  The ability of an insulating solid to conduct heat is rarely effected by the
application of a magnetic field. We have found, however, that the low
temperature heat conduction of some solids increases by more than a factor of
two with the application of a modest magnetic field. The effect occurs in
low-dimensional magnetically ordered materials when a small gap, \delta, in the
acoustic magnon (spin wave) spectra is closed using a magnetic field H >
\delta/g\mu_B. Since all magnetically ordered materials must have a gap in the
magnon spectra for magnons with k = 0, this may be a very general effect. Extra
heat is carried through the solid only when the magentic field exceeds the
critical value (H > \delta/g\mu_B). At this critical field the tiny atomic
magnets in the solid abruptly change the direction they point which results in
more heat flowing through the material. The magnetic field thus acts as a heat
switch. We have observed this effect in three quite different magnetically
ordered materials: K_2V_3O_8, Nd_2CuO_4 and Pr_2CuO_4. Several possible
explanations for these effects will be discussed.
