JETP Letters: issues online

The heat capacity in a La_0.85Ag_0.15MnO₃ manganite was measured near Curie temperature T_C under the applied magnetic fields up to 26 kOe to study scaling critical behavior and obtain universality class. The conventional scaling fails in application to the manganites with a hysteresis and the strong sensitivity of T_C to a magnetic field. But the application of the improved scaling procedure designed by us allows of yielding the good scaling the magnetic heat capacity in La_0.85Ag_0.15MnO₃, which may belong to a new universality class for systems with the strong spin-orbital coupling of t_2g-electrons - Double 3D-Heisenberg with the critical exponent of the heat capacity α=-0.23 and the critical exponent of the correlation radius ν=0.7433. This new universality class is consistent with the crystal, magnetic and orbital symmetries for the La_0.85Ag_0.15MnO₃. Scaling failure in the vicinity of T_C in the range of $t/H^{1/2\nu}\approx[-0.033;0.024]$ is understood by finite-size and other disordering effects when T→ T_C. It is remarkable that finite-size effect is consistent with grain size, $L\approx50 \mu$ m, in the La_0.85Ag_0.15MnO₃. The correlation radius, $Lt^\nu\approx30.28$ Å, estimated from the finite-size effect is of the same order of magnitude with the sizes of the ferromagnetic fluctuations and drops in manganites.