  Exploring novel strategies to manipulate the order parameter of magnetic
materials by electrical means is of great importance, not only for advancing
our understanding of fundamental magnetism, but also for unlocking potential
practical applications. A well-established concept to date uses gate voltages
to control magnetic properties, such as saturation magnetization, magnetic
anisotropies, coercive field, Curie temperature and Gilbert damping, by
modulating the charge carrier population within a capacitor structure. Note
that the induced carriers are non-spin-polarized, so the control via the
electric-field is independent of the direction of the magnetization. Here, we
show that the magnetocrystalline anisotropy (MCA) of ultrathin Fe films can be
reversibly modified by a spin current generated in Pt by the spin Hall effect.
The effect decreases with increasing Fe thickness, indicating that the origin
of the modification can be traced back to the interface. Uniquely, the change
in MCA due to the spin current depends not only on the polarity of the charge
current but also on the direction of magnetization, i.e. the change in MCA has
opposite sign when the direction of magnetization is reversed. The control of
magnetism by the spin current results from the modified exchange splitting of
majority- and minority-spin bands, and differs significantly from the
manipulation by gate voltages via a capacitor structure, providing a
functionality that was previously unavailable and could be useful in advanced
spintronic devices.
