Nanomagnetismus a spintronika

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Enhanced magnetic field concentration using windmill-like ferromagnets
(AIP Publishing, 2024-02-01) Bort-Soldevila, Natanael; Cunill-Subiranas, Jaume; Barrera, Aleix; Del-Valle, Nuria; Silhanek, Alejandro V.; Uhlíř, Vojtěch; Bending, Simon; Palau, Anna; Navau, Carles
Magnetic sensors are used in many technologies and industries, such as medicine, telecommunications, robotics, the Internet of Things, etc. The sensitivity of these magnetic sensors is a key aspect, as it determines their precision. In this article, we investigate how a thin windmill-like ferromagnetic system can hugely concentrate a magnetic field at its core. A magnetic sensor combined with such a device enhances its sensitivity by a large factor. We describe the different effects that provide this enhancement: the thickness of the device and its unique windmill-like geometry. An expression for the magnetic field in its core is introduced and verified using finite-element calculations. The results show that a high magnetic field concentration is achieved for a low thickness-diameter ratio of the device. Proof-of-concept experiments further demonstrate the significant concentration of the magnetic field when the thickness-diameter ratio is low, reaching levels up to 150 times stronger than the applied field. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Zero-field spin wave turns
(AIP Publishing, 2024-03-11) Klíma, Jan; Wojewoda, Ondřej; Roučka, Václav; Molnár, Tomáš; Holobrádek, Jakub; Urbánek, Michal
Spin-wave computing, a potential successor to CMOS-based technologies, relies on the efficient manipulation of spin waves for information processing. While basic logic devices such as magnon transistors, gates, and adders have been experimentally demonstrated, the challenge for complex magnonic circuits lies in steering spin waves through sharp turns. In this study, we demonstrate with micromagnetic simulations and Brillouin light scattering microscopy experiments, that dipolar spin waves can propagate through 90 degrees turns without distortion. The key lies in carefully designed in-plane magnetization landscapes, addressing challenges posed by anisotropic dispersion. The experimental realization of the required magnetization landscape is enabled by spatial manipulation of the uniaxial anisotropy using corrugated magnonic waveguides. The findings presented in this work should be considered in any magnonic circuit design dealing with anisotropic dispersion and spin wave turns.
Accelerating the Laser-Induced Phase Transition in Nanostructured FeRh via Plasmonic Absorption
(WILEY-V C H VERLAG GMBH, 2024-08-01) Mattern, Maximilian; Pudell, Jan Etienne; Arregi Uribeetxebarria, Jon Ander; Zlámal, Jakub; Kalousek, Radek; Uhlíř, Vojtěch; Rössle, Matti; Bargheer, Matias
By ultrafast x-ray diffraction (UXRD), it is shown that the laser-induced magnetostructural phase transition in FeRh nanoislands proceeds faster and more complete than in continuous films. An intrinsic 8 ps timescale is observed for the nucleation of ferromagnetic (FM) domains in the optically excited fraction of both types of samples. For the continuous film, the substrate-near regions are not directly exposed to light and are only slowly transformed to the FM state after heating above the transition temperature via near-equilibrium heat transport. Numerical modeling of the absorption in the investigated nanoislands reveals a strong plasmonic contribution near the FeRh/MgO interface. The larger absorption and the optical excitation of the electrons in nearly the entire volume of the nanoislands enables a rapid phase transition throughout the entire volume at the intrinsic nucleation timescale. Nanostructuring FeRh thin films by solid state dewetting make the laser-induced antiferromagnetic to ferromagnetic phase transition more efficient and speed the switching up to the intrinsic timescale. Ultrafast x-ray diffraction experiments directly measure the structural order parameter averaged over the entire film. Finite element modeling reveals the enhanced plasmonic light absorption near the substrate as the crucial factor. image
Dimensional crossover of microscopic magnetic metasurfaces for magnetic field amplification
(AIP Publishing, 2024-07-01) Lejeune, Nicolas; Fourneau, Emile; Barrera, Aleix; Morris, Oliver; Leonard, Oscar; Arregi Uribeetxebarria, Jon Ander; Navau, Carles; Uhlíř, Vojtěch; Bending, Simon; Palau, Anna; Silhanek, Alejandro Vladimiro
Transformation optics applied to low frequency magnetic systems have been recently implemented to design magnetic field concentrators and cloaks with superior performance. Although this achievement has been amply demonstrated theoretically and experimentally in bulk 3D macrostructures, the performance of these devices at low dimensions remains an open question. In this work, we numerically investigate the non-monotonic evolution of the gain of a magnetic metamaterial field concentrator as the axial dimension is progressively shrunk. In particular, we show that in planar structures, the role played by the diamagnetic components becomes negligible, whereas the paramagnetic elements increase their magnetic field channeling efficiency. This is further demonstrated experimentally by tracking the gain of superconductor-ferromagnet concentrators through the superconducting transition. Interestingly, for thicknesses where the diamagnetic petals play an important role in the concentration gain, they also help to reduce the stray field of the concentrator, thus limiting the perturbation of the external field (invisibility). Our findings establish a roadmap and set clear geometrical limits for designing low dimensional magnetic field concentrators.
Speed limits of the laser-induced phase transition in FeRh
(AIP Publishing, 2024-05-01) Mattern, Maximilian; Jarecki, Jasmin; Arregi Uribeetxebarria, Jon Ander; Uhlíř, Vojtěch; Rössle, Matti; Bargheer, Matias
We use ultrafast x-ray diffraction and the polar time-resolved magneto-optical Kerr effect to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic timescale for the light-induced nucleation of ferromagnetic (FM) domains in the antiferromagnetic material of 8ps, which is substantially longer than the time it takes for strain waves to traverse the film. For the inhomogeneously excited thicker film, only the optically excited near-surface part transforms within 8ps. For strong excitations, we observe an additional slow rise of the FM phase, which we experimentally relate to a growth of the FM phase into the depth of the layer by comparing the transient magnetization in frontside and backside excitation geometry. In the lower lying parts of the film, which are only excited via near-equilibrium heat transport, the FM phase emerges significantly slower than 8ps after heating above the transition temperature.

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