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Ultrafast spin dynamics and transport in magnetic metallic heterostructures

Abstract : The control of magnetization, and thus spin, at the shortest timescale, is a fundamental subject for the development of faster data storage devices. The capability to encode information with femtosecond laser pulses on magnetic metals such as GdFeCo or MnRuGa within a few picoseconds was a significant step towards the realization of such a technology. However, the reversal of magnetization observed in these materials upon a single laser pulse irradiation, called All Optical Helicity Independent Switching (AO-HIS), is still limited to a small class of ferrimagnetic materials and its physical mechanism is not completely understood.In this work, we study AO-HIS in magnetic thin films composed of a single or two GdFeCo layers with different alloy compositions. We show that these layers generate spin currents that can affect the AO-HIS of these materials. In particular, we can use such spin currents to reverse the magnetization of various ferromagnetic multilayers, with a single femtosecond laser pulse, which would otherwise only demagnetize and never switch. Playing with the GdFeCo alloy concentration and the ferromagnetic multilayer Curie temperature, we can tune the energy required to observe single shot reversal of the ferromagnet. In addition, we show that neither AO-HIS of the GdFeCo layer is actually required nor direct light illumination of the ferromagnetic multilayer. It is then possible to reverse the magnetization of ferromagnets using only ultrashort heat and spin currents which are generated by the partial ultrafast demagnetization of GdFeCo and transported via a thick metallic copper spacer. These experimental results were successfully understood using semiclassical transport equations for electrons, phonons and quantum spins based on exchange of angular momentum between localized and itinerant spins.Finally, we were able to measure the dynamics of the ferromagnetic multilayer magnetization reversal which is shown to happen in less than a picosecond, being the fastest magnetization reversal ever observed. The action of the external spin current is shown to have an ultrafast cooling effect on the spin which is visible at the sub-picosecond timescale and which can enhance the transient magnetization by up to thirty percent. These results are also understood using our model of heat and angular momentum transport.
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Submitted on : Monday, February 7, 2022 - 5:23:55 PM
Last modification on : Friday, February 18, 2022 - 2:02:01 PM
Long-term archiving on: : Sunday, May 8, 2022 - 7:16:01 PM

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  • HAL Id : tel-03560783, version 1

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Quentin Remy. Ultrafast spin dynamics and transport in magnetic metallic heterostructures. Physics [physics]. Université de Lorraine, 2021. English. ⟨NNT : 2021LORR0191⟩. ⟨tel-03560783⟩

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