Jungiusstr. 11, Hörsaal AP
SFB 668 - Kolloquium
Prof. P. Vavassori (University of Ferrara (Italy) and CIC Nanogune, San Sebastian -Basque-):
Dipolar-coupled nanomagnet arrays
Interacting arrays of nanoscale magnets allow the creation of artificial metamaterials where the properties are designed in and arise due to the engineering of mesoscale properties, like material, size, shape, and placement of the nanomagnets. For instance, the so-called artificial spin-ice (ASI) structures rely on specific arrangements of Ising-like single domain nanomagnets that are designed to create model systems to study fundamental aspects of competing interactions and frustration phenomena . We present a novel methodology [2,3] to explore experimentally the formation of thermally-induced long range ground-state ordering in ASI systems. This methodology is based on the thermalisation of a ASI array of nanoislands made of a NiFe alloy, which composition has been modified to achieve a Curie temperature TC about 100K lower than that of conventionally used Permalloy. Due to the lower TC, the ASI can be heated-up (ASI melting) and cooled down (ASI freezing) across the nanostructures blocking temperature (TB≈TC) without damaging the sample. We discuss the resulting metastable magnetization configurations that have been analyzed by means of magnetic force microscopy. The thermalisation protocol can be repeated as many times as desired on the same sample, and the heating/cooling parameters can be varied at will. Thereby, this approach opens the pathway to the systematic experimental study of the thermally induced frozen states in ASI systems, which has been subject of many recent theoretical studies.
We eventually investigated a slightly modified ASI geometry in which the nanoislands are arranged in chiral square units forming a checkerboard pattern for the purpose of studying the effects of non-uniformity and eventual asymmetry of dipolar interactions. We demonstrate experimentally and confirmed by theoretical modeling that the interplay between magnetization dynamics and localized magnetic field sources can be used to actively induce and finely control the magnetization states and reversal paths of nano-magnets beyond what achievable via lateral confinement and anisotropy engineering. For example, subjecting a nanomagnet to a properly designed asymmetric and inhomogeneous dipolar near-field leads to a breakdown of the generally anticipated single-domain behavior, even for surprisingly small sizes of the nano-element . In more detail, we show that by proper design the asymmetry character of the dipolar near-field we succeeded to induce a single vortex state with predetermined chirality and stable in zero field, even in nanomagnets with size and shape for which the magnetic vortex is an extremely unfavorable configuration as compared to the single domain one.
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