We report inelastic neutron scattering results that reveal an hour-glass magnetic excitation spectrum in La1.75Sr0.25CoO4. The magnetic spectrum is similar to that observed previously in La1.67Sr0.33CoO4, but the spectral features are broader. We sho
w that the spectrum of La1.75Sr0.25CoO4 can be modeled by the spin dynamics of a system with a disordered cluster spin glass ground state. Bulk magnetization measurements are presented which support the proposed glassy ground state. The observations reiterate the importance of quasi-one-dimensional magnetic correlations and disorder for the hour-glass spectrum, and suggest that disordered spin and charge stripes exist at lower doping in La2-xSrxCoO4 than previously thought.
