Novel ordering of the pyrochlore Heisenberg antiferromagnet with the ferromagneticnext-nearest-neighbour interaction

Abstract

The ordering property of the classical pyrochlore Heisenberg antiferromagnet with theferromagnetic next-nearest-neighbour interaction is investigated by means of a Monte Carlosimulation. The model is found to exhibit a first-order transition at a finite temperatureinto a peculiar ordered state. While the spin structure factor, i.e. the thermal average of thesquared Fourier amplitude of the spin, exhibits a finite long-range order characterized bythe commensurate spin order of period four, the thermal average of the spin itselfalmost vanishes. This means that although the amplitude of the spin Fouriercomponent is long-range ordered the associated phase degree of freedom remainsfluctuating.