Magnetricity and Monopoles Displacement vectors in water Magnetic Monopoles in Spin Ice Potential magnetic monopole capacitor? ice Spin vectors in Spin Ice (Bramwell & Harris 1997) • Spin ice materials like Ho2Ti2O7 are simple Artificial System: Nanomagnets atomic spins transparent crystals that include atoms of •Magnetic charge Q at each vertex in honeycomb “rare earth” elements arranged in corner- •Q=3 monopole defects linked tetrahedra. The atomic magnetic moments or “spins” point into or out of Q= +1 or Q= -1 Ice the tetrahedral (arrows). H2O - + • Magnetic monopoles - the magnetic - + + version of a charged particle like electrons - Spin flips make magnetic or protons – have recently been shown to monopoles analogous to water exist in spin ice. We have shown that ice’s ionic defects monopoles form a magnetic version of Q= +3 or Q= -3 electricity, or “magnetricity” at very low T. + - + - - + 2H2O = [H3O +OH ] = H3O +OH • We have fabricated arrays of nanomagnets in a spin ice geometry, to + - create magnetic monopole defects at room temperature Physics of Bulk Spin Ice Materials Monopoles in Artificial Spin Ice Nanostructures H •The spin ice state has been confirmed by neutron •Scanning electron scattering to be a vacuum for magnetic charge. SEM (a) (d) micrograph of the cobalt •Magnetic monopoles live in this vacuum. They honeycomb nanostructure.Q=+3 on are analogous to water’s ionic defects. Q=+1 site •Magnetic force micrograph H =-52.4 mT •Different spin ice materials have different Hx=-48.8mT x showing a negativelyQ=+1 on (b) (e) monopole concentrations. High pressure has been charged magnetic monopoleQ=-1 site used to create a new spin ice, Dy2Ge2O7 with the defect (bright yellow). The highest monopole concentration yet discovered. Neutron Scattering ice rule in the planar Hx=-54.7 mT MFM structure gives trapped Hx=-50 mT (c) (f) Magnetic Charge magnetic charge of alternating sign at the other measured by µSR vertices seen a weak yellow H =-62.2 mT (negative) and red (positive) Hx=-51.2mT x contrast. The flow of magnetic charge: • Schematic of the magnetic •Mobile defects have effective charge ±2q spins (arrows) and charges (Domain wall). spheres after formation of a • Positive and negative charged defects move in pair of oppositely charged opposite directions. monopole defects (large • “String” of head-tail dipoles created with defects spheres). at the ends. Field Control - Magnetricity Nanostructures Key Publications Bulk Monopole defects under magnetic pressure H STXM at ALS • T. Fennell, et al…; S. T. Bramwell Science, 326 415-417 (2009) • S. T. Bramwell, S. R. Giblin, S. Calder, et al. Nature, 461 956-959 (2009) • S. Ladak, D. Read, G.K. Perkins, L.F.Cohen, W.R. Branford Nature Coulomb blockade of two magnetic charges Physics 6, 359 (2010) Unbinding of monopole pairs at the same vertex • S. R. Giblin, S. T. Bramwell, et al. Nature Physics 7, 252-258 (2011) – “the Wien effect.” • S. Ladak, D. Read, L.F.Cohen, T. Tyliszczak, W. R. Branford, NJP 13, 023023 (2011). (a-f) Scanning Transmission Xray • H.D. Zhou, S.T. Bramwell et al. Nature Comm. 2, 478 1483 (2011) Micrograph of a -3q • S. Ladak , D. Read, L.F.Cohen, W. R. Branford, NJP 13 063032 (2011). monopole defect. (g-h) Micromagnetic simulations of a -3q monopole defect in field close to Relaxing “polarization” caused by depinning field Hd. monopole currents LCN team - Academics in charge: Steve Bramwell (UCL) and Will Branford (Imperial) Acknowledgements - UCL team: S.T. Bramwell, T. Fennell, D. F. McMorrow, R. Aldus, S. Calder, J.A. Bloxsom, A. Harman-Clarke, L. Bovo. - Imperial team: W. R. Branford, S. Ladak, D. Read, G.K. Perkins, K. Zeissler, S. K. Walton, A.M. Gilbertson and L.F. Cohen .