Correlation theory applied to the static and dynamic properties of EuO and EuS

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Abstract
The paramagnetic scattering was recently measured for EuO.1) It was found that spin-wave-like excitations develop for wave vectors approaching the zone boundary. The spectrum was found to be well described by damped harmonic oscillators (also called the two-pole approximation). This approximation was used previously in the correlation theory2) primarily to calculate static properties. Self-consistent dynamic and static calculations have here been performed for EuO, which is an ideal Heisenberg magnet with significant second-nearest-neighbor interaction (J2 = J1/5). The two-pole approximation describes accurately the correlation range, the static susceptibility, and the qualitative behavior of the dynamic properties (i.e., the wave vector at which peaks appear in the spectrum as a function of temperature). However, in order to also obtain the correct frequency scale it is necessary to use a cutoff of the spectrum at high frequencies, which cannot be seen experimentally, but which significantly influences the frequency moments. It was found that the finite J2 has significant importance for a comparison between theory and experiment. It is concluded that the calculation for a simple cubic n.n. magnet by Hubbard3) does not describe the EuO data accurately, neither with respect to line shape nor frequency scale. Significant differences are to be expected between EuO and EuS having opposite sign for J2.

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