  The torque generated by the transfer of spin angular momentum from a
spin-polarized current to a nanoscale ferromagnet can switch the orientation of
the nanomagnet much more efficiently than a current-generated magnetic field,
and is therefore in development for use in next-generation magnetic random
access memory (MRAM). Up to now, only DC currents and square-wave current
pulses have been investigated in spin-torque switching experiments. Here we
present measurements showing that spin transfer from a microwave-frequency
pulse can produce a resonant excitation of a nanomagnet and lead to improved
switching characteristics in combination with a square current pulse. With the
assistance of a microwave-frequency pulse, the switching time is reduced and
achieves a narrower distribution than when driven by a square current pulse
alone, and this can permit significant reductions in the integrated power
required for switching. Resonantly excited switching may also enable
alternative, more compact MRAM circuit architectures.
