The impact of fourth-order exchange interactions on the thermal variation of the order parameter

The thermal decrease of the order parameter can empirically be described by a single T-epsilon power law with an exponent epsilon which depends on the dimensionality of the magnetic interactions and on whether the spin quantum number is integral or half-integral. We present experimental examples in which the order parameter shows a crossover between different T-epsilon power laws as a function of temperature. This indicates that the magnetic interactions can change their dimensionality as a function of temperature.