Nanomagnetismus a spintronika

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    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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    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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    Research Update: Focused ion beam direct writing of magnetic patterns with controlled structural and magnetic properties
    (AIP Publishing, 2018-06-01) Urbánek, Michal; Flajšman, Lukáš; Křižáková, Viola; Gloss, Jonáš; Horký, Michal; Schmid, Michael; Varga, Peter
    Focused ion beam irradiation of metastable Fe78Ni22 thin films grown on Cu(100) substrates is used to create ferromagnetic, body-centered cubic patterns embedded into paramagnetic, face-centered-cubic surrounding. The structural and magnetic phase transformation can be controlled by varying parameters of the transforming gallium ion beam. The focused ion beam parameters such as the ion dose, number of scans, and scanning direction can be used not only to control a degree of transformation but also to change the otherwise four-fold in-plane magnetic anisotropy into the uniaxial anisotropy along a specific crystallographic direction. This change is associated with a preferred growth of specific crystallographic domains. The possibility to create magnetic patterns with continuous magnetization transitions and at the same time to create patterns with periodical changes in magnetic anisotropy makes this system an ideal candidate for rapid prototyping of a large variety of nanostructured samples. Namely, spin-wave waveguides and magnonic crystals can be easily combined into complex devices in a single fabrication step
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    Magnetization reversal and confinement effects across the metamagnetic phase transition in mesoscale FeRh structures
    (IOP PUBLISHING LTD, 2018-03-14) Arregi Uribeetxebarria, Jon Ander; Horký, Michal; Fabianová, Kateřina; Tolley, Robert; Fullerton, E.E.; Uhlíř, Vojtěch
    The effects of mesoscale confinement on the metamagnetic behavior of lithographically patterned FeRh structures are investigated via Kerr microscopy. Combining the temperatureand field-dependent magnetization reversal of individual sub-micron FeRh structures provides specific phase-transition characteristics of single mesoscale objects. Relaxation of the epitaxial strain caused by patterning lowers the metamagnetic phase transition temperature by more than 15 K upon confining FeRh films below 500 nm in one lateral dimension. We also observe that the phase transition becomes highly asymmetric when comparing the cooling and heating cycles for 300 nm-wide FeRh structures. The investigation of FeRh under lateral confinement provides an interesting platform to explore emergent metamagnetic phenomena arising from the interplay of the structural, magnetic and electronic degrees of freedom at the mesoscopic length scale.
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    The growth of metastable fcc Fe78Ni22 thin films on H-Si(100) substrates suitable for focused ion beam direct magnetic patterning
    (Elsevier, 2019-03-01) Gloss, Jonáš; Horký, Michal; Křižáková, Viola; Flajšman, Lukáš; Schmid, Michael; Urbánek, Michal; Varga, Peter
    We have studied the growth of metastable face-centered-cubic, non-magnetic Fe78Ni22 thin films on silicon substrates. These films undergo a magnetic (paramagnetic to ferromagnetic) and structural (fcc to bcc) phase transformation upon ion beam irradiation and thus can serve as a material for direct writing of magnetic nanostructures by the focused ion beam. So far, these films were prepared only on single-crystal Cu(1 0 0) substrates. We show that transformable Fe78Ni22 thin films can also be prepared on a hydrogen-terminated Si(1 0 0) with a 130-nm-thick Cu(1 0 0) buffer layer. The H-Si(1 0 0) substrates can be prepared by hydrofluoric acid etching or by annealing at 1200 degrees C followed by adsorption of atomic hydrogen. The Cu(1 0 0) buffer layer and Fe78Ni22 fcc metastable thin film were deposited by thermal evaporation in ultra-high vacuum. The films were consequently transformed in-situ by 4 keV Ar+ ion irradiation and ex-situ by a 30 keV Ga+ focused ion beam, and their magnetic properties were studied by magneto-optical Kerr effect magnetometry. The substitution of expensive copper single crystal substrate by standard silicon wafers dramatically expands application possibilities of metastable paramagnetic thin films for focused-ion-beam direct magnetic patterning.