The isotherms of the field dependences of the magnetic moment M(H) are measured at temperatures above and below the magnetic phase transition (Tc = 125 K) in single-crystal lanthanum manganite with excess oxygen (LaMnO3+δ). An analysis of these magnetic isotherms with Arrott plots and invoking the Banerjee criterion shows that the magnetic phase transition in this single crystal is a second order phase transition.

The magnetic moment of the LaMnO3+δ single crystal was measured in a wide range of temperatures and magnetic fields under different cooling and measuring conditions. As a result, the nature of the magnetic state of LaMnO3+δ was defined: spin glass clusters form in it.

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This paper deals with the magnetic state of the weakly anion-excess lanthanum manganite LaMnO3+δ (δ ≈ 0.0375). The temperature and field dependences of the magnetic moment of single crystal samples are studied in detail for temperatures of 2–350 K in different fields up to 5 T along the c-axis. The field dependence of the observed singular temperature points, such as those for magnetic ordering, magnetic vitrification, and magnetic phase separation, is recovered. The magnetic field behavior of several of the singular points is found to be interrelated. These results are fully consistent with models of magnetic phase separation that predict the formation of self-localized states such as nanoscale ferromagnetic droplets in a bulk antiferromagnetic matrix.