MAGNETIC ANISOTROPIES IN THE NUCLEAR-MAGNETIC-RESONANCE OF THE HEISENBERG-FERROMAGNET EUO

We investigated the field- and temperature dependence of the nuclear m agnetic resonance of Eu-153 in an EuO single crystal sphere at low tem peratures in fields up to 7 T. The spin-wave analysis yields an ecelle nt, parameter free description of the magnetization with the exchange constants J(1) = 0.55 +/- 1 K, J(2) = 0.15 +/- 1 K and an anisotropy f ield B-alpha = 50 +/- 1 mT from neutron scattering and ferromagnetic r esonance respectively. The influence of a biquadratic exchange within the spin-wave analysis is discussed. The hyperfine field varies with t he orientation of the magnetization by 200 kHz. The temperature and fi eld dependence of this anisotropy indicates that it is due to an aniso tropic hyperfine coupling tenser; The effect was used to observe the r otation of the domain magnetization in small internal fields and deter mine B-alpha with greater accuracy than from the spin-wave analysis.