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Enhanced spin transfer torque effect for transverse domain walls in cylindrical nanowires

Enhanced spin transfer torque effect for transverse domain walls in cylindrical nanowires
Enhanced spin transfer torque effect for transverse domain walls in cylindrical nanowires
Recent studies have predicted extraordinary properties for transverse domain walls in cylindrical nanowires: zero depinning current, the absence of the Walker breakdown, and applications as domain wall oscillators. In order to reliably control the domain wall motion, it is important to understand how they interact with pinning centers, which may be engineered, for example, through modulations in the nanowire geometry (such as notches or extrusions) or in the magnetic properties of the material. In this paper we study the motion and depinning of transverse domain walls through pinning centers in ferromagnetic cylindrical nanowires. We use (i) magnetic fields and (ii) spin-polarized currents to drive the domain walls along the wire. The pinning centers are modelled as a section of the nanowire which exhibits a uniaxial crystal anisotropy where the anisotropy easy axis and the wire axis enclose a variable angle ?P. Using (i) magnetic fields, we find that the minimum and the maximum fields required to push the domain wall through the pinning center differ by 30%. On the contrary, using (ii) spin-polarized currents, we find variations of a factor 130 between the minimum value of the depinning current density (observed for ?P=0?, i.e., anisotropy axis pointing parallel to the wire axis) and the maximum value (for ?P=90?, i.e., anisotropy axis perpendicular to the wire axis). We study the depinning current density as a function of the height of the energy barrier of the pinning center using numerical and analytical methods. We find that for an industry standard energy barrier of 40 kBT, a depinning current of about 5 ?A (corresponding to a current density of 6×1010 A/m2 in a nanowire of 10 nm diameter) is sufficient to depin the domain wall. We reveal and explain the mechanism that leads to these unusually low depinning currents. One requirement for this depinning mechanism is for the domain wall to be able to rotate around its own axis. With the right barrier design, the spin torque transfer term is acting exactly against the damping in the micromagnetic system, and thus the low current density is sufficient to accumulate enough energy quickly. These key insights may be crucial in furthering the development of novel memory technologies, such as the racetrack memory, that can be controlled through low current densities.

1550-235X
94409
Franchin, Matteo
9e00aaa2-959e-420f-854c-3b43aece85e3
Albert, Max
aa9f90ba-ac29-42f4-8485-8d8d7b221ae4
Knittel, Andreas
b2d73a26-4349-4491-90d6-55b78ddd2cea
Chernyshenko, D.
8ff59c7e-7d2c-4188-94e9-fe9a13733903
Fischbacher, Thomas
d3282f31-0a6a-4d19-80d0-e3bebc12f67a
Fangohr, Hans
9b7cfab9-d5dc-45dc-947c-2eba5c81a160
Prabhakar, A
dd188e7d-1e2e-4b42-8c61-e094e0678375
Franchin, Matteo
9e00aaa2-959e-420f-854c-3b43aece85e3
Albert, Max
aa9f90ba-ac29-42f4-8485-8d8d7b221ae4
Knittel, Andreas
b2d73a26-4349-4491-90d6-55b78ddd2cea
Chernyshenko, D.
8ff59c7e-7d2c-4188-94e9-fe9a13733903
Fischbacher, Thomas
d3282f31-0a6a-4d19-80d0-e3bebc12f67a
Fangohr, Hans
9b7cfab9-d5dc-45dc-947c-2eba5c81a160
Prabhakar, A
dd188e7d-1e2e-4b42-8c61-e094e0678375

Franchin, Matteo, Albert, Max, Knittel, Andreas, Chernyshenko, D., Fischbacher, Thomas, Fangohr, Hans and Prabhakar, A (2011) Enhanced spin transfer torque effect for transverse domain walls in cylindrical nanowires. Physical Review B, 84 (9), 94409. (doi:10.1103/PhysRevB.84.094409).

Record type: Article

Abstract

Recent studies have predicted extraordinary properties for transverse domain walls in cylindrical nanowires: zero depinning current, the absence of the Walker breakdown, and applications as domain wall oscillators. In order to reliably control the domain wall motion, it is important to understand how they interact with pinning centers, which may be engineered, for example, through modulations in the nanowire geometry (such as notches or extrusions) or in the magnetic properties of the material. In this paper we study the motion and depinning of transverse domain walls through pinning centers in ferromagnetic cylindrical nanowires. We use (i) magnetic fields and (ii) spin-polarized currents to drive the domain walls along the wire. The pinning centers are modelled as a section of the nanowire which exhibits a uniaxial crystal anisotropy where the anisotropy easy axis and the wire axis enclose a variable angle ?P. Using (i) magnetic fields, we find that the minimum and the maximum fields required to push the domain wall through the pinning center differ by 30%. On the contrary, using (ii) spin-polarized currents, we find variations of a factor 130 between the minimum value of the depinning current density (observed for ?P=0?, i.e., anisotropy axis pointing parallel to the wire axis) and the maximum value (for ?P=90?, i.e., anisotropy axis perpendicular to the wire axis). We study the depinning current density as a function of the height of the energy barrier of the pinning center using numerical and analytical methods. We find that for an industry standard energy barrier of 40 kBT, a depinning current of about 5 ?A (corresponding to a current density of 6×1010 A/m2 in a nanowire of 10 nm diameter) is sufficient to depin the domain wall. We reveal and explain the mechanism that leads to these unusually low depinning currents. One requirement for this depinning mechanism is for the domain wall to be able to rotate around its own axis. With the right barrier design, the spin torque transfer term is acting exactly against the damping in the micromagnetic system, and thus the low current density is sufficient to accumulate enough energy quickly. These key insights may be crucial in furthering the development of novel memory technologies, such as the racetrack memory, that can be controlled through low current densities.

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Published date: 13 September 2011
Organisations: Computational Engineering & Design Group

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Local EPrints ID: 335934
URI: https://eprints.soton.ac.uk/id/eprint/335934
ISSN: 1550-235X
PURE UUID: 04bb2a3a-2640-4669-bbfa-abfe4d6bc2f4

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Date deposited: 14 Mar 2012 15:47
Last modified: 18 Jul 2017 06:10

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