La0.95Mg0.05MnO3: an ideal ferromagnetic system?

Detailed measurements of the field- and temperature-dependent ac susceptibility and magnetization of LaMnO3 substituted with 5 at.% of the divalent cation Mg are presented. This system is a semiconducting ferromagnet at low temperature; in particular, analysis of these data yields Tc = 147.2±0.2 K, with critical exponents δ = 4.75±0.15 (from the field dependence of the susceptibility along the `crossover' line, and confirmed by measurements along the critical isotherm), γ + β = 1.75±0.05 (from the temperature dependence of the crossover line) and γ = 1.39±0.05 (from the temperature dependence of the susceptibility along the same line). Within experimental uncertainty these exponent values agree with those predicted by the isotropic, three-dimensional Heisenberg model. Nevertheless this system exhibits some unusual characteristics, specifically in the temperature dependence of the (low-) field-cooled and zero-field-cooled magnetization. The latter have been modelled by a Preisach-based approach, which helps to resolve a ubiquitous dilemma in the doped manganites, viz. the appearance of a technical hardness irreconcilable with low coercive field values. A possible origin for the large, reversible component in the response below Tc is presented.