Andrew J. Steele

Control of the competition between a magnetic phase and a superconducting phase in cobalt-doped and nickel-doped NaFeAs using electron count.

Dinah R. Parker, Matthew J. P. Smith, Tom Lancaster, Andrew J. Steele, Isabel Franke, Peter J. Baker, Francis L. Pratt, Michael J. Pitcher, Stephen J. Blundell, ∗ and Simon J. Clarke † Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, United Kingdom Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom ISIS Facility, STFC-Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, OX11 0QX, United Kingdom (Dated: September 16, 2010)

Compositional Control of the Superconducting Properties of LiFeAs

The response of the superconducting state and crystal structure of LiFeAs to chemical substitutions on both the Li and the Fe sites has been probed using high-resolution X-ray and neutron diffraction measurements, magnetometry, and muon-spin rotation spectroscopy. The superconductivity is extremely sensitive to composition: Li-deficient materials (Li(1-y)Fe(1+y)As with Fe substituting for Li) show a very rapid suppression of the superconducting state, which is destroyed when y exceeds 0.02, echoing the behavior of the Fe(1+y)Se system. Substitution of Fe by small amounts of Co or Ni results in monotonic lowering of the superconducting transition temperature, T(c), and the superfluid stiffness, rho(s), as the electron count increases. T(c) is lowered monotonically at a rate of 10 K per 0.1 electrons added per formula unit irrespective of whether the dopant is Co and Ni, and at higher doping levels superconductivity is completely suppressed. These results and the demonstration that the superfluid stiffness in these LiFeAs-derived compounds is higher than in all of the iron pnictide materials underlines the unique position that LiFeAs occupies in this class.

Characterization of the Antiferromagnetism in Ag(pyz)2(S2O8) (pyz = Pyrazine) with a Two-Dimensional Square Lattice of Ag2+ Ions

X-ray powder diffraction and magnetic susceptibility measurements show that Ag(pyz)(2)(S(2)O(8)) consists of 2D square nets of Ag(2+) ions resulting from the corner-sharing of axially elongated AgN(4)O(2) octahedra and exhibits characteristic 2D antiferromagnetism. Nevertheless, mu(+)SR measurements indicate that Ag(pyz)(2)(S(2)O(8)) undergoes 3D magnetic ordering below 7.8(3) K.

Gradual destruction of magnetism in the superconducting family NaFe1−xCoxAs

The interplay and coexistence of superconducting, magnetic and structural order parameters in NaFe{1-x}Co{x}As has been studied using SQUID magnetometry, muon-spin rotation and synchrotron x-ray powder diffraction. Substituting Fe by Co weakens the ordered magnetic state through both a suppression of T_N and a reduction in the size of the ordered moment. Upon further substitution of Fe by Co the high sensitivity of the muon as a local magnetic probe reveals a magnetically disordered phase, in which the size of the moment continues to decrease and falls to zero around the same point at which the magnetically-driven structural distortion is no longer resolvable. Both magnetism and the structural distortion are weakened as the robust superconducting state is established.

[Cu(HF2)2(pyrazine)]n: A Rectangular Antiferromagnetic Lattice with a Spin Exchange Path Made Up of Two Different FHF− Bridges†

The two-dimensional antiferromagnet [Cu(HF{sub 2}){sub 2}(pyz)]{sub n} (pyz=pyrazine) has a rectangular lattice (see picture) displaying two types of FHF{sup -} bridging modes. The spin exchange through Cu-(FHF){sub 2}-Cu is about 90% stronger than through Cu-pyz-Cu, and the {mu}{sub 1,1}-coordinated FHF{sup -}, which is close in nature to F{sup -}---HF, is largely responsible for the exchange. C black, Cu red, F green, H cyan.

Dimensionality selection in a molecule-based magnet.

Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy. Here we demonstrate a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S=1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two- to quasi-one-dimensional while effectively maintaining the nearest-neighbor coupling strength.

Low-moment magnetism in the double perovskites Ba2M OsO6 (M =Li,Na)

The magnetic ground states of the isostructural double perovskites Ba2NaOsO6 and Ba2LiOsO6 are investigated with muon-spin rotation. In Ba2NaOsO6 long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below Tc = 7.2±0.2 K, while in Ba2LiOsO6 a static but spatially-disordered internal field is found below 8 K. A novel probabilistic argument is used to show from the observed precession frequencies that the magnetic ground state in Ba2NaOsO6 is most likely to be low-moment (� 0.2µB) ferromagnetism and not canted antiferromagnetism. Ba2LiOsO6 is antiferromagnetic and we find a spin-flop transition at 5.5T. A reduced osmium moment is common to both compounds, probably arising from a combination of spin-orbit coupling and frustration.

Magnetic order in quasi-two-dimensional molecular magnets investigated with muon-spin relaxation

We present the results of a muon-spin relaxation ({mu}+SR) investigation into magnetic ordering in several families of layered quasi-two-dimensional molecular antiferromagnets based on transition-metal ions such as S = 1/2 Cu{sup 2+} bridged with organic ligands such as pyrazine. In many of these materials magnetic ordering is difficult to detect with conventional magnetic probes. In contrast, {mu}{sup +}SR allows us to identify ordering temperatures and study the critical behavior close to TN. Combining this with measurements of in-plane magnetic exchange J and predictions from quantum Monte Carlo simulations we may assess the degree of isolation of the 2D layers through estimates of the effective inter-layer exchange coupling and in-layer correlation lengths at TN. We also identify the likely metal-ion moment sizes and muon stopping sites in these materials, based on probabilistic analysis of the magnetic structures and of muon-fluorine dipole-dipole coupling in fluorinated materials.

Relaxation of muon spins in molecular nanomagnets

We address the cause of the unusual muon spin relaxation (muSR) results on molecular nanomagnets (MNMs). Through measurements on protonated and deuterated samples of the MNMs Cr7Mn (S=1) and Cr8 (S=0), we show that the muon spin for

Ag(nic)2 (nic = Nicotinate): A Spin-Canted Quasi-2D Antiferromagnet Composed of Square-Planar S = 1/2 AgII Ions

S \neq 0