Field Sweep Rate Research Articles

Metastable magnetization response of the vortex state due to patterned blind hole pins

We show that nano-patterning a superconductor with a hexagonally ordered array of blind holes produces significant magnetic field sweep rate dependent metastable bulk magnetization response. Our results are explained on the basis of a collective action of blind hole pinning centers which creates a barrier against vortex redistribution inside the patterned area. Evidence of this barrier is found via magneto-optical imaging.

The effect of columnar defects on the pinning properties ofNdFeAsO0.85 conglomerate particles

An oxypnictide superconductorNdFeAsO0.85 sample with transition temperature of 46 K was irradiated with 2 GeV Ta ions at a fluence of5 × 1010 ions cm−2. High resolution transmission electron microscopy study revealed that theirradiation produced columnar-like defects. The effect of these defects on theirreversible magnetization in polycrystalline randomly oriented fragments wasstudied as a function of field angle and field sweep rate. We find that the criticalcurrent density is moderately enhanced at fields below the matching field (∼1 T). The pinning enhancement is anisotropic and maximum along the defectdirection at 35 K, but becomes more isotropic as the temperature is decreased.The creep rate is suppressed at 35 K and at fields below the matching field,indicating that the columnar defects are efficient pinning sites under theseH andT conditions.

Superparamagnetic Behavior of Pt/CoFe/Pt Nanowires With Decreasing Wire Width

We present experimental demonstration that the thermal unstability grows with reducing the width of Pt/CoFe/Pt nanowires on the way to the superparamagnetic transition. The magnetic hysteresis loops measured at room temperature using a magneto-optical Kerr effect system reveal that the coercive field decreases drastically when the nanowire width is reduced smaller than 160 nm. To understand the origin of the coercive field reduction, the coercive field is measured with varying the sweep rate of the external magnetic field and then, analyzed with a thermally-activated magnetization reversal model. It is revealed that the reduction of the coercive field is mainly ascribed to the decrement of the thermal stability parameter, which in turn induces the abrupt reduction of the retention time down to a few seconds.

The role of thermal coupling on avalanches in manganites

We report here a study on the environmental dependence of the occurrence, at lowtemperature, of ultra-sharp field induced avalanches in phase separated manganites.Despite the high reproducibility of avalanches, it has already been observed that the criticalfields shift with the magnetic field sweep rate and that different sample sizes lead todifferent ignition fields for the avalanches. Critical growing rates have been suggested todescribe the avalanche ignition though the role of thermal coupling has hardly beenconsidered. We qualitatively analyze here a set of experimental data on avalanches inmanganites and discuss the role of thermal coupling as a key parameter of the instability ina dynamical system.

Magnetoelectric properties of CoFe 2O 4–BaTiO 3 core-shell structure composite studied by a magnetic pulse method

A new method using a magnetic pulse field in order to measure the ME effect of CoFe 2O 4–BaTiO 3 core-shell structure composite has been developed. This new method combines advantages of both quasi-static and dynamic ones and allows quantifying the whole ME response of the materials. The maximum ME coefficient of this composite is 5.5 and 4.2 mV/cm Oe for longitudinal and transverse measurements, respectively. These values are bigger than those measured by the lock-in technique and can be attributed to the fact that the pulse field method determines the full magnetoelectric coefficient whereas the AC method determines only the part that corresponds to the amplitude of the AC field. Both pulse duration and field sweep rate dH/ dt are found to affect the ME response of the composite.

Magnetic shielding properties of high-Tc superconducting hollow cylinders: model combining experimental data for axial andtransverse magnetic field configurations

Magnetic shielding efficiency was measured on high-Tc superconducting hollow cylinders subjected to either an axial ora transverse magnetic field in a large range of field sweep rates,dBapp/dt. The behaviour of the superconductor was modelled in order to reproduce themain features of the field penetration curves by using a minimum number of freeparameters suitable for both magnetic field orientations. The field penetrationmeasurements were carried out on Pb-doped Bi-2223 tubes at 77 K by applyinglinearly increasing magnetic fields with a constant sweep rate ranging between10 µT s−1 and10 mT s−1 for bothdirections of the applied magnetic field. The experimental curves of the internal field versus the appliedfield, Bin(Bapp), show that, at a given sweep rate, the magnetic field for which the penetration occurs,Blim, is lower forthe transverse configuration than for the axial configuration. A power law dependence with large exponent,n′, is foundbetween Blim and dBapp/dt. Thevalues of n′ are nearly the same for both configurations. We show that the main features of the curvesBin(Bapp) can be reproduced using a simple 2D model, based on the method of Brandt, involving aE(J) power lawwith an n-exponent and a field-dependent critical current density,Jc(B), (following theKim model: Jc = Jc0(1+B/B1)−1). In particular, a linear relationship between the measuredn′-exponents andthe n-exponentof the E(J) power law is suggested by taking into account the field dependence of the critical currentdensity. Differences between the axial and the transverse shielding properties can be simplyattributed to demagnetizing fields.

Hysteresis Effects in Magnetovoltage Measurements in Superconducting ${\rm MgB}_{2}$

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Hysteresis effects in superconducting <formula formulatype="inline"> <tex Notation="TeX">${\rm MgB}_{2}$</tex></formula> have been studied systematically by Magnetovoltage Measurements (<formula formulatype="inline"><tex Notation="TeX">$V-H$</tex> </formula> curves) as a function of transport current <formula formulatype="inline"> <tex Notation="TeX">$(I)$</tex></formula>, temperature <formula formulatype="inline"> <tex Notation="TeX">$(T)$</tex></formula> and sweep rate of external magnetic field <formula formulatype="inline"><tex Notation="TeX">$(H)$</tex></formula>, <formula formulatype="inline"><tex Notation="TeX">$({\rm d}H/{\rm d}t)$</tex></formula>. It was observed that time dependent effects appear as the field sweep rate <formula formulatype="inline"><tex Notation="TeX">$({\rm d}H/{\rm d}t)$</tex></formula> varies. This reflects itself as a decrease in the width of <formula formulatype="inline"> <tex Notation="TeX">$V-H$</tex></formula> loops with decreasing of <formula formulatype="inline"><tex Notation="TeX">${\rm d}H/{\rm d}t$</tex></formula>. The dissipation measured in <formula formulatype="inline"><tex Notation="TeX">$V-H$</tex> </formula> curves does not change as the sweep rate of external magnetic field is varied. It was observed that the measured dissipation in <formula formulatype="inline"> <tex Notation="TeX">$V-H$</tex></formula> curves is independent of field orientation of <formula formulatype="inline"><tex Notation="TeX">$H$</tex></formula> with respect to <formula formulatype="inline"><tex Notation="TeX">$I$</tex></formula>. Furthermore, it was observed that there is a significant asymmetry in <formula formulatype="inline"><tex Notation="TeX">$V-H$</tex></formula> loops between forward and reverse regions. The physical observations were interpreted mainly in terms of flux de-trapping/trapping and thus the difference in the local fields trapping in the grains between increasing and decreasing branches. </para>

Dynamics of Magnetization in Frustrated Spin-Chain Systems: Ca&lt;sub&gt;3&lt;/sub&gt;Co&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt;

A two-dimensional Ising-like model for the triangular spin-chain lattice, where each spin chain is treated as a rigid superspin, is proposed to investigate the dynamics of magnetization in frustrated triangular spin-chain systems. The superspins are assumed to interact with the nearest neighbours and external agency (heat reservoir and external magnetic field) that causes them to change their states randomly with time. A probability of a single spin-flip process is assumed in a Glauber-like form. This technique allows describing the steps in the magnetization curves observed in Ca3Co2O6 and their dependence on the magnetic field sweep rate and temperature.

Thermal stability of low dose Ga+ ion irradiated spin valves

The thermal stability of low dose Ga+ ion irradiated spin valves has been investigated and compared with that of the as-prepared ones. The dependences of exchange field, measured using vibrating sample magnetometer at room temperature, on magnetic field sweep rate and time spent at negative saturation of the pinned ferromagnetic layer, and training effect were explored. The training effect is observed on both the irradiated spin valves and the as-prepared ones. The magnetic field sweep rate dependence of the exchange bias field of the irradiated spin valves is nearly the same as that of the as-prepared ones. For the as-prepared structure thermal activation has been observed, which showed that holding the irradiated structure at negative saturation of the pinned ferromagnetic layer for up to 28 hours results in no change in the exchange field. The results indicate that the thermal stability of the ion irradiated spin valves is the same as or even better than the as-prepared ones.

Propagation of Magnetic Avalanches inMn12Acat High Field Sweep Rates

Time-resolved measurements of the magnetization reversal in single crystals of Mn12Ac in pulsed magnetic fields, at magnetic field sweep rates from 1.5 kT/s up to 7 kT/s, suggest a new process that cannot be scaled onto a deflagrationlike propagation driven by heat diffusion. The sweep rate dependence of the propagation velocity, increasing from a few 100 m/s up to the speed of sound in Mn12Ac, indicates the existence of two new regimes at the highest sweep rates, with a transition around 4 kT/s that can be understood as a magnetic deflagration-to-detonation transition.

Population transfer for signal enhancement in pulsed EPR experiments on half integer high spin systems

High resolution pulse EPR techniques applied to half integer high spin systems, such as Mn(2+) (S = 5/2), usually focus only on the central |-1/2--> |1/2 transition. The reason is that at high fields, where the zero field splitting is considerably smaller than the Zeeman interaction, the spectrum of this transition is intense and narrow. However, because the experiments are carried out at low temperatures, the low lying levels are heavily populated and the signal of the central transition is nevertheless diminished. This, in turn affects the sensitivity of the pulse EPR technique applied. A transfer of populations from the lower lying levels, which for Mn(2+) are the |-3/2 and |-5/2 levels, to the |-1/2 level will therefore increase the sensitivity. Here we describe such an experiment, where a rapid magnetic field sweep over the |-3/2--> |-1/2 sub-spectrum is carried out, concomitantly with a low power microwave (mw) irradiation, which results in population inversion. After this sweep any pulsed EPR sequence can be applied to the central transition that now has a population difference that deviates from the equilibrium value. The feasibility of the experiment is demonstrated at W-band (95 GHz) on Mn(2+) doped in MgO for echo-detected EPR measurements and the dependence of the signal enhancement on the rate and range of the magnetic field sweep and on the mw power is described. The results are then accounted for theoretically by considering a simple fictitious spin 1/2 system. In addition, preliminary enhanced (55)Mn pulse ENDOR electron nuclear double resonance (ENDOR) spectra are presented.

Transport and magnetic critical currents of Cu-stabilized monofilamentaryMgB2 conductors

We present a comparison between the field dependence of transport(Jc,t) and magnetic(Jc,m) critical current densitieson metal/MgB2 wires atdifferent temperatures. Jc,m was measured using SQUID and vibrating sample magnetometers atdifferent field sweep rates and measurement conditions. In order tofacilitate the comparison, we have chosen Cu-stabilized monofilamentMgB2 wires withnon-magnetic (Cu/Ti/MgB2) orslightly magnetic (SS/Cu/Nb/MgB2) sheaths. For some samples, we have obtained a very good agreement betweenJc,t andJc,m, while for otherwires Jc,t is lower thanJc,m for all fields bydifferent factors, f, typically ranging from 0.5 to 0.7 for the analysed samples butindependent of the temperature (for the analysed temperature range,approximately from 15 to 30 K). The similarity between the scaled curvesf·Jc,m(B) andthe Jc,t(B) ones is remarkable. The main causes of the discrepancies are discussed. The analysis of then-value characterizing the electric field–current density curves,E∼Jn, and its effecton the estimated Jc-values is also presented.

Spontaneous and field-induced magnetic transitions in YBaCo 2O 5.5

A detailed study of magnetic properties of cobaltite YBaCo 2O 5.5 has been performed in high (up to 35 T) magnetic fields and under hydrostatic pressure up to 0.8 GPa. The temperatures of paramagnet–ferromagnet (PM–FM) and ferromagnet–antiferromagnet (FM–AF) phase transitions and their pressure derivatives have been determined. It has been revealed that in the compound with yttrium, in contrast to those with magnetic rare earth atoms, the AF–FM field-induced magnetic phase transition is accompanied by a considerable field hysteresis below 240 K, and the magnetic field of 35 T is not sufficient to complete this transition at low temperatures. The hysteresis value depends on the magnetic field sweep rate, which considered as an evidence of magnetic viscosity that is especially strong in the region of coexistence of the FM and AF phases. High values of susceptibility for the field-induced FM phase show that Co spin state in these compounds changes in strong magnetic field.

Stability of Nb-Ti Rutherford Cables Exhibiting Different Contact Resistances

Dipole magnets for the so-called SIS-300 heavy-ion synchrotron at GSI are designed to generate 6 T with a field sweep rate of 1 T/s. It is foreseen to wind the magnets with a 36 strands Nb-Ti Rutherford cable. An important issue in the cable design is sufficiently low AC loss and stability as well. In order to keep the AC loss at low level, the contact resistance between crossing strands R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is kept high by putting a stainless steel core in the cable. The contact resistance between adjacent strands R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> is controlled by oxidation of the Sn-Ag coating of the strands, like in the LHC. In order to investigate the effect of Ra on the stability of the cable, we prepared four samples with different Ra by varying the heat treatment and applying a soldering technique, resulting in values between 1 muOmega to 9 mOmega. The stability of each sample against transient point-like heat pulses was measured. The results of the stability experiments and a comparison with calculations using the network model CUDI are presented. It is concluded that variation of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> has a strong influence on cable stability and that optimization of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> is mandatory to properly design the cable for the SIS-300 magnets, or likewise for similar magnets that might be used at CERN for a possible LHC injector upgrade.

Transport relaxation measurements and glassy state effects in superconducting MgB 2

Time dependent effects in superconducting MgB 2 have been studied systematically for the first time by transport relaxation measurements ( V– t curves) as a function of transport current ( I), temperature ( T) and external magnetic field ( H). At very low dissipation levels (below ∼1 μV), it was observed that the sample voltage grows up smoothly in time by exhibiting the details of initial stage of relaxation process. At high dissipation levels, steady state corresponding to constant flow rate is maintained within a very short time and monitoring of details of flux dynamic evolving along sample becomes difficult on long time scales. Another interesting behavior is the appearance of voltage peak when the transport current was reduced to a finite value. After peak, it was observed that the sample voltage relaxes smoothly by leveling off within a very short time. The evolution of V– t curves suggests that formation of resistive flow channels along sample develops easily, which is quite similar to that of obtained for the superconducting ceramic samples whose grain boundaries are improved. Time dependent effects were also observed in magnetovoltage measurements ( V– H curves) as the field sweep rate (d H/d t) varies. The observations were interpreted mainly in terms of flux trapping in grains.

Field-induced magnetostructural transition inGd5Ge4studied by pulsed magnetic fields

The field-induced magnetostructural transformation in Gd{sub 5}Ge{sub 4} was examined by magnetization measurements in pulsed magnetic fields. The low-temperature irreversibility of the transition can be destroyed by the magnetocaloric effect, and depending on the heat exchange between the sample and its surroundings, the irreversibility (or kinetic arrest) can also be retained. Measurements by using various magnetic-field sweep rates were conducted to examine the dynamic response of the system in the transition region. The critical fields for the magnetostructural transition below 20 K are field sweep rate dependent--the larger the field sweep rate, the higher the critical field. However, this rate dependence is readily suppressed with increasing temperature.

Dynamic magnetization reversal inCoPt3dots: Magnetic force microscopy measurements at remanence

We have studied the magnetization reversal of ${\mathrm{Co}}_{28}{\mathrm{Pt}}_{72}$ single crystal dots down to $0.2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ diameter with perpendicular anisotropy under slow magnetic field sweep rates $({10}^{4}\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}∕\mathrm{s})$ as well as with field pulses $(6.7\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}∕\mathrm{s})$. The magnetic state of the dots in the remanent state is observed by magnetic force microscopy. After application of short field pulses, $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ dots are frequently multidomain, whereas $0.2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ dots are always single domain. The nucleation volume is estimated either from aftereffect measurements or by fitting the increase of the dynamic coercive field versus field sweep rate. Its value amounts to a few ${10}^{\ensuremath{-}19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}$. We show that the wall velocity around nucleation centers is larger than in the rest of the sample.

Dependency of Flux Jump on Temperature and Field-Sweep Rate for Textured (Nd1/3Eu1/3Gd1/3)Ba2Cu3O7-δ Bulk Superconductor with Gd-211 Particles

Anisotropy flux jumps in the mixed state of a textured (Nd0.33Eu0.33Gd0.33)Ba2Cu3O7-δ bulk superconductor with Gd-211 doping particles have been studied by means of magnetization measurements in the model of the magnetic field paralleling and perpendicular to the c axis. A typical anisotropy flux jump was observed at a temperature ranging from 2.0 to 3.0 K. Under the magnetic field perpendicular to c axis, no flux jump was found at whole temperature range until the sweep rate of 200 Oe/sec. For the magnetic field paralleling to the c axis, the number of flux jumps decreased with the increase of temperature, and the third quadrant of the M–H curve is the most flux-instability quadrant. The magnetic field sweep rate dependences of flux jumps were also studied and the influence of flux creep on flux jumps was also discussed.

Magnetism and the effect of anisotropy with a one-dimensional monatomic chain of cobaltusing a Monte Carlo simulation

The magnetic properties of a one-dimensional (1D) monatomic chain of Co reported in aprevious experimental work are investigated by a classical Monte Carlo simulation based onthe anisotropic Heisenberg model. In our simulation, the effect of the on-site uniaxial anisotropy,Ku, on each individual Co atom and the nearest neighbour exchange interaction,J, are accounted for. The normalized coercivityHC(T)/HC(TCL) is found to show auniversal behaviour, HC(T)/HC(TCL) = h0(eTB/T−e) inthe temperature interval TCL<T≤TBCal, arising from the thermal activation effect. In the above expression,h0 is aconstant, TBCal is the blocking temperature determined by the calculation, andTCL is the temperature above which the classical Monte Carlo simulation gives a gooddescription of the investigated system. The present simulation has reproducedthe experimental features, including the temperature dependent coercivity,HC(T), and the angular dependence of the remanent magnetization,MR(θ, ϕ), upon the relativeorientation (θ, ϕ) of theapplied field H. In addition, the calculation reveals that the ferromagnetic-likeopen hysteresis loop is a result of a slow dynamical process atT<TBCal. The dependence of thedynamical TBCal on the fieldsweeping rate R, the on-siteanisotropy constant Ku, and the number of atoms in the atomic chain,N, has been investigated in detail.

Common reversal mechanisms and correlation between transient domain states and field sweep rate in patterned Permalloy structures

Complicated domain configurations generated during the magnetization reversal of Permalloy elements patterned in various shapes are the results of a surprisingly few switching mechanisms. By comparing the results from micromagnetic simulations with the experimentally observed switching events imaged by Lorentz microscopy, we found that nucleation and annihilation of vortices and vortex/antivortex pairs, and the propagation of Néel and cross-tie walls are common mechanisms for magnetization reversal in Permalloy elements with dimensions favorable to domain formation. In addition, for a given element, different mechanism combination(s) may transpire, depending on the sweep rate in the externally applied field. This is because energy maxima and minima are generally field dependent and that different field sweep rate alters the time allotted for thermally assisted transition between states.