Heat causes atoms in the magnet to move faster. If the magnet is heated enough, it‘s atoms may be moving fast enough to jostle the domains out of alignment. Then the permanent magnet loses it‘s magnetic field and is no longer a magnet.
Earth's magnetic field is created by the dynamo effect. A conductive fluid (liquid iron outer core) is in motion (convection currents). This creates a magnetic field. It is not associated with the core. (See also the magnetic field of Jupiter, Saturn and even Europa.)
the earth is not a permanent magnet, to put it simply. That is, the strong magnetic field is not the product of permanent magnetism but is instead a generated field. Of course, the strong field has imposed itself on rock materials that have gone from above to below curie temperature, so there is a permanent field associated with earth materials in lower temperature domains, but it is not a homogeneous field nor a strong field. There is identifiable residual permanent magnetism associated with the planetary bodies like moon and mars that no longer have a generated field.
