Modeling ultrafast all-optical switching in synthetic ferrimagnets

S. Gerlach, L. Oroszlany, D. Hinzke, S. Sievering, S. Wienholdt, L. Szunyogh, and U. Nowak

Phys. Rev. B 95, 224435 – Published 28 June 2017

Abstract

Based on numerical simulations, we demonstrate thermally induced magnetic switching in synthetic ferrimagnets composed of multilayers of rare-earth and transition metals. Our findings show that deterministic magnetization reversal occurs above a certain threshold temperature if the ratio of transition-metal atoms to rare-earth atoms is sufficiently large. Surprisingly, the total thickness of the multilayer system has little effect on the occurrence of switching. We further provide a simple argument to explain the temperature dependence of the reversal process.

Received 7 March 2017
Revised 24 May 2017

DOI:https://doi.org/10.1103/PhysRevB.95.224435

Physics Subject Headings (PhySH)

Ferrimagnetism Magnetization switching

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Gerlach^1,*, L. Oroszlany², D. Hinzke¹, S. Sievering¹, S. Wienholdt¹, L. Szunyogh³, and U. Nowak¹

¹Fachbereich Physik, Universität Konstanz, D-78457 Konstanz, Germany
²Department of Physics of Complex Systems, Eötvös University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary
³MTA-BME Condensed Matter Research Group and Department of Theoretical Physics, Budapest University of Technology and Economics, Budafoki út 8, HU-1111 Budapest, Hungary

^*stefan.gerlach@uni-konstanz.de

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Issue

Vol. 95, Iss. 22 — 1 June 2017

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Images

Figure 1
Temperature-dependent equilibrium magnetization for a bilayer with three monolayers of Fe and two monolayers of Gd. The magnetization of the element with the stronger intralayer exchange interaction, in this case Fe, closely follows the shape of a ferromagnet with its Curie temperature $T_{C}^{Fe}$ close to the Curie temperature of the coupled system. The element with the weaker intralayer exchange interaction, in this case Gd, exhibits magnetic ordering above its own bulk Curie temperature, $T_{C}^{Gd}$ , due to the interaction with the other sublattice. The chosen ratio of layer thicknesses leads to a larger magnetic moment of the Gd layer at low temperatures and, consequently, to a magnetic compensation point at a temperature $T_{comp}$ which is slightly above the bulk Curie temperature of the Gd.
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Figure 2
The three possible time-relaxation scenarios for the sublattice magnetizations of a RE-TM layered system. The chosen values for T and Gd magnetization are indicated in Fig. 3. Top: Magnetization switches with respect to the initial configuration. Center: Both sublattice magnetizations change their signs twice, ending up back in the initial state. Bottom: No switching, i.e., the rare-earth layer magnetization does not change sign. Also shown are the transverse components (dotted and dashed lines). For the case of backswitching and no switching, these components are of the order of the magnitude of the magnetization.
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Figure 3
Switching in a 3:2 layer sample after 40 ps. Color coding: Magnetization switches (cyan-blue), magnetization does not switch and returns to the initial state (red), and both magnetizations change their sign twice, ending up in the initial state (backswitching) (yellow-orange). The compensation temperature $T_{comp}$ is indicated by a solid black line; the Curie temperature $T_{C}^{Gd}$ of the isolated Gd layer is indicated by a dashed black line. The three points indicate the chosen values for the scenarios in Fig. 2.
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Figure 4
Strong dissipation ( $α あるふぁ = 1$ ) in a 3:2 layer sample, implying no conservation of angular momentum. The relaxation does not lead to a TFMLS either at low temperature where the Gd magnetization increases or at high temperature where it decreases. Top: $T < T_{C}^{Gd}$ . Bottom: $T_{C}^{Gd} < T < T_{C}^{Fe}$ . Also shown are the transverse components (dotted and dashed lines) which remain small for both cases.
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Figure 5
Layer-resolved magnetization dynamics for switching in a 3:2 layer sample. The Gd layer is demagnetized to 50% of its zero-temperature value. The bold lines correspond to the layers at the Gd-Fe interface and the dashed lines show the average value for Gd or Fe. The temperature is $T = 0.5 J_{Fe} / k_{B}$ .
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