Specific heat and magnetic order inLaMnO3+δ

Abstract

Magnetic and specific-heat measurements are performed in three different samples of LaMnO 3+δ , with δ = 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 δ = 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, δ = 0.26, the disorder is sufficient to suppress even short-range ferromagnetic order and yield a spin-glass-like state.75.40. Cx, 75.30.Ds

Disorder effects at low temperatures in La0.7−xYxCa0.3MnO3 manganites

“…The most puzzling issue which emerged from previous investigations of the specific heat is the observation of a large linear term in insulating manganites [5,6,7,9]. The nature of the excitations giving rise to this contribution remain unknown.…”

“…A cluster-glass structure has already been observed in several manganites [18,5,17] and related compounds [19,20]. Such a structure corresponds to microscopic magnetic moments which align ferromagnetically within finite-size clusters, with a random orientation of the cluster moments with respect to each other.…”

“…The temperatures and coefficients are much higher in the present case, undoubtedly related to the fact that here we are dealing with a system of large (cluster) magnetic moments rather than a dilute spin system. It is worth noticing that a large linear term in the specific heat was previously observed in another cluster-glass manganite compound [5].…”

“…Starting with Ba-and Ca-doped LaMnO 3 , initial measurements at higher temperatures identified specific-heat anomalies associated with phase transitions [1,2], while low-temperature data [2,3,4] showed a linear term associated with an electronic contribution, and a cubic phonon term. Subsequent studies were performed on a variety of samples, including cation-deficient LaMnO 3+δ [5], Nd- [6] and Pr- [7,8,9,10] based compounds, or layered manganites [11]. Recent studies [12,13] addressed the issue of isotope effect in the specific heat of manganites.…”

“…The nature of the excitations giving rise to this contribution remain unknown. Explanations based on charge localization and/or spin disorder [5,9] have been put forward. An additional anomalous contribution, arising from excitations following a dispersion relation ∆ + ξk 2 , has also been proposed [9,13], and it has been suggested that these excitations are related to charge ordering, although this interpretation still lacks confirmation.…”