Crossover from Positive to Negative Spin Hall Signal in a Ferromagnetic Metal Induced by the Magnetization Modulated Interface Effect

The spin Hall effect and its inverse (SHE/ISHE), describing the charge-to-spin interconversion, are of critical importance to the fundamental physics involving spin-orbit coupling and the application of spintronic devices. These phenomena in nonmagnetic materials are reasonably understood after extensive studies. In ferromagnetic metals, however, the fundamental issue of generation and transport of pure spin current, especially the interplay of charge-to-spin interconversion and magnetization, is as-yet poorly understood. Here, direct experimental evidence for the sign change in ISHE by injecting a spin current from Y3Fe5O12 into a Fe film via a Cu spacer is reported. The sizable negative ISHE signal (VISHE) in nanometer-thick Fe films reverses its sign with a comparable magnitude when Fe magnetization is varied from a longitudinal to transverse orientation with respect to spin current polarization. With decreasing Fe thickness, this negative longitudinal VISHE increases rapidly to a larger positive value in magnitude. The opposite spin Hall angles for longitudinally and transversely polarized spin currents are reproduced by first-principles transport calculations and the sign change is attributed to the anisotropic contribution at the ferromagnetic interface. This work lays a firm foundation for manipulating spin-to-charge interconversion with an extra degree of freedom of magnetization orientation through ferromagnetic metals. The interplay between the charge-to-spin conversion and the magnetism has been a long-standing unsettled issue in spintronics. This work demonstrates the spin Hall angle of a ferromagnetic metal (Fe) can be gradually reversed by controlling its magnetization for the first time in an experiment. First-principles calculations provide an in-depth microscopic understanding involving the ferromagnetic interface. image