Assertion (A) : The earth's magnetic field is due to iron present in its core. 

Reason(R) : At low temperature, magnet loses its magnetic property or magnetism.

Both A and R are correct and R is NOT a correct explanation of A

The temperature inside the earth is so high that it is impossible for the iron core to behave as a magnet and act as a source of magnetic field. The magnetic field of earth is considered to be due to circulating electric current in molten iron and other conducting materials inside the earth.