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 dependence of these phase boundaries are obtained as Tβγ(H)=43.8-1.55×10-3H2 K and Tαβ(H)=23.9-0.73×10-3H2 K. The magnetic Clausius-Clapeyron equation quantitatively explains the H dependence of Tβγ, but does not Tαβ. The MCE curve at Tβγ is of typical first order, while the curve at Tαβ 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 behaviors.