Short Answer 
Ferromagnetic materials, such as iron and cobalt, contain aligned magnetic domains that respond to external magnetic fields, allowing them to retain magnetization even after the field is removed. However, heating these materials above their Curie temperature disrupts domain alignment, resulting in a loss of their magnetic properties.
Step 1: Understanding Ferromagnetic Materials
Ferromagnetic materials, such as iron, cobalt, nickel, and gadolinium, have unique magnetic properties due to the arrangement of their atoms. Each atom acts like a small magnet because of its internal currents and quantum mechanical spin. In ferromagnetic materials, these atomic magnets can group together to form magnetic domains: regions where atomic dipoles align in the same direction.
Step 2: The Role of Magnetic Domains
In an unmagnetized state, the magnetic domains within a ferromagnetic material are oriented randomly, neutralizing any net magnetic field. When exposed to an external magnetic field, these domains begin to align with the field, which leads to the material becoming magnetized. This magnetization is often retained even after removing the external magnetic field due to the hysteresis property of ferromagnets.
Step 3: Effects of Temperature on Magnetism
The magnetic properties of ferromagnetic materials can be influenced by temperature. Heating a material in the presence of a magnetic field can help align the domains more effectively. However, if the material is heated above its Curie temperature, the thermal energy disrupts the alignment of the domains, causing it to lose its magnetic properties entirely.