α-β and β-γ phase boundaries of solid oxygen observed by adiabatic magnetocaloric effect

The magnetic-field-temperature phase diagram of solid oxygen is investigated by the adiabatic magnetocaloric effect (MCE) measurement with pulsed magnetic fields. Relatively large temperature decrease with hysteresis is observed at just below the β-γ and α-β phase transition temperatures owing to the field-induced transitions. The magnetic field dependences of these phase boundaries are obtained as T_βγ(H) = 43.8 − 1.55 × 10⁻³H² K and T_αβ(H) = 23.9 − 0.73 × 10⁻³H² K. The magnetic Clausius-Clapeyron equation quantitatively explains the H dependence of T_βγ, meanwhile, does not T_αβ. The MCE curve at T_βγ is of typical first-order, while the curve at T_αβ seems to have both characteristics of first- and second-order transitions. We discuss the order of the α-β phase transition and propose possible reasons for the unusual behavior.