Monopolar and dipolar relaxation in spin ice Ho ₂ Ti ₂ O ₇

Abstract

Ferromagnetically interacting Ising spins on the pyrochlore lattice of corner-sharing tetrahedra form a highly degenerate manifold of low-energy states. A spin flip relative to this “spin-ice” manifold can fractionalize into two oppositely charged magnetic monopoles with effective Coulomb interactions. To understand this process, we have probed the low-temperature magnetic response of spin ice to time-varying magnetic fields through stroboscopic neutron scattering and SQUID magnetometry on a new class of ultrapure Ho2Ti2O7 crystals. Covering almost 10 decades of time scales with atomic-scale spatial resolution, the experiments resolve apparent discrepancies between prior measurements on more disordered crystals and reveal a thermal crossover between distinct relaxation processes. Magnetic relaxation at low temperatures is associated with monopole motion through the spin-ice vacuum, while at elevated temperatures, relaxation occurs through reorientation of increasingly spin-like monopolar bound states. Spin fractionalization is thus directly manifest in the relaxation dynamics of spin ice.

Persistent dynamic magnetic state in artificial honeycomb spin ice

“…Lastly, the q m relaxation rate in the 6 nm thick honeycomb lattice is comparable to that found in bulk spin ice material, ~5 ps 18 , 28 . This suggests a correspondence between the dynamic behavior of magnetic charge defects and the monopolar dynamics in the atomistic spin ice.…”

“…Unequivocal evidence of magnetic charges has been obtained using magnetic force microscopy in honeycomb lattices made of permalloy (Ni 0.81 Fe 0.19 ) and cobalt magnets 12 – 15 . However, they are not known to be dynamic in the absence of external tuning agents (magnetic, thermal, or current) 8 , 9 , 15 , 16 , which is in strong contrast to the well-established dynamic properties of magnetic charges in analogous spin ice compounds that exhibit persistent or continuous relaxation at low enough temperature 6 , 17 , 18 . Magnetic charges relax by emitting or absorbing net charge defects, q m , of magnitude 2 Q , also termed effective monopoles 2 , 4 , 8 , 12 , 19 .…”