Magnetic objects can be demagnetized by heat, stress and large magnetic fields. Heating the magnet over its Curie temperature, the temperature in which its magnetic properties change, is the most effective way of demagnetizing, as it randomizes the orientation of the magnetic domains. Subjecting objects to a reversing, decreasing magnetic field, which returns the dipoles to an almost nearly random orientation, will also demagnetize them.Continue Reading
As heating objects above their Curie temperature is often inconvenient, the method of demagnetizing with a decaying alternative field is more commonly used. This can be achieved by pulling out the object from a coil while AC current passes through it. An electromagnetic yoke with AC setting can also be used for demagnetizing.
The relationship of temperature and relaxation time is exploited for thermal demagnetization. Time taken by an assemblage of SD grains to lose remnant magnetization is called relaxation time. Unblocking temperature is a temperature below Curie's temperature where the relaxation time is a only few 100 seconds. At this point, there is no net magnetization if an external field is not present. The relaxation time starts growing as the temperature is lowered, and when the temperature reaches room temperature, the relaxation time grows exponentially and the moments become fixed, resulting in demagnetization.Learn more about Magnetism