The atomic theory of magnetism was given by Weber and modified by Ewing. According to this theory:

• Each and every molecule of a magnetic substance is a complete magnet in itself, having a north pole and a south pole of equal strength.
• In an unmagnetized substance, the molecular magnets are randomly oriented such that they form closed chains.
• When the substance is magnetized, the molecular magnets are realigned so that north poles of all molecular magnets point in one direction and south poles of all molecular magnets point in the opposite direction.
• When all the molecular magnets are fully aligned, the substance is said to be saturated with magnetism
• During heating the magnetized specimen, molecular magnets acquire some kinetic energy. Some of the molecules may get back to the closed chain arrangement. That is why magnetism of the specimen would reduce on heating.

Reference: This article is referred fro my authored book “Electrical Engineering Materials” having ISBN 978-81-272-5069-0