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Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators.

Chung S, Eklund A, Iacocca E, Mohseni SM, Sani SR, Bookman L, Hoefer MA, Dumas RK, Åkerman J - Nat Commun (2016)

Bottom Line: Static and dynamic magnetic solitons play a critical role in applied nanomagnetism.Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers.Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden.

ABSTRACT
Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current-field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

No MeSH data available.


Related in: MedlinePlus

Droplet nucleation boundary.(a) Nucleation boundary found from the field sweeps (empty triangles), current sweeps (solid triangles) and low-frequency signals (solid circles) for devices with different NC radii RNC. Fits using equation (1) are shown by solid lines using the same colour code for each RNC. (b,c) The coefficients  and  are shown in b and c, respectively, as a function of NC area utilizing the same colour code as shown in a. Linear fits in b and c, shown by red dashed lines, are used to calculate the spin-torque asymmetry and efficiency.
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f3: Droplet nucleation boundary.(a) Nucleation boundary found from the field sweeps (empty triangles), current sweeps (solid triangles) and low-frequency signals (solid circles) for devices with different NC radii RNC. Fits using equation (1) are shown by solid lines using the same colour code for each RNC. (b,c) The coefficients and are shown in b and c, respectively, as a function of NC area utilizing the same colour code as shown in a. Linear fits in b and c, shown by red dashed lines, are used to calculate the spin-torque asymmetry and efficiency.

Mentions: All nucleation and collapse points extracted from Fig. 2a,b can now be plotted in Fig. 3a together with the same analysis carried out on three additional NCs with nominal radii ranging from 35 to 50 nm. Clearly, the field-dependent scans (hollow triangles pointing sideways) and current-dependent scans (solid triangles pointing up) complement each other smoothly, justifying the uniqueness of the droplet nucleation conditions. Two trends can be noticed: at low fields, the nucleation current decreases as a function of field, which is consistent with previous reports19 in which the nucleation current was shown to be dominated by the perpendicular component of spin-polarized current, obeying a 1/H dependence; at high fields, the dependence is linear—in strong contrast to the predicted 1/H dependence19 but in good agreement with recent results23. A similar behaviour is obtained from current-sweep measurements. At intermediate fields, both trends are smoothly connected, suggesting that both regimes originate from the physical characteristics of the orthogonal NC-STO. In Fig. 3a, we have also added shaded areas corresponding to regions where the microwave noise power is higher than 0.3 pW as an additional indication of the presence of a droplet. It is interesting to note that for the largest NC, there is a wide region where the microwave noise, but not the resistance, indicates the presence of a droplet. As the resistance is a time-averaged measurement, this apparent discrepancy indicates that, for most of the time, there is no droplet present. In other words, the droplet is highly unstable and the time it takes for the droplet to drift away from the NC is shorter than the re-nucleation time. The greater drift instability is likely a consequence of the larger Oersted field and the higher local temperature, both of which are due to the much higher nucleation current necessary for droplet nucleation in the largest NC.


Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators.

Chung S, Eklund A, Iacocca E, Mohseni SM, Sani SR, Bookman L, Hoefer MA, Dumas RK, Åkerman J - Nat Commun (2016)

Droplet nucleation boundary.(a) Nucleation boundary found from the field sweeps (empty triangles), current sweeps (solid triangles) and low-frequency signals (solid circles) for devices with different NC radii RNC. Fits using equation (1) are shown by solid lines using the same colour code for each RNC. (b,c) The coefficients  and  are shown in b and c, respectively, as a function of NC area utilizing the same colour code as shown in a. Linear fits in b and c, shown by red dashed lines, are used to calculate the spin-torque asymmetry and efficiency.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4837446&req=5

f3: Droplet nucleation boundary.(a) Nucleation boundary found from the field sweeps (empty triangles), current sweeps (solid triangles) and low-frequency signals (solid circles) for devices with different NC radii RNC. Fits using equation (1) are shown by solid lines using the same colour code for each RNC. (b,c) The coefficients and are shown in b and c, respectively, as a function of NC area utilizing the same colour code as shown in a. Linear fits in b and c, shown by red dashed lines, are used to calculate the spin-torque asymmetry and efficiency.
Mentions: All nucleation and collapse points extracted from Fig. 2a,b can now be plotted in Fig. 3a together with the same analysis carried out on three additional NCs with nominal radii ranging from 35 to 50 nm. Clearly, the field-dependent scans (hollow triangles pointing sideways) and current-dependent scans (solid triangles pointing up) complement each other smoothly, justifying the uniqueness of the droplet nucleation conditions. Two trends can be noticed: at low fields, the nucleation current decreases as a function of field, which is consistent with previous reports19 in which the nucleation current was shown to be dominated by the perpendicular component of spin-polarized current, obeying a 1/H dependence; at high fields, the dependence is linear—in strong contrast to the predicted 1/H dependence19 but in good agreement with recent results23. A similar behaviour is obtained from current-sweep measurements. At intermediate fields, both trends are smoothly connected, suggesting that both regimes originate from the physical characteristics of the orthogonal NC-STO. In Fig. 3a, we have also added shaded areas corresponding to regions where the microwave noise power is higher than 0.3 pW as an additional indication of the presence of a droplet. It is interesting to note that for the largest NC, there is a wide region where the microwave noise, but not the resistance, indicates the presence of a droplet. As the resistance is a time-averaged measurement, this apparent discrepancy indicates that, for most of the time, there is no droplet present. In other words, the droplet is highly unstable and the time it takes for the droplet to drift away from the NC is shorter than the re-nucleation time. The greater drift instability is likely a consequence of the larger Oersted field and the higher local temperature, both of which are due to the much higher nucleation current necessary for droplet nucleation in the largest NC.

Bottom Line: Static and dynamic magnetic solitons play a critical role in applied nanomagnetism.Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers.Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden.

ABSTRACT
Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current-field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

No MeSH data available.


Related in: MedlinePlus