A.C. susceptibility as a probe of low-frequency magnetic dynamics.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2018) 013001-

CV Topping, SJ Blundell

The experimental technique of a.c. susceptibility can be used as a probe of magnetic dynamics in a wide variety of systems. Its use is restricted to the low-frequency regime and thus is sensitive to relatively slow processes. Rather than measuring the dynamics of single spins, a.c. susceptibility can be used to probe the dynamics of collective objects, such as domain walls in ferromagnets or vortex matter in superconductors. In some frustrated systems, such as spin glasses, the complex interactions lead to substantial spectral weight of fluctuations in the low-frequency regime, and thus a.c. susceptibility can play a unique role. We review the theory underlying the technique and magnetic dynamics more generally and give applications of a.c. susceptibility to a wide variety of experimental situations.

Hyperfine interaction of individual atoms on a surface.

Science (New York, N.Y.) 362 (2018) 336-339

P Willke, Y Bae, K Yang, JL Lado, A Ferrón, T Choi, A Ardavan, J Fernández-Rossier, AJ Heinrich, CP Lutz

Taking advantage of nuclear spins for electronic structure analysis, magnetic resonance imaging, and quantum devices hinges on knowledge and control of the surrounding atomic-scale environment. We measured and manipulated the hyperfine interaction of individual iron and titanium atoms placed on a magnesium oxide surface by using spin-polarized scanning tunneling microscopy in combination with single-atom electron spin resonance. Using atom manipulation to move single atoms, we found that the hyperfine interaction strongly depended on the binding configuration of the atom. We could extract atom- and position-dependent information about the electronic ground state, the state mixing with neighboring atoms, and properties of the nuclear spin. Thus, the hyperfine spectrum becomes a powerful probe of the chemical environment of individual atoms and nanostructures.

Molecular electronic spin qubits from a spin-frustrated trinuclear copper complex.

Chemical communications (Cambridge, England) 54 (2018) 12934-12937

B Kintzel, M Böhme, J Liu, A Burkhardt, J Mrozek, A Buchholz, A Ardavan, W Plass

The trinuclear copper(ii) complex [Cu3(saltag)(py)6]ClO4 (H5saltag = tris(2-hydroxybenzylidene)triaminoguanidine) was synthesized and characterized by experimental as well as theoretical methods. This complex exhibits a strong antiferromagnetic coupling (J = -298 cm-1) between the copper(ii) ions, mediated by the N-N diazine bridges of the tritopic ligand, leading to a spin-frustrated system. This compound shows a T2 coherence time of 340 ns in frozen pyridine solution, which extends to 591 ns by changing the solvent to pyridine-d5. Hence, the presented compound is a promising candidate as a building block for molecular spintronics.

Extreme Sensitivity of a Topochemical Reaction to Cation Substitution: SrVO2H versus SrV1- xTi xO1.5H1.5.

Inorganic chemistry 57 (2018) 2890-2898

M Amano Patino, D Zeng, SJ Blundell, JE McGrady, MA Hayward

The anion-ordered oxide-hydride SrVO2H is an antiferromagnetic insulator due to strong correlations between vanadium d electrons. In an attempt to hole-dope SrVO2H into a metallic state, a strategy of first preparing SrV1- xTi xO3 phases and then converting them to the corresponding SrV1- xTi xO2H phases via reaction with CaH2 was followed. This revealed that the solid solution between SrVO3 and SrTiO3 is only stable at high temperature. In addition, reactions between SrV0.95Ti0.05O3 and CaH2 were observed to yield SrV0.95Ti0.05O1.5H1.5 not SrV0.95Ti0.05O2H. This dramatic change in reactivity for a very modest change in initial chemical composition is attributed to an electronic destabilization of SrVO2H on titanium substitution. Density functional theory calculations indicate that the presence of an anion-ordered, tetragonal SrMO2H phase is uniquely associated with a d2 electron count and that titanium substitution leads to an electronic destabilization of SrV1- xTi xO2H phases, which, ultimately, drives further reaction of SrV1- xTi xO2H to SrV1- xTi xO1.5H1.5. The observed sensitivity of the reaction products to the chemical composition of initial phases highlights some of the difficulties associated with electronically doping metastable materials prepared by topochemical reactions.

Comparative study of the magnetic properties of La3Ni2B ' O-9 for B ' = Nb, Ta or Sb


C-M Chin, PD Battle, SJ Blundell, E Hunter, F Lang, M Hendrickx, RP Sena, J Hadermann

Two-gap superconductivity with line nodes in CsCa2Fe4As4F2

PHYSICAL REVIEW B 97 (2018) ARTN 060506

FKK Kirschner, DT Adroja, Z-C Wang, F Lang, M Smidman, PJ Baker, G-H Cao, SJ Blundell

Electrically controlled nuclear polarization of individual atoms.

Nature nanotechnology 13 (2018) 1120-1125

K Yang, P Willke, Y Bae, A Ferrón, JL Lado, A Ardavan, J Fernández-Rossier, AJ Heinrich, CP Lutz

Nuclear spins serve as sensitive probes in chemistry1 and materials science2 and are promising candidates for quantum information processing3-6. NMR, the resonant control of nuclear spins, is a powerful tool for probing local magnetic environments in condensed matter systems, which range from magnetic ordering in high-temperature superconductors7,8 and spin liquids9 to quantum magnetism in nanomagnets10,11. Increasing the sensitivity of NMR to the single-atom scale is challenging as it requires a strong polarization of nuclear spins, well in excess of the low polarizations obtained at thermal equilibrium, as well as driving and detecting them individually4,5,12. Strong nuclear spin polarization, known as hyperpolarization, can be achieved through hyperfine coupling with electron spins2. The fundamental mechanism is the conservation of angular momentum: an electron spin flips and a nuclear spin flops. The nuclear hyperpolarization enables applications such as in vivo magnetic resonance imaging using nanoparticles13, and is harnessed for spin-based quantum information processing in quantum dots14 and doped silicon15-17. Here we polarize the nuclear spins of individual copper atoms on a surface using a spin-polarized current in a scanning tunnelling microscope. By employing the electron-nuclear flip-flop hyperfine interaction, the spin angular momentum is transferred from tunnelling electrons to the nucleus of individual Cu atoms. The direction and magnitude of the nuclear polarization is controlled by the direction and amplitude of the current. The nuclear polarization permits the detection of the NMR of individual Cu atoms, which is used to sense the local magnetic environment of the Cu electron spin.

Emergence, causation and storytelling: Condensed matter physics and the limitations of the human mind

Philosophica 92 (2017) 139-164

SJ Blundell

The physics of matter in the condensed state is concerned with problems in which the number of constituent particles is vastly greater than can be easily comprehended. The inherent physical limitations of the human mind are fundamental and restrict the way in which we can interact with and learn about the universe. This presents challenges for developing scientific explanations that are met by emergent narratives, concepts and arguments that have a nontrivial relationship to the underlying microphysics. By examining examples within condensed matter physics, and also from cellular automata, I show how such emergent narratives efficiently describe elements of reality.

Quantum magnetism in molecular spin ladders probed with muonspin spectroscopy


T Lancaster, F Xiao, BM Huddart, RC Williams, FL Pratt, SJ Blundell, SJ Clark, R Scheuermann, T Goko, S Ward, JL Manson, C Ruegg, KW Kramer

Multigap Superconductivity in RbCa2Fe4As4F2 Investigated Using mu SR Measurements


DT Adroja, FKK Kirschner, F Lang, M Smidman, AD Hillier, Z-C Wang, G-H Cao, GBG Stenning, SJ Blundell

Comparative study of the magnetic properties of La<inf>3</inf>Ni<inf>2</inf>B′O<inf>9</inf>for B′ = Nb, Taor Sb

Journal of Solid State Chemistry 258 (2018) 825-834

CM Chin, PD Battle, SJ Blundell, E Hunter, F Lang, M Hendrickx, R Paria Sena, J Hadermann

© 2017 Elsevier Inc. Polycrystalline samples of La 3 Ni 2 NbO 9 and La 3 Ni 2 TaO 9 have been characterised by X-ray and neutron diffraction, electron microscopy, magnetometry and muon spin relaxation (µSR); the latter technique was also applied to La 3 Ni 2 SbO 9 . On the length scale of a neutron diffraction experiment, the six-coordinate sites of the monoclinic perovskite structure are occupied in a 1:1 ordered manner by Ni and a random ⅓Ni/⅔B′ mixture. Electron microscopy demonstrated that this 1:1 ordering is maintained over microscopic distances, although diffuse scattering indicative of short-range ordering on the mixed site was observed. No magnetic Bragg scattering was observed in neutron diffraction patterns collected from La 3 Ni 2 B′O 9 (B′ = Nb or Ta) at 5 K although in each case µSR identified the presence of static spins below 30 K. Magnetometry showed that La 3 Ni 2 NbO 9 behaves as a spin glass below 29 K but significant short-range interactions are present in La 3 Ni 2 TaO 9 below 85 K. The contrasting properties of these compounds are discussed in terms of their microstructure.

Magnetic phases of skyrmion-hosting GaV4S8-ySey (y=0, 2, 4, 8) probed with muon spectroscopy

PHYSICAL REVIEW B 98 (2018) ARTN 054428

KJA Franke, BM Huddart, TJ Hicken, F Xiao, SJ Blundell, FL Pratt, M Crisanti, JAT Barker, SJ Clark, A Stefancic, MC Hatnean, G Balakrishnan, T Lancaster

Low-field spin dynamics of Cr7Ni and Cr7Ni-Cu-Cr7Ni molecular rings as detected by mu SR

PHYSICAL REVIEW B 96 (2017) ARTN 184403

S Sanna, P Arosio, L Bordonali, F Adelnia, M Mariani, E Garlatti, C Baines, A Amato, KPV Sabareesh, G Timco, REP Winpenny, SJ Blundell, A Lascialfari

Coexistence of magnetism and superconductivity in separate layers of the iron-based superconductor Li1-xFex(OH)Fe1-ySe

PHYSICAL REVIEW B 95 (2017) ARTN 134419

CV Topping, FKK Kirschner, SJ Blundell, PJ Baker, DN Woodruff, F Schild, H Sun, SJ Clarke

Room-temperature helimagnetism in FeGe thin films.

Scientific reports 7 (2017) 123-

SL Zhang, I Stasinopoulos, T Lancaster, F Xiao, A Bauer, F Rucker, AA Baker, AI Figueroa, Z Salman, FL Pratt, SJ Blundell, T Prokscha, A Suter, J Waizner, M Garst, D Grundler, G van der Laan, C Pfleiderer, T Hesjedal

Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain α intr = 0.0036 ± 0.0003 at 310 K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.

Quantum Griffiths Phase Inside the Ferromagnetic Phase of Ni_{1-x}V_{x}.

Physical review letters 118 (2017) 267202-267202

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

We study by means of bulk and local probes the d-metal alloy Ni_{1-x}V_{x} close to the quantum critical concentration, x_{c}≈11.6%, where the ferromagnetic transition temperature vanishes. The magnetization-field curve in the ferromagnetic phase takes an anomalous power-law form with a nonuniversal exponent that is strongly x dependent and mirrors the behavior in the paramagnetic phase. Muon spin rotation experiments demonstrate inhomogeneous magnetic order and indicate the presence of dynamic fluctuating magnetic clusters. These results provide strong evidence for a quantum Griffiths phase on the ferromagnetic side of the quantum phase transition.

Local magnetism and spin dynamics of the frustrated honeycomb rhodate Li2RhO3

PHYSICAL REVIEW B 96 (2017) ARTN 094432

P Khuntia, S Manni, FR Foronda, T Lancaster, SJ Blundell, P Gegenwart, M Baenitz

Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet.

Physical review letters 119 (2017) 147701-

M Mergenthaler, J Liu, JJ Le Roy, N Ares, AL Thompson, L Bogani, F Luis, SJ Blundell, T Lancaster, A Ardavan, GAD Briggs, PJ Leek, EA Laird

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (circuit QED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.

Quantum-critical spin dynamics in a Tomonaga-Luttinger liquid studied with muon-spin relaxation

PHYSICAL REVIEW B 95 (2017) ARTN 020402

JS Moller, T Lancaster, SJ Blundell, FL Pratt, PJ Baker, F Xiao, RC Williams, W Hayes, MM Turnbul, CP Landee

Spin Resonance Clock Transition of the Endohedral Fullerene ^{15}N@C_{60}.

Physical review letters 119 (2017) 140801-

RT Harding, S Zhou, J Zhou, T Lindvall, WK Myers, A Ardavan, GAD Briggs, K Porfyrakis, EA Laird

The endohedral fullerene ^{15}N@C_{60} has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.