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Contemporary Physics (2015)

SJ Blundell

Magnetic phase diagram of La2-xSrxCoO4 revised using muon-spin relaxation

PHYSICAL REVIEW B 93 (2016) ARTN 140406

RC Williams, F Xiao, T Lancaster, R De Renzi, G Allodi, S Bordignon, PG Freeman, FL Pratt, SR Giblin, JS Moeller, SJ Blundell, AT Boothroyd, D Prabhakaran

Magnetization dynamics and frustration in the multiferroic double perovskite Lu2MnCoO6

PHYSICAL REVIEW B 93 (2016) ARTN 134431

VS Zapf, BG Ueland, M Laver, M Lonsky, M Pohlit, J Mueller, T Lancaster, JS Moeller, SJ Blundell, J Singleton, J Mira, S Yanez-Vilar, M Antonia Senaris-Rodriguez

Magnetic ground state of the two isostructual polymeric quantum magnets [Cu(HF2)(pyrazine)(2)]SbF6 and [Co(HF2)(pyrazine)(2)]SbF6 investigated with neutron powder diffraction

PHYSICAL REVIEW B 92 (2015) ARTN 134406

J Brambleby, PA Goddard, RD Johnson, J Liu, D Kaminski, A Ardavan, AJ Steele, SJ Blundell, T Lancaster, P Manuel, PJ Baker, J Singleton, SG Schwalbe, PM Spurgeon, HE Tran, PK Peterson, JF Corbey, JL Manson

Three-terminal graphene single-electron transistor fabricated using feedback-controlled electroburning


P Puczkarski, P Gehring, CS Lau, J Liu, A Ardavan, JH Warner, GAD Briggs, JA Mol

Spectroscopy Methods for Molecular Nanomagnets


ML Baker, SJ Blundell, N Domingo, S Hill

Engineering coherent interactions in molecular nanomagnet dimers


A Ardavan, AM Bowen, A Fernandez, AJ Fielding, D Kaminski, F Moro, CA Muryn, MD Wise, A Ruggi, EJL McInnes, K Severin, GA Timco, CR Timmel, F Tuna, GFS Whitehead, REP Winpenny

Surface acoustic wave devices on bulk ZnO crystals at low temperature


EB Magnusson, BH Williams, R Manenti, M-S Nam, A Nersisyan, MJ Peterer, A Ardavan, PJ Leek

Evidence for magnetic clusters in Ni1-xVx close to the quantum critical concentration


R Wang, S Ubaid-Kassis, A Schroeder, PJ Baker, FL Pratt, SJ Blundell, T Lancaster, I Franke, JS Moeller, T Vojta

Anisotropic local modification of crystal field levels in Pr-based pyrochlores: a muon-induced effect modeled using density functional theory.

Physical review letters 114 (2015) 017602-

FR Foronda, F Lang, JS Möller, T Lancaster, AT Boothroyd, FL Pratt, SR Giblin, D Prabhakaran, SJ Blundell

Although muon spin relaxation is commonly used to probe local magnetic order, spin freezing, and spin dynamics, we identify an experimental situation in which the measured response is dominated by an effect resulting from the muon-induced local distortion rather than the intrinsic behavior of the host compound. We demonstrate this effect in some quantum spin ice candidate materials Pr(2)B(2)O(7) (B=Sn, Zr, Hf), where we detect a static distribution of magnetic moments that appears to grow on cooling. Using density functional theory we show how this effect can be explained via a hyperfine enhancement arising from a splitting of the non-Kramers doublet ground states on Pr ions close to the muon, which itself causes a highly anisotropic distortion field. We provide a quantitative relationship between this effect and the measured temperature dependence of the muon relaxation and discuss the relevance of these observations to muon experiments in other magnetic materials.

Robustness of superconductivity to structural disorder in Sr-0.3(NH2)(y)(NH3)(1-y)Fe2Se2

PHYSICAL REVIEW B 92 (2015) ARTN 134517

FR Foronda, S Ghannadzadeh, SJ Sedlmaier, JD Wright, K Burns, SJ Cassidy, PA Goddard, T Lancaster, SJ Clarke, SJ Blundell

Soft chemical control of superconductivity in lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se.

Inorganic chemistry 54 (2015) 1958-1964

H Sun, DN Woodruff, SJ Cassidy, GM Allcroft, SJ Sedlmaier, AL Thompson, PA Bingham, SD Forder, S Cartenet, N Mary, S Ramos, FR Foronda, BH Williams, X Li, SJ Blundell, SJ Clarke

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se (x ∼ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li(1-x)Fe(x)(OH) reservoir layer to fill vacancies in the selenide layer.

Transverse field muon-spin rotation signature of the skyrmion-lattice phase in Cu2OSeO3

Phys Rev B. Solid State 91 (2015) 224408

T Lancaster, RC Williams, IO Thomas, F Xiao, FL Pratt, SJ Blundell, JC Loudon, T Hesjedal, SJ Clark, PD Hatton, M Ciomaga Hatnean, DS Keeble, G Balakrishnan

We present the results of transverse field (TF) muon-spin rotation (μ+SR) measurements on Cu2OSeO3, which has a skyrmion-lattice (SL) phase. We measure the response of the TF μ+SR signal in that phase along with the surrounding ones, and suggest how the phases might be distinguished using the results of these measurements. Dipole field simulations support the conclusion that the muon is sensitive to the SL via the TF line shape and, based on this interpretation, our measurements suggest that the SL is quasistatic on a time scale τ>100 ns.

Spin diffusion in the low-dimensional molecular quantum Heisenberg antiferromagnet Cu(pyz)(NO3)(2) detected with implanted muons

PHYSICAL REVIEW B 91 (2015) ARTN 144417

F Xiao, JS Moeller, T Lancaster, RC Williams, FL Pratt, SJ Blundell, D Ceresoli, AM Barton, JL Manson

Electron paramagnetic resonance of individual atoms on a surface.

Science (New York, N.Y.) 350 (2015) 417-420

S Baumann, W Paul, T Choi, CP Lutz, A Ardavan, AJ Heinrich

We combined the high-energy resolution of conventional spin resonance (here ~10 nano-electron volts) with scanning tunneling microscopy to measure electron paramagnetic resonance of individual iron (Fe) atoms placed on a magnesium oxide film. We drove the spin resonance with an oscillating electric field (20 to 30 gigahertz) between tip and sample. The readout of the Fe atom's quantum state was performed by spin-polarized detection of the atomic-scale tunneling magnetoresistance. We determine an energy relaxation time of T1 ≈ 100 microseconds and a phase-coherence time of T2 ≈ 210 nanoseconds. The spin resonance signals of different Fe atoms differ by much more than their resonance linewidth; in a traditional ensemble measurement, this difference would appear as inhomogeneous broadening.

Magnetostructural relationship in the tetrahedral spin-chain oxide CsCoO2

PHYSICAL REVIEW B 91 (2015) ARTN 024419

NZ Ali, RC Williams, F Xiao, SJ Clark, T Lancaster, SJ Blundell, DV Sheptyakov, M Jansen

A spin-frustrated trinuclear copper complex based on triaminoguanidine with an energetically well-separated degenerate ground state.

Inorganic chemistry 54 (2015) 3432-3438

ET Spielberg, A Gilb, D Plaul, D Geibig, D Hornig, D Schuch, A Buchholz, A Ardavan, W Plass

We present the synthesis and crystal structure of the trinuclear copper complex [Cu3(saltag)(bpy)3]ClO4·3DMF [H5saltag = tris(2-hydroxybenzylidene)triaminoguanidine; bpy = 2,2'-bipyridine]. The complex crystallizes in the trigonal space group R3̅, with all copper ions being crystallographically equivalent. Analysis of the temperature dependence of the magnetic susceptibility shows that the triaminoguanidine ligand mediates very strong antiferromagnetic interactions (JCuCu = -324 cm(-1)). Detailed analysis of the magnetic susceptibility and magnetization data as well as X-band electron spin resonance spectra, all recorded on both powdered samples and single crystals, show indications of neither antisymmetric exchange nor symmetry lowering, thus indicating only a very small splitting of the degenerate S = (1)/2 ground state. These findings are corroborated by density functional theory calculations, which explain both the strong isotropic and negligible antisymmetric exchange interactions.

Transverse field muon-spin rotation signature of the skyrmion-lattice phase in Cu2OSeO3

PHYSICAL REVIEW B 91 (2015) ARTN 224408

T Lancaster, RC Williams, IO Thomas, F Xiao, FL Pratt, SJ Blundell, JC Loudon, T Hesjedal, SJ Clark, PD Hatton, MC Hatnean, DS Keeble, G Balakrishnan

Local magnetism and spin correlations in the geometrically frustrated cluster magnet LiZn <inf>2</inf> Mo<inf>3</inf> O<inf>8</inf>

Physical Review B - Condensed Matter and Materials Physics 89 (2014)

JP Sheckelton, FR Foronda, L Pan, C Moir, RD McDonald, T Lancaster, PJ Baker, NP Armitage, T Imai, SJ Blundell, TM McQueen

LiZn2Mo3O8 has been proposed to contain S=Mo3O13 magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbor interactions. Here, microwave and terahertz electron spin resonance, Li7 nuclear magnetic resonance, and muon spin rotation spectroscopies are used to characterize the local magnetic properties of LiZn2Mo3O8. These results show the magnetism in LiZn2Mo3O8 arises from a single isotropic S=electron per cluster and that there is no static long-range magnetic ordering down to T = 0.07 K. Further, there is evidence of gapless spin excitations with spin fluctuations slowing down as the temperature is lowered. These data indicate strong spin correlations, which, together with previous data, suggest a low-temperature resonating valence-bond state in LiZn2Mo3O8. © 2014 American Physical Society.

Stripe disorder and dynamics in the hole-doped antiferromagnetic insulator la 5 / 3 Sr 1 / 3 CoO 4

Physical Review B - Condensed Matter and Materials Physics 89 (2014)

T Lancaster, SR Giblin, G Allodi, S Bordignon, M Mazzani, R De Renzi, PG Freeman, PJ Baker, FL Pratt, P Babkevich, SJ Blundell, AT Boothroyd, JS Möller, D Prabhakaran

We investigate the magnetic ordering and dynamics of the stripe phase of La5/3Sr1/3CoO4, a material shown to have an hourglass magnetic excitation spectrum. A combination of muon-spin relaxation, nuclear magnetic resonance, and magnetic susceptibility measurements strongly suggest that the physics is determined by a partially disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are dynamic at high temperature and which undergo an ordering transition around 35 K with coexisting dynamics that freeze out in a glassy manner as the temperature is further reduced. © 2014 American Physical Society.