Magnetization reversal induced by spin polarized currents
J.P. Attané, C. Beigné, A. Marty, A. Mihai, J.-C. Pillet, Y. Samson, L. Vila, P. Warin

Near field microscopy images of a FePt/Pt surface. (Left) The observed topography on the AFM image reveals the presence of a quasi-periodic network of microtwins. (Right) The MFM image shows that the domain walls are pinned on this defect network.

We are studying the magnetization reversal, nanomagnetism and spin dependent properties of epitaxial thin films of alloys having a strong perpendicular magnetic anisotropy. In these materials, the magnetic domain walls are thin and thus strong magneto-electronic effects are expected. The structural defects of these epitaxial layers control the magnetization reversal through domain wall pining. In our systems, we manage to measure the time taken by a domain wall to depin from a single structural defect. As this depinning phenomenon is thermo activated, the pinning time varies from one measurement to another: it is stochastic, following a quasi-exponential probability law.

We are studying the domain wall propagation along a nanowire induced by a magnetic field and the depinning of the domain wall by the injection of a spin polarized current.

 

(a) SEM image of a nanostructured thin film of FePt with 3 Hall crosses devoted for domain wall propagation studies. (b) Hysteresis cycle obtained by the extraordinary Hall effect measurement. The observed steps correspond to Barkhausen jumps, ie at the elementary jumps of the domain wall from one defect to the other.

Multi-parameters studies (temperature, field, geometry, defect characteristics etc.) are used to better understand and further control the magnetization reversal in these systems, emphasis is put on the effect of the spin torque on the domain wall. By using nano lithography a single domain wall can be isolated in the device (basically a nanowire). The movement of the domain wall is controlled through the extraordinary Hall Effect and the magnetoresistance.

 

Fractals geometry of domains and percolation. The movement of the domain wall in our samples is similar to a mechanism of percolation by invasion without pinning: the reversed domain (black) has a fractal geometry, its fractal dimension is of 1,896.

We have also an activity on the mechanism of domain wall displacement in continuous films. The domain wall is model system of an elastic interface propagating into a disordered media. We have shown that the initial step of the reversal corresponds to a critical phenomena: the growing of a reversed domain is similar to the mechanism of percolation by invasion without pinning. The resulting geometry is fractal; its dimension depends on universal parameters of the percolation, so it is independent of the nature of the pinning defects.

 

 


References :

 

“Thermally activated depinning of a narrow domain wall from a single defect”
J.P. Attane, D. Ravelosona, A. Marty, Y. Samson, C. Chappert
Physical Review Letters 96, 147204 (2006)

« Magnetic domain wall propagation unto the percolation threshold across a pseudo rectangular disordered lattice” J.P. Attané, Y. Samson, A. Marty, J.C. Toussaint, G. Dubois, A. Mougin, J.P. Jamet
Physical Review Letters 93, 257203 (2004)

« Individual domain wall resistance as a probe of magnetization reversal in submicron ferromagnétic structures »
R. Danneau, P. Warin, J. P. Attané, I. Petej, C. Beigné, C. Fermon, O. Klein, A. Marty, F. Ott, Y. Samson, M. Viret
Physical Review Letters 88, 157201 (2002)

“Domain wall pinning on strain relaxation defects in FePt(001)/Pt thin films”
J.P. Attane, Y. Samson, A. Marty, D. Halley, C. Beigné
Applied Physics Letters 79, 794 (2001)

Collaborators:
D. Ravesolona (IEF, Orsay), S. Mangin (LPS, Nancy), L. Buda (Spintec, CEA-Grenoble)

 

Last update : 10/17 2013 (441)

 

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