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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: 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.

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.


Schematic images of the calculation model (a) Stoner-Wohlfarth grain and (b) ECC grain.
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Figure 1: Schematic images of the calculation model (a) Stoner-Wohlfarth grain and (b) ECC grain.

Mentions: The magnetization mechanisms of the Stoner-Wohlfarth and ECC structured grains were studied by numerically solving the LLG equation. The effective field in the LLG equation was the vector sum of the anisotropy field, magnetostatic field, exchange field, and external dc and microwave fields. Here, the exchange field was not included in the calculation of magnetization behavior for the Stoner-Wohlfarth grain. Rectangular grains were modeled as shown in Figure 1. The grain dimensions are based on recording media of hard disk drives. The thickness of the Stoner-Wohlfarth single spin grain was 5 nm, and those of the soft and hard magnetic sections of the ECC grain were 7 and 5 nm, respectively. The thickness of the soft layer is more than its exchange length (approximately 4 nm). The ECC grain was discretized into 1-nm equilateral cubic prisms, and each prism was assumed to have a single magnetization vector. The uniaxial anisotropy axes of these grains lay in the z-direction. The anisotropy field of the Stoner-Wohlfarth grain was 60 kOe, and those of the soft and hard sections for the ECC grain were 10 and 60 kOe, respectively. In the ECC grain, the magnetizations of the soft and hard magnetic sections were ferromagnetically coupled at their interfaces through exchange interaction (1.0 × 10−6 erg/cm). All magnetizations were initially arranged in the positive z-direction. The dc pulse field, Hdc, was applied in the negative z-direction and had a pulse width of 10 ns with a rise/fall time of 1 ns. The circularly polarized microwave field with the strength of Hac was also applied in the x-y plane, where the dc field was constant. These external fields were assumed to be uniformly distributed in the magnetic grains. For all presented results, the exchange stiffness constants for the soft and hard sections were 1.0 × 10−6 erg/cm; the dimensionless Gilbert damping constant was 0.05. The saturation magnetization for the Stoner-Wohlfarth grain was 800 emu/cm3, and those for the soft and hard sections of the ECC grain were 1,200 and 800 emu/cm3, respectively.


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)

Schematic images of the calculation model (a) Stoner-Wohlfarth grain and (b) ECC grain.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: Schematic images of the calculation model (a) Stoner-Wohlfarth grain and (b) ECC grain.
Mentions: The magnetization mechanisms of the Stoner-Wohlfarth and ECC structured grains were studied by numerically solving the LLG equation. The effective field in the LLG equation was the vector sum of the anisotropy field, magnetostatic field, exchange field, and external dc and microwave fields. Here, the exchange field was not included in the calculation of magnetization behavior for the Stoner-Wohlfarth grain. Rectangular grains were modeled as shown in Figure 1. The grain dimensions are based on recording media of hard disk drives. The thickness of the Stoner-Wohlfarth single spin grain was 5 nm, and those of the soft and hard magnetic sections of the ECC grain were 7 and 5 nm, respectively. The thickness of the soft layer is more than its exchange length (approximately 4 nm). The ECC grain was discretized into 1-nm equilateral cubic prisms, and each prism was assumed to have a single magnetization vector. The uniaxial anisotropy axes of these grains lay in the z-direction. The anisotropy field of the Stoner-Wohlfarth grain was 60 kOe, and those of the soft and hard sections for the ECC grain were 10 and 60 kOe, respectively. In the ECC grain, the magnetizations of the soft and hard magnetic sections were ferromagnetically coupled at their interfaces through exchange interaction (1.0 × 10−6 erg/cm). All magnetizations were initially arranged in the positive z-direction. The dc pulse field, Hdc, was applied in the negative z-direction and had a pulse width of 10 ns with a rise/fall time of 1 ns. The circularly polarized microwave field with the strength of Hac was also applied in the x-y plane, where the dc field was constant. These external fields were assumed to be uniformly distributed in the magnetic grains. For all presented results, the exchange stiffness constants for the soft and hard sections were 1.0 × 10−6 erg/cm; the dimensionless Gilbert damping constant was 0.05. The saturation magnetization for the Stoner-Wohlfarth grain was 800 emu/cm3, and those for the soft and hard sections of the ECC grain were 1,200 and 800 emu/cm3, respectively.

Bottom Line: 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.

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.