There's a physical limit to the number of magnetic bits that you can stuff onto a given area on a traditional hard drive, and we're pretty close to it right now. For hard drives to get bigger without getting, you know, bigger, we're going to have to get creative, and one research team has done this by taking hard drives into the third dimension.
Traditional magnetic platter hard drives consist of flat discs that you can think of as being essentially two-dimensional. There are lots of parallel microscopic tracks, and these tracks are separated into little dots which store the magnetic ones and zeros that make up data. We're getting to the point where we can't make those dots any smaller or the tracks any closer together, but a research team from SPINTEC has figured out that by building the dots up, it's possible to double the capacity of existing hard drives.
Here's how it works: instead of relying on binary magnetic dots that can either be a one or a zero, researchers built up little "pillars" consisting two magnetic dots each. The bottom dot has a magnetic field that can be read as either up or down, while the top dot has a magnetic field that can be read as either left or right. A pillar of two dots, each encoding a different bit, gives you twice the amount of information in the same amount of space. By taking a measurement above the pillar, the top dot can be read, and by taking a measurement between pillars, the bottom dot can be read.
This research was just published in the Journal of Applied Physics, which means that while it technically works, it's nowhere close to being ready for the commercial market. But, it's the first crack at breaking that one terabit per square inch barrier that we're butting our heads against right now.