Hiroko Tamaki

Ferrimagnetic Mixed-Metal Assemblies {NBu4[MFe(ox)3]}x

Abstract Hetero-metal assemblies, (NBu4=tetra(n-butyl)ammonium ion; ox=oxalate ion; M(II)=Ni(1), Fe(2), Mn(3), Zn(4)) have been synthesized by the use of [Fe(ox)3]3- as the building block. 1 and 2 are ferrimagnets of TN=43 and 28 K, respectively. 3 is an antiferromagnet. 4 shows a paramagnetic behavior.

Dinuclear manganese(II) complexes of 2,6-Bis[2-(dialkylamino)ethyliminomethyl]-4-methylphenolate(1–): synthesis, structure, and magnetism

2,6-Bis[2-(dialkylamino)ethyliminomethyl]-4-methylphenolate(1–)[alkyl = methyl (L1) or ethyl (L2)] forms two types of dinuclear manganese(II) complexes [Mn2L(RCO2)2(NCS)](L = L1 or L2, R = CH3 or C6H5) and [Mn2LCl3](L = L1 or L2). The crystal structure of [Mn2L1(CH3CO2)2(NCS)]·H2O·CH3OH has been determined: monoclinic, space group P21/n, a= 17.700(3), b= 12.516(2), c= 15.263(3)A, β= 107.16(1)° and Z= 4. The X-ray analysis reveals a dinuclear structure bridged by the phenolic oxygen of L1 and two acetate groups. The thiocyanate group co-ordinates to one Mn, resulting in different co-ordination geometries about the two manganese ions, i.e. trigonal bipyramidal and pseudo-octahedral. Magnetic susceptibility measurements over the temperature range 4.2–300 K indicate weak antiferromagnetic interaction (J=–2 to –5 cm–1) for [Mn2L(RCO2)2(NCS)] and no appreciable interaction for [Mn2LCl3].

Moessbauer Spectroscopic Study of Magnetic Ordering in Oxalate-Bridged Cr(III)-Fe(II) and Fe(III)-Fe(II) Systems

Abstract Magnetic ordering phenomena in mixed-metal assemblies {NBu4[Fe(II)M(III)(ox)3]}3∞ (M=Cr (1), Fe (2)) were studied by using 57Fe Moessbauer spectroscopy, where NBu4 +=tetra(n-butyl)ammonium ion, ox2-=oxalate ion. Nuclear Zeeman splittings were observed in the Moessbauer spectra of both 1 and 2 at low temperatures (below 10 K for 1 and 41 K for 2), which indicated the occurrence of three-dimensional magnetic ordering in these complexes under zero applied field.

Heat capacities of quasi-two-dimensional hetero-spin honeycomb magnets {NBu4[CuIICrIII(ox)3]}n and {PPh4[MnIICrIII(ox)3]}n (Bu=n-butyl, Ph=phenyl, H2ox=oxalic acid): High-temperature series expansion analysis

Heat capacities of two metal-assembled complexes {NBu4[CuIICrIII(ox)3]}n and {PPh4[MnIICrIII(ox)3]}n (Bu=n-butyl, Ph=phenyl, H2ox=oxalic acid) were measured by adiabatic calorimetry in the 0.5–300 K temperature range. A ferromagnetic phase transition was detected at Tc=6.98 K for {NBu4[CuCr(ox)3]}n and Tc=5.59 K for {PPh4[MnCr(ox)3]}n, above which a remarkable heat capacity tail suggesting the short-range order effects was observed. Furthermore, a lambda-type heat-capacity anomaly due to a structural phase transition was found at Ttrs=226.9 K for {NBu4[CuCr(ox)3]}n and at Ttrs=71.3 K for {PPh4[MnCr(ox)3]}n. The observed entropy gains due to the magnetic phase transitions are very close to the theoretical values, R ln(2×4) for {NBu4[CuCr(ox)3]}n and R ln(6×4) for {PPh4[MnCr(ox)3]}n, expected from the spin multiplicities (CuII, s=1/2; MnII, s=5/2; CrIII, s=3/2). Since this series of metal oxalato assemblies can crystallize in either 2D honeycomb or 3D helical hetero-spin lattices, the theoretical magnetic h...

Calorimetry of Low-Dimensional Magnets

Abstract Heat capacity is very sensitive to the change in the long- and short-range order effects, which crucially depend on lattice dimensionality and spin-spin interaction. Dimensional crossover is often encountered in actual molecule-based or metal-assembled magnets and thus phase transitions due to the spin ordering may occur at low temperatures even for low-dimensional magnets. The important factor governing the short-range order effect due to magnetic fluctuation is the number of the nearest neighbor magnetic ions as well as the lattice dimensionality.