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Ultrasonic toggling of magnetization

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Switching magnetization rapidly, locally, and under weak or zero magnetic field are some of the major stakes for improving the storage and manipulation of magnetically coded information. One approach is to use inverse magnetrostriction, i.e the modification of the direction of the magnetization induced by a strain. This process is very efficient when the strain is produced dynamically by an acoustic wave. A group of researchers from the Institut des Nanosciences de Paris has shown that acoustic waves are indeed capable of toggling the magnetization between two positions of equivalent energy, under zero field, and several millimeters away from where the wave was generated.

In a thin film evidencing a preferred magnetic axis, the two opposite magnetization directions +M and –M are of equal energy, making it an ideal system to code information as “0s” and “1s”, in a binary fashion. It is possible to go from one to the other precessionnally : under a given stimulus, the magnetization first gradually rotates out its initial position until it passes the energy saddle point and over to the other equilibrium valley and finally rotates down to its final equilibrium position in an orbit of decreasing radius. Generally, it is a static magnetic field that lowers the energy barrier and a radio-frequency field or DC electrical current that drive the precession, requiring a local excitation. In a magnetostrictive material, an effective radio-frequency field triggering magnetization precession may be generated by a surface acoustic wave (Rayleigh wave), some sort of micronic earthquake at a few hundreds of MHz. The full and remote acoustic reversal of magnetization had thus already been evidenced at INSP in thin 50nm layers of (Ga,Mn)As and (Ga,Mn)(As,P) [2,3], a model system working at low temperature (<130K, -143°C).

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Figure 1
Interdigitated transducers supplied with an rf voltage generate a surface acoustic wave by piezo-electric effect. At low acoustic amplitude, the magnetization precesses in a cone. At larger amplitude, it oscillates between two opposite states, +M and –M. Using magneto-optical effects to follow the local magnetic state of regions of a few microns square, we evidenced the local and reversible toggling of under more than 20 acoustic pulses.

Up to now, a static magnetic field remained necessary for this switching, in order to adjust the eigenfrequency of the magnetization to that of the acoustic wave (500MHz-1GHz). In that case, minute misorientations of the field with the sample axes strongly favored one of the orientations +M or –M, prohibiting the toggling between these 2 states and thus any possible application of this effect [4]. An option that was recently proposed and demonstrated at INSP is to work at temperatures such that the zero-field magnetic eigenfrequency is naturally close to that of the acoustic wave, while remaining far enough from the Curie temperature, so that reversal doesn’t simply occur by thermal activation. We thus observed localized magnetic reversal at 100K induced by 250ns long acoustic pulses, as well as the toggling between opposite +M/-M states by over 20 successive acoustic pulses [1]. While these results remain to be confirmed on magnetic materials functioning at room temperature, they bode well for the engineering of novel magnetic memories based on remote and field-less acoustic switching of magnetic bits.

References
[1] « Field-Free Magnetization Switching by an Acoustic Wave ». I.S. Camara, J.-Y. Duquesne, A. Lemaître, C. Gourdon, L. Thevenard , Physical Review Applied 11 014045 (2019)
https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.11.014045

[2] « The 2019 Surface Acoustic Waves Roadmap ». Delsing, Per, Cleland, Andrew N., Schuetz, M. J. A., Knörzer, J., Giedke, G., Cirac, J. I., Srinivasan, Kartik, Wu, Marcelo, Balram, Krishna Coimbatore, Bäuerle, Christopher, Meunier, Tristan, Ford, Christopher J. B., Santos, Paulo V., Cerda-Méndez, Edgar, Wang, Hailin, Krenner, Hubert J., Nysten, Emeline D. S., Weiß, Matthias, Nash, G. R., Thevenard, L., Gourdon, C., Rovillain, P., Marangolo, M., Duquesne, J.-Y., Fischerauer, Gerhard, Reiner, Alexander, Paschke, Ben, Denysenko, Dmytro, Volkmer, Dirk, Wixforth, Achim, Bruus, Henrik, Wiklund, Martin, Reboud, Julien, Cooper, Jonathan M., Fu, Yong Qing, Brugger, Manuel S., Rehfeldt, Florian and Westerhausen, Christoph (2019) The 2019 Surface Acoustic Waves Roadmap. Journal of Physics D : Applied Physics. ISSN 0022-3727

[3] « Precessional magnetization switching by a surface acoustic wave ». L. Thevenard, I. S. Camara, J-Y. Prieur, P. Rovillain, A. Lemaître, C. Gourdon, and J.-Y. Duquesne, Physical Review B 93 134430 (2016)

[4] « Resonant magneto-acoustic switching : influence of Rayleigh wave frequency and wavevector ». P. Kuszewski, I.S. Camara, N. Biarrotte, L. Becerra, J. von Bardeleben, W Savero Torres, A. Lemaître, C. Gourdon, J.-Y. Duquesne, L. Thevenard, Journal of Physics : Condensed Matter 30 244003 (2018)

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