Superimposed DC Magnetic Field Research Articles

Low-temperature high-frequency dynamic magnetic susceptibility of classical spin-ice Dy2Ti2O7

Radio-frequency (14.6 MHz) AC magnetic susceptibility, , of Dy2Ti2O7 was measured using self-oscillating tunnel-diode resonator. Measurements were made with the excitation AC field parallel to the superimposed DC magnetic field up to 5 T in a wide temperature range from 50 mK to 100 K. At 14.6 MHz, a known broad peak of from kHz—range audio-frequency measurements around 15 K for both [111] and [110] directions shifts to 45 K, continuing the Arrhenius activated behavior with the same activation energy barrier of E a ≈ 230 K. Magnetic field dependence of along [111] reproduces previously reported low-temperature two-in-two-out to three-in-one-out spin configuration transition at about 1 T, and an intermediate phase between 1 and 1.5 T. The boundaries of the intermediate phase show reasonable overlap with the literature data and connect at a critical endpoint of the first order transition line, suggesting that these features are frequency independent. An unusual upturn of the magnetic susceptibility at T → 0 was observed in magnetic fields between 1.5 T and 2 T for both magnetic field directions, before fully polarized configuration sets in above 2 T.

FeSiBCrC amorphous magnetic powder fabricated by gas-water combined atomization

Amorphous soft magnetic composites have been attracting attention because of its low core loss under high frequency application. It is demanded that the powder for fabricating amorphous soft magnetic composites has amorphous structure and nearly spherical shape, which facilitates the uniform coating of insulator on the surface of powder particles. In this paper, amorphous Fe73.5Si13B11Cr1C1.5 (at%) powder was produced by gas-water combined atomization process which takes advantages of both gas atomization and water atomization, and its properties was compared with powder produced by gas atomization and water atomization. Gas-water combined atomization powder with particle size of −60 μm were found to be amorphous with low coercivity, and the shape of particles was nearly spherical. The permeability of magnetic powder cores prepared with three kinds of powder first increased and then decreased with annealing temperature, and the core loss first decreased and then increased. The cores prepared with gas-water combined atomization powder showed the best overall properties, with an effective permeability of 26.9, a relative permeability of 83.3% under a superimposed DC magnetic field of 7.96 kA/m, a core loss of 258 mW/cm3 under a frequency of 100 kHz, and a magnetization intensity Bm of 0.05 T after annealing at 793 K in nitrogen gas.

Mixed state properties analysis in AC magnetic field of strong pinning Fe(Se,Te) single crystal

The fundamental harmonic of the AC magnetic susceptibility of a Fe(Se,Te) single crystal iron based superconductor has been studied at different AC field amplitudes and frequencies with and without a superimposed DC magnetic field with the aim of analyzing its superconducting properties that can be also suitable for power applications such as the critical current density and the pinning energy in AC regime. From the analysis of the fundamental Cole-Cole plots a finite influence of flux creep phenomena has been detected. Its study, by means of the Arrhenius plots, evidences thermal activation energy U values as high as 103 K, whose dependence on the DC and AC fields, and of the temperature, has been also obtained. The magnetic field dependence shows the existence of the strong pinning regime characterized by plastic deformations of the vortex lattice, whereas the temperature dependence of the critical current density Jc(T) is consistent with the δl pinning model. Finally, the irreversibility field values Hirr(T) have been extracted from the Jc(T) at the different AC frequencies showing very high values if compared with the literature.

Magnetic Field Dependence of Ni Nanorod Brownian Relaxation

AC magnetic susceptibility and optical transmission measurements were performed on Ni nanorod suspensions for ac and superimposed dc magnetic fields up to 9 mT. With the comparably large magnetic moment per nanorod, these field values correspond to Langevin parameters $\xi$ up to about 80. These wide range of $\xi$ values, the narrow size distribution, and the absence of Neel relaxation make these single-domain ferromagnetic nanorods ideally suited for the experimental verification of the dependence of the Brownian relaxation time on ac and dc magnetic fields. Consistent results for the magnetic moment m and the zero-field Brownian relaxation time $\tau _{B,0}$ were obtained for all measurement modalities and the valid field range of alternative models for the ac susceptibility was evaluated.

Influence of static magnetic field strength on the temperature resolution of a magnetic nanoparticle thermometer

This paper investigates the influence of dc magnetic field strength on the resolution of a magnetic nanoparticle (MNP) thermometer, which employs the fundamental f0 and 2f0 harmonics of the MNP magnetization induced by ac and superimposed dc magnetic fields. In ac and parallel dc magnetic fields, the strength of dc magnetic field modulates the harmonics of the MNP magnetization, which affects their temperature sensitivities and measurement signal-to-noise ratios (SNRs). A temperature-adjustable fluxgate-based magnetic particle spectrometer was used to measure the spectra of the MNP magnetization at different temperatures. To determine the temperature, the amplitudes of the measured f0 and 2f0 harmonics were modeled based on the static Langevin function. AC susceptibility measurements on a MNP sample demonstrate the applicability of the static Langevin function for the description of the MNP magnetization spectra at a low frequency ac magnetic field without taking into account the MNP dynamics. Our simulations and experiments show that with increasing dc magnetic field from 0.2 mT to 2.0 mT, both the amplitude of the 2f0 harmonic and the temperature sensitivity of the amplitude ratio of the 2f0 to f0 harmonics increase by a factor of about 10 in an ac magnetic field with a frequency of 70 Hz and an amplitude of 1 mT. Concomitantly, the SNR of the 2f0 harmonic significantly increases by about 20 dB. Consequently, the temperature resolution of the MNP thermometer is improved from 1.97 K to 0.26 K.

Effect of superimposed DC magnetic field on an AC induction semi-levitated molten copper droplet

Effect of superimposed DC magnetic field on an AC induction semi-levitated molten copper droplet

Alternating current magnetotransport in Sm0.1La0.6Sr0.3MnO3

We report frequency dependent (f = 0.1 to 5 MHz) electrical resistivity in Sm0.1La0.6Sr0.3MnO3 in superimposed dc magnetic fields (H = 0–1 kOe). Resistive (ρ′) and reactive (ρ″) components of the complex resistivity (ρ = ρ′+iρ″) were measured simultaneously. With increasing f, both ρ′ and ρ′′ increase abruptly at the onset of ferromagnetic transition (Tc = 348 K) and exhibit an anomaly at T *≈ 147 K ≪ Tc in H = 0 kOe. The observed features in ρ′ and ρ′′ are depressed with increasing H leading to 35% magnetoresistance for ΔH = 300 Oe and f = 3 MHz. It is suggested that the frequency dependent magnetotransport is dominated by the field and frequency dependences of the ac permeability.

Oscillations of weakly viscous conducting liquid drops in a strong magnetic field

We analyse small-amplitude oscillations of a weakly viscous electrically conducting liquid drop in a strong uniform DC magnetic field. An asymptotic solution is obtained showing that the magnetic field does not affect the shape eigenmodes, which remain the spherical harmonics as in the non-magnetic case. A strong magnetic field, however, constrains the liquid flow associated with the oscillations and, thus, reduces the oscillation frequencies by increasing effective inertia of the liquid. In such a field, liquid oscillates in a two-dimensional (2D) way as solid columns aligned with the field. Two types of oscillations are possible: longitudinal and transversal to the field. Such oscillations are weakly damped by a strong magnetic field – the stronger the field, the weaker the damping, except for the axisymmetric transversal and inherently 2D modes. The former are overdamped because of being incompatible with the incompressibility constraint, whereas the latter are not affected at all because of being naturally invariant along the field. Since the magnetic damping for all other modes decreases inversely with the square of the field strength, viscous damping may become important in a sufficiently strong magnetic field. The viscous damping is found analytically by a simple energy dissipation approach which is shown for the longitudinal modes to be equivalent to a much more complicated eigenvalue perturbation technique. This study provides a theoretical basis for the development of new measurement methods of surface tension, viscosity and the electrical conductivity of liquid metals using the oscillating drop technique in a strong superimposed DC magnetic field.

Photoacoustic based technique for measuring the magnetocaloric effect

This paper presents a method for detecting the magnetocaloric effect (MCE), based on the acoustic detection. Small temperature oscillations, due to the application of a modulated magnetic field, are detected by a microphone in a closed cell. The continuous scanning of a superimposed dc magnetic field allows, by numerical calculation, the determination of large temperature variations caused by magnetic field steps from zero to tens of kOe. Measurements were performed in Gd and Gd5(SixGe1-x)4 compounds. The obtained results show the efficiency of the technique, which is suitable for the investigation of materials undergoing both purely magnetic phase transitions and magnetic-crystallographic first order ones.

The anomalous anisotropy in the ac susceptibility of La1.45Nd0.4Sr0.15CuO4 single crystal

The dependencies of ac susceptibility on the superimposed dc magnetic fields for the stripe-ordered La1.45Nd0.4Sr0.15CuO4 single crystal with two superconducting transitions have been studied in the 3D and 2D superconducting ranges for the fields along different orientations of the crystal. The results show that with increasing fields the interlayer Josephson coupling and the in-plane superconductivity are suppressed orderly for the fields perpendicular to CuO2 planes. The influences of the field parallel to CuO2 planes on the 3D and 2D superconductivity are much weaker than those of the field parallel to the c-axis. The irreversibility lines for the 3D and 2D superconducting states are also studied.

Acoustic detection of the magnetocaloric effect: Application to Gd andGd5.09Ge2.03Si1.88

In this paper we present a simple method for the determination of the total magnetocaloric effect based on the acoustic detection of the adiabatic temperature rise caused by the application of an ac magnetic field of small amplitude. The continuous scanning of a superimposed dc magnetic field allows, by numerical integration, the determination of large temperature variations caused by magnetic field steps from zero to tens of kOe. Absolute values of temperature rise are easily acquired after the calibration of the microphone signal using an appropriate reference sample. Once the calibration is done, no further information about the sample's thermal properties is necessary since the measured signal is directly proportional to the temperature variation. Measurements were made in Gd and ${\text{Gd}}_{5.09}{\text{Ge}}_{2.03}{\text{Si}}_{1.88}$ samples in the temperature range from 240 to 320 K. The technique shows to be suitable for the investigation of materials undergoing both purely magnetic phase transitions, as in the case of Gd, and magnetic-crystallographic first-order ones, as observed for ${\text{Gd}}_{5.09}{\text{Ge}}_{2.03}{\text{Si}}_{1.88}$. Besides the ability to determine the temperature variation due to a large magnetic field step through the continuous scanning of the magnetic field, the technique is also very suitable for measuring the magnetocaloric effect under very small magnetic field steps since it has sensitivity below millikelvin. Moreover, it is able to detect temperature variations in very small amount of sample, leading to its potential application in magnetocaloric thin films.

Ac magnetic behavior of large-grain magnetoresistiveLa0.78Ca0.22Mn0.90Oxmaterials

We report a detailed set of ac magnetic measurements carried out on bulk large grain La-Ca-Mn-O samples extracted from a floating zone method-grown rod. Three samples with ${\mathrm{La}}_{0.78}{\mathrm{Ca}}_{0.22}{\mathrm{Mn}}_{0.90}{\mathrm{O}}_{x}$ stoichiometry but differing in their microstructure were investigated by electrical resistivity and ac susceptibility measurements: (i) a single grain sample, (ii) a sample containing two grains, and (iii) a polycrystalline sample. We show that the superimposition of dc magnetic fields during ac magnetic susceptibility measurements is an efficient way for characterizing the magnetic transition of samples with different microstructures. Whereas both single grain and polycrystalline samples display a single susceptibility peak, an additional kink structure is observed in the case of the double grain sample. The temperature dependence of the ac susceptibility measured with superimposed dc magnetic fields is analyzed in the framework of second-order phase transition ideas. The relations between the critical exponents $(\ensuremath{\beta}+\ensuremath{\gamma}\ensuremath{\sim}1.5,$ $\ensuremath{\delta}\ensuremath{\sim}2.5)$ are found to be close to those of the mean-field model for all samples. This is attributed to the disordering caused by unoccupied Mn sites.

Effect of magnetic field on g-jitter induced convection and solute striation during solidification in space

A 2-D finite element model is presented for the melt growth of single crystals in a microgravity environment with a superimposed DC magnetic field. The model is developed using deforming finite element methodology and predicts steady and transient convective flows, heat transfer, solute distribution, and solidification interface morphology associated with the melt growth of single crystals in microgravity with and without an applied magnetic field. Numerical simulations were carried out for a wide range of parameters including idealized microgravity condition, synthesized g-jitter and real g-jitter data taken by on-board accelerometers during space flights. The results reveal that the time varying g-jitter disturbances, although small in magnitude, cause an appreciable convective flow in the liquid pool, which in turn produces detrimental effects during the space processing of single crystal growth. An applied magnetic field of appropriate strength, superimposed on the microgravity, can be very effective in suppressing the deleterious effects resulting from g-jitter disturbances.

Temporal relaxation of plasma electrons acted upon by direct current electric and magnetic fields

On the basis of the time-dependent electron Boltzmann equation the temporal relaxation of the electrons in the presence of electric and magnetic fields in weakly ionized, collision dominated plasmas has been studied. The relaxation process is treated by using a strict time-dependent two-term approximation of the velocity distribution function expansion in spherical harmonics. A new technique for solving the time-dependent electron kinetic equation in this two-term approximation for arbitrary angles between the electric and magnetic fields has been developed and the main aspects of the efficient solution method are presented. Using this new approach and starting from steady-state plasmas under the action of time-independent electric fields only, the impact of superimposed DC magnetic fields on the electron relaxation is analyzed with regard to the control of a neon plasma. The investigations reveal an important effect of the magnetic field on the temporal relaxation process. In particular, it has been found that the relaxation time of the electron component with respect to the establishment of steady-state can be enlarged by some orders of magnitude when increasing the magnetic field strength.

Giant magnetoimpedance in CoFeBSi wires and polycrystalline ferrites

Measurements of Giant magnetoimpedance (GMI) in Co-rich amorphous wires are compared with magnetic measurements in polycrystalline ZnMn ferrites as a function of frequency (5 Hz-13 MHz) and a superimposed dc magnetic field, aiming to simulate GMT conditions. It is found that, except for the scale differences expected in these two different materials, frequency phenomena are essentially similar. In particular, both properties set can be represented by using the same equivalent-circuit approach. These results point to a domain wall damping effect of the dc field, and a reduced role of macroscopic eddy currents.

Critical current anisotropy and AC losses in Bi(Pb)SrCaCuO-2223 and TlBaCaCuO-2212 Ag sheathed superconducting tapes

The dependence of the critical current as well as the AC self-field losses on the orientation of the external DC magnetic field of Bi(Pb)SrCaCuO-2223 and TlBaCaCuO-2212 Ag sheathed superconducting tapes prepared by the powder-in tube method was studied at 77 K. The self-field losses were measured using different voltage-tap configurations and different electrical measuring methods in the frequency range 5-400 Hz and superimposed DC magnetic fields up to 0.2 T. The emphasis has been laid on the field 0.05 T where B(Pb)SCCO samples exhibited a strong critical current anisotropy while TBCCO ones were practically isotropic. The interpretation of the experimental results is presented.

No phase transition in a magnetic field in the ising spin glass Fe0.5Mn0.5TiO3.

The dynamic scaling behavior of the Ising spin glass F${\mathrm{e}}_{0.5}$M${\mathrm{n}}_{0.5}$Ti${\mathrm{O}}_{3}$ in superimposed dc magnetic fields has been studied. The dynamic freezing temperatures ${T}_{f}\left(\ensuremath{\omega},H\right)$ were determined as a function of field $0\ensuremath{\le}H\ensuremath{\le}3.1$ T and frequency $0.017\ensuremath{\le}\ensuremath{\omega}/2\ensuremath{\pi}\ensuremath{\le}5100$ Hz. The in-field scaling behavior is strikingly different from the zero-field behavior and is well described by activated dynamics and a correlation length that depends on field as $\ensuremath{\xi}\ensuremath{\propto}{H}^{\ensuremath{-}{\ensuremath{\nu}}_{H}}$. The results are in agreement with the droplet model predictions and strongly suggest that there is no phase transition in a magnetic field in 3D Ising spin glasses.

Magnetic shielding and harmonic generation in high-Tc superconducting tubes

Magnetic shielding and harmonic generation in a sintered Y1Ba2Cu3O7-x tube, subjected to an incident AC magnetic field, has been studied. This study has shown that the onset of superconductivity in the specimen introduces a magnetic shielding of about 25 dB. Odd harmonics are generated during the propagation of the AC magnetic field. Flux trapping occurs in the specimen for frequencies less than 10 kHz. This trapped flux behaves as a superimposed DC magnetic field and results in a low shielding effectiveness and generation of even harmonics in the transmitted waveform.

AC susceptibility with superimposed DC magnetic field in a current biased granular high Tc superconductor

We have investigated the effect in AC magnetic susceptibility, ϰ ac = ϰ′+j ϰ″, of both DC transport current, J t , and DC magnetic field, H dc , in polycristalline HTSC. Two regimes are observed: at low DC fields critical state is modified by the current self field; but as H dc is increased, J t becomes dissipative ( V dc ≠0) and ϰ ac is determined by the samples's “resistance”.

Hysteretical temperature dependence of AC susceptibility in granular high- Tc superconductors

The AC susceptibility, χ AC( T), with superimposed DC magnetic field, H DS a, has been measured as a function of temperature in bulk sintered LaSrCuO, for zero-field cooled (ZFC) and field-cooled (FC) samples. Results are analyzed within Kim's critical-state model extended to a granular superconductor, where the role of grains is to determine the temperature-dependent effective permeability, μ eff( T), and the effective intergranular local field, H eff j ( H DC a, T), which depends on the applied field, H DC a and the thermal and magnetic history, accounting for the observed hysteresis. A method to determine field-independent intergranular critical-state parameters (critical current at zero field and temperature-dependent density of pinning forces as well as μ eff( T)) and the proper scaling of H eff j ( H DC a, T) for ZFC and FC conditions is described. The calculated AC susceptibility for the whole temperature range for both magnetic conditions reproduces the main features of the experimental results.