  In this letter, we show that Giant Spin Hall Effect (GSHE) MRAM can enable
better energy- delay and voltage performance than traditional MTJ based spin
torque devices at scaled nanomagnet dimensions (10-30 nm). Firstly, we derive
the effect of dimensional scaling on spin injection efficiency, voltage-delay
and energy-delay of spin torque switching using MTJs and GSHE and identify the
optimum electrode geometry for low operating voltage (<0.1 V), high speed (>10
GHz) operation. We show that effective spin injection efficiency >100 % can be
obtained using optimum spin hall electrode thickness for 30 nm nanomagnet
widths. Finally, we derive the energy-delay trajectory of GSHE and MTJ devices
to calculate the energy-delay product of GSHE and MTJ devices with an energy
minimum at the characteristic time of the magnets. Optimized GSHE devices when
combined with PMA can enable MRAM with scaled nanomagnets (30 nm X 60 nm),
ultra-low voltage operation (< 0.1 V), fast switching times (10 ps) and
switching energy as low as 100 aJ/bit.
