Nuclear spin driven quantum relaxation in LiY_0.998Ho_0.002F_4

Staircase hysteresis loops of the magnetization of a LiY_0.998Ho_0.002F_4 single crystal are observed at subkelvin temperatures and low field sweep rates. This behavior results from quantum dynamics at avoided level crossings of the energy spectrum of single Ho^{3+} ions in the presence of hyperfine interactions. Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings. At faster sweep rates, non-equilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps.Comment: 5 pages, 5 eps figures, using RevTe

Magnetic Ordering in CoCl₂·2P(C₆H₅)₃ and CoBr₂·2P(C₆H₅)₃

The crystal structures of CoCl2·2P(C6H5)3 and CoBr2·2P(C6H5)3 are reported. Both crystals belong to the space group P2/c with Z = 2: for the chloride, a = 11.764 (2) Å, b = 8.250 (3) Å, c = 17.254 (7) Å, and β = 106.57 (4)°; for the isostructural bromide, a = 11.828 (2) Å, b = 8.325 (2) Å, c = 17.365 (5) Å, and β = 106.58 (3)°. The crystal susceptibilities of the compounds have been measured over the temperature interval 40 mK-4 K. Antiferromagnetic ordering is observed at 0.21 ± 0.01 K (Cl) and 0.25 ± 0.01 K (Br). The specific heats have also been measured, and the ordering has been confirmed. Both compounds order with lattice dimensionality between 1 and 2, and all the data may be fit by a theoretical calculation for the rectangular Ising lattice, with Jx/Jy = 0.31 ± 0.02 for the chloride and 0.10 ± 0.02 for the bromide. These results require that the |±3/2) component of the 4A2 level be the ground state in both systems. © 1982 American Chemical Society