Universal behaviour of the magnetic heat capacity for T→0

Experimental data for the heat capacity of a number of magnetic and non-magnetic materials are analysed taking into account that T=0 is a stable fixed point (SFP) according to renormalization group theory. In the vicinity of SFP's single power functions with universal exponents hold over a considerable temperature range. This applies also to the heat capacity which is given by one Tε function for T→0. The exponent ε is defined by the relevant contribution to the heat capacity which can be either the lattice or the magnetic subsystem. Only for some magnets with a large spin quantum number the magnetic heat capacity dominates. The exponent ε of the heat capacity is then identical with the exponent of the magnetic order parameter. For most of the magnetic materials the magnetic heat capacity is not relevant and the heat capacity starts with the Debye T3 function of the lattice. The magnetic contribution is then contained in the pre-factor of the T3 function. For the lattice heat capacity the usual exponent is ε=3 but ε=4 and 2 can also occur. T=∞ seems to be another SFP. This can be concluded from the power function behaviour by which the heat capacity of the non-magnetic materials saturates towards the Dulong-Petit asymptotic value. We have identified exponents of −1 and ∼− 43 for T→∞. In the intermediate range between the T→0 power function and the T→∞ power function a linear crossover region occurs. The heat capacity of the non-magnetic materials is therefore completely described by three exponents.