Heat capacities of the spin quantum number S = 1/2 two-dimensional Heisenberg antiferromagnet bis(2-amino-5-chloropyridinium) tetrabromocuprate(II) [(5CAP)2CuBr4] crystal and its nonmagnetic analogue, the bis(2-amino-5-chloropyridinium) tetrabromozincate(II) [(5CAP)2ZnBr4] crystal, were measured by adiabatic calorimetry. For the (5CAP)2ZnBr4 crystal, single-crystal X-ray diffraction was also performed. The (5CAP)2ZnBr4 crystal belongs to the orthorhombic space group Pbca, with a = 16.074(2) Å, b = 7.688(2) Å, c = 30.538(6) Å, and Z = 8. In the (5CAP)2CuBr4 crystal, an antiferromagnetic phase transition occurred at T N = 5.08 K, and a thermal anomaly arising from the short-range order characteristic of two-dimensional magnetic substances was found above T N; the heat capacity of the (5CAP)2ZnBr4 crystal showed no thermal anomaly. The enthalpy and entropy gains due to the magnetic transition were estimated to be 49.3 J mol-1 and 5.65 J K-1 mol-1, respectively. The value of the entropy gain coincides well with the R ln 2 (5.76 J K-1 mol-1, R stands for the gas constant) expected for S = 1/2 spin systems. The thermal anomaly observed above T N is well accounted for in terms of the S = 1/2 two-dimensional antiferromagneic Heisenberg model of a square lattice with J/k B = −4.3 K. Spin wave analysis of the magnetic heat capacities below T N suggests that the (5CAP)2CuBr4 crystal is in a three-dimensional antiferromagnetic state, which is realized by a weak interlayer magnetic interaction between the two-dimensional layers.

The magnetic susceptibility and magnetization of two new layered S= 1 2Heisenberg antiferromagnets with moderate exchange are reported.The two isostructural compounds, (2-amino-5-chloropyridinium)2CuBr4 ((5CAP)2CuBr4) and (2-amino-5-methylpyridinium)2CuBr4 ((5MAP)2CuBr4), contain S= , Cu(II) ions related by C-centering, yielding four equivalent nearest neighbors. The crystal structure of newly synthesized compound, (5CAP)2CuBr4, shows the existence of layers of distorted copper(II)-bromide tetrahedra parallel to the ab plane, separated by the organic cations along the c axis. Magnetic pathways are available through the bromide-bromide contacts within the layers and provide for moderate antiferromagnetic exchange. Susceptibility measurements indicate interaction strengths to be 8.5(1) K, and 6.5(1) K and ordering temperatures of 5.1(2) K and 3.8(2) K for (5CAP)2CuBr4 and (5MAP)2CuBr4 respectively. High field magnetization experiments on both compounds show upward curvature of M(H,T). Magnetization measurements made at T = 1.3 K show saturation occurs in (5MAP)2CuBr4 at 18.8 T and in (5CAP)2CuBr4 at 24.1 T. The magnetization curves are consistent with recent theoretical predictions. Single crystal magnetization measured at 2.0 K indicate a spin flop transition at 0.38 T and 0.63 T for (5CAP)2CuBr4 and (5MAP)2CuBr4 respectively.

The magnetic materials ͑5CAP͒ 2 CuCl 4 and ͑5MAP͒ 2 CuCl 4 form effective square planar lattices of CuCl 4 2Ϫ anions. The Cu 2ϩ ions have weak antiferromagnetic interactions in the plane through Cl-Cl contacts. These planes are well separated by the 5MAP and 5CAP organic groups. High temperature susceptibility measurements give Jϭ0.57 K for ͑5CAP͒ 2 CuCl 4 and Jϭ0.38 K for ͑5MAP͒ 2 CuCl 4 . (T,Hϭ0) for these materials has a rounded maximum at T peak ϭ1.1 K for ͑5CAP͒ 2 CuCl 4 and T peak ϭ0.62 K for ͑5MAP͒ 2 CuCl 4 . A kink in (T,Hϭ0) for the ͑5MAP͒ 2 CuCl 4 suggests a phase transition to three-dimensional long range order at T C ϭ0.44 K. Measurements of (T,H) show that the saturation field H C for the transition to a fieldinduced saturated state is 3.78 T for ͑5CAP͒ 2 CuCl 4 and 2.17 T for ͑5MAP͒ 2 CuCl 4 . Thus these materials are excellent model systems in which to explore the full H -T phase diagram of the two-dimensional Sϭ1/2 Heisenberg antiferromagnet.

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As part of a program to create new low dimensional S=1/2 antiferromagnetic Heisenberg compounds with moderate exchange, we have prepared four new copper complexes with square magnetic lattices. The complexes L2[Cu&] have been prepared (L = 2-amino-5-methylpyridiniuq X = C1, Br; L = 2-amino-5chloropyriduq X = Br). The compounds are isomorphous, giving monoclinic crystals; space group C2/c. The tetrahalometallate ions lie in C-centered layers, forming a square magnetic lattice. The layers are separated by the organic cations. P r e l i i analysis of the magnetic data shows that the exchange constant, J, increases in the order Br > C1 and 2-amino-5-chloropyridinium > 2-amino-5-methylpyridinium. A new member of the pyrazine bridged family M(pyrazinehXz has also been prepared and studied. Cu(p+ (BF& also crystallizes in the space group C2/c. Analysis of the temperature dependence of the susceptibility yields and exchange value of -7.98 K., similar to that found for the known Cu(pzh(C10,h complex. 333 Downloaded by [Heriot-Watt University] at 00:57 27 December 2014 334 A.S. ALBRECHT el al.

An amorphous phase of Co3BTCA2(H2O)6 has been prepared (BTCA=1,3,5-benzenetri- carboxylate). The static properties (dc susceptibility, hysteresis) of Co3BTCA2(H2O)6 are similar to those of other ‘‘glassy’’ magnetic systems, but the dynamics of Co3BTCA2(H2O)6 (absence of long relaxation times, disappearance of frequency dependence at low temperatures) are unusual. The amorphous phase is characterized by a narrow hysteresis loop at low temperature. For the hysteresis curve at 4.2 K, the coercive field is several hundred Oersted and the remnant magnetization is on the order of 10−2 Msat. The temperature dependence of the dc magnetic susceptibility shows an unusual field dependence below 30 K, with the largest effective moments being obtained in the smallest applied fields. The freezing temperature, Tf, in zero field, as determined by both dc and ac susceptibilities, has been found to be Tf=23 K. The ac susceptibility shows the characteristic spin glass frequency dependence, yet loses all frequency dependence for temperatures below 9 K.

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