S/F结构中的临界电流突变
Sudden Critical Current Drops Induced in S/F Structures
摘要：
在寻找S/F结构新的物理性质的过程中，我们发现超导临界电流可以由相邻铁磁体的畴态控制。超导体是一种厚度为d_{s} ~ 2 xi_{S}的细线。Nb/Co和Nb/Py（坡莫合金Ni_{80} Fe_{20}）双层结构具有显著的磁各向异性。当沿硬磁轴方向（即平行于硬磁各向异性轴方向的电流轨迹）进行测量时，铁磁厚度d_{F} > 30 nm的Nb/Co结构的临界电流测量显示出在两个非常确定的步骤中的突然下降。当沿易磁化轴方向或铁磁层厚度小于30 nm时，这些液滴消失。液滴伴随着涡旋通量流。此外，磁阻测量接近Tc的铁磁体的饱和磁场附近的急剧增加。在Nb/Py双层结构中，沿易各向异性轴的铁磁厚度d_{F} ~ 50 nm，再现了类似的结果。这些结果被解释为是由于自发的涡旋形成和流动引起的布洛赫域壁的铁磁体下面。我们认为这些布洛赫域壁产生的二维涡旋-反涡旋晶格结构。
英文摘要：
In the search for new physical properties of S/F structures, we have found that the superconductor critical current can be controlled by the domain state of the neighboring ferromagnet. The superconductor is a thin wire of thickness d_{s} ~ 2 xi_{S}. Nb/Co and Nb/Py (Permalloy Ni_{80}Fe_{20}) bilayer structures were grown with a significant magnetic anisotropy. Critical current measurements of Nb/Co structures with ferromagnet thickness d_{F} > 30nm show sudden drops in two very defined steps when the measurements are made along the hard axes direction (i.e. current track parallel to hard anisotropy axes direction). These drops disappear when they are made along the easy axis direction or when the ferromagnet thickness is below 30nm. The drops are accompanied by vortex flux flow. In addition magnetorestistance measurements close to Tc show a sharp increase near saturation fields of the ferromagnet. Similar results are reproduced in Nb/Py bilayer structure with the ferromagnet thickness d_{F} ~ 50nm along the easy anisotropy axes. These results are explained as being due to spontaneous vortex formation and flow induced by Bloch domain walls of the ferromagnet underneath. We argue these Bloch domain walls produce a 2D vortex-antivortex lattice structure.
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