Artificial Spin Ice - Slow Dynamics

Our initial research on artificial spin ice concentrated on field-driven experiments [3-5]. Employing x-ray photoemission electron microscopy (PEEM) to resolve the exact arrangement of the nanomagnet moments, we investigated the possibility to achieve with demagnetization procedures the low-energy configurations in the building blocks of artificial kagome ice [3,4], and we observed the creation and propagation of emergent magnetic monopoles in extended arrays [5].

Subsequently we fabricated artificial spin ice structures that exhibit fluctuating magnetic moments at experimentally accessible temperatures (see Fig. 2). This was achieved either by patterning Permalloy (Ni₈₀Fe₂₀) wedge films [6,7] or 𝛿-doped Pd(Fe) films in collaboration with the group of Björgvin Hjovarrson at Uppsala University [8]. In artificial square ice, we were able to follow in real time a thermally-driven relaxation process from an initial high energy configuration of the magnetic moments to a ground state configuration [6] (see Fig. 1). In building blocks of artificial kagome ice, our simple thermal annealing procedure, that involves a moderate heating above the nanomagnet blocking temperatures (T_B = 320-330 K), proved to be highly effective in achieving the low-energy configurations [7]. However, with increasing systems size, it becomes more and more difficult to access the predicted ground state configurations. 

We have also investigated an artificial spin ice system with a modified geometry, derived from the square ice [9]. Employing PEEM to measure the evolution of a thermally-active system at room temperature, we have established that the average magnetization displays chiral dynamics: it consistently rotates in a unique direction following saturation with an external field (see Fig. 3). Here, the topology of the magnetostatic interactions effectively creates a collective ratchet-like behaviour. This implementation of a 'chiral ice', demonstrates how artificial spin ice can be tailored into a functional material, which converts energy into a well-defined rotation of the magnetization. The fact that this system behaves like a ratchet opens the possibility of using artificial spin ice in nanoscale devices such as nanomotors, actuators or sensors.

In a collaboration with the Laboratory for Multifunctional Ferroic Materials, we have modified a square ice structure so that every nanomagnet in the array is replaced by two nanomagnets. Such an artificial magneto-toroidal crystal can be poled with a magnetic tip [10] and, by altering the geometry, can give insight into how the intermagnet interactions influence the magnetic domain structure [11],