Electrical trapping of a magnetic domain wall

In Spintronic applications, information is written and read on magnetic metals using electrical currents. In contrast, the electric field which is used to control the state of transistors, is not exploited yet in spintronics applications despite the lower power consumption expected compared to current based devices. We have made an important step toward this direction with the first demonstration of a reversible electrical pinning of magnetic domain walls. This demonstration opens the way for controlled domain wall propagation which is essential in future logic or memory applications.

Electric field control of magnetism has been overlooked for a long time in conventionnal ferromagnetic metals. Indeed, electric-field cannot penetrate in a metal as it is screened at the surface by charges which accumulates on the few topmost atomic layers. However, if you consider an ultrathin film of 1nm or less, this charge accumulation at the surface can influence the properties of the whole film. At Institut Néel, we have elaborated in collaboration with SPINTEC an ultrathin 0.6 nm cobalt film sandwitched between a metalic nonmagnetic platinium and a dielectric alumina layer. This magnetic material possess a strong anisotropy i.e. it is more energetically favorable for the magnetisation to align out of plane than in plane. We have carried out the following experiement : the magnetisation of the sample is visualized using a microscope with polarized light (Kerr microscopy). We obtain grey contrast images where the bright regions corresond to magnetization down and dark regions to magnetisation up (Figure1.b). We apply the electric field using a patterned electrode made of a tranparent material (ITO) which alows us to vizualize the magnetization even below the electrode (Figure1.a).
Figure : a. Schématic representation of the sample. b,c. Kerr microscopy image of the sample. When the electrid field is "on" the magnetization is reversing from down (blue) to up (red). The region below the electrode is left down (blue) as the electric- field is blocking the domain wall propagation. c. When the electric field is turned off the domain all is released and the whole magnetization is reversed.
The magnetization reversal is measured by applying a magnetic field oposite to the magnetization and recording images at different times. The applied magnetic field is reversing the magnetisation direction in the following way : a small region with oposite magnetization is first created near a defect (nucleation) and this region is later extending. Our sample can therefore present a region with up and another with down magnetisation.The frontier between those two regions is called a magnetic domain wall and inside, the magnetisation is rotating progressively to minimize the energy (figure 1.a). We have demonstrated that the domain wall is pinned when it reaches the electrode side (Figure 1.a), allowing to leave the magnetization unreversed bellow the electrode while the rest of the sample is reversed (Figure 1.b). When the electrode is turned off the domain wall is released and the magnetization reversed everywhere(Figure 1.c). The origin of this domain wall pinning lies in an energy step which is present at the edge of the electrode. This energy difference comes from the anisotropy difference induced by the electric field. This controlable pinning of a domain wall can be of key importance for applications in magnetic logic and domain wall based memory.

Contributors

  • Institut Néel (France) : A. Bernand-Mantel, L. Herrera-Diez, L. Ranno, S. Pizzini, J. Vogel, D. Givord
  • SPINTEC/CEA(France) : O. Boulle, I. M. Miron, S. Auffret, G. Gaudin

Our related publication :

  • Electric-field control of domain wall nucleation and pinning in a metallic ferromagnet A. Bernand-Mantel, L. Herrera-Diez, L. Ranno, S. Pizinni, J. Vogel, D. Givord, O. Boulle, I. M. Miron, S. Auffret, G. Gaudin [apl.aip.org/resource/1/applab/v102/i12/p122406_s1]

Corresponding author  :

A. Bernand-Mantel

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