  Generation of perpendicular effective magnetic field or perpendicular spins
({\sigma}z) is central for the development of energy-efficient, scalable, and
external-magnetic-field-free spintronic memory and computing technologies.
Here, we report the first identification and the profound impacts of a
significant effective perpendicular magnetic field that can arise from
asymmetric current spreading within magnetic microstrips and Hall bars. This
effective perpendicular magnetic field can exhibit all the three
characteristics that have been widely assumed in the literature to "signify"
the presence of a flow of {\sigma}z, i.e., external-magnetic-field-free current
switching of uniform perpendicular magnetization, a sin2{\phi}-dependent
contribution in spin-torque ferromagnetic resonance signal of in-plane
magnetization ({\phi} is the angle of the external magnetic field with respect
to the current, and a {\phi}-independent but field-dependent contribution in
the second harmonic Hall voltage of in-plane magnetization. This finding
suggests that it is critical to include current spreading effects in the
analyses of various spin polarizations and spin-orbit torques in magnetic
heterostructure. Technologically, our results provide perpendicular effective
magnetic field induced by asymmetric current spreading as a novel, universally
accessible mechanism for efficient, scalable, and external-magnetic-field-free
magnetization switching in memory and computing technologies.
