  Progress in the last two decades has effectively integrated spintronics and
nanomagnetics into a single field, creating a new class of spin-based devices
that are now being used both to Read (R) information from magnets and to Write
(W) information onto magnets. Many other new phenomena are being investigated
for nano-electronic memory as described in Part II of this book. It seems
natural to ask whether these advances in memory devices could also translate
into a new class of logic devices.
  What makes logic devices different from memory is the need for one device to
drive another and this calls for gain, directionality and input-output
isolation as exemplified by the transistor. With this in mind we will try to
present our perspective on how W and R devices in general, spintronic or
otherwise, could be integrated into transistor-like switches that can be
interconnected to build complex circuits without external amplifiers or clocks.
We will argue that the most common switch used to implement digital logic based
on complementary metal oxide semiconductor (CMOS) transistors can be viewed as
an integrated W-R unit having an input-output asymmetry that give it gain and
directionality. Such a viewpoint is not intended to provide any insight into
the operation of CMOS switches, but rather as an aid to understanding how W and
R units based on spins and magnets can be combined to build transistor-like
switches. Next we will discuss the standard W and R units used for magnetic
memory devices and present one way to integrate them into a single unit with
the input electrically isolated from the output. But we argue that this
integrated W-R unit would not provide the key property of gain. We will then
show that the recently discovered giant spin Hall effect could be used to
construct a W-R unit with gain and suggest other possibilities for spin
switches with gain.
