THERMODYNAMICS IN THE HIGH-FIELD PHASES OF (TMTSF)(2)CLO4

We report the results of magnetocaloric-effect measurements of the fie ld-induced spin-density-wave (FISDW) state in the Bechgaard salt (TMTS F)(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 i nsights concerning the unusual low-temperature transition between FISD W phases at 27 T. The magnetocaloric signature of this transition appe ars as two coupled first-order transitions. partial derivative S/parti al derivative H oscillates with period H-1 = (255 T)(-1) for samples w here the ClO4 anions have ordered. Anion disorder suppresses the oscil latory behavior, but not the high-field inter-FISDW phase transition.