Öner Y, Boyraz C
Faculty of Science and Letters, Department of Physics Engineering, Istanbul Technical University, 34469, Maslak, Istanbul, Turkey.
J Phys Condens Matter. 2019 Apr 17;31(15):155801. doi: 10.1088/1361-648X/ab0061. Epub 2019 Feb 19.
We present the field and temperature dependence of the magnetizations of BaFePtAs and BaLaFePtAs samples synthesized by solid-state reaction method. The samples were formed as a single phase in the ThCrSi-type structure. Replacing Ba with the smaller La atom results in a lattice shrinkage. The critical current, J (H, T) has been determined (using Bean's critical state model) from magnetic hysteresis loops in a temperature range between T = 5 K and the superconducting transition temperatures (20 K), in fields up to H = 9 T. We find a nonmonotonic 'fishtail' shape (exhibiting a second peak) of the magnetization loops as well as a very large irreversibility. We observe a remarkable flux jump at T = 5 K for BaFePtAs due to magneto-thermal instability, but a very sharp magnetization peak for BaLaFePtAs near H = 0, which corresponds to a much-reduced relaxation rate of vortices. J decreases exponentially with temperature as well as with field in lower temperatures and fields ranges. La doping causes a considerable increase in the irreversibility, leading to a significant enhancement of J . The analysis shows that the high J is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. The normalized pinning force F /F as a function of the reduced magnetic field h = H/H is also obtained. Using the approaches of Dew-Hughes (1974 Phil. Mag. 30 293) and Kramer (1973 J. Appl. Phys. 44 1360), we determine the nature of the pining sources. It is found that many different pinning mechanisms are active simultaneously. The modified expression of F /F based on collective pinning theory enables us to determine the field dependence of the relaxation rate S(H, T = 5 K) indirectly instead of using more difficult relaxation measurements. Finally, all drastic changes with La doping are clearly demonstrated and investigated under different models introduced in the literature.
我们展示了通过固态反应法合成的BaFePtAs和BaLaFePtAs样品的磁化强度随磁场和温度的变化关系。样品形成了ThCrSi型结构的单相。用较小的La原子取代Ba会导致晶格收缩。利用Bean临界态模型，在温度范围T = 5 K至超导转变温度（20 K）、磁场高达H = 9 T的条件下，通过磁滞回线确定了临界电流J(H, T)。我们发现磁化回线呈现非单调的“鱼尾”形状（有第二个峰值）以及非常大的不可逆性。由于磁热不稳定性，我们观察到BaFePtAs在T = 5 K时出现明显的磁通跳跃，但BaLaFePtAs在H = 0附近有非常尖锐的磁化峰值，这对应于涡旋弛豫率的大幅降低。在较低温度和磁场范围内，J随温度以及磁场呈指数下降。La掺杂导致不可逆性显著增加，从而使J显著增强。分析表明，高J主要归因于密集微观点缺陷对涡旋的集体（弱）钉扎，同时也有强钉扎机制的一定贡献。还获得了归一化钉扎力F/F作为约化磁场h = H/H的函数。使用Dew - Hughes（1974年，《哲学杂志》30卷，293页）和Kramer（1973年，《应用物理杂志》44卷，1360页）的方法，我们确定了钉扎源的性质。发现许多不同的钉扎机制同时起作用。基于集体钉扎理论的F/F修正表达式使我们能够间接确定弛豫率S(H, T = 5 K)的磁场依赖性，而无需进行更困难的弛豫测量。最后，在文献中引入的不同模型下，清楚地展示并研究了La掺杂带来的所有剧烈变化。
