R 2 Ti 2 O 7 (R = Y and various rare earth elements) has pyrochlore type structure, which consists of two kinds of three-dimensional networks individually formed by the corner-sharing R 4 -and Ti 4 -tetrahedra, respectively. Due to this structural characteristic, magnetic moments at the R sites are expected to be frustrated, if their nearest neighbor interaction is antiferromagnetic.For R = Tb, the value of Weiss temperature w is À19 K, indicating the Tb 3þ moments have antiferromagnetic nearest neighbor interaction and the system does not exhibit clear evidence for magnetic ordering. Previously, we investigated both the dynamical and static magnetic properties of Tb 2 Ti 2 O 7 by means of neutron scattering on a single crystal. 1-3) Based on results of measurements of the specific heat C and ac magnetic susceptibility , low temperature state of Tb 2 Ti 2 O 7 was discussed in ref. 4, too.The frustration is also expected even for the system with the ferromagnetic nearest neighbor interaction, if the moments have strong uniaxial anisotropy, where each moment lies along the local principal axis corresponding to the line which connects the site with the center of gravity of the tetrahedron. (There are four principal axes along [111] and other crystallographically equivalent directions.) Such the situation can be found in R 2 Ti 2 O 7 with R = Dy 5) and Ho, 6) which are called ''spin ice''.For Yb 2 Ti 2 O 7 , w is equal to be $0:53 K as shown in the inset of Fig. 1, indicating the nearest neighbor interaction between the Yb 3þ moments is ferromagnetic. The electronic ground state of Yb 3þ ion was reported to be a Kramers doublet with relatively small planar anisotropy, g ? ¼ 4:27 and g k ¼ 1:70, where g ? and g k are the g-values perpendicular to and along the local principal axis, respectively. 7) Sengupta et al. reported that the system has uniaxial anisotropy, g ? ¼ 0 and g k ¼ 3:4. 8) A sharp peak of C-T curve was reported at 0.24 K, indicating the existence of the phase transition. 9) However, Hodges et al. did not observe magnetic reflection except the small angle diffuse scattring. 7) In order to identify the specific heat anomaly at 0.24 K, we have carried out neutron diffraction and other magnetic measurements on a single crystal of Yb 2 Ti 2 O 7 down to 0.03 K by using dilution refrigerator. Here, we report that the system exhibits ferromagnetic transition at T C ¼ 0:24 K. We also discuss the low temperature behavior of the moments.A single crystal of Yb 2 Ti 2 O 7 was grown by a floating zone (FZ) method. The magnetization M was measured by using a SQUID magnetometer. The method of the ac magnetic susceptibility is described in ref. 4. Neutron measurements were carried out by using the triple axis spectrometer HQR (T1-1) installed at the thermal guide of JRR-3 of JAERI in Tokai. The crystal was oriented with [hh0] and [00l] axes in the scattering plane. Figure 1 shows the M-H curves of Yb 2 Ti 2 O 7 obtained at 5 K with the magnetic fields along [001], [110] and [111], where the anisotropy of the curves is ...

Two-dimensional metal–organic frameworks (2D MOFs) have attracted much attention, as they are the crystalline materials that exhibit both conductivity and microporosity. Numerous efforts have been made to advance their application as chemiresistive sensors or electrochemical capacitors. However, the intrinsic physical properties and spin states of these materials remain poorly understood. Most of these 2D MOFs possess a honeycomb lattice, with a Kagomé lattice arrangement of metal cations. These structural characteristics suggest that these MOFs would be candidates for geometrically frustrated spin systems with unprecedented magnetic phenomena. Herein, by performing magnetic susceptibility and specific heat measurements at an ultralow temperature down to 38mK on a 2D semiconductive MOF, Cu3(HHTP)2, a quantum spin liquid state that arises from the geometrical frustration was suggested. This result illustrates the potential of strongly correlated MOFs as systems with emergent phenomena induced by unusual structural topologies.

Using dc magnetization, ac susceptibility, specific heat, and neutron diffraction, we have studied the magnetic properties of Mn[N(CN)2]2(pyz) (pyz = pyrazine) in detail. The material crystallizes in the monoclinic space group P2(1)/n with a = 7.3248(2), b = 16.7369(4), and c = 8.7905 (2) A, beta = 89.596 (2) degrees, V = 1077.65(7) A(3), and Z = 4, as determined by Rietveld refinement of neutron powder diffraction data at 1.35 K. The 5 K neutron powder diffraction data reflect very little variation in the crystal structure. Interpenetrating ReO3-like networks are formed from axially elongated Mn(2+) octahedra and edges made up of mu-bonded [N(CN)2](-) anions and neutral pyz ligands. A three-dimensional antiferromagnetic ordering occurs below T(N) = 2.53(2) K. The magnetic unit cell is double the nuclear one along the a- and c-axes, giving the (1/2, 0, 1/2) superstructure. The crystallographic and antiferromagnetic structures are commensurate and consist of collinear Mn(2+) moments, each with a magnitude of 4.15(6) mu(B) aligned parallel to the a-direction (Mn-pyz-Mn chains). Electronic structure calculations indicate that the exchange interaction is much stronger along the Mn-pyz-Mn chain axis than along the Mn-NCNCN-Mn axes by a factor of approximately 40, giving rise to a predominantly one-dimensional magnetic system. Thus, the variable-temperature magnetic susceptibility data are well described by a Heisenberg antiferromagnetic chain model, giving g = 2.01(1) and J/k(B) = -0.27(1) K. Owing to single-ion anisotropy of the Mn(2+) ion, field-induced phenomena ascribed to spin-flop and paramagnetic transitions are observed at 0.43 and 2.83 T, respectively.