arXiv subjects

Björn Erik Skovdal

Publications and source records attributed to Björn Erik Skovdal.

5 recordsLinked to original sources

arXiv2022

Emergent anisotropy in magnetic metamaterials

We demonstrate directional dependence of the self-modification of internal mesospin textures in magnetic metamaterials, arising from the coupling of the atomic- and mesoscopic length-scales. Dressing the mesospins in different directions enables the quantification of the degree of texture in the internal magnetization and its impact on the interaction energy of the mesospins. The emerging anisotropy is manifested in a directional dependence of the remanent magnetization with temperature.

cond-mat.mes-hall↗

arXiv2022

Thermal excitations within and among mesospins in artificial spin ice

We provide experimental and numerical evidence for thermal excitations within and among magnetic mesospins, forming artificial spin ice structures. At low temperatures, a decrease in magnetization and increase in susceptibility is observed with increasing temperature, interpreted as an onset of thermal fluctuations of the magnetic texture within the mesospins. At elevated temperatures a pronounced susceptibility peak is observed, related to thermally induced flipping of the mesospins and a collapse of the remanent state. The fluctuations, while occurring at distinct length- and energy-scales, are shown to be tunable by the interaction strength of the mesospins.

cond-mat.mes-hall↗

arXiv2022

Emergent tri-criticality in magnetic metamaterials

Metallic discs engineered on the 100 nm scale have an internal magnetic texture which varies from a fully magnetized state to a vortex state with zero moment. The interplay between this internal structure and the inter-disc interactions is studied in magnetic metamaterials made of square arrays of the magnetic discs. The texture is modeled by a mesospin of varying length with O(2) symmetry and the inter-disc interaction by a nearest neighbour coupling between mesospins. The thermodynamic properties of the model are studied numerically and an ordering transition is found which varies from Kosterlitz-Thouless to first order via an apparent tri-critical point. The effective critical exponent characterising the finite size magnetization evolves from the value for the 2D-XY model to less than half this value at the tri-critical point. The consequences for future experiments both in and out of equilibrium are discussed.

cond-mat.mes-hall↗

arXiv2021

Temperature induced collapse of spin dimensionality in magnetic metamaterials

Spin and spatial dimensionalities are universal concepts, essential for describing both phase transitions and dynamics in magnetic materials. Lately, these ideas have been adopted to describe magnetic properties of metamaterials, as well as to replicate and explore ensembles of mesospins belonging to different universality classes. Here, we take the next step by investigating magnetic metamaterials not conforming to the conventional framework of continuous phase transitions. Instead of a continuous decrease in the moment with temperature, discrete steps are possible, resulting in a binary transition in the interactions of the elements. The transition is enabled by nucleation and annihilation of vortex cores, shifting topological charges between the interior and the edges of the elements. Consequently, the mesospins can be viewed as shifting their spin dimensionality, from 2 (XY-like) to 0, (vortices) at the transition. The results provide insight into how dynamics at different length-scales couple, which can lead to thermally driven topological transitions in magnetic metamaterials.

cond-mat.mes-hall↗

arXiv2020

Precise control of $J_\mathrm{eff}=1/2$ magnetic properties in Sr$_2$IrO$_4$ epitaxial thin films by variation of strain and thin film thickness

We report on a comprehensive investigation of the effects of strain and film thickness on the structural and magnetic properties of epitaxial thin films of the prototypal $J_\mathrm{eff}=1/2$ compound Sr$_2$IrO$_4$ by advanced X-ray scattering. We find that the Sr$_2$IrO$_4$ thin films can be grown fully strained up to a thickness of 108 nm. By using X-ray resonant scattering, we show that the out-of-plane magnetic correlation length is strongly dependent on the thin film thickness, but independent of the strain state of the thin films. This can be used as a finely tuned dial to adjust the out-of-plane magnetic correlation length and transform the magnetic anisotropy from two-dimensional (2D) to three-dimensional (3D) behavior by incrementing film thickness. These results provide a clearer picture for the systematic control of the magnetic degrees of freedom in epitaxial thin films of Sr$_2$IrO$_4$ and bring to light the potential for a rich playground to explore the physics of $5d$-transition metal compounds.

cond-mat.str-el↗