Quantum-dot-based device for high-performance magnetic microscopy and spin filtering in the Kondo regime

We propose a nanoscale device consisting of a double quantum dot with a full exchange and pair-hopping interaction. In this design, the current can only flow through the upper dot but is sensitive to the spin state of the lower dot. The system is immersed in a highly inhomogeneous magnetic field and only the bottom dot feels a substantial magnetic field, while the top dot experiences only a residual one. We show that our device exhibits very interesting magnetic field--dependent transport properties at low temperatures. The Kondo effect partially survives the presence of the magnetic field and allows conductances that differ by several orders of magnitude to be obtained for the two spin types across the top dot. Interestingly, as a function of the magnetic field, our two-dot device changes from a spin singlet state to a spin triplet state, in which the amplitudes of the spin-dependent conductances are reversed. Our device is able to discriminate between positive and negative magnetic fields with a high sensitivity and is, therefore, particularly interesting for imaging the surface of antiferromagnetic insulating materials with alternated surface magnetic field, as well as for spin-filtering applications.