The reason of using laminated cores in the stator of an alternator is to reduce the eddy current loss.
The cores of magnetic devices working with alternating or pulsed current have to be non-conductive in order to avoid the formation of eddy currents within them.
The cores of soft iron used in electromagnetic machines are made of thin sheets insulated from each other by paint or sometimes paper. This breaks conductivity in the direction in which eddy currents would be induced.
Ferrite materials are made of powder bonded by cured resin and are inherently non-conductive.