Excitations in Fe1-xCoxCl2: a randomly mixed antiferromagnet with competing spin anisotropies

Magnons, phonons and electronic excitations in the mixed crystal Fe1-xCoxCl2 have been studied by Raman scattering spectroscopy. Magnon-like excitations have been observed for T<TN in crystals with 0<or=x<0.03 and 0.9<X<or=1. The temperature dependences of the one-magnon peak position, linewidth and intensity have also been studied. For FeCl2 an interacting magnon theory provides an excellent description of the peak position and linewidth results for TN<or approximately=0.7 TN. The FeCl2 and CoCl2 magnon frequencies and linewidths are relatively insensitive to addition of impurities, but their intensities are drastically diminished such that the magnon Raman scattering is unobservable for 0.03<x<0.9. Ferromagnetic resonance experiments on iron-rich samples produced similar results. The reason for this loss of intensity is unknown but it may indicate a reduction in the density of stats at k=0. The magnetic ordering is also reflected in changes in the electronic Raman spectrum of Co2+ and Fe2+ ions. The authors' results for the concentration and temperature dependence of this scattering suggest that the magnetic ions may experience random fields in the mixed phase. The temperature and concentration dependences of the two Raman-active k=0 phonons have been analysed over the whole concentration range. Both phonons exhibit the expected linear concentration dependence of their frequencies provided allowance is made for Fe2+-electron-Eg-phonon and Co2+-electron-A1g-phonon coupling.