We present new upper critical field Hc2(T ) data in a broad temperature region 0.3K ≤ T ≤ Tc for LuNi2B2C and YNi2B2C single crystals with well characterized low impurity scattering rates.The absolute values for all T , in particular Hc2(0), and the sizeable positive curvature (PC) of Hc2(T ) at high and intermediate T are explained quantitatively within an effective two-band model. The failure of the isotropic single band approach is discussed in detail. Supported by de Haas van Alphen data, the superconductivity reveals direct insight into details of the electronic structure. The observed maximal PC near Tc gives strong evidence for clean limit type II superconductors. 74.60.Ec, 74.70.Ad, 74.72Ny The discovery [1,2] of superconductivity in transition metal borocarbides has generated large interest due to their relatively high transition temperatures T c ∼ 15 to 23 K and due to the relation between the mechanisms of superconductivity in these compounds, in cuprates, and in ordinary transition metals. Another highlight is the coexistence of magnetism and superconductivity in some of these compounds containing rare earth elements [3][4][5]. A study of the non-magnetic compounds such as LNi 2 B 2 C, with L=Lu,Y,Th,Sc [6], is a prerequisite for the understanding of their magnetic counterparts. Experimental data for LuNi 2 B 2 C [7] demonstrate beside a maximal positive curvature (PC) of H c2 (T ) near T c , observed also for YNi 2 B 2 C [8,4,9], a weak T -dependent anisotropy within the tetragonal basal plane and a Tindependent out-of-plane anisotropy of the upper critical field H c2 . Both anisotropies have been described [7] in terms of nonlocal corrections to the Ginzburg-Landau (GL) equations. In this picture the PC of H c c2 ( H to the tetragonal c-axis) is caused, almost purely, by the basal plane anisotropy. However, it should be noted that the reported anisotropy of H c2 for YNi 2 B 2 C is significantly smaller than for LuNi 2 B 2 C [7,9,10] whereas its PC is comparable or even larger. Further explanations of the unusual PC of H c2 (T ), such as quasi-2D fluctuations [11], are excluded by the underestimation of H c2 (T ) at low-T [9] and the observed weak anisotropy. The quantum critical point scenario [12] as well as the bipolaronic one [13] can be disregarded because the slope of H c2 (T ) decreases for T →0 (see Fig. 1). Local density approximation (LDA) band structure calculations [14,15] predict a nearly isotropic electronic structure with rather complicated bands near the Fermi level E F . However, in analyzing the superconductivity in terms of an isotropic single-band (ISB) Eliashberg model, the multi-band character and the anisotropic Fermi surface have been widely ignored so far.Here we present and analyze theoretically new data of H c2 (T ) in a broad interval 0.3K≤ T ≤ T c for high purity LuNi 2 B 2 C and YNi 2 B 2 C single crystals. We show that typical features of both compounds, such as H c2 (0) ∼ 8 to 10 T and the unusual PC of H c2 (T ) for T > ∼ 0.5T c , cannot in any way be explained ...

The interaction of rare-earth magnetism and superconductivity has been a topic of interest for many years. In classical magnetic superconductors (Chevrel phases, ternary rhodium borides, etc) as well as in the high-T c cuprates the superconducting state usually coexists with antiferromagnetic order on the rare-earth sublattice. In these compounds the magnetic ordering temperature T N is much below the superconducting transition temperature T c . The discovery of superconducting borocarbides RT 2 B 2 C with R = Sc, Y, La, Th, Dy, Ho, Er, Tm or Lu and T = Ni, Ru, Pd or Pt (where not all of these combinations of R and T result in superconductivity) has reanimated the research on the coexistence of superconductivity and magnetic order. Most of these borocarbides crystallize in the tetragonal LuNi 2 B 2 C type structure which is an interstitial modification of the ThCr 2 Si 2 type. Contrary to the behaviour of Cu in the cuprates Ni does not carry a magnetic moment in the borocarbides. Various types of antiferromagnetic structures on the rare-earth sublattice have been found to coexist with superconductivity in RNi 2 B 2 C for R = Tm, Er, Ho and Dy. Particularly of interest is the case of HoNi 2 B 2 C for which three different types of antiferromagnetic structures have been observed: (i) a commensurate one with Ho moments aligned ferromagnetically within layers perpendicular to the tetragonal c axis where consecutive layers are aligned in opposite directions, (ii) an incommensurate spiral along the c axis and (iii) an incommensurate a-axismodulated structure with a modulation vector τ ≈ (0.55, 0, 0). This wave vector emerges in various RNi 2 B 2 C compounds with magnetic as well as nonmagnetic R elements and is connected with Fermi surface nesting. Both incommensurate magnetization structures have been shown to be related to the near-reentrant behaviour observed in HoNi 2 B 2 C whereas the commensurate structure coexists well with the superconducting state in this compound. The variation of T N and T c with the de Gennes factor can roughly be drawn on straight lines from Lu to Gd and from Lu to Tb, respectively, with the exception of Yb. Consequently, T c > T N holds for Tm, Er, Ho and T c < T N for Dy. However, the study of various pseudoquaternary (R, R )Ni 2 B 2 C compounds has shown that this so-called de Gennes scaling is not universal for the borocarbides and it breaks down in some cases, which is attributed to effects of details

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Highly dense nanocrystalline MgB 2 bulk superconductors with distinctly improved pinning were prepared by mechanical alloying of Mg and B powders and hot compaction at ambient temperatures. The nanocrystalline samples reveal high j c = 10 5 A/cm 2 at 20 K and 1 T together with a strongly shifted irreversibility line towards higher fields resulting in H irr (T) ∼ 0.8 H c2 (T), whereas typically H irr (T) ∼ 0.5 H c2 (T) is observed for bulk untextured samples. These values exceed that of all other so far reported bulk samples and are in the range of the values of thin films. The improved pinning of this material which mainly consists of spherical grains of about 40-100 nm in size is attributed to the large number of grain boundaries.

High trapped fields were found in zinc-doped, bulk melt-textured YBa2Cu3O7−x (YBCO) material showing a pronounced peak effect in the field dependence of the critical current density. Trapped fields up to 1.1 T were found at 77 K at the surface of a YBCO disk (diameter 26 mm, height 12 mm). Very high trapped fields up to 14.35 T were achieved at 22.5 K for a YBCO disk pair (diameter 26 mm, height 24 mm) by the addition of silver and using a bandage made of stainless steel. The pinning forces and trapped fields obtained in bulk YBCO material are compared with results reported for melt-processed NdBa2Cu3O7−x and SmBa2Cu3O7−x.

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Improved trapped fields are reported for bulk melt-textured YBa2Cu3O 7−δ (YBCO) material in the temperature range between 20 K and 50 K. Trapped fields up to 12.2 T were obtained at 22 K on the surface of single YBCO disks (with Ag and Zn additions). In YBCO mini-magnets, maximum trapped fields of 16 T (at 24 K) and of 11.2 T (at 47 K) were achieved using (Zn + Ag) and Zn additions, respectively. In all cases, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process and to avoid cracking. We observed cracking not only during the magnetizing process, but also as a consequence of flux jumps due to thermomagnetic instabilities in the temperature range betweeen 20 K and 30 K. 7460Ge, 7462Bf, 7480BjBulk type II superconductors can trap high magnetic fields that are generated by superconducting persistent currents circulating macroscopically within the superconductor. The main features of the resulting field distribution are a maximum trapped field B 0 in the center of the superconducting domain and a field gradient towards the sample edge which is determined by the critical current density j c of the supercurrents. Therefore, high trapped fields B 0 in bulk superconductors require a high critical current density and a large size of the current loops. Large single-domain bulk YBa 2 Cu 3 O 7−δ based (YBCO) samples can be produced by melt texture processing, especially by using SmBa 2 Cu 3 O 7−δ (Sm-123) as a seed crystal.[1] The pinning effect in melt textured samples can be improved by irradiation methods [2,3] and alternatively, by zinc-doping. [4,5] Cracking of the samples was found to limit the trapped field of bulk YBCO at temperatures below 77 K which can be explained by tensile stresses that occur during the magnetization process due to the stored flux density gradient and may exceed the tensile strength of the material.[6] The mechanical properties of bulk YBCO and its tensile strength can be improved considerably by the addition of Ag. [7,8] Another possibility to avoid cracking during magnetizing is to encapsulate the bulk YBCO disks in steel tubes. [9] A steel tube leads to stress compensation by generating a compressive stress on YBCO after cooling from 300 K to the measuring temperature which is due to the higher thermal expansion coefficient of steel compared to that of YBCO in the a, b-plane. Maximum trapped fields B 0 of 11.5 T at 17 K and of 14.4 T at 22 K were reported for single YBCO disks and mini-magnets consisting of two single YBCO disks, respectively. [9] In both cases, Ag was added to the YBCO disks which were placed into austenite Cr-Ni steel tubes. Previous attempts to combine the two beneficial effects of Ag and Zn additions failed due to the solubility of Zn in Ag in oxygen atmosphere. High trapped fields have also been reported for bulk Sm-123 with additions of Ag. At the surface of Sm-123 disks, trapped fields of 2.1 T at 77 K and of 8 T at 40 K have been observed. [10] In the present paper, t...

The FePt alloys have recently attracted considerable attention due to their excellent intrinsic magnetic, chemical and mechanical properties. Their possible usage ranges from permanent magnets for special applications (e.g. in micro‐electro‐mechanical systems, magnetic MEMS, and in aggressive environments) to ultra‐high density magnetic storage media. The article describes general aspects concerning the phase formation and magnetic properties of materials based on the L10 FePt phase. Both thin film and bulk approaches are considered. The production of bulk nanocrystalline Fe100‐xPtx powders by mechanical alloying and subsequent annealing is described. Various combinations of phases, away from thermodynamic equilibrium, have been obtained using this technique.