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Introduction to Superconducting Devices
Published in Raja Sekhar Dondapati, High-Temperature Superconducting Devices for Energy Applications, 2020
Superconductors lose their superconductivity when the magnetic field strength exceeds a certain value in the external magnetic field. The magnetic field strength which causes a superconductor to lose its superconductivity is called critical field strength and is denoted by Hc. When the temperature is below Tc, Hc is a function of temperature and continuously increases with a decrease in temperature. With a similar behavior as Tc, there is also a field transition width when the superconductors transfer from a normal state to a superconducting state. For a practical superconductor, there are usually two critical fields, namely, lower critical field Hc1 and upper critical field Hc2. When the external field is less than Hc1, the superconductor is in Meissner state; however, when the external field is larger than Hc2, the superconductor is in normal state. While the field is between Hc1 and Hc2, the superconductor is in mixed state (more details provided in Section 1.5).
Superconductivity
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
Elaine A. Moore, Lesley E. Smart
It follows that a superconducting material can be made nonsuperconducting by the application of a large enough magnetic field. The minimum value of the field strength required to bring about this change is called the critical field strength (HC), its value depending on the material in question and on the temperature. Type I superconductors, which include most of the pure metal superconductors, have a single critical field. Type II superconductors, which include alloys and the high-TC superconductors, allow some penetration of the magnetic field into the surface above a critical field, HC1, but do not return to their nonsuperconducting state until a higher field, HC2, is reached. Type II superconductors tend to have higher critical temperatures than Type I superconductors.
An effective spin Hamiltonian approach to heavy electron metamagnetism
Published in Philosophical Magazine, 2020
P. Kumar, B. S. Shivaram, V. Celli
We have also briefly discussed the inclusion of other degrees of freedom, to the extent that they modify the results of the simple one spin localised model. Interactions with spins in other sites are treated in the mean field approximation. They can significantly shift the critical field for metamagnetism and even lead to a phase transition that is a mix of meta and ferro (or anti-ferro) magnetism. We plan to examine this mixed transition in future work. All other effects are accounted for, phenomenologically by a parameter w. Thus in the final analysis the full model involves several parameters, in addition to Δ, and γ that are absorbed by rescaling into ‘universal’ plots. These are λ (for the mean field strength) and w and it is possible that additional parameters (for instance an anisotropy of the g-factor) will be required to fully describe all the complexities of heavy fermion materials. In the end, it will be left to the experimentalists to determine, when all presented aspects are considered, the extent to which this model falls short in describing their data. Whether useful predictions to aid in the discovery of future heavy fermion materials and their unique properties that can arise from this model remains to be investigated.
The investigation of magnetic phase transitions and magnetocaloric properties in high-pressure annealed MnNiFeGe alloy
Published in Philosophical Magazine, 2021
Chuancong Wang, Qiubo Hu, Lei Zhang, Mingzhen Wei
To further investigate the magnetisation behaviours of the high-pressure annealed MnNiFeGe alloy, a series of magnetisation isotherms (M-H) are measured with a magnetic field up to 1.0 T. Figure 4(a) shows the isothermal M-H curves for this alloy around the TC of austenite. These M-H curves exhibit weak ferromagnetic character at first and then have a rise with the increase of magnetic field, suggesting a field-induced metamagnetic phase transition [26,27]. One explanation about this metamagnetic behaviour is the magnetic-field-induced MT from FM austenite to FM martensite, for the reason that the magnetic moment of martensite is larger than that of austenite [17,28]. However, here the metamagnetic phase transition takes place at a much lower magnetic field, which is not consistent with the fact that the critical field for driving the MT is usually higher than 2.0 T [9,16]. As we know, the martensite of MnNiGe alloy is spirally AFM in its nature, only when the Fe doped content increases to a certain extent, FM arrangement increases and forms larger zones, so it is transformed into FM state [16]. Furthermore, AFM-FM conversion and competition in the martensite also have been observed in the Mn(Fe)NiGe ribbon systems [29]. Therefore, a more reasonable explanation about this metamagnetic behaviour is the field-induced conversion from the spirally AFM state to the FM state of the martensite [19]. Moreover, detailed investigation indicates that there is no magnetic hysteresis in these curves of Figure 4(a), showing the second-order character of the ferromagnetic phase transition of the austenite.
Study of the critical current density and the thermodynamic critical field in deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br organic superconductor
Published in Phase Transitions, 2018
Youssef Ait Ahmed, Ahmed Tirbiyine, Ahmed Taoufik, Hassan El Ouaddi, Habiba El Hamidi, Abdelhalim Hafid, Abdelaziz Labrag, Hassan Chaib
In this paper, we have studied the critical current density JC and the thermodynamic critical field HC as a function of temperature in deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br organic superconductor. JC and HC are deduced from the irreversible and the reversible part of the hysteresis loops respectively. Our results show that the thermodynamic critical field HC and the critical current density JC decrease with increasing temperature and magnetic field. This can be explained in term of the pinning strength of vortices wich decreases with magnetic field and temperature.