Experimental critical exponents of pure ferromagnets:: The cost of excessive proximity to TC may be too high

The limited purity of available ferromagnetic materials may turn into a pitfall in experimental studies of critical phenomena. In pursuit of asymptotic critical behaviour it is not uncommon to approach the Curie point too closely, violating the condition |epsilon| >> Delta T/T-C, epsilon = ( T - T-C)/ T-C ( here Delta T similar to Delta x x 10(3) K is the width of the critical region,. x being the fractional impurity content). If this happens, the presence of what is normally regarded as a tiny amount of impurity can no longer be neglected. One of the consequences is that the critical exponent beta is noticeably over-estimated in comparison with its value in the pure material ( which is the quantity sought in such studies). This problem does not occur if the excessive proximity to the Curie point is avoided.