Kyong O. Joung

Field‐induced magnetic ordering in uniaxial nickel systems: A second example

The zero‐field susceptibilities parallel and perpendicular to the tetragonal axis of NiCl2⋅4SC(NH2)2 have been measured between 1.2 and 30 K and fitted to the usual spin Hamiltonian. The resulting parameters are D/kB = 7.6±0.4 K, z0J/kB = −4.5±0.3 K, g∥ = 2.26±0.03 and g⊥ = 2.34±0.03. The susceptibility has also been measured parallel to the tetragonal axis at constant temperatures between 0.3 and 2 K as a function of external field. Magnetic ordering is found to occur, and portions of the phase boundary between paramagnetic and antiferromagnetic states have been determined.

Field-induced level-crossing and crystal structure of the singlet ground-state system (Ni(C5H5NO)6)(NO3)2

The compound (Ni(C5H5NO)6)(NO3)2 crystallises in the space group R3 with the metal atom at the centre of the unit cell, which imposes a centre of inversion, and a three-fold rotation axis at the metal atom. The NO3- groups are positionally disordered. Susceptibility measurements at 1.60K on oriented single crystals of (Ni(C5H5NO)6)(NO3)2 in applied fields clearly shows a singlet-doublet crossing at H1c=46.2+or-1.5 kOe. Susceptibilities at zero field are also presented. The data analyses yield D/kB approximately=5.69K for the singlet-doublet separation and zJ/kB approximately=-1.45K for the (subcritical) antiferromagnetic interaction.

Structural and magnetic properties of Co(urea)2Cl2⋅2H2O: A two‐dimensional Ising system with hidden canting

The characterization of Co(urea)2Cl2⋅2H2O is reported. The crystal structure at ambient temperatures yields the following information: the chlorine, water, and urea group are coordinated, the urea being bound via the oxygen, to form an octahedral environment about the metal atom. Intermolecular hydrogen bonds between the coordinated chlorine and water ligands produce parallel infinite sheets which provide a two‐dimensional exchange pathway. The sheets are separated by the urea ligands and may be considered as magnetically isolated from one another. Crystal data: space group P21/c, Z = 2, a = 7.397 (3) A, b = 7.795(1) A, c = 9.764(2) A, β = 116.23(5) A, V = 505 A3, ρcalc = 1.87 g cm−3, ρobs = 1.88 g cm−3, R = 4.1% for 692 reflections. A λ anomaly in the specific heat was observed at 2.585±0.005 K, and the critical parameters are characteristic of the two‐dimensional Ising model. The zero‐field susceptibilities have been measured over the temperature interval 1.2–30 K, and are consistent with antiferromagne...

Magnetic ordering in GdH3

Abstract In contrast to a previous report, GdH 3 is found to order antiferromagnetically at 1.8 K.

Magnetic ordering in [Co(γ-Ch3C5H4NO)6](ClO4)2: I. Crystal structure and superexchange pathways

Abstract The crystal structure of the title compound is reported, where the ligand is γ-methylpyridine N-oxide. The crystal data are: space group P 2 1 / c , Z = 2, a = 9.498(2) A , b = 10.761(3) A , c = 20.523(7) A , and β = 92.46(2)° . The structure consists of octahedrally-coordinated [Co(γ-CH3C5H4NO)6]2+ ions in a largely two-dimensional network, together with perchlorate ions which exhibit two-fold positional disorder. Likely superexchange paths are discussed.

Magnetic Ordering in [Co(γ-Ch₃C₅H₄NO)₆](ClO₄)₂ I

Abstract The crystal structure of the title compound is reported, where the ligand is γ-methylpyridine N-oxide. The crystal data are: space group P 2 1 / c , Z = 2, a = 9.498(2) A , b = 10.761(3) A , c = 20.523(7) A , and β = 92.46(2)° . The structure consists of octahedrally-coordinated [Co(γ-CH3C5H4NO)6]2+ ions in a largely two-dimensional network, together with perchlorate ions which exhibit two-fold positional disorder. Likely superexchange paths are discussed.

Magnetic ordering in [Co(γ-CH3C5H4NO)6](ClO4)2: II. Analysis of susceptibilities and specific heat in terms of the two-dimensional rectangular Ising model; evidence for methyl-group rotational tunneling

Abstract The single-crystal susceptibilities of [Co(γ-CH3C5H4NO)6](ClO4)2 have been measured over the temperature interval 40 mK-4.2 K, and antiferromagnetic ordering is found to occur at 0.49±0.02 K. Specific heat data (50 mK-1.2 K) exhibit a λ-anomaly at 0.480±0.015 K. Both sets of measurements have been analyzed in terms of the rectangular 2d Ising lattice for S = 1 2 , with the parameters |Ja/kB| = (0.55±0.02)K, Jb/Ja = 0.6±0.2 from the specific heat, and g c′ = 5.48±0.03, Ja/kB = (−0.54±0.05) K, Jb/Ja = 0.6±0.2 from the susceptibility. At the lowest temperatures reached (0.05–0.3 K), the specific heats of both [Co(γ-CH3C5H4NO)6](ClO)4)2 and the Cu2+-doped related Zn compound, [Zn(γ-CH3C5H4NO )6](ClO4)2 show contributions which probably arise from rotational tunneling of the CH3 groups attached to the pyridine N-oxide rings. Assuming a 3-fold rotational barrier, the height of the potential barriers opposing CH3-rotations is estimated as V 3 ⋍ 430 K .

A two-dimensional ising system with canting: Co(urea)2Cl2.2H2O

Abstract The crystal structure, specific heat and magnetic susceptibilities of Co(urea)2Cl2.2H2O are reported. The crystal contains planar trans-CoCl2(H2O)2 units, and undergoes long range antiferromagnetic order at (2.585 ± 0.005) K. The data are consistent with the planar Ising S = 1 2 model with g|| = (7.03 ± 0.05) and J/k = (-2.16 ± 0.05) K.