Mixed valence manganese oxides R_1-xA_xMnO₃ (R = rare-earth cation, A = alkali or alkaline earth cation) have been heavily investigated in recent years [see review by Coey, Viret and von Molnár, Advances in Physics 48 (1999) 167]. These materials show a variety of interesting crystallographic, electronic and magnetic properties and phases. Their extraordinary properties led to the development of new physical concepts such as double exchange and the Jahn-Teller polaron in the 1950's and 60's.

In the 1990's renewed interest in these materials awoke when new phenomena as, e.g, the colossal magnetoresistance near the Curie temperature or electronic phase separation, were found in thin films.

In the parent compound, LaMnO₃ (insulating antiferromagnet, superexchange), Mn is in a 3+ valence state. Upon doping by a divalent element (Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺) the Mn is driven into a mixed valence state Mn³⁺/Mn⁴⁺. For appropriate doping the ground state of the material is aferromagnetic metal with the transport properties governed by double exchange. It is one of our main results that there is a surprising similarity between divalent and tetravalent doping leading to a Mn³⁺/Mn⁴⁺ and Mn²⁺/Mn³⁺ mixed valency, respectively.

Energy can be gained by a small lattice distortion in case of the Mn³⁺. Here, one electron with d_z² orbital occupies the two e_g states (this may lead to charge and orbital ordering for certain doping). In the other cases, there is either no electron (Mn⁴⁺) or two electrons (Mn²⁺) in the e_g states with no Jahn-Teller distortion.

The transport properties of the mixed valence manganites are thought to be brought about by a double exchange mechanism. Hence, conductivity is enhanced if the magnetic moments of the Mn are aligned, i.e. in the ferromagnetic state. At around the transition temperature their is a maximum in the resistance since the alignment of the magnetic moments is destroyed. This alignment of the moments can be partially re-established by an external magnetic field resulting in the colossal magnetoresistance properties. Well above Tc these materials are polaronic semiconductors, i.e., the electrical conductance is due to polaron hopping with the formation of a gap at the Fermi level. This leads to an increasing conductance for increasing temperature.

The manganites show a high degree of spin polarization at E_F making them suitable for innovative device concepts. Early spin-resolved photoemission measurements on La_0.7Sr_0.3MnO₃ [J.-H. Park et al., Nature 392 (1998) 794] showed a nearly complete spin polarization. However, point contact Andreev reflection measurements gave about 73% spin polarization for La_0.7Sr_0.3MnO₃, in agreement with the finding of minority spin carriers by tunneling experiments using a superconducting electrode [Worledge et al., 2000] and band structure calculations [Pickett and Singh, 1996].

There is growing evidence for phase separation in the hole doped manganites: an electrically well conducting phase can coexist on a nanometer scale with a second phase of lesser conductivity in chemically homogeneous material [E. Dagotto, T. Hotta and A. Moreo, Phys. Reports 344 (2001) 1]. Within this scenario the metal-insulator-transition can be described by percolation assuming a spacial growth of the conducting phase for decreasing temperature. If regions of this pahse overlap a continuous current may form resulting in a sudden drop of the resistivity. It is believed that this phenomenon relies on the similarity of the relevant energy scales in the manganites.

The percolative behavior in hole doped manganites could be demonstrated by noise measurements: noise is enhanced close to the percolation threshold [29]. A more direct method to visualize the phase separation is Scanning Tunneling Microscopy [M. Fäth et al., Science 285 (1999) 1540; T. Becker et al., Phys. Rev. Lett. 89 (2002) 237203].

Early magnetization measurements (started by Wollan and Koehler 1955) showed that the spin-only ferromagnetic moment of Mn is rarely attained. Several explanations were subsequently given: a canted or spiral spin structure, magnetic inhomogeneities and disorder on the A-site sublattice. The conduction electrons should be highly spin polrized due to the double-exchange mechanism in which the e_g electrons of Mn³⁺ are delocalized and interact with the localized Mn⁴⁺ spins.

We performed magnetization measurements on manganites (R_0.7A_0.3)MnO₃ with R = La³⁺, Pr³⁺, Nd³⁺ and A = Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺ in high magnetic fields (up to 25 T) [23]. Only for La_0.7Sr_0.3)MnO₃ a negligible high field slope of the B(H)-curves was found, i.e. the ground state of this compound is a fully aligned ferromagnet, in agreement with other measurements. For the other La-compounds, the high-field slopes (0.1 - 10 × 10^-3 μ_B, typical for weak ferromagnetism) and the reduced saturation magnetization point towards a cation deficiency.

(Pr_0.7Ca_0.3)MnO₃ does not appear to be ferromagnetic in its ground state but shows two first-order magnetization processes [23]. The first one (at around 5 T) leads from an insulating, canted antiferromagnetic to the metallic, ferromagnetic, charge disordered state. The second transition is likely a crossing of the Pr crystal field levels. The Nd compounds appear to be charge ordered.

For (Nd_0.7Ca_0.3)MnO₃, there is an insulating, nonferromagnetic ground state. The transition to ferromagnetism is associated with a melting of local charge order of the Mn ions. For this compound, the transition is strongly temperature and sweep-rate dependent [P1]. Assuming a thermally activated process for the melting / freezing of the charge ordered state we found an activation energy of 130 K. The thermal activation involves clusters of about 8 Mn ions in size.