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.
We present a general theoretical framework, based on the Preisach model of
hysteresis, for describing irreversible phenomena in magnetically ordered
materials, which includes the effects of the critical ordering temperature
TC. The model is based on the premise that the free energy landscape can be
decomposed into an ensemble of bistable Barkhausen elements, each
characterized by two energy barriers which are defined by local anisotropy and
interaction fields. The free energy landscape has an explicit temperature
dependence which originates from the critical growth of the spontaneous moment
below the critical temperature TC, and thermal overbarrier fluctuations are
responsible for relaxing the system towards equilibrium. The theory identifies
certain fundamental characteristic energies which play a primary role in
determining the principal features of the magnetic response. The model is able
to replicate a broad spectrum of behaviour observed experimentally in the
field and temperature dependence of magnetic response functions such as the
field cooled and zero-field cooled moment and the major hysteresis
loop, in a wide variety of magnetic materials, including very specific
experimental anomalies and trends, and provides a rigorous theoretical
framework which quantifies the interpretation of the experimental data.
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