M. Orendáč

Cyanocomplexes with one-dimensional structures: preparations, crystal structures and magnetic properties

The review surveys the preparation methods, crystallochemistry and magnetic properties of one-dimensional cyanocomplexes. Their preparation methods are mainly based on reactions in solutions containing suitable building blocks as precursors of the polymeric structure. The analysis of published data on crystal structures is given, as the knowledge of the crystal structure is essential for the study of magneto-structural correlation. Published data on magnetic properties are discussed along with the methods used for the study of magnetic properties.

Preparation, crystal structure and magnetic properties of Cu(en)2Pd(CN)4

The compound Cu(en) 2 Pd(CN) 4 was prepared and characterized. Its crystal structure is constructed of chains running along the body diagonal to the unit cell. The building elements are [Cu(en) 2 ] 2+ cations and square-planar [Pd(CN) 4 ] 2− anions, which are linked by the bridging μ 2 -CN − groups. These bridging cyano groups are in trans positions in the cation as well as in the anion. The axial CuN(C) coordination bonds are rather long (2.544(2) A). The chains are connected through weak hydrogen bonds of the type NH⋯N(C), thereby forming an infinite plane-like arrangement. Susceptibility and magnetization studies revealed the presence of a weak antiferromagnetic exchange coupling in the studied material.

Chain-like crystal structure and magnetic properties of [catena-bis(1,2- diaminoethane)copper(II)-μ-dicyano-argentate]-dicyanoargentate

Abstract Cu(en) 2 Ag 2 (CN) 4 (en = 1,2-diaminoethane) crystallizes in the orthorhombic space group Pnnm with cell parameters a = 6.316(1), b = 9.018(2), c = 13.199(3) A , Z = 2 . The structure is formed of free linear [Ag(CN) 2 ] − anions and infinite cationic chains [-CU(en) 2 NCAgCN-] − , containing paramagnetic copper atoms bridged by a second kind of linear dicyanoargentate species. The coordination geometry of the copper atoms corresponds to an elongated tetragonal bipyramid with two chelating en molecules in the equatorial positions and N-bonded bridging cyano groups in the axial positions. The Ag…Ag distances exhibit short value of 3.1580(5) A. Weak bifurcated hydrogen bonds of the N(3)H(2)…N(2)(C(2))…H(2)N(3) type, symmetrical about twofold axis, are present (N(3)…N(2) 3.195(3) A). These hydrogen bonds form two symmetrically related infinite 2D patterns in the xz plane at y = 0 and y = 1 2 connecting free dicyanoargentate anions and Cu(en) 2 2− cations. The phase identity between single crystal and bulk sample was evidence by powder X-ray diffractometry and Rietveld profile refinement. The study of magnetic properties by magnetization and specific heat measurements reveals that despite the chain structure the title compound may represent an S = 1 2 3 D magnetic system characterized by a low value of the exchange coupling constant. [ J / k b ] ⪡ 60 mK.

Switching of the Magnetocaloric Effect of MnII Glycolate by Water Molecules

The transformation of Mn(II) glycolates (glc) between the three-dimensional coordination polymer [Mn(glc)2]n (1) and discrete mononuclear phase [Mn(glc)2 (H2O)2] (2) can be reversibly switched by water molecules, which dramatically change the magnetocaloric effect (MCE) of Mn(II) glycolates from the maximum of 6.9 J kg(-1)  K(-1) in 1 to 60.3 J kg(-1)  K(-1) in 2. This case example reveals that the effect of magnetic coupling on MCE plays a dominant role over that of other factors such as magnetic density for 3d-type magnetic refrigerants.

Magneto-structural correlations in one-dimensional Ni(en)2Pd(CN)4: magnetic properties and redetermination of the crystal structure at two temperatures

Abstract The magneto-structural correlations in Ni(en)2Pd(CN)4 (NEPC) have been studied. The crystal structure is formed by infinite electroneutral 2,2-TT type chains [Ni(en)2NCPd(CN)2CN] (en=1,2-diaminoethane) in which the paramagnetic nickel atoms are bridged by diamagnetic tetracyanopalladate anions. Contrary to the previous room temperature X-ray analysis of NEPC, the results of the current structure determinations at both 293 and 150 K showed the nickel atom to be coordinated in the form of slightly elongated octahedron with two chelate bonded and disordered en molecules in the equatorial plane (mean NiN distance of 2.096 A at both temperatures) and N-bonded bridging cyano groups in the axial positions (NiN distances of 2.117 and 2.108 A at 293 and 150 K, respectively). Consequently, the zero field splitting parameter D is finite and positive. The chains are held together by weak hydrogen bonds of the NH⋯N(C) type. Magnetisation, susceptibility and specific heat measurements indicate that NEPC represents an S=1 Heisenberg magnetic chain characterised by strong planar anisotropy, D/kB≈5 K, weak in-plane anisotropy, E/D=0.1, and by a small exchange coupling constant, J/kB≈0.25 K. These properties are similar to those exhibited by an analogous compound Ni(en)2Ni(CN)4.

Magnetocaloric study of spin relaxation in dipolar spin ice Dy2Ti2O7

The magnetocaloric effect of polycrystalline samples of pure and Y-doped dipolar spin ice Dy2Ti2O7 was investigated at temperatures from nominally 0.3 to 6 K and in magnetic fields of up to 2 T. As well as being of intrinsic interest, it is proposed that the magnetocaloric effect may be used as an appropriate tool for the qualitative study of slow relaxation processes in the spin ice regime. In the high-temperature regime the temperature change on adiabatic demagnetization was found to be consistent with previously published entropy vs temperature curves. At low temperatures (T < 0.4 K) cooling by adiabatic demagnetization was followed by an irreversible rise in temperature that persisted after the removal of the applied field. The relaxation time derived from this temperature rise was found to increase rapidly down to 0.3 K. The data near 0.3 K indicated a transition into a metastable state with much slower relaxation, supporting recent neutron-scattering results. In addition, magnetic dilution of 50% concentration was found to significantly prolong the dynamical response in the mK temperature range, in contrast with results reported for higher temperatures at which the spin correlations are suppressed. These observations are discussed in terms of defects and loop correlations in the spin ice state.

Specific heat study of magnetic excitations in a one-dimensional S=1 Heisenberg magnet with strong planar anisotropy

The results of experimental studies of the specific heat of the magnetic chain compounds Ni(C2H8N2)2Ni(CN)4, Ni(C11H10N2O)2Ni(CN)4, and Ni(C10H8N2)2Ni(CN)4⋅H2O are reported. All compounds are identified as S=1 planar Heisenberg magnetic chains with large planar anisotropy and different values of the in-plane anisotropy constant. The low-temperature specific heat data are interpreted assuming the existence of noninteracting excitons and antiexcitons as elementary excitations from the singlet-ground state. The extended strong-coupling model is used for analysis of the data at higher temperatures. The applicability of the models used with respect to the value of the in-plane anisotropy is discussed.

S=1 Heisenberg chain Ni(C2H8N2)2Ni(CN)4 in the frame of strong coupling theory

Our previous experimental study has identified Ni(C2H8N2)2Ni(CN)4 (NENC) as an S=1 antiferromagnetic (AF) Heisenberg chain with strong planar anisotropy D/kB≈6 K and D/|J|≈7.5. The position of the system in the (D,λ) phase diagram ((7.5,1) in units J) has enabled the study of excitons as elementary excitations from the singlet ground state (SGS). Strong coupling theory for the thermodynamics of 1d S=1 large—D systems is used for the re—analysis of specific heat and susceptibility data. The influence of in—plane anisotropy and exciton interactions is discussed.

Interplay between mesoscopic phase separation and bulk magnetism in the layered Na x CoO 2

Specific heat of layered

Nonlinear excitations in CsNiF 3 in magnetic fields perpendicular to the easy plane

{\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}