Controlling supercurrents and their spatial distribution in ferromagnets

Spin-triplet Cooper pairs induced in ferromagnets form the centerpiece of the emerging field of superconducting spintronics. Control over the magnetization of the stacked ferromagnetic layers that generate the triplets allows, for instance, making controllable pi-junctions. In such stacks, crucial for triplet generation is the presence of different uniform magnetization directions, The mechanism at work is that the spin-dependent scattering of a singlet in one layer generates an m_S = 0 component of the triplet state, which, in the next ferromagnet with a different magnetization direction, manifest itself as an m_S = 1, or ‘equal-spin’ triplet. There are other configurations, however, which give rise to controllable triplet currents. Using more intricate magnetic texture even permits control over the spatial distribution of supercurrent. Here we discuss two types of experiments, both based on a disk-shaped ferromagnetic layer as supercurrent carrier. The in-plane magnetization forms a vortex, with in the center a core where the magnetic flux is forced out. In the first experiment, two superconductor / ferromagnet contacts on top of the disk allow us to generate a triplet supercurrent in the disk, and by using superconducting quantum interferometry, we show the existence of two channels. Moreover, we show how the supercurrent can be controlled by moving the vortex with an in-plane magnetic field. Micromagnetic simulations are used to make the connection between the behavior of the supercurrent and the magnetic texture of the disk. In the second experiment, we show that a triplet current can even be generated by placing the superconducting contacts directly on top of the disk, without the presence of the second ferromagnet. Again, the supercurrent is sensitive to the position of the magnetic vortex core. The rotating magnetization, combined with the edges of the structure, yields the correct kind of inhomogeneity. This is not entirely trivial as will be discussed.