Peculiarities in the properties of some rare-earth compounds with orthorhombic structures

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.