PHENOMENOLOGY, SPECIFIC-HEAT, AND CORRECTIONS TO SCALING IN COPPER AMMONIUM BROMIDE

Experimental data near the critical point for the spontaneous magnetiz ation, the zero-field susceptibility, and the specific heat of the (S = 1/2) insulating Heisenberg ferromagnet Cu (NH4)(2)Br(4)2H(2)O are co mpared with two extensions of asymptotic scaling. One is a previously described phenomenology which is equivalent to the predictions of the renormalization group (RG) when all irrelevant variables are neglected . The second results if one adds to the asymptotic scaling formulas th e confluent singularities due to the leading irrelevant variable. We f ind that both sets of formulas are consistent with the data for all th ree quantities: the spontaneous magnetization, the aero-field suscepti bility, and the specific heat, but that overall, the phenomenology req uires fewer parameters. A discrepancy between the experimental and the oretical values for the susceptibility exponent. is discussed.