I don't have too much of a math background; I'm in calculus AB and AP physics. In my physics class, the teacher said that, to cool something below absolute zero, you would have to slow the particles until they're not moving, then slow them even more. That got me thinking, if you could theoretically give particles velocity with components that contain imaginary numbers, could you create a vector with negative magnitude? I've tried to find some combination that would make the math work, but I couldn't find a solution. However, I only did try in three dimensions, and I don't really have a good methodology to approaching this problem. I've asked my physics teacher and calculus teacher, and neither knew.
Well, vectors are defined in order to have a real, positive magnitude. This is because we need a metric in the vector spaces they live in, and a metric is a quantity with various properties, the most basic one is that they have to be nonnegative. So the answer is no, the object you're imagining is not defined as far as I know. But keep thinking out of the box!