  Rare-earth manganites are fascinating, because they display a wide variety of
fundamental properties from magnetism to ferroelectricity, from colossal
magnetoresistance to semi-metallicity, and because they can be used in a number
of important technological applications such as controlling a magnetic memory
by an electric field or vice versa, new types of attenuators or transducers
etc. In this paper, we present our investigation on monocrystal samples with an
orthorhombic structure, grown in two different space groups: D2h(16) for
La0.78Pb0.22MnO3 and Pr0.7Sr0.3MnO3 and D2h(9) for HoMn2O5 and TbMn2O5. The
doped perovskite manganites Ln1-x Ax MnO3 (where Ln is a rare-earth ion and A
is a divalent ion) from the group D2h(16), which crystallized in different
modifications of the perovskite structure, characterized by the parameter
deformation of the type c/&#8730;2<b<a. Many properties of these compounds
(especially the giant magnetoresistance GMR, being interesting for practical
applications) depend strongly on the carrier density, on the specific zone
structure, on the type and the quantity of dopants, on the defects of the
crystal and their magnetic structure, or on the applied magnetic fields.
