  We report the first observation of soft-x-ray scattering from spin
fluctuations in a high-Tc parent compound. An antiferromagnetic charge transfer
insulator, CaCuO2, was irradiated by Cu M-edge soft x-rays. Ultra-high
resolution measurements of scattered intensity revealed magnon-magnon
excitations, due to spin exchange scattering, as low-energy loss features. The
process is analogous to optical Raman scattering. The spectra provide the first
measurement of the two-magnon excitation energy and the antiferromagnetic
exchange parameter in infinite-layer CaCuO2. The results reveal resonant
inelastic soft x-ray scattering as a novel probe of the spin dynamics in
cuprates.
