Magnetization and magnon excitation energies of the magnetic semiconductors EuTe and EuO on the basis of the renormalized spin wave theory

Abstract

We present analytic expressions for the temperature dependent magnetization and magnon dispersion relation of the antiferromagnetic (AFM) EuTe and the ferromagnetic (FM) EuO. These bulk semiconductors represent concentrated spin systems for which the interaction between magnons has to be taken into account. We do this using the renormalized spin wave theory. A higher order Green's function according to Tjablikov is used for their description. As a result, we obtain a modified Bloch- T3/2 law, for low temperatures and high magnetic fields. A full analytic expression is given for the sublattice magnetization of the AFM EuTe (B0=0), with a Néel temperature of TN = 9.81 K. The magnetization curves σN(T) of EuO (for 0 \leq T \leq 0.68TC) and σR(T) (for 0.82TC < T < TC) agree well with experiment. The spin wave excitation energy perfectly matches experimental data from inelastic neutron scattering. In the case of EuTe the magnon excitation energies exhibit a characteristic maximum. For q = 0, an energy gap Eg occurs in the spin wave spectrum, a consequence of the AFM exchange interaction J2(T). Such energy gaps exist in the spectrum of magnon excitation energies only in systems with AFM interactions.