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Magnetic vortex core reversal by excitation of spin waves.

Kammerer M, Weigand M, Curcic M, Noske M, Sproll M, Vansteenkiste A, Van Waeyenberge B, Stoll H, Woltersdorf G, Back CH, Schuetz G - Nat Commun (2011)

Bottom Line: Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited.These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism.Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified.

View Article: PubMed Central - PubMed

Affiliation: Max-Planck-Institut für Metallforschung, Heisenbergstraße 3, 70569 Stuttgart, Germany. kammerer@mf.mpg.de

ABSTRACT
Micron-sized magnetic platelets in the flux-closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified.

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Model for spin wave-induced vortex core reversal.This sketch shows the origin of the out-of-plane magnetization near the vortex core and illustrates the origin of the observed asymmetry in the 'dip' formation for CW versus CCW spin-wave excitation. To the left, the basic shape of the vortex core and the bipolar amplitude of the spin wave are shown. The magnetization change as a result of the gyrofield of the moving vortex core differs for CW and CCW rotation senses (middle sketches). The resulting structures (right sketches) agree qualitatively with results from the micromagnetic simulations as shown in Figures 3,4 and 6.
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f5: Model for spin wave-induced vortex core reversal.This sketch shows the origin of the out-of-plane magnetization near the vortex core and illustrates the origin of the observed asymmetry in the 'dip' formation for CW versus CCW spin-wave excitation. To the left, the basic shape of the vortex core and the bipolar amplitude of the spin wave are shown. The magnetization change as a result of the gyrofield of the moving vortex core differs for CW and CCW rotation senses (middle sketches). The resulting structures (right sketches) agree qualitatively with results from the micromagnetic simulations as shown in Figures 3,4 and 6.

Mentions: The gyrofield, as used by Guslienko et al. to describe the 'dip' formation in the gyrotropic mode1916, can also be taken to explain the difference in 'dip' formation for the (m=+1) and (m=−1) modes, as is sketched in Figure 5. This effective field originates from the movement of the vortex and results in out-of-plane contributions to the left and right side of the core (with respect to the direction of motion), which are negative and positive, respectively (middle sketch in Fig. 5). This velocity dependent behaviour breaks the symmetry between counter rotating vortices. If we now combine the spin wave amplitude with the effect of the gyrofield, we can understand the 'dip' formation for any of the counter rotating modes (right sketch in Fig. 5). For a vortex up and the mode (m=−1) with a CW vortex motion, the gyrofield acts constructively on the spin wave amplitude. For the (m=+1) mode, it acts destructively, causing the double 'dip' configuration as seen in Figure 4. Alternatively, these 'dip' structures can be regarded as a hybridization between the gyrotropic mode and the azimuthal modes32. Kravchuk et al.22 also presented a 'dip' formation mechanism for spin-wave excitation under the condition of an artificially fixed vortex core. However, our results show that this condition can not be used to predict the reversal in free vortices, as in realistic structures. According to their analysis, the polarization direction of the 'dip' is given by the rotation sense of the external field only. This would mean that reversal of a given core polarization is only possible for one of the two azimuthal modes. For example, the reversal at the (m=+1) as presented in Figure 2 can not be explained with this assumption, as the 'dip' polarization would be the same as the core polarization. The dynamic origin of the 'dip' structure clearly explains this contradiction. For a free vortex, the 'dip' formation seems to be dominated by the gyrofield.


Magnetic vortex core reversal by excitation of spin waves.

Kammerer M, Weigand M, Curcic M, Noske M, Sproll M, Vansteenkiste A, Van Waeyenberge B, Stoll H, Woltersdorf G, Back CH, Schuetz G - Nat Commun (2011)

Model for spin wave-induced vortex core reversal.This sketch shows the origin of the out-of-plane magnetization near the vortex core and illustrates the origin of the observed asymmetry in the 'dip' formation for CW versus CCW spin-wave excitation. To the left, the basic shape of the vortex core and the bipolar amplitude of the spin wave are shown. The magnetization change as a result of the gyrofield of the moving vortex core differs for CW and CCW rotation senses (middle sketches). The resulting structures (right sketches) agree qualitatively with results from the micromagnetic simulations as shown in Figures 3,4 and 6.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3104515&req=5

f5: Model for spin wave-induced vortex core reversal.This sketch shows the origin of the out-of-plane magnetization near the vortex core and illustrates the origin of the observed asymmetry in the 'dip' formation for CW versus CCW spin-wave excitation. To the left, the basic shape of the vortex core and the bipolar amplitude of the spin wave are shown. The magnetization change as a result of the gyrofield of the moving vortex core differs for CW and CCW rotation senses (middle sketches). The resulting structures (right sketches) agree qualitatively with results from the micromagnetic simulations as shown in Figures 3,4 and 6.
Mentions: The gyrofield, as used by Guslienko et al. to describe the 'dip' formation in the gyrotropic mode1916, can also be taken to explain the difference in 'dip' formation for the (m=+1) and (m=−1) modes, as is sketched in Figure 5. This effective field originates from the movement of the vortex and results in out-of-plane contributions to the left and right side of the core (with respect to the direction of motion), which are negative and positive, respectively (middle sketch in Fig. 5). This velocity dependent behaviour breaks the symmetry between counter rotating vortices. If we now combine the spin wave amplitude with the effect of the gyrofield, we can understand the 'dip' formation for any of the counter rotating modes (right sketch in Fig. 5). For a vortex up and the mode (m=−1) with a CW vortex motion, the gyrofield acts constructively on the spin wave amplitude. For the (m=+1) mode, it acts destructively, causing the double 'dip' configuration as seen in Figure 4. Alternatively, these 'dip' structures can be regarded as a hybridization between the gyrotropic mode and the azimuthal modes32. Kravchuk et al.22 also presented a 'dip' formation mechanism for spin-wave excitation under the condition of an artificially fixed vortex core. However, our results show that this condition can not be used to predict the reversal in free vortices, as in realistic structures. According to their analysis, the polarization direction of the 'dip' is given by the rotation sense of the external field only. This would mean that reversal of a given core polarization is only possible for one of the two azimuthal modes. For example, the reversal at the (m=+1) as presented in Figure 2 can not be explained with this assumption, as the 'dip' polarization would be the same as the core polarization. The dynamic origin of the 'dip' structure clearly explains this contradiction. For a free vortex, the 'dip' formation seems to be dominated by the gyrofield.

Bottom Line: Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited.These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism.Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified.

View Article: PubMed Central - PubMed

Affiliation: Max-Planck-Institut für Metallforschung, Heisenbergstraße 3, 70569 Stuttgart, Germany. kammerer@mf.mpg.de

ABSTRACT
Micron-sized magnetic platelets in the flux-closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified.

Show MeSH
Related in: MedlinePlus