Investigations of the sublattice magnetizations M-sub(T) in antiferromagnets with fourth-order exchange interactions: EuxSr1-xTe

We present a neutron scattering study of the temperature and composition de pendence of the MnO-type superstructure reflection intensities in the diama gnetically diluted antiferromagnetic compounds EuxSr1-x,Te. In these materi als antiferromagnetic biquadratic and ferromagnetic three-spin interactions have been identified recently. These fourth-order non-Heisenberg interacti ons are able to create their own order parameter which is believed to gover n the order of the transverse moment components and which, hence, is direct ed perpendicular to the common Heisenberg order parameter. The observed MnO -type diffraction intensities originate in the sublattice magnetizations, M -sub(T); of both order parameters. Due to the different composition depende ncies for biquadratic interaction processes (similar to x) and three-spin i nteraction processes (similar to x(2)), the ferromagnetic three-spin intera ctions dominate for x > x(c) = 0.85, while for x < 0.85 the antiferromagnet ic biquadratic interactions dominate. Associated with this sign change in t he 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 cl early a ferromagnetic component for x > 0.85 no ferromagnetic Bragg intensi ties were observed in standard neutron scattering spectra using EuTe powder samples. We explain this by the competition of antiferromagnetic biquadrat ic and ferromagnetic three-spin interactions whereby a disturbed ferromagne tic superstructure may be generated which gives rise also to weak MnO-type diffraction lines. It is found that the resulting M-sub(T) obeys a T-2 law until a temperature as large as 0.75T(N) irrespective of the nature of the transverse order parameter. The T-2 law must, hence, be common to both type s of order parameter showing that the fourth-order interactions re-define t he spin dynamics of both completely. From the linear composition dependence of the normalized T-2 coefficient the existence of three-spin interactions is again confirmed.