising from anisotropy (most probably spin-orbit coupling) are also revealed. For the first time, the leading term in the non-linear response is shown to display a distinct anomaly at low temperatures. Although not divergent (as in Ising model predictions) the temperature at which this anomaly occurs in the 8 at.% Zr sample agrees with the transverse spin-freezing temperature estimated recently from in-field Mossbauer measurements. While detailed realistic three-dimensional model predictions describing sequential phase transitions in systems with competing exchange interactions are currently unavailable, the present data, nevertheless, support the possibility of a second 're-entrant' transition in this system.

Frequency dependence of AC susceptibility due to the viscous motion of flux lines

The authors studied the influence of viscous flux flow on the field distribution and hysteresis losses in high-T/sub c/ superconductors with field-dependent critical current density J/sub c/. The results are compared with measurements of AC susceptibility on ceramic YBaCuO and BiSrCaCuO as well as on single-crystal YBaCuO samples. Clear evidence for flux flow effects is seen for frequencies above approximately 1 kHz for all samples. The extreme flux flow model indicates that the flux flow should play an important role at much lower frequencies. The deviations from these results could be caused to some extent by the field dependence of J/sub c/, but at lower frequencies the flux creep seems to be dominant. This is confirmed by the current-voltage characteristics of analogous samples. It is shown that some important differences appear in the frequency dependences of the real and imaginary parts of the AC susceptibility for fields of harmonic and triangular shape. These differences, as well as the weaker flux flow effects in BiSrCaCuO as compared with YBaCuO, offer support for the conclusion that the flux flow effects are generally less important than supposed by some theories.