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Field demagnetised artificial square ice magnets: long-range interactions, origin of stochasticity and effective thermodynamics
Brunn, Ondřej ; Lacour, Daniel (oponent) ; Lassailly,, Yves (oponent) ; Kolařík, Vladimír (vedoucí práce)
In the past two decades, artificial spin ice systems have become a powerful experimental platform to investigate cooperative magnetic phenomena often associated with highly frustrated magnets. Compared to their natural counterparts, artificial spin ice systems made of interacting magnetic nanostructures offer several key advantages. Being engineered through nanofabrication processes, an extensive palette of geometries can be designed. In addition, their magnetic configuration can be visualised directly, at the scale of the spin degree of freedom, using magnetic imaging techniques. Local and global quantities can then be measured conveniently, in real space and time, at almost any desired temperature. This PhD work focuses on such artificial spin systems, and more specifically on the square geometry, which was initially proposed as a two-dimensional (2D) counterpart of the three-dimensional (3D) pyrochlore crystal structure. However, this 2D approach removes the magnetic frustration present in 3D, and the system orders in a conventional antiferromagnetic fashion rather than exhibiting a highly degenerate, liquid-like ground state. Following a strategy proposed in the literature, arrays of nanostructures consisting of two vertically offset sub-lattices were fabricated to restore frustration, enabling to reach a spin liquid regime experimentally. Imaging the magnetic configurations obtained after a field demagnetisation protocol, the analysis of the spin-spin correlations reveals deviations from what is predicted by the (short-range) square ice model. Comparing the experimental findings to Monte Carlo simulations, our results indicate that long-range magnetostatic interactions are not washed out in our arrays, contrary to what was initially thought. Then, these artificial square ice structures were used to understand to what extent the field demagnetisation protocol we apply is a stochastic process. To do so, we studied the magnetic configurations obtained after successive field protocols. Our results show that each captured magnetic micro-state differs substantially from the previous one, but not entirely. Analysing the corresponding spin and vertex configurations, we demonstrate that our field protocol is a stochastic process, although we also observe unambiguous signatures of magnetic determinism that we attribute to the presence of quenched disorder. The possible sources of randomness in our experiment are discussed. Finally, we explore the behaviour of a series of field-demagnetised conventional (non-offset) square arrays, in which the lattice parameter is gradually varied to tune the interaction strengths. Comparing the experimental vertex populations and spin-spin correlations to Monte Carlo predictions, we show that the lattice series is well approximated by a unique short-range spin Hamiltonian probed at different effective temperatures. In other words, the lattice parameter can serve as a knob to probe the thermodynamics of a given spin model.
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Magnetic arctic circle physics in an artificial square ice magnet
Ondříšková, Martina ; Lacour, Daniel (oponent) ; Rougemaille, Nicolas (vedoucí práce)
Under specific boundary conditions, the square ice model exhibits phase separation, with a disordered core and an ordered outer region, known as the ’arctic circle’ phenomenon. Inspired by recent experimental realizations using programmable lattices, we aim to observe and investigate these properties in a square lattice of interacting nanomagnets, achieving the domain wall boundary conditions (DWBC) necessary for the arctic circle and exploring their overall impact on the system. In this work, we will use magnetic force microscopy on lithographically fabricated arrays to directly observe the topological nature of the Coulomb phase and the monopole segregation mechanism. Employing an innovative approach, we will engineer the boundary conditions of the lattice through specific wiring of the nanomagnets at the edges. Our work will not only focus on the spin liquid nature of the disordered region within the arctic curve and monopole segregation by magnetic charge and moment but also on the propagation of constraints from the DWBC and their overall impact on the system.
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