Anisotropic superconducting and normal state magnetic properties of single crystals of RNi2B2C compounds (R = Y, Gd, Dy, Ho, Er, and Tm)

The interaction of superconductivity with magnetism has been one of the most interesting and important phenomena in solid state physics since the 1950\u27s when small amounts of magnetic impurities were incorporated in superconductors. The discovery of the magnetic superconductors RNi[subscript]2B[subscript]2C (R = rare earth, Y) offers a new system to study this interaction. The wide ranges of superconducting transition (T[subscript] c) and antiferromagnetic (AF) ordering temperatures (T[subscript] N) ( 0 K ≤ T[subscript]c ≤ 16 K, 0 K ≤ T[subscript]N ≤ 20 K) give a good opportunity to observe a variety of interesting phenomena;Single crystals of high quality with appropriate size and mass are crucial in examining the anisotropic intrinsic properties. Single crystals have been grown successfully by an unusual high temperature flux method and characterized thoroughly by X-ray, electrical transport, magnetization, neutron scattering, scanning electron microscopy, and other measurements. Superconductivity in single crystal DyNi[subscript]2B[subscript]2C is discovered and exhibits T[subscript]c \u3c T[subscript]N, which is the first observation of this ordering of transition temperatures in a crystallographically ordered compound except heavy fermion superconductors. The crystals reveal highly anisotropic behavior for R = Tb, Dy, Ho, Er, and Tm, but not for Gd, in the temperature dependent normal state magnetic susceptibility for 2 K ≤ T ≤ 300 K, which implies that the crystalline electric field (CEF) interaction plays a key role in the anisotropy. Nice fitting of anisotropic magnetization data of HoNi[subscript]2B[subscript]2C was achieved by a detailed CEF analysis, and CEF parameters (B[subscript]sp n m), energy levels, and corresponding eigenstates were determined. The determined data can be used for analyzing magnetism and superconductivity below T[subscript] N and T[subscript] c. The long range magnetic ordering in the magnetic sublattices, investigated by magnetization, specific heat, and magnetoresistivity, are found to be sensitive to the orientation of the applied field. Superconductivity and local magnetic moment ordering coexist below T[subscript] N for R = Dy, Ho, Er, and Tm. The interplay between the two phenomena appears in anisotropic reentrant behavior of the superconductivity and anomalous suppression of the upper critical field (H[subscript] c2)