The magnetic phases of the ideal spin-1/2 triangular-lattice antiferromagnet Ba3CoSb2O9 are identified and studied using 135,137 Ba nuclear magnetic resonance (NMR) spectroscopy in magnetic fields ranging to 30T, oriented parallel and near perpendicular to the crystallographic ab-plane. For both directions, the saturation field is approximately 33T. Notably, the NMR spectra provide microscopic evidence for the stabilization of an up-up-down spin configuration for in-plane fields, giving rise to an one-third magnetization plateau (Msat/3), as well as for a higher field phase transition near to ∼ (3/5)Msat for both field orientations. Phase transitions are signaled by the evolution of the NMR spectra, and in some cases through spin-lattice relaxation measurements. The results are compared with expectations obtained from a semi-classical energy density modeling, in which quantum effects are incorporated by effective interactions extracted from the spin-wave analysis of the two-dimensional model. The interlayer coupling also plays a significant role in the outcome. Good agreement between the model and the experimental results is achieved, except for the case of fields approaching the saturation value applied along the c-axis.

We report on the high-pressure syntheses and detailed characterizations of two effective pseudospin S-1/2 XY pyrochlores Er 2 Pt 2 O 7 andYb 2 Pt 2 O 7 via X-ray/neutron powder diffraction, dc and ac magnetic susceptibility, and specific-heat measurements down to 70 mK. We found that both compounds undergo long-range magnetic transition at T N,C ≈ 0.3 K, which are ascribed to an antiferromagnetic-and ferromagnetic-type order for Er 2 Pt 2 O 7 and Yb 2 Pt 2 O 7 , respectively, based on the field dependence of their transition temperature as well as the systematic comparisons with other similar pyrochlores R 2 B 2 O 7 (R = Er, Yb; B = Sn, Ti, Ge). The observed T N of Er 2 Pt 2 O 7 is much lower than that expected from the relationship of T N versus the ionic radius of B 4+ derived from the series of Er 2 B 2 O 7 , while the T C of Yb 2 Pt 2 O 7 is the highest among the series of ferromagnetic Yb 2 B 2 O 7 (B = Sn, Pt, Ti). Given the monotonic variation of the lattice constant as a function of the B-cation size across these two series of R 2 B 2 O 7 (R = Er, Yb), the observed anomalous values of T N,C in the Pt-based XY pyrochlores imply another important factor beyond the nearest-neighbor R-R distance is playing a role. In light of the anisotropic exchange interactions J ex = {J zz , J ± , J ±± , J z± } for the S-1/2 XY pyrochlores, we have rationalized these observations by considering a weakened (enhanced) antiferromagnetic planar J ± (ferromagnetic Ising-like J zz ) due to strong Pt 5d-O 2p hybridization within the plane perpendicular to the local [111] direction.

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Abstract:Although 2011 marks the 50th anniversary of Nb 3 Sn as the first high field superconductor, real understanding of its upper critical field behavior µ 0 H c2 is incomplete. Here we show surprising µ 0 H c2 data on highly homogeneous bulk samples examined both by small-current, transport and by volumetric-averaging specific heat and the reversible magnetization techniques, which exhibit identical upper critical field µ 0 H c2 (0.3 K) ~ 29± 0.2 T with or without undergoing the cubic to tetragonal transition, a result in strong contrast to widely used multiple-source data compilations that show a strong depression of µ 0 H c2 (0K) from 29 T to 21.4 T in the tetragonal state.Nb 3 Sn is the most widely used superconductor for generating fields above ~10 T because it is easily and economically fabricated in round-wire, multifilament forms that lend themselves both to laboratory magnets and to the cabled, high-current conductors needed for accelerator and fusion uses. More than 600 tons of Nb 3 Sn will be used in the International Tokomak Experimental Reactor (ITER). This wide and long-standing use [1] makes it all the more surprising that there is no agreed data set that shows the variation of the upper critical field µ 0 H c2 across the variable composition of the A15 phase of Nb 3 Sn. Rather homogeneous bulk Nb 3 Sn was made by Devantay et al. [7] by heating samples into the melt phase and by Goldacker et al. [8] in a Hot Isostatic Press (HIP) at 1100 ºC. To further reduce inhomogeneity, we used a HIP capable of reaching up to 2200 °C. Here we report on bulk Nb 3 Sn with nominally stoichiometric 25at% Sn and Sn-rich 27at% Sn so as to ensure the most Sn-rich composition of the A15 phase. About 45 g samples were synthesized by combining Nb (-325 mesh, 99.8%, Alfa Aesar) and Sn (-325 mesh, 99.8%, Alfa Aesar) powders in a high energy ball mill. Mixing and powder packing was performed in a dedicated glove box filled with Ar gas to minimize oxidation. After 60 minutes of ball milling, the mixed powders were pressed in a Cold Isostatic Press (CIP) to form a hard pellet, then wrapped in Ta foil and put into a steel tube with one closed end. This HIP tube was then evacuated and the open end sealed by welding. The sealed can was pressurized at 2 kbar during both a pre-anneal at 650 °C for 16 hr and during the main A15 phase reaction at 1200 °C for 72 or 160 hr.The central reacted A15 part of each of the cans was cut into 2 pieces using a precision diamond saw, one piece being then re-sealed in an evacuated Ta-lined Nb tube for a 2 nd HIP homogenization and reaction-continuation anneal at 1400 °C, 1600 °C or 1800 °C for 24 hr. One piece of the 27 at% Sn 3 sample annealed at 1800 o C was further annealed in a Ta-lined Nb tube for 30 days at 1200 °C. In this report we describe samples by their nominal or overall atomic %Sn content, followed by the final heat treatment and time (if not specified, 24 hr). For example, 25Sn_1800 means the sample finally annealed at 1800 °C for 24 hr after pre-annealing at 650 °C for 16 hr ...