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Stable microwave-assisted magnetization switching for nanoscale exchange-coupled composite grain.

Tanaka T, Kashiwagi S, Furomoto Y, Otsuka Y, Matsuyama K - Nanoscale Res Lett (2013)

Bottom Line: Magnetization mechanisms of nanoscale magnetic grains greatly differ from well-known magnetization mechanisms of micrometer- or millimeter-sized magnetic grains or particles.These studies imply that the switching behavior of microwave-assisted magnetization switching of the ECC grain can be divided into two groups: stable and unstable regions, similar to the case of the Stoner-Wahlfarth grain.This switching behavior is preferred for the practical applications of microwave-assisted magnetization switching.

View Article: PubMed Central - HTML - PubMed

Affiliation: Graduate School of Information Science and Electrical Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan. t-tanaka@ed.kyushu-u.ac.jp.

ABSTRACT
Magnetization mechanisms of nanoscale magnetic grains greatly differ from well-known magnetization mechanisms of micrometer- or millimeter-sized magnetic grains or particles. Magnetization switching mechanisms of nanoscale exchange-coupled composite (ECC) grain in a microwave field was studied using micromagnetic simulation. Magnetization switching involving a strongly damped or precessional oscillation was studied using various strengths of external direct current and microwave fields. These studies imply that the switching behavior of microwave-assisted magnetization switching of the ECC grain can be divided into two groups: stable and unstable regions, similar to the case of the Stoner-Wahlfarth grain. A significant reduction in the switching field was observed in the ECC grain when the magnetization switching involved precessional oscillations similar to the case of the Stoner-Wohlfarth grain. This switching behavior is preferred for the practical applications of microwave-assisted magnetization switching.

No MeSH data available.


Switching field of the ECC grain. The dc field incident angles are (a) 0°, (b) 15°, (c) 30°, and (d) 45°.
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Figure 7: Switching field of the ECC grain. The dc field incident angles are (a) 0°, (b) 15°, (c) 30°, and (d) 45°.

Mentions: During the magnetization switching process of the ECC grain, the magnetization of the soft layer will rotate first under the external field while providing an exchange field to the hard layer to effectively rotate its magnetization, thereby achieving a lower switching field. Soft magnetic layers thicker than their exchange length induce complex incoherent magnetization switching. This means that magnetization mechanisms in the ECC grain cannot be analyzed using the theoretical treatment. Therefore, micromagnetic calculations are required to analyze the stability of magnetization switching in the ECC grain. Figure 7 presents the switching field of the ECC grain with incident angles of 0°, 15°, 30°, and 45° when applying a microwave frequency of 15 GHz. In comparison with the switching field of the Stoner-Wohlfarth grain, a significant reduction in switching fields is obtained in the calculated Hac field range. The switching field is minimum when the incident angle is 30°, which is smaller than that for the Stoner-Wohlfarth grain. This tendency is a well-known characteristic in ECC grains in the absence of microwave fields. The abrupt change in HSW is also clearly seen at Hac = 0.6 kOe when the incident angle is 0°. This implies that the magnetization behavior of the ECC grain can be classified into the three solution regions of the stability matrix, which is similar to the case of Stoner-Wohlfarth grains. The magnetization switching behaviors were also computed to analyze the switching processes in the stable and unstable switching regions for the ECC grain. Figure 8 shows the trajectories of the magnetization at the top of the hard layer projected onto the x-z plane when the dc and microwave fields are (a) Hdc = 16.6 kOe, Hac = 0.5 kOe and (b) Hdc = 11.4 kOe, Hac = 0.6 kOe at an angle of incidence of 0°. Figure 8a shows magnetization switching induced by large damping in the early stage of the switching process. The magnetization switching process seems to be an unstable switching according to the comparison between theoretical analysis and micromagnetic simulation as shown in Figures 2 and 3, respectively. On the other hand, the precessional oscillation is observed at Hdc = 11.4 kOe with Hac = 0.6 kOe. Magnetization switching involving precessional oscillation was also observed in the stable switching of the Stoner-Wohlfarth grains. This implies that unstable and stable switching occurs under the conditions (a) and (b), respectively, in the ECC grains, indicating that the microwave-assisted switching behavior of the ECC grains qualitatively agrees with the theory predicted by Bertotti [21,22] and micromagnetic simulation by Okamoto [14].


Stable microwave-assisted magnetization switching for nanoscale exchange-coupled composite grain.

Tanaka T, Kashiwagi S, Furomoto Y, Otsuka Y, Matsuyama K - Nanoscale Res Lett (2013)

Switching field of the ECC grain. The dc field incident angles are (a) 0°, (b) 15°, (c) 30°, and (d) 45°.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4228253&req=5

Figure 7: Switching field of the ECC grain. The dc field incident angles are (a) 0°, (b) 15°, (c) 30°, and (d) 45°.
Mentions: During the magnetization switching process of the ECC grain, the magnetization of the soft layer will rotate first under the external field while providing an exchange field to the hard layer to effectively rotate its magnetization, thereby achieving a lower switching field. Soft magnetic layers thicker than their exchange length induce complex incoherent magnetization switching. This means that magnetization mechanisms in the ECC grain cannot be analyzed using the theoretical treatment. Therefore, micromagnetic calculations are required to analyze the stability of magnetization switching in the ECC grain. Figure 7 presents the switching field of the ECC grain with incident angles of 0°, 15°, 30°, and 45° when applying a microwave frequency of 15 GHz. In comparison with the switching field of the Stoner-Wohlfarth grain, a significant reduction in switching fields is obtained in the calculated Hac field range. The switching field is minimum when the incident angle is 30°, which is smaller than that for the Stoner-Wohlfarth grain. This tendency is a well-known characteristic in ECC grains in the absence of microwave fields. The abrupt change in HSW is also clearly seen at Hac = 0.6 kOe when the incident angle is 0°. This implies that the magnetization behavior of the ECC grain can be classified into the three solution regions of the stability matrix, which is similar to the case of Stoner-Wohlfarth grains. The magnetization switching behaviors were also computed to analyze the switching processes in the stable and unstable switching regions for the ECC grain. Figure 8 shows the trajectories of the magnetization at the top of the hard layer projected onto the x-z plane when the dc and microwave fields are (a) Hdc = 16.6 kOe, Hac = 0.5 kOe and (b) Hdc = 11.4 kOe, Hac = 0.6 kOe at an angle of incidence of 0°. Figure 8a shows magnetization switching induced by large damping in the early stage of the switching process. The magnetization switching process seems to be an unstable switching according to the comparison between theoretical analysis and micromagnetic simulation as shown in Figures 2 and 3, respectively. On the other hand, the precessional oscillation is observed at Hdc = 11.4 kOe with Hac = 0.6 kOe. Magnetization switching involving precessional oscillation was also observed in the stable switching of the Stoner-Wohlfarth grains. This implies that unstable and stable switching occurs under the conditions (a) and (b), respectively, in the ECC grains, indicating that the microwave-assisted switching behavior of the ECC grains qualitatively agrees with the theory predicted by Bertotti [21,22] and micromagnetic simulation by Okamoto [14].

Bottom Line: Magnetization mechanisms of nanoscale magnetic grains greatly differ from well-known magnetization mechanisms of micrometer- or millimeter-sized magnetic grains or particles.These studies imply that the switching behavior of microwave-assisted magnetization switching of the ECC grain can be divided into two groups: stable and unstable regions, similar to the case of the Stoner-Wahlfarth grain.This switching behavior is preferred for the practical applications of microwave-assisted magnetization switching.

View Article: PubMed Central - HTML - PubMed

Affiliation: Graduate School of Information Science and Electrical Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan. t-tanaka@ed.kyushu-u.ac.jp.

ABSTRACT
Magnetization mechanisms of nanoscale magnetic grains greatly differ from well-known magnetization mechanisms of micrometer- or millimeter-sized magnetic grains or particles. Magnetization switching mechanisms of nanoscale exchange-coupled composite (ECC) grain in a microwave field was studied using micromagnetic simulation. Magnetization switching involving a strongly damped or precessional oscillation was studied using various strengths of external direct current and microwave fields. These studies imply that the switching behavior of microwave-assisted magnetization switching of the ECC grain can be divided into two groups: stable and unstable regions, similar to the case of the Stoner-Wahlfarth grain. A significant reduction in the switching field was observed in the ECC grain when the magnetization switching involved precessional oscillations similar to the case of the Stoner-Wohlfarth grain. This switching behavior is preferred for the practical applications of microwave-assisted magnetization switching.

No MeSH data available.