Abstract

The scaling of the magnetic heat capacity in the two manganites La_0.85Ag_0.15MnO₃ and Sm_0.55Sr_0.45MnO 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 t_2g 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-T_c 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.

中文翻译：

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与Jahn-Teller失真和双重交换有关的新通用性类

摘要

两种锰矿中La _0.85 Ag _0.15 MnO ₃和Sm _0.55 Sr _0.45的磁热容量成比例MnO分别给出了不属于任何已知通用性类别的热指数的临界指数α= –0.23和ν= 0.7433。这些结果不能归因于化学不均匀性和/或结构缺陷，因为样品质量高。因此，异常的临界指数不仅与化学无序和/或结构缺陷有关，而且与晶格的集体行为有关。磁场和掺杂对过渡金属三元氧化物的影响之间存在类比：磁场通过t _{2 g}的取向影响晶格畸变轨道充当化学掺杂。比例关系似乎比其中的关键指数更稳定。晶格畸变和铁磁性的同步性导致了一种新的临界，但是由磁结构无序引起的它们的失步导致违反了等温和等磁指数之间的比例关系。尽管双交换系统显示出新颖的临界性，但它们满足比例关系，直到磁行为与协同Jahn-Teller失真形式的相干晶格行为同步。双交换键的断裂导致亚磁簇之间形成磁偶极-偶极相互作用，从而使晶格畸变和铁磁性失步，从而导致比例关系遭到破坏。T _c超导体。双交换系统中异常的临界是由于双交换铁磁性的不寻常的半经典性质，这是由实际交换引起的，即，通过自旋守恒而不是通过虚拟交换通过Mn ³⁺ -O-Mn ⁴⁺链的电子流像通常的铁磁体一样 双交换铁磁性的产生仅是因为电子可以自由地进行迭代，使电子将Mn阳离子的磁矩定向在一个方向上。