[Cu(HF2)2 (pyrazine)]n: a rectangular antiferromagnetic lattice with a spin exchange path made up of two different FHF- bridges.

Angew Chem Int Ed Engl 50 (2011) 1573-1576

JL Manson, ML Warter, JA Schlueter, T Lancaster, AJ Steele, SJ Blundell, FL Pratt, J Singleton, RD McDonald, C Lee, M-H Whangbo, A Plonczak

Superconductors: What they're good for

New Scientist 212 (2011) 6-7

S Blundell

Already used in applications as diverse as body scanning and discovering the origin of mass, superconductors hold promise for even greater technologies. © 2011 Reed Business Information Ltd, England.

Superconductors: Cold beginnings

New Scientist 212 (2011)

S Blundell

It took more than half a century to figure out how superconductivity might work and how to make it useful. © 2011 Reed Business Information Ltd, England.

Superconductors: Getting warmer

New Scientist 212 (2011) 4-5

S Blundell

What we really want is a superconductor that operates at room temperature. Might one be within our grasp? © 2011 Reed Business Information Ltd, England.

Superconductors: 100 years on

New Scientist 212 (2011) 8-

S Blundell

Applications may have been slow in coming, yet how can you fail to be impressed when you look at an MRI scanner? © 2011 Reed Business Information Ltd, England.

The forgotten brothers

Physics World 24 (2011) 26-29

S Blundell

Critical behavior in the inhomogeneous ferromagnet SrFe <inf>0.80</inf>Co<inf>0.20</inf>O<inf>3.0</inf>

Physical Review B - Condensed Matter and Materials Physics 83 (2011)

J Lago, MJ Rosseinsky, SJ Blundell, PD Battle, M Diaz, I Uriarte, T Rojo

A detailed muon spin relaxation (μSR) and magnetization study is presented of the paramagnetic-to-ferromagnetic transition in the compound SrFe0.80Co0.20O3. The critical exponents derived from the static critical analysis are close to the theoretical predictions for the Heisenberg model in three dimensions. However, a small drift toward mean-field values is interpreted as arising from the presence of long-range dipolar interactions between the Fe(IV) centers. The evolution of spin dynamics across the transition determined from the μSR study is consistent with this interpretation. μSR and magnetization data also provide evidence of an inhomogeneous magnetic state both above and below Tc, placing this system in line with other double-exchange materials such as manganites and cobaltites where spontaneous electronic and magnetic phase separation appears conspicuous and explains much of the encountered experimental phenomenology. © 2011 American Physical Society.

Photochemical stability of N@C-60 and its pyrrolidine derivatives


G Liu, AN Khlobystov, A Ardavan, GAD Briggs, K Porfyrakis

Probing magnetic order in LiMPO<inf>4</inf> (M= Ni, Co, Fe) and lithium diffusion in Li<inf>x</inf>FePO<inf>4</inf>

Physical Review B - Condensed Matter and Materials Physics 84 (2011)

PJ Baker, I Franke, FL Pratt, T Lancaster, D Prabhakaran, W Hayes, SJ Blundell

Muon spin relaxation measurements are reported on three members of the LixMPO4 series. The magnetic properties of stoichiometric samples with M= Ni, Co, Fe were investigated at low temperature. In LiNiPO 4 we observe different forms of the muon decay asymmetry in the commensurate and incommensurate antiferromagnetic phases, accompanied by a change in the temperature dependence of the muon oscillation frequency. In LiCoPO4 the form of the muon decay asymmetry indicates that the correlation between layers decreases as the Néel temperature is approached from below. LiFePO4 shows more conventional behavior, typical for a three-dimensional antiferromagnet. Measurements on Li xFePO4 with x=0.8,0.9, and 1 show evidence for lithium diffusion below ∼250 K and muon diffusion dominating the form of the relaxation at higher temperature. The thermally activated form of the observed hopping rate suggests an activation barrier for lithium diffusion of ∼100 meV and a diffusion constant of DLi∼10-10to10 -9 cm2 s-1 at room temperature. © 2011 American Physical Society.

Measurement of the internal magnetic field in the correlated iridates Ca4IrO6, Ca5Ir3O12, Sr3Ir2O7 and Sr2IrO4

Physical Review B - Condensed Matter and Materials Physics 83 (2011)

I Franke, PJ Baker, SJ Blundell, T Lancaster, W Hayes, FL Pratt, G Cao

Oxides containing iridium ions display a range of magnetic and conducting properties that depend on the delicate balance between interactions and are controlled, at least in part, by the details of the crystal architecture. We have used muon spin rotation (μSR) to study the local field in four iridium oxides, Ca4IrO6, Ca5Ir3O 12, Sr3Ir2O7, and Sr 2IrO4, which show contrasting behavior. Our μSR data on Ca4IrO6 and Ca5Ir3O12 are consistent with conventional antiferromagnetism where quasistatic magnetic order develops below TN=13.85(6) and 7.84(7) K, respectively. A lower internal field is observed for Ca5Ir3O12, as compared to Ca4IrO6, reflecting the presence of both Ir4+ and Ir5+ ions and resulting in a more magnetically dilute structure. Muon precession is only observed over a restricted range of temperature in Sr3Ir2O7, while the Mott insulator Sr2IrO4 displays more complex behavior, with the μSR signal containing a single, well-resolved precession signal below T N=230 K, which splits into two precession signals at low temperature following a reorientation of the spins in the ordered state. © 2011 American Physical Society.

Observation of a level crossing in a molecular nanomagnet using implanted muons.

J Phys Condens Matter 23 (2011) 242201-

T Lancaster, JS Möller, SJ Blundell, FL Pratt, PJ Baker, T Guidi, GA Timco, REP Winpenny

We have observed an electronic energy level crossing in a molecular nanomagnet (MNM) using muon spin relaxation. This effect, not observed previously despite several muon studies of MNM systems, provides further evidence that the spin relaxation of the implanted muon is sensitive to the dynamics of the electronic spin. Our measurements on a broken ring MNM [H(2)N(t)Bu(is)Pr][Cr(8)CdF(9)(O(2)CC(CH(3))(3))(18)], which contains eight Cr ions, show clear evidence for the S = 0 --> S = 1 transition that takes place at B(c) = 2.3 T. The crossing is observed as a resonance-like dip in the average positron asymmetry and also in the muon spin relaxation rate, which shows a sharp increase in magnitude at the transition and a peak centred within the S = 1 regime.

Electron spin ensemble strongly coupled to a three-dimensional microwave cavity


E Abe, H Wu, A Ardavan, JJL Morton

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

Angew Chem Int Ed Engl (2011)

JL Manson, ML Warter, JA Schlueter, T Lancaster, AJ Steele, SJ Blundell, FL Pratt, J Singleton, RD McDonald, C Lee, MH Whangbo, A Plonczak

Superconductivity and fluctuating magnetism in quasi-two-dimensional κ-(BEDT-TTF) 2Cu[N(CN) 2]Br probed with implanted muons

Physical Review B - Condensed Matter and Materials Physics 83 (2011)

T Lancaster, SJ Blundell, FL Pratt, JA Schlueter

A muon-spin relaxation (μ+SR) investigation is presented for the molecular superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br. Evidence is found for low-temperature phase separation throughout the bulk of the material, with only a fraction of the sample showing a superconducting signal, even for slow cooling. Rapid cooling reduces the superconducting fraction still further. For the superconducting phase, the in-plane penetration depth is measured to be λ||=0.47(1)μm, and evidence is seen for a vortex decoupling transition in applied fields above 40 mT. The magnetic fluctuations in the normal state produce a precipitous drop in relaxation rate above 100 K, and we discuss the possible causes for the unusual relaxation that we observe for T>Tc. © 2011 American Physical Society.

Low-moment magnetism in the double perovskites Ba<inf>2</inf>MOsO <inf>6</inf> (M=Li,Na)

Physical Review B - Condensed Matter and Materials Physics 84 (2011)

AJ Steele, PJ Baker, T Lancaster, FL Pratt, I Franke, S Ghannadzadeh, PA Goddard, W Hayes, D Prabhakaran, SJ Blundell

The magnetic ground states of the isostructural double perovskites Ba 2NaOsO6 and Ba2LiOsO6 are investigated with muon-spin relaxation. In Ba2NaOsO6 long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below Tc=7.2±0.2K, while in Ba 2LiOsO6 a static but spatially disordered internal field is found below 8 K. A probabilistic argument is used to show from the observed precession frequencies that the magnetic ground state in Ba 2NaOsO6 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. © 2011 American Physical Society.

Magnetic phase transitions in transition-metal complexes with triazole derivatives

Polyhedron 30 (2011) 3202-3205

T Sugano, M Kurmoo, SJ Blundell, W Hayes, S Vilminot

Three divalent transition-metal (Co, Ni and Cu) complexes with the organic anion, 1,2,4-triazolato (tr), as a ligand molecule were prepared by means of hydrothermal syntheses and their magnetic properties were investigated by SQUID magnetometry. The Co(tr)2 and Cu(tr)2 complexes exhibit long range ordering below 8 and 30 K, respectively, while the Ni(tr)2 complex does not show any magnetic phase transition down to 1.8 K. The magnetization isotherms of Co(tr)2 and Cu(tr)2 measured at 2.0 K show hysteresis loops with the coercive fields of 0.5 and 4.7 kOe, respectively. At temperatures higher than about 50 K, the temperature dependence of the magnetic susceptibility of Co(tr)2, Ni(tr)2 and Cu(tr)2 follows the Curie-Weiss law with the Curie constants of 2.95, 0.945 and 0.420 emu K mol-1 and the Weiss temperatures of -62, -74 and -97 K, respectively. These results suggest that the magnetically ordered phases observed in Co(tr)2 and Cu(tr)2 at low temperatures come from antiferromagnetic interactions resulting in canted arrangements of magnetic moments of the transition-metal cations. We discuss here the magnetic interactions in these transition-metal complexes by referring the results of the magnetization measurements. © 2011 Elsevier B.V. All rights reserved.

Magnetic and non-magnetic phases of a quantum spin liquid.

Nature 471 (2011) 612-616

FL Pratt, PJ Baker, SJ Blundell, T Lancaster, S Ohira-Kawamura, C Baines, Y Shimizu, K Kanoda, I Watanabe, G Saito

A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins. Recently, materials have become available showing persuasive experimental evidence for such a state. Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment. This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators. The layered molecular system κ-(BEDT-TTF)(2)Cu(2)(CN)(3) is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers that provides a prime example of a spin liquid formed in this way. Despite a high-temperature exchange coupling, J, of 250 K (ref. 6), no obvious signature of conventional magnetic ordering is seen down to 20 mK (refs 7, 8). Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose-Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.

[Cu(HF<inf>2</inf>)<inf>2</inf>(pyz)]<inf>n</inf>: A rectangular antiferromagnetic lattice with a spin exchange path made up of two different FHF<sup>-</sup> bridges

Angewandte Chemie - International Edition 50 (2011) 1573-1576

JL Manson, ML Warter, JA Schlueter, T Lancaster, AJ Steele, SJ Blundell, FL Pratt, J Singleton, RD McDonald, C Lee, MH Whangbo, A Plonczak

The two-dimensional antiferromagnet [Cu(HF2)2(pyz)] n (pyz=pyrazine) has a rectangular lattice (see picture) displaying two types of FHF- bridging modes. The spin exchange through Cu-(FHF)2-Cu is about 90% stronger than through Cu-pyz-Cu, and the 1,1-coordinated FHF-, which is close in nature to F - ⋯ HF, is largely responsible for the exchange. C black, Cu red, F green, H cyan. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Electrically detected magnetic resonance in a W-band microwave cavity.

Rev Sci Instrum 82 (2011) 034704-

V Lang, CC Lo, RE George, SA Lyon, J Bokor, T Schenkel, A Ardavan, JJL Morton

We describe a low-temperature sample probe for the electrical detection of magnetic resonance in a resonant W-band (94  GHz) microwave cavity. The advantages of this approach are demonstrated by experiments on silicon field-effect transistors. A comparison with conventional low-frequency measurements at X-band (9.7  GHz) on the same devices reveals an up to 100-fold enhancement of the signal intensity. In addition, resonance lines that are unresolved at X-band are clearly separated in the W-band measurements. Electrically detected magnetic resonance at high magnetic fields and high microwave frequencies is therefore a very sensitive technique for studying electron spins with an enhanced spectral resolution and sensitivity.

The statistical mechanics of community assembly and species distribution

New Phytologist (2011)

CK Kelly, SJ Blundell, MG Bowler, GA Fox, PH Harvey, MR Lomas, F Ian Woodward