An electric current sent through multilayers of normal metals (N) and ferromagnets (F) can excite the ferromagnetic order parameter and even reverse the magnetization [1]. Similarly, a precessing ferromagnet emits spin-currents to adjacent conductors [2]. Additionally, spin current noise in normal electric conductors in contact with nanoscale ferromagnets can be detected by an increase in the magnetization noise by means of a fluctuating spin-transfer torque [3]. Johnson-Nyquist noise in the spin current is related to the increased Gilbert damping due to spin pumping, in accordance with the fluctuation-dissipation theorem. We will discuss all these phenomena in a unified picture using magnetoelectronic circuit theory and the Onsager reciprocity relations [4,5].

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