Ac Magnetic Response Research Articles

Comparison of physical properties of ferrofluids based on mineral transformer oil and bio-degradable gas-to-liquid oil

Ferrofluids based on mineral transformer oil have been intensively studied over two decades owing to their outstanding thermal and electrical insulating properties. However, current environmental demands on transformer operation give priority to the use of biodegradable transformer oils. Clearly, the differences in physical properties of biodegradable oils and those refined from crude oil determine the physical properties of ferrofluids based on the two types of oils. In this study, ferrofluids with various concentrations of iron oxide nanoparticles have been prepared on mineral transformer oil (M−oil) and biodegradable transformer oil based on gas-to-liquid technology (S-oil). The ferrofluids were subjected to experimental investigation of magnetization, AC magnetic susceptibility, thermal conductivity, viscosity and dielectric response. Based on zero-field-cooled and field-cooled magnetization curves, a lower temperature of a specific magnetization maximum associated with the melting of the oil (phase transition) was found for M−samples than for S-samples. The effect of carrier liquid on the onset of extrinsic superparamagnetism in addition to the intrinsic superparamagnetism is observed. Spectra of AC magnetic susceptibility of both types of ferrofluids exhibit quasi constant behaviour with amoderate decrease at high frequencies of amagnetic field. The viscosity of M−samples is slightly higher than that of S-samples. On the other hand, M−samples exhibit lower thermal conductivity than S-samples. In both cases, the thermal conductivity linearly decreases with temperature. Dielectric spectroscopy has revealed remarkable dielectric dispersion below 100 Hz in both ferrofluid types. The relaxation has been ascribed to interfacial polarization, which can be considered as a serious drawback from an electrical engineering application point of view. The measured physical properties of the ferrofluids are analyzed in regard to the density and viscosity of the base oils.

Complex AC Magnetic Susceptibility as a Tool for Exploring Nonlinear Magnetic Phenomena and Pinning Properties in Superconductors.

The versatile AC magnetic susceptibility technique offers a detailed insight into the complex electrodynamic phenomena in superconductors. In the present study, we outline the key effects related to the temperature, AC field amplitude and frequency variations of the fundamental and harmonic components for an investigation of the vortex dynamics in a flux-grown FeSe crystal. By means of higher harmonic (nonlinear) analysis, we have explored certain atypical, asymmetric features in the AC magnetic response. These effects were identified through the detection of an even (second) harmonic and an unusual temperature shift in the odd (third) harmonic, possibly due to the complex interactions related to the composite superconducting/magnetic morphology of the crystal. Using the high-frequency sensitivity of the third harmonic, the basic functional dependencies of the pinning activation energy, as the main mixed state parameter, were determined with the implementation of the Kim-Anderson Arrhenius relation in the framework of the collective creep theory.

Negative resistance and heat dissipation in superconducting wires in an ac magnetic field

A superconducting wire embedded into an alternating magnetic field H∝cos(ωt) can manifest negative resistance. At high enough frequencies ω≳1/τ, where τ is the characteristic time of magnetic relaxation, this effect becomes prominent. As a result, the energy obtained by the wire from a source of magnetic field (external coil) is transformed into the electromotive force which reduces the external voltage required to keep the transport current in the wire. We show that negative resistance appears in line with the out-of-phase regime of the ac magnetic response, and both are related with the crucial inhibition of the vortex dynamics by zero-field (annihilation) lines. We also find a universal (independent of frequency) expression for the mean voltage in a wire.

Anomalous out-of-phase magnetic ac response in superconducting wires

We describe and explain two competing regimes of ac magnetic response in current-carrying type-II superconducting wires which were observed experimentally. In the usual regime, voltage $V(t)$, induced by vortex motion across the wire, is ``in phase'' with the external magnetic field $H(t)\ensuremath{\propto}sin(\ensuremath{\omega}t)$. However, as frequency $\ensuremath{\omega}$ grows up or transport current $I$ decreases, an anomalous, ``out-of-phase'' peak in $V(t)$ appears. If these two regimes coexist, then two peaks in voltage are observed per half period of $H(t)$ in both experiment and numerical simulations. At certain combinations of $\ensuremath{\omega}, I$ and the amplitude of the external field, the out-of-phase mechanism even overwhelms the usual, in-phase one. It is shown that the out-of-phase maximum in $V(t)$ is due to the inhibition effect of zero-field (annihilation) lines on flux motion. Such lines, if present in the sample, significantly decelerate magnetic relaxation and dramatically affect the induced voltage. A phase diagram enabling one to distinguish between the in-phase and out-of-phase regimes is constructed.

Magnetic response of a highly nonlinear soliton lattice in a monoaxial chiral helimagnet

We present the theory of nonlinear ac magnetic response for the highly nonlinear regime of the chiral soliton lattice. Increasing of the dc magnetic field perpendicularly to the chiral axis results in crossover inside the phase, when nearly isolated $2\pi$-kinks are partitioned by vast ferromagnetic domains. Assuming that each of the kink reacts independently from the other ones to the external ac field, we demonstrate that internal deformations of such a kink give rise to the nonlinear response.

Robust cycloid crossover driven by anisotropy in the skyrmion host GaV4S8

We report on the anomalous magnetization dynamics of the cycloidally-modulated spin textures under the influence of uniaxial anisotropy in multiferroic $\mathrm{GaV_4S_8}$. The temperature and field dependence of the linear ac susceptibility [$\chi_{1\omega}^{\prime}(T,H)$], ac magnetic loss [$\chi_{1\omega}^{\prime\prime}(T,H)$], and nonlinear ac magnetic response [$M_{3\omega}(T,H)$] are examined across the magnetic phase diagram in the frequency range $f = 10-10000$ Hz. According to recent theory, skyrmion vortices under axial crystal symmetry are confined along specific orientations, resulting in enhanced robustness against oblique magnetic fields and altered spin dynamics. We characterize the magnetic response of each spin texture and find that the dynamic rigidity of the N\'eel skyrmion lattice appears enhanced compared to Bloch-type skyrmions in cubic systems, even in the multidomain state. Anomalous $M_{3\omega}$ and strong dissipation emerge over the same phase regime where strong variations in the cycloid pitch were observed on lowering temperature in recent small-angle neutron scattering experiments [White et al., Phys. Rev. B 97, 020401(R) (2018)]. Here, we show that strong anisotropy also drives an extended crossover of the zero-field cycloid texture in $\mathrm{GaV_4S_8}$. The frequency dependence of these dynamic signatures is consistent with that of a robust anharmonic spin texture exhibiting a correlated domain arrangement. The results underpin the essential role of magnetic anisotropy in enhancing the rigidity of topological spin textures for diverse applications.

Mapping Dynamical Magnetic Responses of Ultrathin Micron-Size Superconducting Films Using Nitrogen-Vacancy Centers in Diamond.

Two-dimensional superconductors have attracted growing interest because of their scientific novelty, structural tunability, and useful properties. Studies of their magnetic responses, however, are often hampered by difficulties to grow large-size samples of high quality and uniformity. We report here an imaging method that employed NV- centers in diamond as a sensor capable of mapping out the microwave magnetic field distribution on an ultrathin superconducting film of micron size. Measurements on a 33 nm thick film and a 125 nm thick bulklike film of Bi2Sr2CaCu2O8+δ revealed that the alternating current (ac) Meissner effect (or repulsion of ac magnetic field) set in at 78 and 91 K, respectively; the latter was the superconducting transition temperature of both films. The unusual ac magnetic response of the thin film presumably was due to thermally excited vortex-antivortex diffusive motion in the film. Spatial resolution of our ac magnetometer was limited by optical diffraction and the noise level was at 14 μT/Hz1/2. The technique could be extended with better detection sensitivity to extract local ac conductivity/susceptibility of ultrathin or monolayer superconducting samples as well as ac magnetic responses of other two-dimensional exotic thin films of limited lateral size.

Investigation of fundamental and higher harmonic AC magnetic susceptibility of FeSe0.5Te0.5 superconductor

We present the complex harmonic magnetic susceptibilities χn = χ′n-iχ″n (n = 1, 3) of FeSe0.5Te0.5 polycrystalline superconducting sample. The ac magnetic susceptibility is measured as a function of various external perturbations such as temperature T, the ac magnetic field amplitude Hac, frequency υ, and the magnitude of dc bias field Hdc. The in-phase (χ′n) and out of phase (χ″n) components of the fundamental and third harmonics of ac susceptibility are found to vary as a function of ac driven field. Particularly, the curves shift toward lower temperatures with increasing Hac. Contrary to ac magnetic field (Hac), no noticeable change has been observed within the range of applied dc magnetic field (Hdc) of up to 20 Oe. At a fixed ac magnetic field of Hac = 0.5 Oe, both parts of the third harmonics show frequency dependence. The imaginary part of the third harmonics, χ″3 shows a small peak followed by a negative minimum at lower temperatures. The small peak diminishes as the frequency increases. The negative minimum suppresses and shifts towards the higher temperatures as we increase the frequency. To better understand the ac magnetic response under the influence of various perturbations, we have analyzed the polar plots (Cole-Cole) of the complex ac susceptibility for both the harmonics. Our analysis suggests that the studied sample is in a vortex glass state, characterized by a collective flux creep within Bean’s model, while the Kim–Anderson model is ruled out.

Harmonic AC magnetic susceptibility analysis of FeSe crystals with composite morphology

The AC magnetic response of multi-domain FeSe crystals has been explored by means of fundamental and third harmonic AC magnetic susceptibility (ACMS) analysis. Our previous studies have revealed a complex morphology which especially modifies the mixed state properties. The effects were expressed by an additional ‘pseudo’ peak feature of the magnetic hysteresis and in the present study we analyze its vortex dynamics nature and irreversibility behavior in the context of ACMS. We find that the effect is detectable mainly with the sensitive third harmonic component and especially at high AC field frequencies. From the analysis, the temperature dependence of the ‘pseudo’ peak effect at different DC fields ranges was determined, confirming the previously established superconductor–normal metal–superconductor type links between the domains in the crystal. The irreversibility line, the most important parameter for high power applications, shows a typical glass/liquid transition in the vortex matter. In addition we have observed surface barrier effects and vortex avalanche activity, the behavior of which appears to be influenced by the superconducting and magnetic nature coexistence and morphology. The presented results show the effective application of the harmonic AC magnetic susceptibility technique for a versatile analysis of complex nonlinear phenomena in materials with a sophisticated AC magnetic response.

Influence of pH on Dynamic Magnetic Susceptibility of Iron-Oxide Nanoparticles in a Chitosan Hydrogel Matrix

The effect of varying the pH on the ac magnetic response of the synthesized iron-oxide nanoparticles (FeOx NPs) embedded in chitosan hydrogel (CH) is experimentally demonstrated. By utilizing the chemical sensitivity of CH in FeOx-CH, we study the effect of changing the rheological property of the CH on the magnetic properties of FeOx NPs. Here, the FeOx-CH is synthesized via in situ precipitation of FeOx during CH gel formation. The morphology of the FeOx-CH is characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The swelling ratio (SR) and the ac susceptibility (ACS) of the FeOx-CH are also investigated as a function of pH. The XRD and TEM results confirm that the synthesized FeOx has a spinel crystal structure with a particle size of 10-20 nm, whereas SEM shows that the hydrogel has a porous structure. The SR result shows that FeOx-CH swells when the pH decreases (acidic). The ACS result shows that the frequency at the peak of the ACS imaginary part (χ" peak frequency) shifts to higher frequency upon the decrease of pH. From these results, it is inferred that the swelling enhances the Brownian relaxation as suggested by the strong correlation between the SR and χ" peak frequency at different pH conditions. Analytical calculations are also presented to estimate the change in effective viscosity due to the pH change.

AC magnetic response of superconducting single crystals exhibiting a second peak on the DC magnetization curves

The strong modulation of the AC magnetic response in the magnetic field-temperature region of the DC magnetization peak effect (PE) occurring just below the DC irreversibility line (IL) of weakly pinned superconducting single crystals is well known. However, it is not yet clear if the AC magnetic signal registered at usual frequencies and amplitudes is significantly affected by the presence of the second magnetization peak (SMP) in the DC magnetic hysteresis curves, since the SMP appears in specimens with stronger pinning and is located well below the IL. We investigated the AC magnetic response of several La2−xSrxCuO4 and 122-type pnictide superconducting single crystals exhibiting a pronounced DC SMP. It was found that in the case of small demagnetization effects, the AC magnetic signal across the SMP remains in the linear regime, with no detectable distortions in the SMP range (within the high sensitivity of a Physical Property Measurement System). This indicates reduced values of the Campbell penetration depth compared with the London magnetic penetration depth. A clearly nonmonotonic, SMP-related variation has only been obtained for the DC field dependence of the out of phase component of the AC magnetic moment in the nonlinear regime. The nonlinear AC Bean regime far below the IL has been attained in the conditions of enhanced demagnetization effects (plate-like specimens in perpendicular magnetic fields).

Vortex dynamics driven by AC magnetic field in YBCO thin films with complex pinning structures

We investigated the AC magnetic response of a YBa2Cu3O7 film with embedded BaZrO3 nanorods and Y2O3 nanoparticles at a static magnetic field Hdc lower than the matching field HΦ. We proposed a practical formula for the determination of the induced current J during AC susceptibility measurements, which allows us to directly obtain the pinning potential Uc from the characteristics of electric field E versus J. We show that the dynamic critical current Jd induced at the depinning frequency fd can be experimentally obtained by measuring m′(T) and m″(T) at different frequencies and amplitudes. It was found that the value of fd obtained by extrapolation of J to Jd in the PHz domain is much higher than the frequency reported for ordinary YBa2Cu3O7 thin films as determined by microwave impedance measurements.

Magnetic field dependence of nonlinear magnetic response and tricritical point in the monoaxial chiral helimagnet Cr1/3NbS2

We present a comprehensive study of the magnetization dynamics and phase evolution in the chiral helimagnet ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$, which realizes a chiral soliton lattice (CSL). The magnetic field dependence of the ac magnetic response is analyzed for the first five harmonic components, ${M}_{\mathrm{n}\ensuremath{\omega}}(H)\phantom{\rule{0.16em}{0ex}}(n=1--5)$, using a phase sensitive measurement over a frequency range, $f=11--10\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{Hz}$. At a critical field, the modulated CSL continuously evolves from a helicity-rich to a ferromagnetic domain-rich structure, where the crossover is revealed by the onset of an anomalous nonlinear magnetic response that coincides with extremely slow dynamics. The behavior is indicative of the formation of a spatially coherent array of large ferromagnetic domains, which relax on macroscopic time scales. The frequency dependence of the ac magnetic loss displays an asymmetric distribution of relaxation times across the highly nonlinear CSL regime, which shift to shorter time scales with increasing temperature. We experimentally resolve the tricritical point at ${T}_{\mathrm{TCP}}$ in a temperature regime above the ferromagnetic Curie temperature which separates the linear and nonlinear magnetic regimes of the CSL at the phase transition. A comprehensive phase diagram is constructed which summarizes the features of the field and temperature dependence of the magnetic crossovers and phase transitions in ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$.

Short-range order in the quantum XXZ honeycomb lattice material BaCo2(PO4)2

We present observations of highly frustrated quasi two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the S$_{eff}$ = 1/2 compound $\gamma$-BaCo$_2$(PO$_4$)$_2$ ($\gamma$-BCPO). Specific heat shows a broad peak comprised of two weak kink features at $T_{N1} \sim$ 6 K and $T_{N2} \sim$ 3.5 K, the relative weights of which can be modified by sample annealing. Neutron powder diffraction measurements reveal short range quasi-2D order that is established below $T_{N1}$ and $T_{N2}$, at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length $\xi_c = 60\pm2$ \AA \ ($T_{N1}$) and in quasi-2D helical domains with $\xi_h = 350 \pm 11$ \AA \ ($T_{N2}$). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on $\gamma$-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ $J_1$-$J_2$-$J_3$ model with ferromagnetic nearest-neighbor exchange $J_1$ are favored, both near regions of high classical degeneracy. High energy coherent excitations ($\sim 10$ meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above $T_N$. These data show that $\gamma$-BCPO is a rare highly frustrated, quasi-2D S$_{eff}$ = 1/2 honeycomb lattice material which resists long range magnetic order and spin freezing.

Lanthanum ion substituted cobalt ferrite nanoparticles and their hyperthermia efficiency

We investigated the structural, compositional, and magnetic properties as well as the AC magnetic hyperthermic response of CoFe2-xLaxO4(x=0.0,0.2,0.5) nanoparticles. We found that the La3+ ions substituted into the Fe3+ ion sites, and resulted in an increased magnetocrystalline anisotropy with x, and altered the time-dependent magnetism. To provide a better understanding of the AC magnetic hyperthermia response, a series of temperature versus time measurements were done by varying the magnetic field amplitude, the carrier medium viscosity and the concentration of the nanoparticles as parameters that governed the heating efficiency. A decrease of specific loss power was observed with an increase of the viscosity of the carrier medium for x=0 and x=0.25 substituted Co-ferrite nanoparticles, while a small increase was observed with the x=0.1 La3+ substituted Co-ferrite nanoparticles (due to their higher intrinsic magnetocrystalline anisotropy).

Magnetoelectric behaviors in BaTiO3/CoFe2O4/BaTiO3 laminated ceramic composites prepared by spark plasma sintering

The BaTiO3 (BTO) / CoFe2O4 (CFO) / BaTiO3 (BTO) laminated ceramic composites were prepared via spark plasma sintering (SPS). The ceramic composites were highly dense with clear boundaries between the three layers. The short time and low temperature required for densification during the SPS process diminish the possibility for unwanted reactions to occur. The BTO/CFO/BTO laminated ceramic composites exhibit excellent ME properties including: (1) maximal ME voltage coefficient as high as ~ 1.0 V/Oe cm; (2) obvious AC magnetic field frequency response and sensitivity from 102 to 107 Hz; (3) perfect ME anisotropy matches well with the theoretical fitting curve; (4) giant transverse ME voltage coefficient as large as ~ 4.1 V/Oe cm at resonance frequency of ~ 105 Hz. It is possible to fabricate several kinds of ME function devices with the BTO/CFO/BTO laminated ceramics prepared in this work.

Bifunctional CoFe2O4/ZnO Core/Shell Nanoparticles for Magnetic Fluid Hyperthermia with Controlled Optical Response

Conjugation of optical and magnetic responses in a unique system at the nanoscale emerges as a powerful tool for several applications. Here, we fabricated bifunctional CoFe2O4-core/ZnO-shell nanoparticles with simultaneous photoluminescence in the visible range and ac magnetic losses suitable for hyperthermia. The structural characterization confirms that the system is formed by a ≈7 nm CoFe2O4 core encapsulated in a ≈1.5-nm-thick semiconducting ZnO shell. As expected from its high anisotropy, the magnetic losses in an ac magnetic field are dominated by the Brown relaxation mechanism. The ac magnetic response of the core/shell system can be accurately predicted by the linear response theory and differs from that one of bare CoFe2O4 nanoparticles as a consequence of changes in the viscous relaxation process due to the effect of the magnetostatic interactions. Concerning the optical properties, by comparing core/shell CoFe2O4/ZnO and single-phase ZnO nanoparticles, we found that the former exhibits a broade...

Dy3TaO7, A stoichiometric spin glass and the effect of disorder via chemical substitution in the Dy3−xYxTaO7 (0 ≤ x ≤ 3) solid solution

In this work, we present both structural and magnetic (DC magnetization and AC susceptibility) studies of the Dy3−xYxTaO7 solid solution. The structural characterization of samples was performed by Rietveld refinements of the X-ray diffraction data. All compounds crystallized in a weberite-related structure in the orthorhombic C2221 space group (No. 20); the variations of the lattice parameters obey the Vegard´s law in the whole range of composition. DC magnetic measurements of the Dy3−xYxTaO7 system showed a Curie-Weiss paramagnetic behaviour, with antiferromagnetic interactions at T>150K. Below 3K a spin glass behaviour in the 0 ≤ x ≤ 1 range of the solid solution was observed. The stoichiometric Dy3TaO7 compound showed spin glass behaviour although there is no evidence of structural disorder. For some Y3+ doped compounds (x = 0.33, 0.66 and 1.0), chemical disorder reduced the freezing temperature (Tg) values with a ×1/3 dependence. Cole-Cole analysis of the AC magnetic field response showed similar phenomenological parameters for the stoichiometric (x = 0) and the Y3+ doped compounds with spin glassiness, suggesting an analogous mechanism for these compounds. For the Dy3−xYxTaO7 system, in which the spin glass behaviour seems to exhibit a critical concentration, a magnetic phase diagram is proposed.

Demagnetization harmonic effects on the magnetization of granular systems on a macroscopic scale: the superconducting case

A model has been developed to determine the effective ac magnetic response of magnetic systems, taking into account the demagnetization effects arising from the sample geometry which determine the out-of-phase components of the applied fundamental frequency and higher harmonic components. Indeed, demagnetization fields and their intermodulation can significantly affect the ac magnetic response. This approach provides a system of self-consistent linear equations relating the magnetic response to the external magnetic field by means of nonlinear magnetic susceptibility. The model is extended to the magnetic response of granular systems in terms of the contributions of the individual grains and of the whole sample in the presence of demagnetization effects of the whole sample and of the grains on a macroscopic scale. In particular, our model is applied to a granular superconducting system. The comparison between the performed numerical simulations and the experimental data shows that the demagnetization fields of the single grains and of the whole sample, and their intermodulation, are relevant if magnetic measurements are used to extract detailed information about the analyzed material.

Erratum to: Vortex Flux Dynamics and Harmonic ac Magnetic Response of Ba(Fe0.94 Ni0.06)2As2 Bulk Superconductor

Erratum to: Vortex Flux Dynamics and Harmonic ac Magnetic Response of Ba(Fe0.94 Ni0.06)2As2 Bulk Superconductor