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Pushing magnetic textures with current - João SAMPAIO

LPS - Université Paris-Sud


LPS - Bât 510 - 2ème étage, aile sud

Ferromagnetic materials can present different textures of spins at a nanometric scale, such as domain walls (DWs) or the recently discovered magnetic skyrmions. The complex dynamics of these objects underpin several proposed new devices for storing and processing information with high density and low power consumption, such as the DW and Skyrmion Racetracks [1,2], or DW logic circuits [3]. Moving magnetic textures directly with electrical currents is essential to these ideas, and was first demonstrated using the Spin Transfer Torque (STT) [4]. STT occurs when a spin polarized current is injected through a magnetic texture, relaxes its spin and transfers its angular momentum to the local magnetisation, setting the texture in motion. Its magnitude in typical ferromagnets, however, is too small for the envisaged applications. This led the research to new systems, often with unexpected results, of which three examples will be presented.

One path to improve STT is to reduce the magnetisation while at the same time conserving the current spin polarisation. While this is impossible in normal ferromagnets, it is not in ferrimagnetic alloys of the type Rare Earth / Transition Metal. Their two sub-lattices with opposing moments allow the tuning of the net magnetization with alloy composition or temperature. Studying DW dynamics in TbFe tracks [Fig 1], we show how the different effects of current on the material (STT, Joule heating, Oersted…) can be disentangled to determine the STT efficiency versus net magnetisation.

Bi-layers of ferromagnetic and heavy metals showed a surprisingly high STT efficiency when they were first studied in 2010 [5]. The two responsible effects can be seen in our study of current-induced reversal of Pt/[Co/Ni] [6] : the changed DW structure induced by interfacial Dzyaloshinskii-Moriya interaction (DMI), and the generation of pure spin currents in the Pt layer through the Spin Hall effect.

Another promising idea is to use magnetic skyrmions – nanometric chiral spin textures whose motion is predicted to be less affected by defects than DWs [2]. Our recent studies show a new way to stabilise skyrmions with DMI and dipolar interactions, and to nucleate and propagate them using currents [7].

[1] SSP Parkin et al. “Domain wall racetrack memory”, Science 320, 190 (2008)

[2] J Sampaio et al., “Nucleation, stability and current-induced motion of isolated magnetic skyrmions in nanostructures”, Nat. Nano. 8, 839 (2013)

[3] D.A. Allwood et al., “Magnetic domain wall logic”, Science 309, 1688 (2005)

[4] P.P. Freitas & L. Berger, “Observaton of s-d exchange force between domain walls and electric current in very thin Permalloy films”, J. Appl. Phys 57, 1266 (1985)

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