# How do hard drives store information using magnetic layering? (Antiferromagnetic/ferromagnetic films)

I am doing some work with solid state materials in some chemistry courses and I've been doing some research into exactly how a hard drive stores information as 1's and 0's. It seems like a bunch of people know how it functions as an overview but the physics of what's going on is less well known.

So far I can confidently say that a hard drive is composed of many tiny thin films within the sectors on the platter. The film is divided into (at least) two layers. The top layer is the "pinning" layer which is an antiferromagnetic. The bottom layer is a ferromagnetic layer or in some cases a ferrimagnetic layer. The top layer contains magnetic domains which are anti-parallel and therefore contains no overall magnetic moment. The bottom layer being ferromagnetic has all of its magnetic domains aligned with each other, creating a net magnetic moment across the film.

I understand that in order to achieve a 1 or a 0, the magnetic moment of the underlying ferromagnetic layer undergoes a net magnetic moment flip, which is can be read. The pinning layer helps to keep it as a 1 or a 0 by not allowing this moment flip without considerable application of an external force. This force is termed the exchange energy of the film. This however is where I can't figure things out.

How does the "pinning" layer exert any force on the underlying ferromagnetic layer if it has no net magnetic moment? And more importantly how does this exchange energy manifest itself in a "snappy" transition between the 1 and 0 state of the ferromagnetic layer?

Please let me know if there is any more info that I can provide in order to help clarify, I am super interested in the answer to this question and would love to do some more research to figure it out if I can only get pointed in the right direction.