Irreversibility and the anisotropy of the low-temperature magnetization of manganites. Spin-glass polyamorphism
V.A. Sirenko and V.V. Eremenko
B. Verkin Institute for Low Temperature Physics and Engineering National Academy of Sciences of Ukraine 47 Lenin Ave., Kharkov 61103, Ukraine
Received December 18, 2013
The temperature variation of magnetization is considered for manganites with different composition and structural morphology measured in two cooling regimes — in a magnetic field (FC) and without it (ZFC) at two orientations of a magnetic field — along c axis and normal to it. There are found general laws: 1) the difference between MFC and MZFC — magnetizations at temperature Т = 5 K grows with the magne-tic field increase, approaching the maximum value in a magnetic field about 2 kOe, with is followed by decrease in a range 2–5 kOe; 2) within a magnetic-field range 0–5 kOe, The field dependence of the “splitting” temperature T*, below which the difference of MFC and MZFC magnetizations occurs, is satisfactorily described by power law with exponent 2/3, which is predicted by the theory of spin glass. Both results are typical as of single crystals, and of ceramics and films as well. At the same time, the field dependences of anisotropy of magnetic susceptibility differ for the samples with different level of magnetic ordering (Θ/TC). The results agree with the found in this work universality of the line, which marks the low temperature range of irreversibility at the Н–Т phase diagram of manganites. Deviation of the Т*–Н line with exponent 2/3 in a high magnetic field, which is usually explained by the appearance of a transverse to magnetic field magnetization-component, which is typical of the samples with antiferromagnetic-phase content. Consideration takes into account the multiphase state of the system, namely the co-existence of spin glass with fer-ro- and antiferromagnetism. The found change of anisotropy of magnetic susceptibility with a magnetic-field increase, peculiarities of the behavior of magnetic and thermo-magnetic irreversibility are considered as a manifestation of spin-reorientation phase transi-tion in antiferromagnetic medium. Eventually, it induces a spin-glass transformation from Ising-type to Geisenberg one, which results in the change of exponent at Т*–Н diagram from 2/3 to 2. The discovered phenomenon is universal, as it is observed for manganites of various composition and structure morphology, and represents the polyamorphism of some kind — the spin-glass polyamorphism.
PACS: 75.30.–m Intrinsic properties of magnetically ordered material; PACS: 75.30.–c Magnetic properties of nanostruc-tures.
Key words: spin glass, polyamorphism, LaMnO3+x single crystals.