The transition metal coordination compound (pyrimidine)2FeCl2 crystallizes in a chiral lattice, space group I4122 (or I4322). Combined magnetization, Mössbauer spectroscopy and powder neutron diffraction studies reveal that it is a canted antiferromagnet below TN = 6.4 K with an unusually large canting of the magnetic moments of 14• from their general antiferromagnetic alignment, one of the largest reported to date. This results in weak ferromagnetism with a ferromagnetic component of 1 µB. The large canting is due to the interplay between the antiferromagnetic exchange interaction and the local single-ion anisotropy in the chiral lattice. The magnetically ordered structure of (pyrimidine)2FeCl2, however, is not chiral. The implications of these findings for the search of molecule based materials exhibiting chiral magnetic ordering is discussed.

We present measurements of the magnetic susceptibility wðTÞ on Cu 2 Te 2 O 5 Br 2 under externally applied pressure. From our data we extract the pressure response of the antiferromagnetic phase transition at T 0 ¼ 11:6 K and of the overall magnetic coupling strength. Our experiments indicate that with pressure the overall magnetic coupling strength increases by about 25% with applied pressure of only 8 kbar: In contrast, the phase transition temperature T 0 is significantly suppressed and not observable anymore at a pressure of 8.2 kbar.The spin-tetrahedra system Cu 2 Te 2 O 5 Br 2 [1] belongs to a class of quantum magnets which has been in the focus of intense research efforts recently [2][3][4][5][6]. Here, the presence of a spin gap through dimerization for a quantum magnet does not lead to a non-magnetic singlet ground state. Instead, based on thermodynamic and spectroscopic techniques an unusual magnetic ground state has been evidenced. Tetragonal Cu 2 Te 2 O 5 Br 2 contains clusters of Cu 2þ with S ¼ 1 2 in a distorted square planar CuO 3 Br coordination (Fig. 1). These tetrahedra form weakly coupled sheets within the crystallographic a-b plane. Therefore, this system is ideal to study the interplay between the spin frustration of a tetrahedron with localized low-energy excitations and the tendency for a more collective magnetism induced by inter-tetrahedra couplings.The thermodynamic properties of Cu 2 Te 2 O 5 Br 2 are ascribed to two magnetic couplings within the tetrahedra, with the competing exchange constants J 1 and J 2 [2], and an inter-tetrahedra coupling J c [9]. As result of the coupling, the system undergoes a phase transition at T 0 ¼ 11:6 K: Neutron powder diffraction of Cu 2 Te 2 O 5 Br 2 reveals an antiferromagnetically ordered state with a strongly reduced magnetic moment of 0:51ð5Þ m B =Cu 2þ below T 0 [5]. On a microscopic level the cause for the phase transition has not been unambiguously resolved [7][8][9][10][11][12]. In particular, with the existence of low-lying excitations a magnetically ordered state close to quantum criticality has been discussed.Pressure experiments have proven to be a particularly useful tool to study quantum critical behavior. Therefore, in this work we present a pressure study on Cu 2 Te 2 O 5 Br 2 : For our experiments we used a CuBe pressure cell in a commercial SQUID magnetometer to measure wðTÞ at pressures up to 8.2 kbar and in external fields up to 5 T for temperatures 2-40 K. A powder ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ -see front matter r

We present measurements of the magnetic susceptibility ͑T͒ on the pyrimidine bridged transition metal complexes XCl 2 ͑P͒ 2 ͑X =Fe,Co,Ni; P = pyrimidine͒ under externally applied pressure. From our data we extract the pressure response of the antiferromagnetic phase transition at 6.4 K for X = Fe, 4.7 K for X = Co, and 16.3 K for X = Ni. The experiments indicate that with a pressure of ϳ8 kbar the transition temperature in all compounds increases by about 15%. Our data are complemented by electronic structure calculations, based on spin-unrestricted density functional theory. From these calculations we derive a bulk modulus B 0 = 20 GPa for FeCl 2 ͑P͒ 2 , in reasonable agreement with the experimentally determined value of 15 GPa. Further, the calculations reproduce the antiferromagnetically ordered state as the ground state. Finally, we calculate the pressure response of T N for XCl 2 ͑P͒ 2 , X = Fe, Ni, and derive an increase of T N by 16% and 9% for Fe and Ni, respectively, in good agreement with the experiments.

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We present 57 Fe Mössbauer spectroscopy experiments on the cyclic spincluster Fe 6 (tea) 6 (CH 3 OH) 6 (tea = triethanolaminato(-3)). In former studies, the spin cluster has been treated as a homogenous, quasi-one-dimensional spin S = 5/2 Heisenberg antiferromagnet. Our experiments reveal spectra, which consists of two different quadrupolar doublets. In consequence, there are two different Fe sites among the hexanuclear iron spin-cluster.