  Research in spintronics often involves generation of heat in nanoscale
magnetic systems. This heat generation can be intentional, as when studying
effects created by an external applied temperature difference, or
unintentional, coming as a consequence of driving relatively large charge
currents through tiny structures. Understanding and controlling these thermal
gradients can present challenges to experimentalists, which are related at some
level to the fact that heat flow is much more difficult to isolate and
manipulate than charge flow. This paper aims to provide a simple, intuitive
framework to understand the fundamental issues that arise in spintronic
materials and devices involving thermal gradients. The first goal is to provide
simple tools to demonstrate how thermal gradients arise in systems with thin
conducting films on bulk substrates. The main results are that a thermal
gradient pointing perpendicular to the plane of a thin film supported on a
macroscopic substrate is very common, even while the largest temperature drop
in the system will exist across the bulk substrate itself. These results point
to the need to understand the range of thermoelectric and magnetothermoelectric
effects that can generate voltage signals and other responses to thermal
gradients. I provide a brief review of these, along with relevant spin effects.
The review concludes with examples and comments on several important ongoing
issues in spintronics where thermal gradients play key roles.
