Irreversibility and anisotropy of the low-temperature magnetization in manganites. Spin-glass polyamorphism

Abstract

The temperature dependences of the magnetization in manganites of different composition and structural morphology were measured in two cooling regimes, field cooling (FC) and zero-field cooling (ZFC), for two different orientations of a magnetic field, parallel and perpendicular to the c-axis. The following general tendencies were found: (1) The difference between the magnetizations MFC and MZFC at T = 5 K increases with increasing magnetic field, reaching the maximum value in a magnetic field of about 2 kOe, and then drops in the range 2–5 kOe; (2) The field dependence of the “splitting” temperature T* below which the difference between the magnetizations MFC and MZFC appears can be reasonably well described by a power law with the exponent 2/3 as predicted by the theory of spin glasses. Both results are characteristic for single crystals, as well as for ceramics and films. On the other hand, the field dependence of the anisotropy of magnetic susceptibility is different for samples with different degrees of magnetic ordering (Θ/TC). These results are consistent with the detected in the present study universality of the line separating the low-temperature region of irreversibility in the H–T phase diagram of manganites. Deviations from the T*–H-line with the exponent 2/3 in strong magnetic fields, which are commonly associated with the appearance of the magnetization component transverse to the magnetic field, are typical for samples containing the antiferromagnetic phase. The interpretation takes into account the multi-phase nature of the systems, i.e., coexistence of spin glass with ferromagnetism and antiferromagnetism. The observed change in the anisotropy of magnetic susceptibility with increasing magnetic field and the behavior of magnetic and thermomagnetic irreversibility are regarded as a manifestation of the spin-reorientation phase transition in an antiferromagnetic environment. This in turn initiates the transformation of the spin-glass—from the Ising- to the Heisenberg-type—which leads to the change in the exponent in the T*–H diagram from 2/3 to 2. The observed phenomenon is universal—it was observed in manganites of different composition and structural morphology—and represents a particular type of polyamorphism, namely, spin-glass polyamorphism.