Specific heat and magnetic order in LaMnO_{3+\delta}

Magnetic and specific-heat measurements are performed in three different samples of LaMnO_{3+\delta}, with \delta=0.11, 0.15 and 0.26, presenting important disorder effects, such as carrier localization, due to high amounts of La and Mn vacancies. For the samples with \delta =0.11 and 0.15, magnetic measurements show signatures of a two-step transition: as the temperature is lowered, the system enters a ferromagnetic phase followed by a disorder-induced cluster-glass state. Spin-wave-like contributions and an unexpected large linear term are observed in the specific heat as a function of temperature. In the sample with the highest vacancy content, \delta=0.26, the disorder is sufficient to suppress even short-range ferromagnetic order and yield a spin-glass-like state.Comment: RevTeX 2-col, 8 pages, 5 ps figures included, submitted to PR