| Abstract: | The scaling of the magnetic heat capacity in the two manganites La0.85Ag0.15MnO3 and Sm0.55Sr0.45MnO has given the critical exponents α = –0.23 and ν = 0.7433 of the heat capacity and correlation radius of the magnetic order parameter, respectively, which do not belong to any known universality class. These results cannot be attributed to chemical inhomogeneities and/or structural imperfections because the samples are of a high quality. Thus, unusual critical exponents can be associated not only with the chemical disorder and/or structural defects but also with the collective behavior of the lattice. An analogy has been revealed between the effects of the magnetic field and doping on ternary oxides of transition metals: the magnetic field affecting lattice distortions through the orientation of t2g orbitals acts as chemical doping. It seems that scaling relations are more stable than critical exponents in them. The synchronism of lattice distortions and ferromagnetism leads to a novel criticality, but their desynchronization induced by magnetostructural disorder results in the violation of scaling relations between isothermal and isomagnetic exponents. Although double-exchange systems demonstrate novel criticality, they satisfy scaling relations until the magnetic behavior is synchronized with the coherent lattice behavior in the form of cooperative Jahn–Teller distortions. Breaking of double exchange bonds leads to the formation of metamagnetic clusters with magnetic dipole–dipole interaction between them, which desynchronizes lattice distortions and ferromagnetism, resulting in the violation of scaling relations. The proposed new universality class includes diverse materials such as manganites, cobaltites, crystalline Fe–Pt and amorphous Fe–Mn alloys, and high-Tc superconductors. Unusual criticality in double-exchange systems is due to an unusual semiclassical nature of double-exchange ferromagnetism caused by real exchange, i.e., electron current through Mn3+–O–Mn4+ chains with the conservation of the spin rather than by virtual exchange as in a usual ferromagnet. Double-exchange ferromagnetism arises only because to freely itinerate, electrons orient the magnetic moments of Mn cations in a single direction. |
