References of "Ostler, Thomas"
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See detailUltrafast double magnetization switching in GdFeCo with two picosecond-delayed femtosecond pump pulses
Atxitia, U; Ostler, Thomas ULiege

in Applied Physics Letters (2018), 113

The recently discovered thermally induced magnetization switching (TIMS) inducedby single femtosecond laser pulses in ferrimagnetic GdFeCo alloys proceeds on the picosecond time-scale. The rate at which ... [more ▼]

The recently discovered thermally induced magnetization switching (TIMS) inducedby single femtosecond laser pulses in ferrimagnetic GdFeCo alloys proceeds on the picosecond time-scale. The rate at which data can be changed for use of TIMS in technological devices is limited by the processes leading to thermal equilibrium. In the present work, we address the question of whether it is possible to further excite switching via TIMS well before thermal equilibrium between subsystems is reached. In particular, we investigate the conditions for double thermally induced magnetic switching by the application of two shortly delayed laser pulses. These conditions become relevant for potential applications as it sets both a limit to rewrite data and demonstrates the importance of spatial confinement of a heat pulse to bit size, as neighboring bits may be accidentally re-switched for spatially extended pulse spots. To demonstrate this effect, we theoretically study the switching behavior in a prototypical ferrimagnetic GdFeCo alloy as a function of composition. We use computer simulations based on thermal atomistic spin dynamics and demonstrate the possibility of inducing a second switching event well before thermal equilibrium is reached and define the conditions under which it can occur. Our theoretical findings could serve as a guidance for further understanding of TIMS as well as to act as a guide for future applications. [less ▲]

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See detailRoles of heating and helicity in ultrafast all-optical magnetization switching in TbFeCo
Lu, X; Zou, X; Hinzke, D et al

in Applied Physics Letters (2018), 113

Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual ... [more ▼]

Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual roles of the thermal and nonthermal effects after a single circularly polarized laser pulse. The evolution of this ultrafast switching occurs over different time scales, and a defined magnetization reversal time of 460 fs is shown—the fastest ever observed. Micromagnetic simulations based on a single macro-spin model, taking into account both heating and the inverse Faraday effect, are performed which reproduce HD-AOS demonstrating a linear path for magnetization reversal. Published [less ▲]

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See detailProbability Distribution of Substituted Titanium in RT12 (R = Nd and Sm; T = Fe and Co) Structures
Skelland, C; Ostler, Thomas ULiege; Westmorland, S. C. et al

in IEEE Transactions on Magnetics (2018), 12(April),

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See detailA multiscale model of the effect of Ir thickness on the static and dynamic properties of Fe/Ir/Fe films
Cuadrado, Ramón; Oroszlány, László; Szunyogh, László et al

in Scientific Reports (2018), 8

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See detailCompetition of lattice and spin excitations in the temperature dependence of spin-wave properties
Di Gennaro, Marco ULiege; Miranda, Alonso L.; Ostler, Thomas ULiege et al

in Physical Review. B, Condensed Matter (2018), 97

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See detailSite-Resolved Contributions to the Magnetic-Anisotropy Energy and Complex Spin Structure of Fe/MgO Sandwiches
Cuadrado, R.; Oroszlány, L.; Deák, A. et al

in Physical Review Applied (2018), 9(5),

Fe/MgO-based magnetic tunnel junctions are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent ... [more ▼]

Fe/MgO-based magnetic tunnel junctions are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite-size effects and temperature-dependent magnetic properties which must be carefully controlled for practical applications. In this article, we investigate the electronic, structural, and magnetic properties of MgO/Fe/MgO sandwiches using first-principles calculations and atomistic spin modeling based on a fully parametrized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin-spiral ground-state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layerwise temperature dependence of the magnetization. The different temperature dependences of the interface and bulklike layers results in an unexpected nonmonotonic temperature variation of the effective magnetic-anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure Fe/MgO interface and the role of elevated temperatures providing insight when interpreting experimental data of nanoscale magnetic tunnel junctions. © 2018 American Physical Society. [less ▲]

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See detailConditions for thermally induced all-optical switching in ferrimagnetic alloys: Modeling of TbCo
Moreno, Roberto; Ostler, Thomas ULiege; Chantrell, Roy et al

in Physical Review B (2017), 96

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See detailSubstrate Induced Strain Field in FeRh Epilayers Grown on Single Crystal MgO (001) Substrates
Barton, C. W.; Ostler, Thomas ULiege; Huskisson, D. et al

in Scientific Reports (2017), 7

Equi-atomic FeRh is highly unusual in that it undergoes a rst order meta-magnetic phase transition from an antiferromagnet to a ferromagnet above room temperature (Tr ≈ 370 K). This behavior opens new ... [more ▼]

Equi-atomic FeRh is highly unusual in that it undergoes a rst order meta-magnetic phase transition from an antiferromagnet to a ferromagnet above room temperature (Tr ≈ 370 K). This behavior opens new possibilities for creating multifunctional magnetic and spintronic devices which can utilise both thermal and applied eld energy to change state and functionalise composites. A key requirement in realising multifunctional devices is the need to understand and control the properties of FeRh in the extreme thin lm limit (tFeRh < 10 nm) where interfaces are crucial. Here we determine the properties of FeRh lms in the thickness range 2.5–10 nm grown directly on MgO substrates. Our magnetometry and structural measurements show that a perpendicular strain eld exists in these thin films which results in an increase in the phase transition temperature as thickness is reduced. Modelling using a spin dynamics approach supports the experimental observations demonstrating the critical role of the atomic layers close to the MgO interface. [less ▲]

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See detailModeling the thickness dependence of the magnetic phase transition temperature in thin FeRh film
Ostler, Thomas ULiege; Craig, Barton; Thomas, Thomson et al

in Physical Review. B, Condensed Matter (2017), 95

FeRh and its first-order phase transition can open new routes for magnetic hybrid materials and devices under the assumption that it can be exploited in ultra-thin-film structures. Motivated by ... [more ▼]

FeRh and its first-order phase transition can open new routes for magnetic hybrid materials and devices under the assumption that it can be exploited in ultra-thin-film structures. Motivated by experimental measurements showing an unexpected increase in the phase transition temperature with decreasing thickness of FeRh on top of MgO, we develop a computational model to investigate strain effects of FeRh in such magnetic structures. Our theoretical results show that the presence of theMgO interface results in a strain that changes the magnetic configuration which drives the anomalous behavior. [less ▲]

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See detailTowards Optimization of Thermally Induced Magnetization Switching
Ostler, Thomas ULiege

Conference (2016, November 04)

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See detailFirst Order Phase Transition in FePt/FeRh Bilayers
Ostler, Thomas ULiege; Barton, Craig; Hrkac, Gino et al

Conference (2016, November 01)

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See detailEffects of interactions on the relaxation processes in magnetic nanostructures
Atkinson, Lewis; Ostler, Thomas ULiege; Ondrej, Hovorka et al

in Physical Review. B, Condensed Matter and Materials Physics (2016), 94

Controlling the relaxation of magnetization in magnetic nanostructures is key to optimizing magnetic storage device performance. This relaxation is governed by both intrinsic and extrinsic relaxation ... [more ▼]

Controlling the relaxation of magnetization in magnetic nanostructures is key to optimizing magnetic storage device performance. This relaxation is governed by both intrinsic and extrinsic relaxation mechanisms and with the latter strongly dependent on the interactions between the nanostructures. In the present work we investigate laser induced magnetization dynamics in a broadband optical resonance type experiment revealing the role of interactions between nanostructures on the relaxation processes of granular magnetic structures. The results are corroborated by constructing a temperature dependent numerical micromagnetic model of magnetization dynamics based on the Landau-Lifshitz-Bloch equation. The model predicts a strong dependence of damping on the key material properties of coupled granular nanostructures in good agreement with the experimental data. We show that the intergranular, magnetostatic and exchange interactions provide a large extrinsic contribution to the damping. Finally we show that the mechanism can be attributed to an increase in spin-wave degeneracy with the ferromagnetic resonance mode as revealed by semianalytical spin-wave calculations. [less ▲]

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See detailCompetition of phonon and magnon effects in the temperature dependence of spinwave stiffness
Ostler, Thomas ULiege; Verstraete, Matthieu ULiege; Di Gennaro, Marco ULiege et al

Conference (2016, September 21)

Magnons are the elementary magnetic excitations in ordered solids. Understanding such collective excitations is important for a number of technologically relevant fields, such as, magnonics [1] or spin ... [more ▼]

Magnons are the elementary magnetic excitations in ordered solids. Understanding such collective excitations is important for a number of technologically relevant fields, such as, magnonics [1] or spin caloritronics [2]. The central interactions in spin caloritronics are the couplings of phonons with electrons and spin degrees of freedom. Furthermore, understanding the effects of temperature on the phonon and spin degrees of freedom adds a further complexity. In the present work we have developed a multiscale model of ferromagnetic materials and demonstrate the effect of temperature dependent phonon displacements on the magnons spectra. Our results show that the for Fe and Ni the effect of phonon displacements acts to reduce the Curie temperature, whereas for Permalloy the opposite is true due to an increased long-ranged exchange interaction. This increased exchange interaction results in an increasing spin-wave stiffness with increasing temperature, overcoming the usual decrease due to magnon softening. To determine the effects of increasing the phonon temperature we have developed a multiscale model whereby we begin by calculating the thermal displacement of phonons, $\sqrt{\langle u^2(T) \rangle}$, calculated using the phonopy software package [3] using electronic ground state and phonon properties determined using the ABINIT software package [4]. Then the exchange constants are determined using the SPRKKR package [5]. Finally, we use linear spin wave theory to determine the effect of the phonon temperature on the exchange alone, demonstrating an increasing frequency of the acoustic magnon branch. We take into account the thermal effects of the magnetic system through the use of the atomistic spin dynamics approach. Magnon softening due to thermal effects demonstrates a more modest increase in the exchange stiffness (over the purely phononic effect), however, an overall increase is still observed. \newline \newline [1] A. V. Chumak, V. I. Vasyuchka, A. A. Serga, and B. Hillebrands, Nature Physics, {\bf 11}, 453–461 (2015). \newline [2] G. E. W. Bauer, E. Saitoh, and B. J. van Wees, Nature Materials {\bf 11}, 391 (2012). \newline [3] Atsushi Togo and Isao Tanaka, Scr. Mater., {\bf 108}, 1-5 (2015) \newline [4] X. Gonze \textit{et al.} Computer Physics Communications {\bf 180}, 2582-2615 (2009). \newline [5] T. Huhne \textit{at al.} Physical Review B, {\bf 58}, 10236 (1998). [less ▲]

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See detailUltrafast Spin Dynamics in FeRh
Ostler, Thomas ULiege

Conference (2016, July 16)

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See detailStrain induced vortex core switching in planar magnetostrictive nanostructures
Ostler, Thomas ULiege

Conference (2016, June 26)

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See detailThermally induced magnetization switching in Gd/Fe multilayers
Xu, Chudong; Ostler, Thomas ULiege; Chantrell, Roy

in Physical Review B (2016), 93

A theoretical model of Gd/Fe multilayers is constructed using the atomistic spin dynamics formalism. By varying the thicknesses and number of layers we have shown that a strong dependence of the energy ... [more ▼]

A theoretical model of Gd/Fe multilayers is constructed using the atomistic spin dynamics formalism. By varying the thicknesses and number of layers we have shown that a strong dependence of the energy required for thermally induced magnetization switching (TIMS) is present;with a larger number of interfaces, lower energy is required. The results of the layer resolved dynamics show that the reversal process of the multilayered structures, similar to that of a GdFeCo alloy, is driven by the antiferromagnetic interaction between the transition-metal and rare-earth components. Finally, while the presence of the interface drives the reversal process, we show here that the switching process does not initiate at the surface but from the layers furthest from it, a departure from the alloy behavior which expands the classes of material types exhibiting TIMS. [less ▲]

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See detailOptimal electron, phonon, and magnetic characteristics for low energy thermally induced magnetization switching
Atxitia, U.; Ostler, Thomas ULiege; Chantrell, R. W. et al

in Applied Physics Letters (2015), 107(19), 192402

Using large-scale computer simulations, we thoroughly study the minimum energy required to ther- mally induced magnetization switching (TIMS) after the application of a femtosecond heat pulse in ... [more ▼]

Using large-scale computer simulations, we thoroughly study the minimum energy required to ther- mally induced magnetization switching (TIMS) after the application of a femtosecond heat pulse in transition metal-rare earth ferrimagnetic alloys. We find that for an energy efficient TIMS, a low ferrimagnetic net magnetization with a strong temperature dependence is the relevant factor for the magnetic system. For the lattice and electron systems, the key physics for efficient TIMS is a large electron-phonon relaxation time. Importantly, we show that as the cooling time of the heated elec- trons is increased, the minimum power required to produce TIMS can be reduced by an order of magnitude. Our results show the way to low power TIMS by appropriate engineering of magnetic heterostructures. [less ▲]

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See detailMagnon excitations in atomistic spin dynamics due to spin-lattice coupling
Ostler, Thomas ULiege; Ellis, Matthew; Chantrell, Roy

Conference (2015, October 22)

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See detailUltrafast Magnetism at Complex Interfaces
Ostler, Thomas ULiege

Conference (2015, October 21)

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