Nanomagnetismus a spintronika

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Control of magnetic vortex states in FeGa microdisks: Experiments and micromagnetics
(Vietnam National University, 2023-06-14) Pradhan, Gajanan; Celegato, Federica; Madri, Alessandro; Coisson, Marco; Barrera, Gabriele; Mikuličková, Lenka; Arregi Uribeetxebarria, Jon Ander; Čelko, Ladislav; Uhlíř, Vojtěch; Rizzi, Paola; Tiberto, Paola
Magnetic vortices have been an interesting element in the past decades due to their flux-closure domain structures which can be stabilized at ground states in soft ferromagnetic microstructures. In this work, vortex states are shown to be nucleated and stabilized in Fe80Ga20 and Fe70Ga30 disks, which can be upcoming candidate for applications in strain-induced electric field control of magnetic states owing the high magnetostriction of the alloy. The magnetization reversal in the disks occurs by the formation vortex, double vortex or S-domain state. Micromagnetic simulations have been performed using the FeGa material parameters and the simulated magnetic states are in good agreement with the experi-mental results. The studies performed here can be essential for the use of FeGa alloy in low-power electronics.& COPY; 2023 Vietnam National University, Hanoi. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Preserving Metamagnetism in Self-Assembled FeRh Nanomagnets
(AMER CHEMICAL SOC, 2023-02-15) Motyčková, Lucie; Arregi Uribeetxebarria, Jon Ander; Staňo, Michal; Průša, Stanislav; Částková, Klára; Uhlíř, Vojtěch
Preparing and exploiting phase-change materials in the nanoscale form is an ongoing challenge for advanced material research. A common lasting obstacle is preserving the desired functionality present in the bulk form. Here, we present self-assembly routes of metamagnetic FeRh nanoislands with tunable sizes and shapes. While the phase transition between antiferro-magnetic and ferromagnetic orders is largely suppressed in nanoislands formed on oxide substrates via thermodynamic nucleation, we find that nanomagnet arrays formed through solid-state dewetting keep their metamagnetic character. This behavior is strongly dependent on the resulting crystal faceting of the nanoislands, which is characteristic of each assembly route. Comparing the calculated surface energies for each magnetic phase of the nanoislands reveals that metamagnetism can be suppressed or allowed by specific geometrical configurations of the facets. Furthermore, we find that spatial confinement leads to very pronounced supercooling and the absence of phase separation in the nanoislands. Finally, the supported nanomagnets are chemically etched away from the substrates to inspect the phase transition properties of self-standing nanoparticles. We demonstrate that solid-state dewetting is a feasible and scalable way to obtain supported and free-standing FeRh nanomagnets with preserved metamagnetism.
Magnetic-field-controlled growth of magnetoelastic phase domains in FeRh
(IOP Publishing, 2023-06-01) Arregi Uribeetxebarria, Jon Ander; Ringe, Friederike; Hajduček, Jan; Gomonay, Olena; Molnár, Tomáš; Jaskowiec, Jiří; Uhlíř, Vojtěch
Magnetic phase transition materials are relevant building blocks for developing green technologies such as magnetocaloric devices for solid-state refrigeration. Their integration into applications requires a good understanding and controllability of their properties at the micro- and nanoscale. Here, we present an optical microscopy study of the phase domains in FeRh across its antiferromagnetic-ferromagnetic phase transition. By tracking the phase-dependent optical reflectivity, we establish that phase domains have typical sizes of a few microns for relatively thick epitaxial films (200 nm), thus enabling visualization of domain nucleation, growth, and percolation processes in great detail. Phase domain growth preferentially occurs along the principal crystallographic axes of FeRh, which is a consequence of the elastic adaptation to both the substrate-induced stress and laterally heterogeneous strain distributions arising from the different unit cell volumes of the two coexisting phases. Furthermore, we demonstrate a magnetic-field-controlled directional growth of phase domains during both heating and cooling, which is predominantly linked to the local effect of magnetic dipolar fields created by the alignment of magnetic moments in the emerging (disappearing) FM phase fraction during heating (cooling). These findings highlight the importance of the magnetoelastic character of phase domains for enabling the local control of micro- and nanoscale phase separation patterns using magnetic fields or elastic stresses.
Subpicosecond metamagnetic phase transition in FeRh driven by non-equilibrium electron dynamics
(Nature Portfolio, 2021-08-24) Pressacco, Frederico; Sangalli, Davide; Uhlíř, Vojtěch; Kutnyakhov, Dmytro; Arregi Uribeetxebarria, Jon Ander; Agustsson, Steinn Ymir; Brenner, Günter; Redlin, Harald; Heber, Michael; Vasilyev, Dmitry; Demsar, Jure; Schönhense, Gerd; Gatti, Matteo; Marini, Andrea; Wurth, Wilfried; Sirotti, Fausto
In FeRh, it is possible to optically drive a phase transition between ferromagnetic (FM) and anti-ferromagnetic (AFM) ordering. Here, using a combination of photoelectron spectroscopy and ab-initio calculations, the authors demonstrate the existence of a transient intermediate phase, explaining the delayed appearance of the FM phase. Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350 +/- 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer.
Spin stress contribution to the lattice dynamics of FePt
(AAAS, 2020-06-01) von Reppert, Alexander; Willig, Lisa; Pudell, Jan Etienne; Zeuschner, Steffen Peer; Sellge, Gabriel; Ganss, Fabian; Hellwig, Olaf; Arregi Uribeetxebarria, Jon Ander; Uhlíř, Vojtěch; Crut, Aurelien; Bargheer, Matias
Invar-behavior occurring in many magnetic materials has long been of interest to materials science. Here, we show not only invar behavior of a continuous film of FePt but also even negative thermal expansion of FePt nanograins upon equilibrium heating. Yet, both samples exhibit pronounced transient expansion upon laser heating in femtosecond x-ray diffraction experiments. We show that the granular microstructure is essential to support the contractive out-of-plane stresses originating from in-plane expansion via the Poisson effect that add to the uniaxial contractive stress driven by spin disorder. We prove the spin contribution by saturating the magnetic excitations with a first laser pulse and then detecting the purely expansive response to a second pulse. The contractive spin stress is reestablished on the same 100-ps time scale that we observe for the recovery of the ferromagnetic order. Finite-element modeling of the mechanical response of FePt nanosystems confirms the morphology dependence of the dynamics.

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