Thermodynamics in the high-field phases of (TMTSF)(2)ClO4

We report the results of magnetocaloric-effect measurements of the field-induced spin-density-wave (FISDW) state in the Bechgaard salt (TMTSF)(2)ClO4 in fields up to 30 T at temperatures down to T = 1.6 K. Our results confirm the phase diagram recently presented but add several insights concerning the unusual low-temperature transition between FISDW phases at 27 T. The magnetocaloric signature of this transition appears as two coupled first-order transitions. partial derivative S/partial derivative H oscillates with period H-1 = (255 T)(-1) for samples where the ClO4 anions have ordered. Anion disorder suppresses the oscillatory behavior, but not the high-field inter-FISDW phase transition.