Athermal Effect Research Articles

Modelling exchange bias with MuMax3

The unidirectional shift of the hysteresis loop and the athermal training effect are two key features of the exchange bias phenomenon in most polycrystalline FM/AFM bilayers. We show that, by using MuMax3 which is an open source micromagnetic simulation program, we are able to reproduce experimental data (bias field and coercivity) for a polycrystalline Co/CoO bilayer. We also demonstrate that rotatable uncompensated spins can be responsible for the athermal training effect and that the reversal mechanism between the ascending and descending branch in the first hysteresis loop can be different, as has frequently been noticed in experiments.

Numerical study of the influence of interfacial roughness on the exchange bias properties of ferromagnetic/antiferromagnetic bilayers

Exchange bias and coercivity are both studied numerically in antiferromagnetic/ferromagnetic (AFM/FM) bilayers in the presence of a rough interface. The roughness is modeled by an AFM atomic mesa of variable width, in a periodic bidimensional system. Unlike the flat interface case, roughness can favor the presence of magnetic interfacial frustration or the formation of sharp magnetic domain walls pinned within the first AFM planes, inside the AFM mesa, in a Peierls potential well. We demonstrate by using athermal steepest-descent calculations that irreversible processes can occur during the hysteresis loops, when the AFM mesa width is less than half of the system period. In this case, the depinning of the domain wall from the Peierls potential well during the descending branch is not followed by its rewinding in a certain range of the AFM anisotropy. This leads to a large increase of both exchange bias and coercivity at low temperature and to an athermal training effect. When the thermal activation is taken into account by using Monte Carlo simulations, we show that a random walk of the domain wall occurs within the AFM layer. These processes induce changes in the AFM spin configuration when the system is cycled several times and produce a thermally activated training effect. Our simulations, interpreted in the context of periodic Peierls potential, provide an explanation for two important features of the exchange bias phenomenon, i.e., the thermal variation of its characteristic fields and the different contributions giving rise to the training effect (AFM bulk vs interface). More generally, the presence of interfacial atomic roughness reduces both exchange bias and coercivity with respect to the perfect interface case.

Abnormal texture development in magnesium alloy Mg–3Al–1Zn during large strain electroplastic rolling: Effect of pulsed electric current

Single pass large strain electroplastic rolling (LSER) was conducted on AZ31 alloy, using pulsed electric current of different current densities, to improve the rollability of magnesium alloy sheet. It was found that TD (transverse direction)-split texture developed in the alloy once the current density exceeded a critical value (in the present case 90 A/mm2). Microstructure and texture analysis reveal that deformed grains, rather than the dynamically recrystallized grains, are the major contributor to the overall TD-split texture. Intragranular misorientation axis (IGMA) and viscoplastic self-consistent (VPSC) modeling were employed to analyze the influence of the pulsed electric current on the deformation mechanisms. Prismatic <a> type geometrically necessary dislocations (GNDs) were found to be dominant in the deformed grains of the specimens with TD-split texture. VPSC modeling indicates that the TD-split texture is likely due to the enhanced prismatic <a> activity caused by the electric current. The special athermal effect of the pulsed electric current on the deformation mechanisms makes LSER a promising technique for texture modification in magnesium alloys for improved formability.

Mg–3Al–1Zn alloy strips processed by electroplastic differential speed rolling

The effect of reduction per pass on the mechanical properties, microstructure and texture evolution of Mg–3Al–1Zn magnesium alloy strips processed by electroplastic differential speed rolling has been investigated. With increasing reduction per pass, the mechanical properties of the rolled strips increase. Both the shear effect and the current pulses play an important part in modifying the microstructure and texture by promoting dynamic recrystallisation. During dynamic recrystallisation, nucleus tends to nucleate in the vicinity of the shear bands. And as the progress of dynamic recrystallisation, shear bands are gradually consumed by the newly nucleated grains and become nearly indistinguishable. The athermal effect of current pulses plays an important role in promoting dynamic recrystallisation.

Non-octahedral-like dislocation glides in aluminum induced by athermal effect of electric pulse

Abstract

Experimental investigation on electrically assisted cylindrical deep drawing of AZ31B magnesium alloy sheet

The effect of the electric pulses on the ductility of AZ31B magnesium alloy sheet was studied through cylindrical deep drawing test. In order to show the advantages of the electric pulses during this test, deep drawing tests both at room temperature and at elevated temperatures induced by the electric pulses were carried out using the self-designed device which only allows the electric pulses flow through the flange and die corner regions. The result shows that as temperature rises, the limit of the deep drawing depth increases. The limit of the deep drawing depth increases even faster when the temperature is more than 373 K. Moreover, to find out whether the athermal effect exists or not in this process, experiments with only varying the current frequency and peak current at the same temperature were conducted. The result confirms the existence of the athermal effect. Microstructure evolution analysis shows that the dynamic recrystallization induced by the electric pulses at a relatively lower temperature facilitates the ductility improvement.

Investigation on transient electrically-assisted stress relaxation of QP980 advanced high strength steel

Advanced high strength steels have been increasingly used in auto body manufacturing industry to reduce weight and increase safety. However springback often occurs after stamping and seriously affects the forming accuracy, which is one of the bottle necks of its application. Targeting to control the springback, a novel method which employs the electric pulses to promote the relaxation of residual stress is investigated. In order to achieve this goal, electrically-assisted stress relaxation tests were carried out which shows that at a relative lower temperature, 373 K, the residual stress decreases with increasing electric parameters, and the athermal effect of the electric pulses is observed. A model of electrically-assisted stress relaxation is proposed. By combining the proposed stress relaxation model and the perfect plastic material model, a springback angle prediction model considering electric pulses is also proposed. The proposed models are verified with experimental results.

Investigation on the electrically-assisted stress relaxation of AZ31B magnesium alloy sheet

In this paper, the influence of the electrical parameters on spingback reduction during electrically-assiated sheet V-bending process was investigated. At first, electrically-assisted stress relaxation tests were carried out and the stress relaxation model considering the electric pulses was proposed and validated. In the stress relaxation tests with and without electric pulses at the same temperature, the athermal effect was observed. Based on the stress relaxation model and Mises yield criterion, the spingback angle prediction model was proposed and then verified by the V-bending experimental data. Microstructure analysis shows that detwinning occurs in the electrically-assisted stress relaxation after V-bending, which contributes to spingback reduction.

Electroplastic effect in AZ31B magnesium alloy sheet through uniaxial tensile tests

The electroplastic effect in AZ31B magnesium alloy sheet was investigated through uniaxial tensile tests. In order to show the athermal effect of the electrical pulses, two types of uniaxial tensile tests at the same testing temperature were carried out: uniaxial tension in environmental cabinet and uniaxial tension with electrical pulses. In addition, the distribution of temperature field in the cross-section area during uniaxial tension with electrical pulses was simulated. The results show that the distribution of temperature field along the cross-section area is homogeneous. By comparing the true stress–true strain curves of AZ31B alloy under uniaxial tensile tests, the athermal effect with electrical pulses was confirmed. The microstructure evolution after the uniaxial tension was studied by optical microscopy. The results indicate that the electrical pulses induced dynamic recrystallization plays an important role in the decrease of flow stress. Finally, a flow stress model of AZ31B sheet taking the influence of electroplastic effect into account was proposed and validated. The results demonstrate that the calculated data fit the experimental data well.

Experimental investigation on electroplastic effect of DP980 advanced high strength steel

This paper investigated the influence of the electric pulses on the flow behavior and plasticity of the advanced high strength steel (AHSS) Dual Phase, DP980. In order to isolate the thermal effect of the electric pulses, two kinds of uniaxial tensile tests at the same testing temperature were carried out: (1) isothermal tensile test carried out in an environment cabinet and (2) electrically-assisted isothermal tensile test. The stress–strain curves were recorded and compared. The results indicate that at the same testing temperature, the stress–strain curves obtained by test (1) are generally lower than curves deserved by test (2). It demonstrates that electric pulses can not reduce the flow stress when compared with the case that without electric pulse which is contrary to the traditional electroplastic effect. Another result is that when the testing temperature is not more than 573K, the stress–strain curves obtained by both tests are higher than the tension curve in room temperature and the cases are opposite when testing temperature is more than 573K. In addition, the elongation improvement is not observed as well in the stress–strain curves. The difference between the two tests when compared the cross section shrinkage rate and the fracture elongation rate is not obvious. From another aspect to study the athermal effect of the electric pulse, tests with only varying the peak current density or pulse frequency at the same testing temperature were conducted. The results once again show that no evident difference between the stress–strain curves. It confirms that no athermal effect exists in DP980. However, with temperature elevated to 473K, the material is strengthened and then weakened as temperature exceeds 473K. Scanning electron microscope (SEM) analysis was adopted to better understand the observed phenomena. The results show that the decomposion of the martensite and the thermal effect are the main reason that attributes to the strength variation. Fracture surface morphology analysis indicates that the appearance of the shallower and smaller dimples as well as fracture platform represents the bad plasticity when increasing temperature. Meanwhile, fracture pattern is transformed from brittle fracture in test (1) to ductile fracture in test (2) when tensioned at 673K.

Mapping motion of antiferromagnetic interfacial uncompensated magnetic moment in exchange-biased bilayers.

In this work, disordered-IrMn3/insulating-Y3Fe5O12 exchange-biased bilayers are studied. The behavior of the net magnetic moment ΔmAFM in the antiferromagnet is directly probed by anomalous and planar Hall effects, and anisotropic magnetoresistance. The ΔmAFM is proved to come from the interfacial uncompensated magnetic moment. We demonstrate that the exchange bias and rotational hysteresis loss are induced by partial rotation and irreversible switching of the ΔmAFM. In the athermal training effect, the state of the ΔmAFM cannot be recovered after one cycle of hysteresis loop. This work highlights the fundamental role of the ΔmAFM in the exchange bias and facilitates the manipulation of antiferromagnetic spintronic devices.

Investigation of influence of direct-current pulses on springback during V-bending of AZ31B magnesium alloy sheet

The influence of electrical parameters pulse frequency (f) and peak current density (J) on the springback of AZ31B magnesium alloy sheet was studied through electrical pulse-assisted (EPA) V-bending at 373K. To do this, a current pulse generator and a bending setup were designed and manufactured. In the test, controlling variable method in which only f or J can be changed while other testing parameters were kept constant was adopted. The results show that at the same testing temperature 373K, varying f or J, different springback angle can be deserved: the higher the f or J, the smaller the springback angles. In addition, at 373K, increase of effective current also leads to evident reduction in springback angle. It demonstrates that athermal effect of the electrical pulses exists in EPA V-bending of this material. To understand the mechanism of the EPA V-bending, microstructure evolution in the bent part was studied through optical microscope. The results indicate that the electrical parameters f and J not only decrease the grain size at the bent part, the higher the f or J, the finer the crystals but also decrease the number of twinning crystals during EPA V-bending. Both the decrease of grain size and the number of twinning crystals are considered to contribute to springback reduction.

Recrystallization and microstructure evolution of the rolled Mg–3Al–1Zn alloy strips under electropulsing treatment

The microstructural evolution of rolled Mg–3Al–1Zn (AZ31) alloy during electropulsing treatment (EPT) was analyzed. The results indicate that EPT accelerates tremendously the recrystallization and the microstructure of the rolled Mg–3Al–1Zn alloy is refined. A fine grain microstructure with an average grain size about 2μm is obtained by the EPT with low duty ratio and high current density (LDRHCD-EPT) at low temperature below 359K for a short time of 320s. The EPT has two main effects on the samples, the Joule heat and the athermal effect. Based on the experimental data and the theory analysis, we think that the athermal effect plays a key role in the recrystallization of the deformed alloys under EPT and accelerates the nucleation rate of recrystallization during EPT. Furthermore, the subsequent growth of the recrystallization grains is more dependent on the Joule heat than the athermal effect. As a result, in order to obtain refined recrystallization grains, the athermal effect should be strengthened while the thermal effect should be weakened during EPT.

Reduced Threshold Current in NbO2 Selector by Engineering Device Structure

Abstract —The leakage current scaling issues for NbO 2 selec-tor devices are investigated. By introducing a rough Pt bot-tom electrode (BE) (RMS roughness ∼ 2.5 nm) and insertinga 20-nm-thick dielectric layer (Nb 2 O 5 and HfO 2 ) between theBE and NbO 2 layer, we show that the threshold current forthe insulator-metal-transition in microscale devices ( ∼ 150 µ m)can be reduced to ∼ 20 µ A, close to that realized in nanoscale( ∼ 10 nm) 3-D vertical ReRAM. This could be attributed to athermal confinement effect caused by the presence of a permanentconductive filament in dielectric layer. The experimental resultsare supported by finite element simulation. Index Terms —Non-volatile memory, resistive switching,threshold switching, niobium oxide, cross-point memory. I. I NTRODUCTION R ECENTLY, hybrid 1 selector + 1 resistor (1S1R) mem-ory devices based on insulator-metal-transition (I-M-T)selectors have been shown to have performance suitable for usein next generation non-volatile memory [1]–[6]. The advan-tage of threshold-switching selector elements is that they canoperate at current densities that are compatible with resistive-switching memory devices under high-voltage operation [2].However, they have the disadvantage that they operate atcurrents in the range 0.1–10 mA in the low voltage regime,much higher than the 1–10

RETRACTED-Effect of electropulsing treatment on the microstructure, texture, and mechanical properties of cold-rolled Ti–6Al–4V alloy

Abstract

Influence of electropulsing treatment on the recrystallization and texture of Ni9W alloy strip

Abstract

Dependence of Interaction Free Energy between Solutes on an External Electrostatic Field

To explore the athermal effect of an external electrostatic field on the stabilities of protein conformations and the binding affinities of protein-protein/ligand interactions, the dependences of the polar and hydrophobic interactions on the external electrostatic field, −Eext, were studied using molecular dynamics (MD) simulations. By decomposing Eext into, along, and perpendicular to the direction formed by the two solutes, the effect of Eext on the interactions between these two solutes can be estimated based on the effects from these two components. Eext was applied along the direction of the electric dipole formed by two solutes with opposite charges. The attractive interaction free energy between these two solutes decreased for solutes treated as point charges. In contrast, the attractive interaction free energy between these two solutes increased, as observed by MD simulations, for Eext = 40 or 60 MV/cm. Eext was applied perpendicular to the direction of the electric dipole formed by these two solutes. The attractive interaction free energy was increased for Eext = 100 MV/cm as a result of dielectric saturation. The force on the solutes along the direction of Eext computed from MD simulations was greater than that estimated from a continuum solvent in which the solutes were treated as point charges. To explore the hydrophobic interactions, Eext was applied to a water cluster containing two neutral solutes. The repulsive force between these solutes was decreased/increased for Eext along/perpendicular to the direction of the electric dipole formed by these two solutes.

Kinetic Modeling for Microwave-Enhanced Degradation of Methylene Blue Using Manganese Oxide

This study was originally performed to compare the MnO<sub >2</sub>-based degradation of aqueous methylene blue (MB) under microwave irradiation- (MW-) enhanced and conventional heating- (CH-) enhanced conditions. The degradation process and kinetics were investigated to elucidate the microwave effect on the reaction. The results showed that all three tested conditions, sole MnO<sub >2</sub>, MnO<sub >2</sub>/CH, and MnO<sub >2</sub>/MW, followed the third-order (second upon MB and first upon MnO<sub >2</sub>) kinetic model. However, a higher degradation rate of MB was available under the MW-enhanced process, which implies that the &#x201c;athermal effect&#x201d; of MW might be of more benefit for the generation of electrophilic oxygen ions (<svg style="vertical-align:-3.27605pt;width:26.762501px;" id="M1" height="15.7" version="1.1" viewBox="0 0 26.762501 15.7" width="26.762501" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,11.55)"><path id="x4F" d="M381 665q131 0 226.5 -93t95.5 -239q0 -158 -96 -253t-238 -95q-137 0 -231 95t-94 238q0 142 92.5 244.5t244.5 102.5zM359 629q-89 0 -151 -74.5t-62 -208.5q0 -141 69 -233t175 -92q90 0 150.5 74t60.5 211q0 152 -69 237.5t-173 85.5z" /></g> <g transform="matrix(.012,-0,0,-.012,12.763,15.637)"><path id="x32" d="M412 140l28 -9q0 -2 -35 -131h-373v23q112 112 161 170q59 70 92 127t33 115q0 63 -31 98t-86 35q-75 0 -137 -93l-22 20l57 81q55 59 135 59q69 0 118.5 -46.5t49.5 -122.5q0 -62 -29.5 -114t-102.5 -130l-141 -149h186q42 0 58.5 10.5t38.5 56.5z" /></g><g transform="matrix(.012,-0,0,-.012,19.1,3.4)"><path id="x2212" d="M535 230h-483v50h483v-50z" /></g> </svg>, <svg style="vertical-align:-0.20473pt;width:20.424999px;" id="M2" height="11.875" version="1.1" viewBox="0 0 20.424999 11.875" width="20.424999" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,11.55)"><use xlink:href="#x4F"/></g> <g transform="matrix(.012,-0,0,-.012,12.763,3.4)"><use xlink:href="#x2212"/></g> </svg>, and <svg style="vertical-align:-0.20473pt;width:26.137501px;" id="M3" height="16.1" version="1.1" viewBox="0 0 26.137501 16.1" width="26.137501" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,15.775)"><use xlink:href="#x4F"/></g> <g transform="matrix(.012,-0,0,-.012,12.763,7.613)"><use xlink:href="#x32"/></g><g transform="matrix(.012,-0,0,-.012,18.474,7.613)"><use xlink:href="#x2212"/></g> </svg>) to degrade MB. The results showed that the degradation percentage of MB could reach 100&#x25;, corresponding to 92&#x25; total organic carbon (TOC) removal under microwave irradiation at pH 7.20 for 10&#x2009;min.

The origin of athermal training in polycrystalline metallic exchange bias thin films

The phenomenon of training in exchange bias systems has no clear and unambiguous explanation. It is known that the training effect consists of an athermal and a thermal component where the latter derives from the well understood thermal instability in the bulk of the antiferromagnetic (AF) grains. In this work, we report on a study of the magnetic properties of a tri-layer system consisting of F/AF/F where reversal of the individual ferromagnetic (F) layers shows that the athermal training effect derives solely from the interface spin configuration and not from the bulk of the AF grains. We propose a model whereby the origin of the training derives from the degree of order in interfacial spin clusters which lie in a thermo-remanent state after the AF is set and whose order is disrupted by the application of a reverse field resulting in a reduced coercivity and the characteristic change in the loop shape.

The influence of the energy absorbed from microwave pretreatment on biogas production from secondary wastewater sludge

In this study, microwave treatment is analyzed as a way to accelerate the hydrolysis in anaerobic digestion of municipal wastewater sludge. The influence of the absorbed energy, power and athermal microwave effect on organic matter solubilization and biogas production has been studied. In addition, a novel method that considers the absorbed energy in the microwave system is proposed, in order to obtain comparable experimental results. The absorbed energy is calculated from an energy balance.The highest solubilization was achieved using 0.54kJ/ml at 1000W, where an increment of 7.1% was observed in methane production, compared to the untreated sample. Using a higher energy value (0.83kJ/ml), methane production further increased (to 15.4%), but solubilization decreased. No power influence was found when 0.54kJ/ml was applied at 1000, 600 and 440W. Microwave heating was compared to conventional heating in two different experimental setups, providing similar methane yields in all cases.