Thermodynamic stability of a regular black hole in nonlinear electrodynamics

By the turning point method, we study the thermodynamic stability of the a regular black hole in nonlinear electrodynamics. The microcanonical, canonical, and grand canonical ensembles are analyzed; the phase diagrams (m, beta(m)) and (q, beta(q)) (microcanonical ensemble) show that the regular black hole is stable at all the equilibrium sequence, but for the phase diagram (beta(m), -m) (canonical and grand canonical ensembles) the black hole shows a turning point. This means that the black hole is thermodynamically unstable. The temperature of the regular black hole has behavior that resembling Reissner-Nordstrom behavior: the temperature grows with the mass as in an ordinary thermodynamical system, but there is a maximum value of mass beyond which the temperature diminishes. The heat capacity displays a divergence with a change of sign that occurs precisely at the turning point obtained by the turning point method in the canonical ensemble.