Changes in the magnetic structure of (La0.25Pr0.75)0.7Ca0.3MnO3 upon the isotopic substitution of 18O for 16O

Phase separation, percolation and giant isotope effect in manganites

“…Thus the situation close to doping x = 0.5 resembles that at small x: the system away from particular commensurate doping tends to phase separate into a commensurate (here charge-ordered) state and FM droplets. As we will see below this picture permits to explain many experimental results in the system (La 1−y Pr y ) 1−x Ca x MnO 3 and is confirmed by direct experiments [11,[19][20][21].…”

“…There appear at present more and more indications that the formation of inhomogeneous states with concomitant percolation behaviour is an intrinsic feature of manganites with the colossal magnetoresistance (CMR): percolation picture quite naturally explains many features of manganites in a wide concentration range, and may even lie at the core of the very phenomenon of CMR [16,17]. In this article I will give a short summary of the theoretical situation with the phase separation in manganites and of some of the experimental con-sequences and evidences of it, based mostly on the experimental results of Moscow groups (Babushkina, Balagurov, Fisher et al)-see [18][19][20][21] and also other papers in these Proceedings [22,23]. In particular, giant isotope effect which is characteristic of this situation was studied in details in these works and will be discussed shortly at the end of this paper.…”

“…In the last part of the paper I will shortly discuss experimental evidences in favour of the theoretical picture described above. One of the most convenient systems to study these effects, especially the interplay of CO insulating and FM states, is the system (La 1−y Pr y ) 1−x Ca x MnO 3 studied in details in [11,12,[18][19][20][21][22][23]. As already mentioned above, La 1−x Ca x MnO 3 has a ferromagnetic metallic state with the colossal magnetoresistance for 0.16 ≤ x ≤ 0.5.…”

Anomalous temperature behavior of the resistivity in lightly doped manganites around a metal-insulator phase transition

“…The observed phenomenon is thought to arise from competition between substitution induced strengthening of potential barriers (which hamper the charge hopping between neighboring M n sites) and weakening of carrier's kinetic energy. The data are well fitted assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites.PACS numbers: 71.30.+h, 75.50.Cc, 71.27.a To clarify the underlying microscopic transport mechanisms in exhibiting colossal magnetoresistance manganites, numerous studies (both experimental and theoretical) have been undertaken during the past few years [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] which revealed a rather intricate correlation of structural, magnetic and charging properties in these materials based on a crucial role of the M n 3+ −O−M n 4+ network. In addition to the so-called double-exchange (DE) mechanism (allowing conducting electrons to hop from the singly occupied e 2g orbitals of M n 3+ ions to empty e 2g orbitals of neighboring M n 4+ ions), these studies emphasized the important role of the Jahn-Teller (JT) mechanism associated with the distortions of the network's bond angle and length and leading to polaron formation and electron localization in the paramagnetic insulating region.…”

“…In turn, the onset of ferromagnetism below Curie point increases the effective bandwidth with simultaneous dissolving of spin polarons into band electrons and rendering material more metallic. To modify this network, the substitution effects on the properties of the most popular La 0.7 Ca 0.3 M nO 3 manganites have been studied including the isotopic substitution of oxygen ("giant" isotope effect [8,9]), rare-earth (RE) [10][11][12][13][14] and transition element (TE) [15][16][17] doping at the M n site. In particular, an unusually sharp decrease of resistivity ρ(T ) in La 0.7 Ca 0.3 M n 0.96 Cu 0.04 O 3 due to just 4% Cu doping has been reported [17] and attributed to the Cu induced weakening of the kinetic carrier's energy E 0 (x).…”