Investigations of the sublattice magnetizations M(T) in antiferromagnets with fourth-order exchange interactions: EuSrTe

Abstract:

We present a neutron scattering study of the temperature and composition dependence of the MnO-type superstructure reflection intensities in the diamagnetically diluted antiferromagnetic compounds Eu_xSr_1-xTe. In these materials antiferromagnetic biquadratic and ferromagnetic three-spin interactions have been identified recently. These fourth-order non-Heisenberg interactions are able to create their own order parameter which is believed to govern the order of the transverse moment components and which, hence, is directed perpendicular to the common Heisenberg order parameter. The observed MnO-type diffraction intensities originate in the sublattice magnetizations, , of both order parameters. Due to the different composition dependencies for biquadratic interaction processes and three-spin interaction processes , the ferromagnetic three-spin interactions dominate for x > x c =0.85, while for x <0.85 the antiferromagnetic biquadratic interactions dominate. Associated with this sign change in the fourth-order interaction sum the transverse order parameter changes from the antiferromagnetic MnO type for x <0.85 to ferromagnetic for x >0.85. This is noticed as a sudden decrease of the low-temperature MnO scattering intensities at x c =0.85. Although susceptibility measurements reveal clearly a ferromagnetic component for x >0.85 no ferromagnetic Bragg intensities were observed in standard neutron scattering spectra using EuTe powder samples. We explain this by the competition of antiferromagnetic biquadratic and ferromagnetic three-spin interactions whereby a disturbed ferromagnetic superstructure may be generated which gives rise also to weak MnO-type diffraction lines. It is found that the resulting obeys a T² law until a temperature as large as 0.75T_N irrespective of the nature of the transverse order parameter. The T² law must, hence, be common to both types of order parameter showing that the fourth-order interactions re-define the spin dynamics of both completely. From the linear composition dependence of the normalized T² coefficient the existence of three-spin interactions is again confirmed.