Abraham said: “And besides all this, between us and you a great chasm has been fixed, so that those who want to go from here to you cannot, nor can anyone cross over from there to us.”
Demons are not bound by such restraints however.
In the story of Lazarus and the rich man, the chasm spoken of was the chasm between the Sheol of the damned where the rich man was, and the Sheol of the blessed (the "bosom of Abraham") where Lazarus was. It was not, in that case, talking about the separation of earth and the afterlife.
As was mentioned before, however, any of these chasms, while not being able to be crossed over at will, can be (as we saw in the example of Samuel) be allowed by God for his own purposes.