The doomsday device on Stanley Kubrick's Dr Strangelove is also a fictional cobalt type bomb, or as quoted [0] in the movie:
"If you take, say, fifty H-bombs in the hundred megaton range and jacket them with Cobalt-Thorium G, when they are exploded they will produce a doomsday shroud. A lethal cloud of radioactivity which will encircle the earth for ninety-three years!"
It's a fantastic movie that I highly recommend to anybody that hasn't seen it yet.
Another evil intention I can recall along this thread is the "neutron" bomb.
The idea being, produce a nuclear weapon that has a extreme radiation pulse during detonation but generates no long lived nuclides. Detonate over enemy territory, everyone dies from radiation sickness from the detonation, then you can move in and all the infrastructure is still in place.
Not much development ever took place, considering such a thing could only ever be an offensive/invasion weapon.
https://en.wikipedia.org/wiki/Cobalt_bomb
A theme for example in Nevil Shute's On the Beach. Where people in Australia spent their last days waiting for the cloud to eventually reach them.