INTRODUCTION AND MOTIVATION
At present, there is much research being conducted on how to replace the standard electronic transistor with an option that is faster and consumes less power. One research group at Argonne National Laboratory has proposed using magnetic vortices in a type II superconductor coupled with thin ferromagnetic stripes whose magnetization can be well controlled with an in-plan magnetic field to be used as such a circuit. By applying magnetic field to a hybrid superconductor/ferromagnetic heterostructure they are able to generate and manipulate the vortices making them candidates for quantum memory and micro-processing.
The hybrid SC/FM devices consisted of superconducting niobium (Nb) films (100nm thick) with ferromagnetic permalloy (Py) strips which were 1.6mm long x 30µm wide x 40nm thick, and spaced 2µm apart on the Nb. The Py strips have preferential in-plane magnetization. The samples were placed in a Montana Instruments cryostat with a castle configuration for integration with an external magnet, and the samples were cooled below the superconducting transition of Nb which is 8.7K. Once below the Tc of 8.7K a normal magnetic field (Hz) was introduced and then increased until vortices appeared. Vortex generation and manipulation were imaged optically with a CCD and then analyzed digitally.
The researchers applied an out-of-plane field (Hz) of varying strength to generate superconducting vortices. The sample’s electromagnetic configuration was manipulated with in-plane Hx and/or Hy magnetic fields. In addition to magnetic field effects, the temperature was varied from below Tc to above Tc. By changing all these factors, they could manipulate and control the vortices either promoting or retarding their motion in the channels between the FM strips. Figure 1a shows that when the sample is cooled across the transition temperature, vortex formation is induced in the superconductor by the applied Hz field and also Hs (stray field) from the FM stripe edges. The Py strips are polarized by the Hx field, and at the edges there exists either a potential barrier where the Hz vortex is positive and the Py strip is also positively charged and a potential well where the Hz vortex is positive and the Py strip is negative. These potential wells allow for vortex entry along the strip edge and provide a channel for their movement. In Figure 1c the magnetic field is applied parallel to the long edge of the Py strips and the magnetic charge (ρM = divM) is concentrated at the narrow ends of the strips; this results in vortices able to enter from the ends, but at a much slower rate than the long edges of the strip as the case in Figure 1b. This is because the stray fields have more of an effect than the in the 1b scenario.
Figure 1: a) Close-up sketch of Py strips (tan) and Nb film (blue) and the result of Hz and Hx (1b) applied when temperature is going from above Tc to below Tc b) Zoomed out view of charges along Py strips resulting from applied Hx field c) Zoomed out view of charges along Py strips resulting from applied Hy field. Source
Figure 2 shows the resultant optical images of the various applied Hx and Hy fields. The Hz magnetic field is held constant at 12.4 Oe and the temperature is also held constant below Tc, at 5K. The Hx and Hy magnetic fields are applied at 150 Oe. In Figure 2a the vortices move along the long edge of the Py strips. In Figure 2b there is an absence of vortices able to penetrate the positive end of the Py strips. Conversely, in Figure 2c the vortices can penetrate through the negative end of the Py strips. When applying both Hx and Hy the combined effect is that there are vortices along the long edges of the strips as well as a block of the vortices at the short end of the strips, a combination of Figure 2a and 2b.
Figure 2: The images of the flux distribution of vortices on the sample T=5K for constant Hz and various directions of Hx and Hy in figures 2a, b, c, and d. The red lines indicate the edge of the Py strips and the blue lines indicate the edge of the Nb film. Source
This group showed that controlling the motion of vortex flow was possible in hybrid superconductor/ferromagnetic devices with various configurations of an applied in-plane magnetic field. This proof-of-principle experiment demonstrates that the development of vortex circuits may be possible.
V.K. Vlasko-Vlasov, F. Colauto, T. Benseman, D. Rosenmann, and W.-K. Kwok Triode for Magnetic Flux Quanta. Sci. Rep. 6, 36847; doi: 10.1038/srep36847 (2016).