In recognition of the excellent contribution that postdocs make to the research in the Physics Department at the University of Warwick, the Head of Department, Prof. David Leadley, has initiated an award for the best paper published by a research fellow this year. Chris Patrick and Santosh Kumar from PRETAMAG have jointly won the first award for their paper “Rare-earth/transition-metal magnetic interactions in pristine and (Ni,Fe)-doped YCo₅ and GdCo₅” Phys.Rev.Materials 1, 02411 (2017). The paper was consider to be well-written and to convey clearly the outcomes of the joint theory-experimental activities. The interpretation of results and insight into the materials that this joint approach has enabled was particularly commended.

Oxides containing 5d ions, particularly Ir4+, offer possibilities for interesting spin-orbit and spin-lattice coupling effects. In these materials, the energy scales for spin-orbit interactions, Coulomb repulsion, and crystalline-electric fields can be very similar. Competition between these three energies can result in exotic magnetic states such as spin-orbit entanglement, extreme magnetic exchange anisotropy leading to, for example, a Kitaev spin liquid, and spin-orbit entangled Mott insulating behaviour.

Using experiments performed at the National High Magnetic Field Laboratory, a team from the USA, UK and South Korea (including Paul Goddard at Warwick) have observed extremely large coercive magnetic fields of up to 55 T in Sr₃NiIrO₆ and 52 T in Sr₃CoIrO₆, with switched magnetic moments of ≈ 1μ_B and 3μ_B per formula unit, respectively. Rather than the switching of magnetic domains, which is the cause of the coercive fields of traditional ferromagnets, the large hysteresis observed in our materials evolves out of a frustrated, antiferromagnetic ground state that incorporates an entangled spin-orbit state on the 5d ion.

Magnetic properties of Sr₃NiIrO₆ and Sr₃CoIrO₆: Magnetic hysteresis with coercive fields of up to 55 T, J. Singleton, J. W. Kim, C. V. Topping, A. Hansen, E.-D. Mun, S. Chikara, I. Lakis, S. Ghannadzadeh, P. Goddard, X. Luo, Y. S. Oh, S.-W. Cheong, and V. S. Zapf, Physical Review B 94, 224408 (2016).

A paper has been published in Nature Physics that reveals that magnetic fragmentation occurs in the spin ice candidate Nd₂Zr₂O₇. The work was carried out in collaboration with colleagues from the CEA in Saclay and the ILL, CEA, and the Institut Neel in Grenoble.

It has recently been suggested that in spin ice, the magnetic moment field can fragment, resulting in a dual ground state consisting of a fluctuating spin liquid, a so-called Coulomb phase, on top of a magnetic monopole crystal. Here we show, by means of neutron scattering measurements, that such fragmentation occurs in Nd₂Zr₂O₇. We observe the spectacular coexistence of an antiferromagnetic order induced by the monopole crystallization and a fluctuating state with ferromagnetic correlations.

Experimentally, this fragmentation manifests itself through the superposition of magnetic Bragg peaks, characteristic of the ordered phase, and a pinch point pattern, characteristic of the Coulomb phase. These results highlight the relevance of the fragmentation concept to describe the physics of systems that are simultaneously ordered and fluctuating.

Observation of magnetic fragmentation in spin ice, S. Petit, E. Lhotel, B. Canals, M. Ciomaga Hatnean, J. Ollivier, H. Mutka, E. Ressouche, A. R. Wildes, M. R. Lees, and G. Balakrishnan, Nature Physics 12, 746, (2016).

Also see ILL highlights.

At temperatures above 300 mK, gradual restoration of kinetic monopole equilibria causes the non-Ohmic current to smoothly evolve into the high-field Wien effect for magnetic monopoles, as confirmed by comparison to a recent and rigorous theory of the Wien effect in spin ice. The results extend the universality of the exponential-root field form into magnetism and illustrate the power of emergent particle kinetics to describe far-from-equilibrium response in complex systems.

Experimental signature of the attractive Coulomb force between positive and negative magnetic monopoles in spin ice, C. Paulsen, S. R. Giblin, E. Lhotel, D. Prabhakaran, G. Balakrishnan, K. Matsuhira, and S. T. Bramwell, Nature Physics 12, 661 (2016).

A paper has been published in JACS which reports the accurate electron density distribution and magnetic properties of two metal–organic polymeric magnets. The work is a collaboration with the Universities of Bern, Eastern Washington, Durham, and Oxford, and the high field laboratory in Los Alamos,

The paper combines high-resolution single-crystal X-ray diffraction, DFT calculations, and magnetic studies to characterize the possible magnetic exchange pathways and establish the relationships between the electron (charge and spin) densities and the exchange-coupling constants in both compounds.

Experimental and Theoretical Electron Density Analysis of Copper Pyrazine Nitrate Quasi-Low-Dimensional Quantum Magnets, L. H. R. Dos Santos, A. Lanza, A. M. Barton, J. Brambleby, W. J. A. Blackmore, P. A. Goddard, F. Xiao, R. C. Williams, T. Lancaster, F. L. Pratt, S. J. Blundell, J. Singleton, J. L. Manson, P. Macchi, Journal of the American Chemical Society 138, 2280 (2016).

A paper has been published in Physical Review Letters reporting our discovery of unconventional superconductivity in La₇Ir₃, one of a new family of noncentrosymmetric superconductors that breaks time reversal symmetry. The study using muon spectroscopy was carried out in collaboration with colleagues from IISER, Bhopal and TIFR, Mumbai in India, and ISIS-RAL.

Unconventional Superconductivity in La₇Ir₃ Revealed by Muon Spin Relaxation: Introducing a New Family of Noncentrosymmetric Superconductor That Breaks Time-Reversal Symmetry, J. A. T. Barker, D. Singh, A. Thamizhavel, A. D. Hillier, M. R. Lees, G. Balakrishnan, D. M. Paul, and R. P. Singh, Physical Review Letters 115, 267001 (2015).

Neutron scattering and magnetization measurements down to 90 mK have been used to determine the magnetic ground state of the spin-ice candidate Nd₂Zr₂O₇. Along with colleagues from Grenoble and Saclay we show Nd₂Zr₂O₇ undergoes a transition at 285 mK towards an all-in–all-out antiferromagnetic state, and establish the H-T phase diagram. We propose that this behaviour results from the peculiar nature of the Nd dipolar-octupolar doublet, revealing the importance of multipolar correlations in pyrochlore oxides.

Fluctuations and All-In All-Out Ordering in Dipole-Octupole Nd₂Zr₂O₇, E. Lhotel, S. Petit, S. Guitteny, O. Florea, M. Ciomaga Hatnean, C. Colin, E. Ressouche, M. R. Lees, and G. Balakrishnan, Physical Review Letters 115, 197202 (2015).

A paper examining the hidden magnetic order in the spin-liquid Gd₃Ga₅O₁₂ has been published in the journal Science. Along with colleagues based in Oxford, the Niels Bohr Institute Copenhagen, Denmark, ESS Lund, Sweden, and the ILL Grenoble, France the work combines neutron scattering with theory to show that the spin-liquid Gd₃Ga₅O₁₂ supports hidden order in which multipoles are formed from 10 spin loops.

Hidden order in spin-liquid Gd₃Ga₅O₁₂, J. A. M. Paddison, H. Jacobsen, O. A. Petrenko, M. T. Fernández-Díaz, P. P. Deen, and A. L. Goodwin, Science 350, 179 (2015).

Please also see ILL Highlights 2015.

A paper demonstrating a new approach that allows air sensitive two-dimensional crystals to be cleaved, transferred, aligned, and encapsulated has been published in the journal Nano Letters. Along with colleagues based in Manchester, Grenoble in France, and Tsukuba in Japan, the work illustrates this technology on black phosphorous and niobium diselenide.

Quality Heterostructures from Two-Dimensional Crystals Unstable in Air by Their Assembly in Inert Atmosphere, Y. Cao, A. Mishchenko, G. L. Yu, E. Khestanova, A. P. Rooney, E. Prestat, A. V. Kretinin, P. Blake, M. B. Shalom, C. Woods, J. Chapman, G. Balakrishnan, I. V. Grigorieva, K. S. Novoselov, B. A. Piot, M. Potemski, K. Watanabe, T. Taniguchi, S. J. Haigh, A. K. Geim, and R. V. Gorbachev, Nano Letters 15, 4914 (2015).

A paper examining the unconventional Fermi surface in an insulating state in hexaboride SmB₆ has been published in the journal Science. Along with colleagues based in Cambridge , the National High Magnetic Field Laboratory in Tallahassee and Los Alamos observation, and the Naval Research Laboratory in Washington DC, the work shows that SmB₆ has an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid.

B. S. Tan, Y.-T. Hsu,1 B. Zeng, M. Ciomaga Hatnean, N. Harrison, Z. Zhu, M. Hartstein, M. Kiourlappou, A. Srivastava, M. D. Johannes, T. P. Murphy, J.-H. Park, L. Balicas, G. G. Lonzarich, G. Balakrishnan, and Suchitra E. Sebastian Unconventional Fermi surface in an insulating state, Science 349, 287 (2015).

A paper entitled Dispersionless Spin Waves and Underlying Field-Induced Magnetic Order in Gadolinium Gallium Garnet has been published in Physical Review Letters. Oleg Petrenko and Nick d’Ambrumenil based here at Warwick, along with Hannu Mutka and Pascale Deen, have used neutron diffraction and inelastic neutron scattering in high magnetic fields to study the unusual magnetic properties of Gd₃Ga₅O₁₂. (Also see ESS website highlights).

N. d’Ambrumenil, O. A. Petrenko, H. Mutka, and P. P. Deen, Physical Review Letters 114, 227203 (2015).

A paper has been published in Nature in which angle-resolved quantum oscillation measurements are used to study the normal-state nodal electronic structure in underdoped high-Tc copper oxides. The work was performed at the High Field Laboratory in Los Alamos by Paul Goddard, along with his colleagues Suchitra Sebastian, N. Harrison, F. F. Balakirev, M. M. Altarawneh, Ruixing Liang, D. A. Bonn, W. N. Hardy and G. G. Lonzarich from Cambridge, Los Alamos, Mu’tah Universit in Jordan, and the University of British Columbia in Canada.

Suchitra E. Sebastian, N. Harrison, F. F. Balakirev, M. M. Altarawneh, P. A. Goddard, Ruixing Liang, D. A. Bonn, W. N. Hardy & G. G. Lonzarich, Nature 511, 61 (2014).

A paper has been published in Angewandte Chemie Int. Ed. describing a new synthesis approach to oxides of ruthenium, including new structures, and materials with unusual magnetic properties. The work was performed by Craig Hiley, a PhD student in the group of Richard Walton, in collaboration with an industrial partner (Johnson Matthey) and co-workers in Physics, as part of an ongoing study into the chemistry of precious-metal containing materials.

A paper describing a systematic investigation of the magnetic properties of the three-dimensional Kondo topological insulator SmB₆ using magnetization and muon-spin relaxation/rotation measurements has been published in Physical Review B. The study was carried out in collaboration with colleagues based at PSI and Zurich in Switerland, and Princeton in the USA.

P. K. Biswas, Z. Salman, T. Neupert, E. Morenzoni, E. Pomjakushina, F. von Rohr, K. Conder, G. Balakrishnan, M. Ciomaga Hatnean, M. R. Lees, D. McK. Paul, A. Schilling, C. Baines, H. Luetkens, R. Khasanov, and A. Amato, Phys. Rev. B 89, 161107(R) (2014).

A paper describing our work on the non centrosymmetric superconductor Re₆Zr has been published in Physical Review Letters. The study using muon spectroscopy was carried out in collaboration with colleagues from Kent, Bristol, and ISIS-RAL.

R. P. Singh, A. D. Hillier, B. Mazidian, J. Quintanilla, J. F. Annett, D. McK. Paul, G. Balakrishnan, and M. R. Lees, Phys. Rev. Lett. 112, 107002 (2014).