Hard magnetic materials are those which have high coercivity, high retentivity and low loss in magnetisation due to mechanical treatment and no effect of temperature. These materials are called hard because these are difficult to magnetise and demagnetise. The higher the value of coercive force the harder is the magnetic material.
Example: The strength of the reverse magnetic field that is coercivity needed to completely demagnetise steel is more than that required for iron.
Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0