We report the first observation of two-dimensional incommensurate magnetic fluctuations in the layered metallic perovskite Sr3Ru2O7. The wavevectors where the magnetic fluctuations are strongest are different from those observed in the superconducting single layer ruthenate Sr2RuO4 and appear to be determined by Fermi surface nesting. No antiferromagnetic ordering is observed for temperatures down to 1.5 K. For temperatures T > ∼ 20 K, the fluctuations become predominately ferromagnetic. Our inelastic neutron scattering measurements provide concrete evidence of the coexistence of competing interactions in Sr3Ru2O7 and of the low energy scale of the fluctuations.78.70. Nx, 75.40.Gb, 74.20.Mn, 75.30.Kz

The conduction electrons in a metal experience competing interactions with each other and the atomic nuclei. This competition can lead to many types of magnetic order in metals. For example, in chromium the electrons order to form a spin-density-wave (SDW) antiferromagnetic state. A magnetic field may be used to perturb or tune materials with delicately balanced electronic interactions. Here, we show that the application of a magnetic field can induce SDW magnetic order in a quasi-2D metamagnetic metal, where none exists in the absence of the field. We use magnetic neutron scattering to show that the application of a large (B ≈ 8 T) magnetic field to the perovskite metal Sr3Ru2O7 (refs 3-7) can be used to tune the material through two magnetically ordered SDW states. The ordered states exist over relatively small ranges in field (≲0.4 T), suggesting that their origin is due to a new mechanism related to the electronic fine structure near the Fermi energy, possibly combined with the stabilizing effect of magnetic fluctuations. The magnetic field direction is shown to control the SDW domain populations, which naturally explains the strong resistivity anisotropy or 'electronic nematic' behaviour observed in this material.

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A small angle neutron scattering study of the flux-line lattice in a large single crystal of untwinned YBa2Cu3O 7−δ is presented. In fields parallel to the c-axis, diffraction spots are observed corresponding to four orientations of a hexagonal lattice, distorted by the a-b anisotropy. A value for the anisotropy, the penetration depth ratio, of λa/λ b =1.18(2) was obtained. The high quality of the data is such that second order diffraction is observed, indicating a well ordered FLL. With the field at 33 • to c a field dependent re-orientation of the lattice is observed around 3T.PACS numbers: 74.60. Ge, 74.72.Bk, 61.12.Ex The remarkable properties of the mixed state in the cuprate high-T c superconductors are of great current interest. In particular, the expectation that high-T c superconductors have an unconventional pairing symmetry has led inevitably to the question: how does the structure of the flux-line lattice (FLL) differ between conventional and unconventional superconductors? The question has been taken up by several recent theoretical contributions [1][2][3][4] which predict a variety of interesting FLL effects all deviating from the benchmark triangular Abrikosov lattice. However, such discussions may presuppose that the crystallographic properties of the FLL are already well understood in the simplest low-field regime where unconventional effects are least prevalent. This has been far from the experimental truth. Observations require a probe sensitive to the microscopic arrangement of fluxlines. Direct imaging [5] and decoration techniques [6] all have inherent drawbacks, and muon spin rotation has not yet achieved the sophistication to resolve the most subtle effects [7]. By comparison, small angle neutron scattering (SANS) provides unrivalled insights into the crystallography of the FLL, and is the only technique capable of unequivocably resolving such questions.The demanding nature of neutron experiments requires large single crystals (masses > ∼ 200mg), and because of this YBa 2 Cu 3 O 7−δ (YBCO) has been the cuprate of choice for SANS experiments [8]. However, the materials properties of YBCO are complicated. The presence of Cu-O chains which are aligned with the crystallographic b direction render the otherwise tetragonal structure orthorhombic. Upon cooling from the growth, twin boundaries form along {110} directions separating domains of interchanged a and b axes. A strong interaction between flux-lines and these twin planes significantly influences FLL properties. There is a further effect of the chains. Although the orthorhombic distortion is only slight (≈ 1%), the electronic structure is markedly affected, and the consequence is anisotropy within the abplane of both superconducting [9,10] and normal state [11] properties. All previous SANS studies have been on heavily twinned crystals [12,13], and although observations of a pattern with four-fold symmetry were claimed to be due to unconventional d x 2 −y 2 pairing [12], it could not be discounted that alignment by twin planes, in comb...

High-purity niobium exhibits a surprisingly rich assortment of vortex lattice ͑VL͒ structures for fields applied parallel to a fourfold symmetry axis, with all observed VL phases made up of degenerate domains that spontaneously break some crystal symmetry. Yet a single regular hexagonal VL domain is observed at all temperatures and fields parallel to a threefold symmetry axis. We report a detailed investigation of the transition between these lush and barren VL landscapes, discovering new VL structures and phase transitions at high fields. We show that the number and relative population of VL domains is intrinsically tied to the underlying crystal symmetry. We discuss how subtle anisotropies of the crystal may generate the remarkable VLs observed.