Neutron scattering study of spin waves and exchange interactions in ferromagnetic and paramagnetic EuS

Abstract

The spin wave excitations of an 153EuS single crystalline sample have been measured by inelastic neutron scattering through the entire 1. Brillouin zone with the momentum Q parallel to the main symmetry directions 〈100〉, 〈110〉, and 〈111〉, and for temperatures ranging from well below (1.3 K) to well above (36 K) the Curie temperature (Tc = 16.6 K). Least square fits of the low-temperature spin wave dispersion curves to the Holstein-Primakoff theory yielded the exchange energy constants Jr of the Heisenberg operator up to the fifth (r = 5) Eu-Eu-neighbour interactions as follows: J1/kB = 0.22(1±3)K, J2/kB = −0.10(0±4)K, J3/kB = 0.00(6±2)K, J4/kB = −0.00(7±2)K, and J5/kB = −0.00(4±2)K. They correspond with Θp = 21.1±0.2 K and also with the theoretically expected ratio of Tc/Θp. The temperature dependence of the spin wave energies, which has been measured for large-q spin-waves up to T/Tc = 1.5, does not agree with the spin-wave renormalization theory. This EuS-result differs from EuO, where the spin-wave renormalization was reported to agree with theory. The variation of the Jr with magnetic dilution is discussed by considering related results on the EuxSr1−xS system.