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Quantum spin decoherence theory of magnetoresistance in mesoscopic ferromagnets and its applications

Quantum decoherence is the key mechanism determining whether quantum effects can manifest in quantum computation and transport, and mastering decoherence is central to designing and operating functional quantum devices. Here, we present a quantum spin decoherence theory of magnetoresistance (MR) from an open-quantum system perspective. Importantly, even when the spin-up and -down species have the same density of states, magnon MR, anisotropic MR, and Hanle MR still emerge in mesoscopic ferromagnets, which arise from the magnon-induced spin flip, spin relaxation anisotropy, and Hanle spin precession of itinerant electrons, respectively. The theory not only predicts the magnetic field and temperature dependencies of MR, which are related to spin relaxation time and spin-exchange field, but also obtains the universal cosine-square law of anisotropic MR. Moreover, we reveal diverse behaviors of the MR effects that enable the simple detection of the spin-exchange coupling strength via an electrical measurement. Our theory advances the understanding of the fundamental physics of MR in mesoscopic ferromagnets, revealing how it enables electrical probing of quantum decoherence-the decisive factor in nanoscience and nanotechnology.