Alternatively, for a single-character symbol, there's either ⤤ (2924 north east arrow with hook) or ⮳ (2bb3 ribbon arrow up right). The combining square isn't exactly centered, depending on where I paste it.
The closest alternative I’ve seen is definitely north east arrow with hook. Personally, I like its simpler appearance. Ideally you would still want to label it on hover with some alt or title attribute text.
I would expect the north east arrow with hook to be used for "go back up", e.g. after having followed a footnote. The hook implies an element of "returning" for me.
I’ve seen it used there too. That’s probably a better use for it. Personally I’ve never had to call out external links before, and if I did, I think including or referencing the domain the link is on is more useful than a generic symbol that might also mean “opens in new window or tab” sometimes.
There's a useful selection of arrows here: http://xahlee.info/comp/unicode_arrows.html