Influence of HF2- geometry on magnetic interactions elucidated from polymorphs of the metal-organic framework [Ni(HF2)(pyz)2]PF6 (pyz = pyrazine).

Dalton Trans 41 (2012) 7235-7243

JL Manson, KE Carreiro, SH Lapidus, PW Stephens, PA Goddard, RE Del Sesto, J Bendix, S Ghannadzadeh, I Franke, J Singleton, T Lancaster, JS Möller, PJ Baker, FL Pratt, SJ Blundell, J Kang, C Lee, M-H Whangbo

A tetragonal polymorph of [Ni(HF(2))(pyz)(2)]PF(6) (designated β) is isomorphic to its SbF(6)-congener at 295 K and features linear Ni-FHF-Ni pillars. Enhancements in the spin exchange (J(FHF) = 7.7 K), Néel temperature (T(N) = 7 K), and critical field (B(c) = 24 T) were found relative to monoclinic α-PF(6). DFT reveals that the HF(2)(-) bridges are significantly better mediators of magnetic exchange than pyz (J(pyz)), where J(FHF) ≈ 3J(pyz), thus leading to quasi-1D behavior. Spin density resides on all atoms of the HF(2)(-) bridge whereas N-donor atoms of the pyz ring bear most of the density.

Monopoles, magnetricity, and the stray field from spin ice.

Phys Rev Lett 108 (2012) 147601-

SJ Blundell

An analysis is presented of the behavior of muons in the low-temperature state in spin ice. It is shown in detail how the behavior observed in some previous muon experiments on spin ice in a weak transverse field may result from the macroscopic stray field of magnetized spin ice. A model is presented which allows these macroscopic field effects to be simulated and the results agree with experiment. The persistent spin dynamics at low temperature originate from the sample and could be a muon-induced implantation effect that is operative in out-of-equilibrium systems with long relaxation times.

Recent Topics of Organic Superconductors


A Ardavan, S Brown, S Kagoshima, K Kanoda, K Kuroki, H Mori, M Ogata, S Uji, J Wosnitza

[Ni(HF2)(3-Clpy)4]BF4 (py = pyridine): evidence for spin exchange along strongly distorted F···H···F- bridges in a one-dimensional polymeric chain.

Inorg Chem 51 (2012) 7520-7528

JL Manson, AG Baldwin, BL Scott, J Bendix, RE Del Sesto, PA Goddard, Y Kohama, HE Tran, S Ghannadzadeh, J Singleton, T Lancaster, JS Möller, SJ Blundell, FL Pratt, VS Zapf, J Kang, C Lee, M-H Whangbo, C Baines

[Ni(HF(2))(3-Clpy)(4)]BF(4) (py = pyridine) is a simple one-dimensional (1D) coordination polymer composed of compressed NiN(4)F(2) octahedra that form chains with bridging HF(2)(-) ligands. In spite of significant distortion of the HF(2)(-) bridge, a quasi-1D antiferromagnetic (AFM) behavior was observed with J(FHF) = 4.86 K.

Proximal magnetometry of monolayers of magnetic moments


Z Salman, SJ Blundell

Chemistry at the nanoscale: synthesis of an N@C60-N@C60 endohedral fullerene dimer.

Angew Chem Int Ed Engl 51 (2012) 3587-3590

BJ Farrington, M Jevric, GA Rance, A Ardavan, AN Khlobystov, GAD Briggs, K Porfyrakis

Persistent dynamics in the S=1/2 quasi-one-dimensional chain compound Rb 4Cu(MoO 4) 3 probed with muon-spin relaxation

Physical Review B - Condensed Matter and Materials Physics 85 (2012)

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

We report the results of muon-spin relaxation measurements on the low-dimensional antiferromagnet Rb 4Cu(MoO 4) 3. No long-range magnetic order is observed down to 50 mK implying a ratio T N/J<0.005 (where J is the principal exchange strength along the spin chains) and an effective ratio of interchain to intrachain exchange of |J ⊥/J|<2×10 -3, making the material an excellent realization of a one-dimensional quantum Heisenberg antiferromagnet. We probe the persistent spin excitations at low temperatures and find that ballistic spin transport dominates the excitations detected below 0.3 K. © 2012 American Physical Society.

Magnetism in crown-ether-substituted nitronyl nitroxide derivatives and their metal complexes

Physica Status Solidi (C) Current Topics in Solid State Physics 9 (2012) 1205-1207

T Sugano, SJ Blundell, W Hayes, H Tajima, H Mori

Rare-earth-metal (M = Nd, Gd and Dy) complexes of the organic monoradicals, 15-crown-5-phenyl nitronyl nitroxide 1 and 15-crown-5- or 18-crown-6-phenyl iminonitroxide 2, were synthesized. We present here magnetic properties of the metal-free radicals and the metal complexes of these organic monoradicals studied using SQUID megnetometry. Almost of all the rare-earth-metal complexes show two-sublattice behavior in a wide range of antiferromagnetic (AFM) intrasublattice and ferromagnetic (FM) intersublattice interactions, Γ and λ, with |CΓ | and |Cλ | ≅ 10 to 100 K (C is the Curie constant) over the temperature range between 1.8 and 300 K, while the metal-free radicals exhibit only weak AFM intermolecular interactions, less than the Weiss constant θ = -1 K. The magnitude of interactions observed in the metal complexes of each radical increase in order from Gd to Dy to Nd complexes. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

[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.

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.

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.

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.

[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.

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