Out-of-equilibrium microstates but effective thermodynamics in artificial kagome ice networks

Abstract

Using magnetic force microscopy and Monte Carlo simulations, we investigate the low-energy properties of two artificial kagome ice structures. The two systems differ in that the first series of lattices consists of an assembly of physically disconnected nanomagnets coupled via magnetostatics, whereas the second series is made of fully connected honeycomb networks. Imaging the microstates resulting from a field demagnetization protocol, and analyzing their magnetic correlations in real and reciprocal space, we observe distinct behaviors between the two lattice types. While the former exhibits properties well described by the dipolar kagome ice model equilibrated at a finite fictional temperature, the latter instead is found systematically out of equilibrium. Remarkably, this out-of-equilibrium physics can be reformulated into an at-equilibrium one by strengthening specific coupling terms in the spin Hamiltonian. We interpret this property as a result of the field-induced domain wall propagation that arises when demagnetizing a connected network, i.e., a field driven kinetic process that competes with the formation of local flux-closure configurations that minimize the magnetostatic energy. Our findings highlight how micromagnetic effects bias the selection of spin liquid microstates during a field demagnetization protocol.Using magnetic force microscopy and Monte Carlo simulations, we investigate the low-energy properties of two artificial kagome ice structures. The two systems differ in that the first series of lattices consists of an assembly of physically disconnected nanomagnets coupled via magnetostatics, whereas the second series is made of fully connected honeycomb networks. Imaging the microstates resulting from a field demagnetization protocol, and analyzing their magnetic correlations in real and reciprocal space, we observe distinct behaviors between the two lattice types. While the former exhibits properties well described by the dipolar kagome ice model equilibrated at a finite fictional temperature, the latter instead is found systematically out of equilibrium. Remarkably, this out-of-equilibrium physics can be reformulated into an at-equilibrium one by strengthening specific coupling terms in the spin Hamiltonian. We interpret this property as a result of the field-induced domain wall propagation that arises when demagnetizing a connected network, i.e., a field driven kinetic process that competes with the formation of local flux-closure configurations that minimize the magnetostatic energy. Our findings highlight how micromagnetic effects bias the selection of spin liquid microstates during a field demagnetization protocol.