Abrahamsen, A.B.

Andersen, N.H.

Canfield, P.C.

Eskildsen, M.R.

Online Research Database In Technology

  General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.   Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal  If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.         Downloaded from orbit.dtu.dk on: Mar 31, 2019Influence of magnetism on flux line lattice in TmNi2B2C superconductorAbrahamsen, Asger Bech; Eskildsen, Morten Ring; Andersen, Niels Hessel; Canfield, P.C.Published in:Superconductivity and magnetism: Materials properties and developments.  Extended abstractsPublication date:2003Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Abrahamsen, A. B., Eskildsen, M. R., Andersen, N. H., & Canfield, P. C. (2003). Influence of magnetism on fluxline lattice in TmNi2B2C superconductor. In N. H. Andersen, N. Bay, J-C. Grivel, P. Hedegård, D. McMorrow, S.Mørup, L. T. Kuhn, A. Larsen, B. Lebech, K. Lefmann, P-E. Lindelof, S. Linderoth, ... N. F. Pedersen (Eds.),Superconductivity and magnetism: Materials properties and developments.  Extended abstracts (pp. 20-21).Roskilde: Risø National Laboratory.INFLUENCE OF MAGNETISM ON FLUX LINE LATTICE IN TmNi2B2C SUPERCONDUCTOR  A. B. Abrahamsen (Materials Research Deparment, Risø National Laboratory, Denmark), M. R. Eskildsen (University of Notre Dame, USA), N. H. Andersen (Risø National Laboratory, Denmark), P. C. Canfield (Ames Laboratory, USA)  We have examined the phase boundary of the square flux line lattice in TmNi2B2C using small angle neutron scattering and the transition into a rhombic lattice for H > 2 kOe is found to scale with the upper critical field Hc2(T) of the superconductor. The influence of the magnetic Tm-ions on the flux line lattice symmetry in the paramagnetic state seems only indirect through the shape of the upper critical field.   A co-existence of superconductivity and magnetic ordering of the rare earth moment in the form of a spin-density wave (SDW) is observed in the borocarbide series RNi2B2C, when the rare earth is R = Tm, Er, Ho or Dy. The presence of the rare earth moments polarises the conduction electrons, which mediates the indirect RKKY interaction between different moments. However, the superconducting state is also formed by the conduction electrons and a strong interplay between magnetism and superconductivity is seen. In the case of R = Tm a quite complicated phase-diagram of the flux line lattice symmetries is found when the applied field is oriented along the c-axis of the tetragonal crystalline unit cell1, as shown on Fig 1. The critical temperature is Tc = 11 K and the maximum upper critical field Hc2 = 10 kOe is found at T = 5 K. At lower temperatures HC2 shows a minimum as the magnetic ordering of the qmI and qmII SDW is setting in at TN = 1.5 K. In the paramagnetic phase TN < T < Tc a smooth transition from a rhombic to a square flux line lattice is seen as the applied field in increased from the lower critical field Hc1. This transition is driven by non-local electrodynamics, which favours a four-fold symmetric current and field pattern, depending on the underlying Fermi surface symmetry, when the field profile of the flux lines is highly overlapping2. Insert D and A illustrates the change from an almost hexagonal to a square flux line lattice, which is common for even the non-magnetic borocarbides R = Y and Lu.3 A transition back into a rhombic symmetry is observed as the applied field is increased towards the upper critical field Hc2. Small angle neutron scattering (SANS) has been used to measure the azimuthal width of the diffraction spots from the square flux line lattice. The transition field Hc2 between the A and B phase was determined as the field, where the width increased above the resolution of the SANS camera and insert B shows two rhombic lattices clearly resolved.  It has been argued that the square flux line lattice would not exist close to Hc2, because the separation between the flux lines is larger for the rhombic than the square lattice, whereby the loss of condensation energy is smaller when the superconducting order parameter is suppressed in between the flux lines as the vortex cores start to overlap.3 Thus the AB phase transition is expected to coincide with the crossover from intermediate  to  high flux  line density, which  is often  defined  by  the field  where the flux line separation a is a factor  larger than the superconducting coherence length ξ.IH 4C ≈4  Flux quantization for the square flux line lattice gives a relation between magnetic flux density B and the separation between the flux lines  20 aB=Φ  The coherence length ξ is connected to the upper critical field BC2 by inserting ξπ2=a , whereby it is seen that the AB phase transition scales with the upper critical field by AB2C2 IH2BB Cπ=  From the AB phase boundary of figure 1 one gets an estimate of CIH = 3.96-3.45, which is in qualitative agreement with the definition of the crossover from intermediate to high flux line density.  It is suggested that the square flux line lattice exist between a lower phase boundary (AD) determined by non-local electrodynamics and an upper boundary (AB), where the vortex cores start to overlap. Thus the influence of the magnetic Tm-ions on the flux line lattice symmetry seems only to be indirect through the shape of the upper critical field Hc2(T) curve in the paramagnetic phase.     Fig. 1. Phase diagram of TmNi2B2C showing square (A) and different rhombic flux line lattices (B, C, D). It is suggested that the AB phase boundary has a positive slope, because it scales with the upper critical field Hc2. Insert D: Magnetic decoration showing rhombic lattice at H = 20 Oe and T = 4.2 K, A: Square lattice at H = 2.0 kOe and T = 2.3 K, B: Rhombic lattice at H = 6.5 kOe and T = 1.7 K.   REFERENCES  1. M.R. Eskildsen et.al., NATURE 393, 242 (1998)  2. V.G. Kogan et.al, Phys. Rev. B. 55, R8693 (1997) 3. M.R Eskildsen et.al, Phys. Rev. Lett. 86, 5148 (2001)  4. E.H. Brandt, Rep. Prog. Phys. 58, 1465(1995) 

Influence of magnetism on flux line lattice in TmNi<sub>2</sub>B<sub>2</sub>C superconductor

http://orbit.dtu.dk/ws/files/171169640/Abstract_16.pdf

Influence of magnetism on flux line lattice in TmNi<sub>2</sub>B<sub>2</sub>C superconductor

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology