The info graphic suggests that they use the different cooling rate of the first and second layer to lock in the applied magnetic field of heat assisted magnetic recording.

They beat both layers

They apply a magnetic field to save a “1” bit

Both layers are magnetized to a “1” bit

The first layer cools down and locks that bit into place permanently.

They apply the magnetic field to save a “0” bit

While the second layer is still hot and accepts the “0” orientation of the magnetic field, the first layer is already too cold and will not change its magnetization.

The second layer cools down and locks that bit into place.

Neat!