Ac Hysteresis Loops Research Articles

Flux-creep mechanism studied in extended windows for the HgBa 2Ca 2Cu 3O y superconductor

Flux-creep mechanisms are examined by the current density J dependence of the effective barriers U( J), which were determined to be U(J) αJ −μ by three independent measurements i.e. AC susceptibility (ACS), magnetic relaxation ( M- t) and hysteresis loop ( M- H) for polycrystalline HgBa 2Ca 2Cu 3O y . It is found that, as J is increased, the U( J) of M- H decreases monotonically and gradually crosses over from μ = 0.43, which connects smoothly with the ACS one with J ≈ 10 2 A/cm 2, to the one with μ = 1.03, which just overlaps with the M- t one with J ≈ 10 6 A/cm 2. This result demonstrates the consistency of the data of the three methods and is considered to be evidence of a crossover between the CDW type and large bundle creep in the vortex-glass phase or collective-creep theories.

Creep-induced magnetic anisotropy in a Co-rich amorphous wire

Magnetic anisotropy of amorphous (Fe0.06Co0.94)72.5Si12.5B15 wire induced by current annealing under applied stress is investigated. The anisotropy constant is determined either from the axial or circular ac hysteresis loop, depending on the sign of the anisotropy constant. At low annealing temperatures and short annealing times weak easy-wire-axis anisotropy is observed. With increasing time and temperature of annealing hard-wire-axis anisotropy develops. Relaxation annealing without stress causes change of the sign of anisotropy constant. The induced anisotropy, however, is fully reversible indicating that plastic strain does not contribute to creep-induced magnetic anisotropy. A simple bond-orientational-anisotropy model, proposed to explain the observed behavior, assumes that competition between opposite contributions of two different kinds of excess atomic bonds is responsible for creep-induced anisotropy of this alloy.

Tensor components of the magnetization in a twisted Fe-rich amorphous wire

Four components of the susceptibility tensor of a high magnetostrictive amorphous Fe77.5Si7.5B15 wire are investigated by means of ac hysteresis loop measurements. The samples have been carefully annealed in order to eliminate the internal stresses produced during the quenching. When tensile stress and/or torsion are applied bistable magnetic behavior with a large Barkhausen jump is observed. The effect of applied torque on the longitudinal and circular components of remanence magnetization and on the longitudinal and circular coercive fields is studied for different constant values of tensile stress. With increasing torque the axial and circular components of remanence decrease and increase, respectively, approaching the limit value of 0.71 Ms. The axial coercive fields for the axial and radial magnetization components coincide. They are nearly constant at low torsion and above same threshold value, depending on applied stress, follow approximately a square-root dependence on applied torque. When tensile stress is applied the circular coercive fields show a well pronounced minimum at the same threshold torque. The experimental results are interpreted by taking into account the axial and helical components of magnetic anisotropy induced by magnetoelastic coupling.

YBa 2Cu 3O 7−δ low field diamagnetic properties : A multiharmonic analysis

Multiharmonic analysis of the magnetic AC susceptibility of a YBa 2Cu 3O 7−δ powder sintered sample has been performed, measuring nine in-phase, χ n ′ ( T), and out-of-phase, χ n ″ ( T), components. Measurements at zero and low bias DC field, H o, have been performed. For even n, both χ n ′ ( T) and χ″ n ( T) are very sensetive to H o and become zero at real zero field. Odd harmonics ( n≥3) strongly depend on H o and on the weak-link nature. Furthermore, from χ n ′ and χ n ″ ( T) AC hysteresis loops have been derived and analyzed. The applicability of available theoretical models in the description of the harmonic components has been analyzed.

Thermomagnetic data in iron-rare-earth-boron alloys

The magnetic phases present in iron-rare-earth-boron alloys were examined with ac and dc susceptibility and hysteresis loops measurements in the temperature range of 4.2 K to 700°C. In FeNdB a shoulder has been observed on the demagnetization curve below 150 K and a peak in ac susceptibility around 140 K. A similar shoulder has been found in FePrB but no peak in ac susceptibility, indicating that its origin is quite different. The relation between the microstructure and magnetic properties is discussed.

A simple apparatus for measurement of small magnetostriction of amorphous alloys

A simple and compact apparatus for measuring small values of magnetostriction of ribbon-shaped ferromagnetic amorphous alloys was built. The apparatus consists of an open flux of highly sensitive and stable AC flux integrator, a set of solenoid coils to magnetise the ribbon and to pick up the flux change off the ribbon and a clamping jig to alloy tensile stress by a spring balance. The AC hysteresis loops along the ribbon axis are stored in a digital memory and recorded on an X-Y recorder. The measuring frequency, a typical field strength and the maximum tensile stress are 8 Hz, 0.8 to 3.2 kA m-1 and 109 N m-2, respectively. The apparatus makes it possible to measure the magnetostriction of the thin amorphous ribbon of the order of 10-6 to 10 -8 with an accuracy of 5 to 10%, by analysing constricted hysteresis loops of the stressed ribbon.

Computer aided measurements of AC-hysteresis loops

An outline of a digital, computer-aided technique for measuring dynamic hysteresis loops of soft magnetic materials in the 10 Hz to 2 kHz frequency range is given. Using a transformer core, the primary current and the voltage induced in the secondary winding are scanned synchronously. A computer determines the induction curve by numerical integration and provides important characterizing data. Examples of measurements under various energizing conditions are given.

Instantaneous reversible permeability in amorphous ferromagnets

The instantaneous reversible permeability is the permeability measured, by an alternating test field of small amplitude and high frequency, on the various points of the hysteresis loop during the slow traversing of the magnetization. We report here the behaviour of this permeability for tape-wound ribbons of amorphous alloys 2605 and 2826 in various conditions. As for polycrystalline materials, this permeability is generally much higher than that measured at the same points of the hysteresis loop under static fields, particularly in the region of the ac hysteresis loop where large irreversible magnetization jumps occur. The characteristics of the real part of the instantaneous permeability and those of the loss factor suggest that the permeability can increase because the test field is able to produce irreversible displacement of some of the walls which are removed from their minimum energy sites during the reversal of the magnetization. The behaviour of the instantaneous permeability compared to that of the gross magnetization rate indicates that an additional contribution to the instantaneous permeability can arise from an increased wall mobility due to the hindering of the after-effect.

Magnetic phases of amorphous FeyNi80-yP14B6alloys

The phase diagram for the amorphous alloys, FeyNI80-yP14B6, has been determined by AC hysteresis loop and AC susceptibility measurements.

Magnetic phases of amorphous transition metal-metalloid alloys

Hysteresis loop and ac susceptibility measurements were performed on three series of amorphous alloys: (A w B 1- w ) 75P 16B 6Al 3, where (A, B) are (Fe, Ni), (Co, Ni) and (Fe, Mn). Upon cooling, low w alloys undergo paramagne t to spin glass transitions. Alloys with higher w first experience a Curie transition to a ferromagnetic state, and then a spin freezing transition to a spin glass state. the T dependence of the width of the ac hysteresis loop is used to determine the spin freezing transition temperature. A magnetic phase diagram is presented for each alloy series and the value of w required for ferromagnetism, w C, is determined. When measured in the presence of small constant fields, the ac susceptibility of alloys with w just above w C has maxima near both transition temperatures. The field and temperature dependences of the peaks are explained by scaling arguments, used to determine the critical exponent δ for the Curie transition, and suggest that a similar scaling law holds for the ferromagnet to spin glass transition.