It is shown experimentally that the temperature dependence of the magnetic order parameter can completely be described by a few temperature power functions with analytical changes (crossover) at the intersections of these functions. The standard behaviour consists in one power function for T-->0 with exponent & and the well known critical power law for T-->Tc with exponent (3. Additional crossover events can occur if the thermal variation of either the absolute value of the interactions or the exchange anisotropy exceeds a threshold value. In the itinerant ferromagnets Fe, Ni and Co a particular crossover due to a changing spin quantum number is observed.