Magnetic storage can be affected by undesirable magnetic fields and by heat. I guess it happened to you, at least once, to discover that the magnetic strip on the back of your badge, credit card... got corrupted and didn't work any more. Sometimes you are advised not to place your cards near the cell phone, because they might be de-magnetised...
To overcome these problems, engineers manufacturing magnetic substrate used for storing bits like to play on the safe side and create "bigger" bits containers to be more resistant. This, unfortunately, hampers a further shrinking of the containers, hence the increase of storage density.
Now in a paper on Applied Physics researchers from the US Army Research Lab present a completely different way to store information on a magnetic media using a laser to heat ferromagnetic material in such a way that it crystallise. This result in a different permeability that can be used to differentiate a "1" from a "0".
The crystal or non crystalline form is very stable and it is not subject to change as it would a normal magnetic substrata.
This approach also overcomes the "superparamagnetic" limits (that constrain the smallest dimension a magnetic surface has to have to store a bit), hence leading to a denser information storage. From my point of view I feel that the real advantage is the resilience provided, rather than the increased (potential) density. The amount of storage capacity is not an issue on credit cards, whilst losing the information (demagnetisation) is an issue.