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

An Oxide-Centered Trinuclear Manganese(III) Compound with a Bulky Phenol-Pyrazolate Ligand

A new oxide-centered trinuclear manganese(III) compound with the formula [Mn3(μ3-O)(naphpz)3(O2CMe)(MeOH)2]n (1) {H2naphpz = 2-[1H-pyrazol-5(3)-yl]naphthalen-1-ol} has been synthesized and characterized. Three bulky doubly deprotonated phenol-pyrazole ligands are coordinated to the three manganese(III) ions with a μ3-oxide bridge in the same plane. Methanol and acetate ligands are at the apical positions of the manganese(III) ions. The trinuclear units are bridged by acetate ligands in the anti-anti mode, leading to the formation of 1-D chains. Temperature-dependent magnetic susceptibility studies indicate the presence of overall antiferromagnetic interactions with magnetic exchange constants of J1 ≈ –1.9 and J2 ≈ –5.5 cm–1 and g = 1.91. Weak antiferromagnetic interactions (zJ′ = –0.37 cm–1) between the trinuclear units in the 1-D chain (and between the chains) in compound 1 are present.This work was financially supported by the EC-RTN “QuEMolNa” (No. MRTN-CT-2003-504880) and the ECNetwork of Excellence “MAGMANet” (No. 515767-2).Peer Reviewe

Ga NMR studies of a superconducting Ga

We present 69,71Ga-NMR experiments on microcrystalline samples of the recently discovered supramolecular compound ${\rm Ga_{84}[N(SiMe_3)_2]_{20}Li_6Br_2(thf)_{20}{\cdot} 2toluene}$, which is composed of ligand-coordinated Ga84 metal clusters, packed together in a fully ordered crystalline matrix. The compound is highly conducting and even shows superconductivity below $T_c \sim 7.2$ K. Our preliminary results between 10–300 K show a metallic-like behavior: the nuclear spin-lattice relaxation rate $T_1^{-1}$ follows the Korringa law $^{69}(T_1T)^{-1}$ = 0.36 ${\rm s^{-1}K^{-1}}$, but with a relaxation rate approximately three times smaller than in bulk α-Ga metal. No quantum-size effects are observed, the Korringa law being followed down to 10 K, whereas the quantum-gaps for individual clusters should amount to $\sim 10^3$ K. These results therefore suggest a transport process based on intermolecular charge transfer, similar as in alkali-doped fullerenes and silicon-clathrates

Low Temperature Magnetic Properties of the Bimetallic Compounds [Co(C₅H₅NO)₆](MX₄) (M = Co²⁺, Zn²⁺; X = Cl⁻, Br⁻)

We have measured the specific heat and suceptibility of the compounds [Co(C5H5NO)6] (MX4) (M = Co, Zn; X = Cl, Br), which are all isostructural and monoclinic. The data show that two magnetic subsystems are present in the Co/Co compounds. The Co2+ ions on the octahedral positions of the chlorine compounds exhibit antiferromagnetic ordering at Tc = 0.95(1) K, the Co2+ ions on the tetrahedral positions remaining paramagnetic down to at least 0.04 K. The paramagnetic ions appear to experience two different molecular fields from the ordered Co2+ ions. The behaviour of the bromine compounds is similar, the ordering temperature being about 0.65 K. Both chlorine compounds are apparently canted antiferromagnets. The bromine compounds show compensated antiferromagnetism. © 1984

Size-dependent magnetic ordering and spin dynamics in DyPO(4) and GdPO(4) nanoparticles

Low-temperature magnetic susceptibility and heat-capacity measurements on nanoparticles (d? 2.6 nm) of the antiferromagnetic compounds DyPO4 (TN=3.4 K) and GdPO4 (TN=0.77 K) provide clear demonstrations of finite-size effects, which limit the divergence of the magnetic correlation lengths, thereby suppressing the bulk long-range magnetic ordering transitions. Instead, the incomplete antiferromagnetic order inside the particles leads to the formation of net magnetic moments on the particles. For the nanoparticles of Ising-type DyPO4 superparamagnetic blocking is found in the ac susceptibility at ?1 K, those of the XY-type GdPO4 analog show a dipolar spin-glass transition at ?0.2 K. Monte Carlo simulations for the magnetic heat capacities of both bulk and nanoparticle samples are in agreement with the experimental data. Strong size effects are also apparent in the Dy3+ and Gd3+ spin dynamics, which were studied by zero-field muon spin rotation (?SR) and high-field 31P-nuclear magnetic resonance (31P-NMR) nuclear relaxation measurements. The freezing transitions observed in the ac susceptibility of the nanoparticles also appear as peaks in the temperature dependence of the zero-field ?SR rates, but at slightly higher temperatures, as to be expected from the higher frequency of the muon probe. For both bulk and nanoparticles of GdPO4, the muon and 31P-NMR rates are for T?5 K dominated by exchange-narrowed hyperfine broadening arising from the electron spin-spin interactions inside the particles. The dipolar hyperfine interactions acting on the muons and the 31P are, however, much reduced in the nanoparticles. For the DyPO4 analogs the high-temperature rates appear to be fully determined by electron spin-lattice relaxation processes.RRR/Radiation, Radionuclides and ReactorsApplied Science