Crossover from one- to three-dimensional behavior in the S=1/2 Heisenberg antiferromagnet Cu(N2H5)2(SO4)2

From experimental and theoretical analyses of magnetic and specific-heat properties, we present the complete magnetic phase diagram of the quasi-1D antiferromagnet Cu(N2H5)(2)(SO4)(2). On cooling and at zero magnetic field this compound enters a 1D regime with short-range magnetic correlations, marked by a broad maximum in the specific heat and magnetic susceptibility at T-max similar to 2 K, followed by a tridimensional antiferromagnetically ordered phase below T-N similar to 1 K induced by small interchain couplings. The intermediate-temperature 1D regime can be modeled using exact quantum-transfer-matrix calculations, which offer a compatible description of the nonmonotonic behavior of T-max as a function of the magnetic field, giving J = 4.25 K for the intrachain exchange parameter. The analysis of magnetic specific-heat and susceptibility data at low temperature indicates that the interchain exchange couplings are an order of magnitude smaller than the coupling inside the chains.