For both applied aspects of magnetism and for understanding the basic mechanisms giving rise to the magnetic phenomena it is necessary to change a number of physical parameters in a systematic manner. The magnetic properties of alloys with localised magnetic moments may be systematically altered by various means, including alloying, mechanical deformation or heat treatment. An example of localized 3d magnetic behaviour is provided by Heusler alloys, e.g. Pd₂MnIn. Within a simplified theoretical framework the influence of alloying is considered and the results are compared to experiments. The change in magnetic properties brought about by disordering the Mn and In atoms by suitable heat treatment of the sample is analysed.
Finally, magnetic systems are considered in which the magnetism originates from itinerant electrons. Some examples are given of itinerant electron magnets with transition temperatures above 300 K. The means of modifying the magnetic properties of these materials is briefly discussed.