Phenomenological Theory of Superconductivity and Magnetism in Ho1−x_{1-x}Dyx_xNi2_2B2_2C

The coexistence of the superconductivity and magnetism in the Ho$_{1-x}$Dy$_x$Ni$_2$B$_2$C is studied by using Ginzburg-Landau theory. This alloy shows the coexistence and complex interplay of superconducting and magnetic order. We propose a phenomenological model which includes two magnetic and two superconducting order parameters accounting for the multi-band structure of this material. We describe phenomenologically the magnetic fluctuations and order and demonstrate that they lead to anomalous behavior of the upper critical field. The doping dependence of $T_c$ in Ho$_{1-x}$Dy$_x$Ni$_2$B$_2$C showing a reentrance behavior are analyzed yielding a very good agreement with experimental data.Comment: 4 pages, 3 figures, REVTeX, submitted to PR

Specific Heat Study of the Magnetic Superconductor HoNi2B2C

The complex magnetic transitions and superconductivity of HoNi2B2C were studied via the dependence of the heat capacity on temperature and in-plane field angle. We provide an extended, comprehensive magnetic phase diagram for B // [100] and B // [110] based on the thermodynamic measurements. Three magnetic transitions and the superconducting transition were clearly observed. The 5.2 K transition (T_{N}) shows a hysteresis with temperature, indicating the first order nature of the transition at B=0 T. The 6 K transition (T_{M}), namely the onset of the long-range ordering, displays a dramatic in-plane anisotropy: T_{M} increases with increasing magnetic field for B // [100] while it decreases with increasing field for B // [110]. The anomalous anisotropy in T_{M} indicates that the transition is related to the a-axis spiral structure. The 5.5 K transition (T^{*}) shows similar behavior to the 5.2 K transition, i.e., a small in-plane anisotropy and scaling with Ising model. This last transition is ascribed to the change from a^{*} dominant phase to c^{*} dominant phase.Comment: 9 pages, 11 figure

Neutron-diffraction study of antiferromagnetic order in the magnetic superconductor ErNi<SUB>2</SUB>B<SUB>2</SUB>C

We have carried out powder-neutron-diffraction studies of the antiferromagnetic order that develops below TN=6.8 K in the magnetic superconductor (Tc=11 K) ErNi2B2C. The antiferromagnetic structure is associated with spins on the Er atoms, which order in a transversely polarized planar sinusoidal structure propagating along the a or b axis in equal domains, with the Er moments parallel to the b or a axis, respectively. Third and fifth harmonics of the sine wave are observed at low temperatures, indicating a squared-up sine wave, with a low-temperature amplitude of 7.8&#956;B/Er atom. There is also evidence of an induced moment of -0.35&#956;B on the Ni ions. A similar study of the isostructural compound YNi2B2C revealed no magnetic peaks (&#956; &lt; 0.13&#956;B) developing down to 0.3 K for Q values less than 1.6 A-1

Structures and magnetic properties of trinuclear copper(II) halide salts

The crystal structures of a series of trinuclear copper(II) halide salts have been determined. These all contain pseudoplanar, symmetrically bibridged Cu3X82- (Ia) or Cu3X7L- (Ib) anions. A2Cu3Cl8 or A2Cu3Cl8 salts are formed with A being the (Chemical Equation Presented) N-methylpiperazinium (NMPZ), 3-methyl-2-aminopyridinium (3MAP), and 5-methyl-2-aminopyridinium (5MAP) cations. A Cu3Br83- analogue is found with the 5-bromo-6-methyl-2-aminopyridinium (5B6MAP) cation. With the methylphenethylammonium (NMPH) cation, a (NMPH)Cu3Cl7·EtOH salt is isolated. The salts are all monoclinic with the following space group and lattice constants: NMPZ, C5H14N2Cu3Cl8, P21/c, a = 6.840 (3) Å, b = 14.321 (9) Å, c = 9.890 (5) Å, β = 102.90 (5)°, Z = 4; 3MAP, C12H18N4Cu3Cl8, C2/c, a = 26.05 (1) Å, b = 13.687 (4) Å, c = 7.099 (3) Å, β = 117.95 (3)°, Z = 4; NMPH, C11H20NOC3Cl7, P21/n, a = 11.843 (4) Å, b = 7.626 (3) Å, c = 23.840 (10) Å, β = 79.61 (3)°, Z = 4; 5B6MAP, C12N10N4-Cu3Br10, P21/n, a = 13.216 (3) Å, b = 4.076 (1) Å, c = 24.614 (6) Å, β = 91.65 (2)°, Z = 2. Each copper ion extends its primary coordination by formation of two semicoordinate bonds to halide ions in adjacent oligomers, yielding the familiar 4+2 coordination geometry for copper(II) complexes. These additional linkages cause the oligomers to aggregate into stacks, yielding stacking patterns of type IIa (NMPZ, 3MAP salts) or type IIb (NMPH, 5MAP, and 5B6MAP). Cu-X distances are approximately 2.3 (Figure Presented) Å (Cl) or 2.4 Å (Br) within the oligomers and 2.7-3.3 Å (Cl) or 3.2-3.3 Å (Br) between oligomers. The bridging Cu-X-Cu angles within the trimer average near 94°. Magnetic susceptibility measurements have been made on the NMPZ, NMPH, 3MAP, and 5MAP chloride salts. In addition, measurements were performed on Cu3Cl6(CH3CN)2, which contains neutral trinuclear oligomers. All compounds have doublet ground states arising from antiferromagnetic exchange coupling between neighboring copper(II) ions. The value of J/k is typically -20 to -35 K, consistent with the structural characteristics of the trimeric specie