Magnetic droplets in nano-contact spin-torque oscillators with

Magnetic droplets in nano-contact spin-torque oscillators with perpendicular magnetic anisotropy

S. M. Mohseni, S.R. Sani, Ye. Pogoryelov, J. Persson, S. Chung, T. N. Anh Nguyen, P. K. Muduli, E. Iacocca, S. Bonetti, A. Eklund, R. K. Dumas, A. M. Deac, M. Hoefer, Johan Åkerman

Material Physics, KTH – Royal Institute of Technology, 164 40 Kista, Sweden

Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden

NanOsc AB, Electrum 205, 164 40 Kista, Sweden

Department of Mathematics, North Carolina State University, Raleigh, NC 27695, USA

Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf e. V., 01314

Domain Microstructure and Dynamics in Magnetic Elements Heraklion, April 4-8, 2013

Johan Åkerman, Heraklion, Greece, April 9, 2013

Outline

• Nano-Contact Spin-Torque Oscillators (NC-STOs)

• NC-STOs with perpendicular free layers • Orthogonal spin valves: perpendicular free layer, in-plane fixed layer

• W. H. Rippard et al, PRB 81, 014426 (2010)

• S. M. Mohseni et al, PSS-RRL 5, 432 (2011)

Co – Fixed layer

Base electrode

Cu - Spacer

NiFe or Co/Ni – Free layer

Cap

Point contact 40 – 400 nm

SiO2

Idc Vrf


Outline





• Ortho-NC-STOs at high fields • In-plane fixed layer tilts out-of-plane

• Dramatic drop in frequency (30 GHz -> 20 GHz)

• Jump in resistance, sign change of magnetoresistance

• Complex intrinsic dynamics – auto-modulation

Co – Fixed layer

Base electrode

Cu - Spacer


Cap


SiO2

Idc Vrf


Outline









• Magnetic Droplet Solitons • Conservative magnon drops in perpendicular thin films with no damping

• Dissipative magnetic droplets in perpendicular thin films with STT

• A novel dynamic nano-magnetic object ready to play with!

Co – Fixed layer

Base electrode

Cu - Spacer


Cap


SiO2

Idc Vrf


Nano-Contact Spin Torque Oscillators (NC-STOs)

>20,000 devices per wafer

Co or CoFe – Fixed layer

Base electrode

Cu - Spacer


Cap


SiO2

Idc Vrf


In-plane NC-STOs as a function of field angle

f

CoFe - Fixed layer

Base electrode

Cu - Spacer

NiFe - Free layer

Cap

Point contact 40 nm

SiO2

Idc Vrf

J. C. Slonczewski, JMMM 159, L261 (1999)

A.N. Slavin and V.S. Tiberkevich, PRL 95, 237201 (2005)

S. Bonetti, V. S. Tiberkevich, G. Consolo, G. Finocchio, P. K. Muduli, F. Mancoff, A. Slavin and J. Åkerman, PRL 105, 217204 (2010)


NC-STOs are central to STT Driven Magnonics

Nature Nanotechnology 6, 635 (2011) Nature Materials 9, 984 (2010)


Outline





Co – Fixed layer

Base electrode

Cu - Spacer


Cap


SiO2

Idc Vrf


Outline

• Nano-Contact Spin-Torque Oscillators

• NC-STOs with perpendicular free layers • Orthogonal spin valves: perpendicular free layer



Co – Fixed layer

Base electrode

Cu - Spacer


Cap


SiO2

Idc Vrf


Orthogonal NC-STOs Zero field operation

Cu3/Pd3

Cu8

Co8

Ta5/Cu15/Ta5

Co0.3/[Ni0.8/Co0.4]x4


Outline






Co – Fixed layer

Base electrode

Cu - Spacer


Cap


SiO2

Idc Vrf


Increasing the applied field -> f drops + sidebands

• Transitions to lower f at 0.65 T • f drops: 30 GHz 20 GHz • Power increases 40x • Appearance of sidebands


Transition in both dynamic and static properties

0 2000 4000 6000 8000 10000 12000

17.200

17.205

17.210

17.215

17.220

17.225

17.230

17.235

17.240

17.245

17.250

I dc

=-6 mA

R (O

hm)

H (Oe)

Increasing

Decreasing

• Transitions to lower f at 0.65 T • f drops: 30 GHz 20 GHz • Power increases 40x • Appearance of sidebands

• Jump in resistance • MR changes sign


Outline










Co – Fixed layer

Base electrode

Cu - Spacer


Cap


SiO2

Idc Vrf


Magnon drops

Lossless Magnon Drops suggested 35 years ago in perpendicularly magnetized thin films with zero damping.

B. A. Ivanov and A. M. Kosevich, Zh. Eksp. Teor. Fiz. 72, 2000 (1977);

A. M. Kosevich, B. A. Ivanov, and A. S. Kovalev, Phys. Rep. 194,117 (1990)


Magnon drops




• Magnon gas

• Easy axis anisotropy (perpendicular)

• Easy axis creates magnon attraction

• Weak = two magnons do not bind, need many

• Magnon Drop = Self-localized bound state of large number of magnons


Magnon drops




• Magnon gas

• Easy axis anisotropy (perpendicular)

• Easy axis creates magnon attraction

• Weak = two magnons do not bind, need many

• Magnon Drop = Self-localized bound state of large number of magnons

“When relaxation processes are taken into account,

magnon drops can exist only under the conditions

of an external influence that guarantees the number,

N, of spin deviations.

Secondly, it is necessary that the mean lifetime of the

magnons be longer than the time required for their

condensation into a drop and that the resorption of

the drop be compensated by the excitation of

magnons by the external influence.”


Outline










• Dissipative magnetic droplets in perpendicular thin films with STT

Co – Fixed layer

Base electrode

Cu - Spacer


Cap


SiO2

Idc Vrf


Magnetic Droplets

• All spins precess in

phase around normal

• Dynamically

stabilized

• Precession around

equator for mz = 0

• Reversal of core is

asymptotic, never

fully reversed





Ordinary vs Droplet precession

• High-frequency ~ FMR

• Maximum precession underneath NC

• MR should decrease with field

• Lower frequency ~ 30 – 60% of FMR

• Maximum precession around NC

• MR could increase with field – if core

is reversed


Experiment – Field dependence

• Lower frequency ~ 30 – 60% of FMR

• Maximum precession around NC

• MR could increase with field – if core

is reversed


Experiment – Field and Current dependence

40x increase in power


Critical current for droplet nucleation

• Nucleation current scales inversely with perpendicular component of the fixed layer magnetization, mz • Spin polarization of the fixed layer, pz ~ mz

• Nucleation current scales linearly with nano-contact area • Nucleation governed by a critical current density for perpendicular polarization


Auto-modulation of droplet signal


Zoology of droplet modes

• Stationary droplet

• Periodic motion

• Spinning

• Breather mode &

wall deformation


Experimental observation of Breather Mode

10 15 20 25 30 35

0

2

4

6

8

10

12

3/2 f: 34.5 GHz1/2 f: 11.5 GHz

STO frequency (GHz)

f: 23 GHz


A new dynamic nano-magnetic object to play with!

• Domain walls (Bloch, Néel) • Static, Non-linear, Topological

• Vortices • Static, Non-linear, Topological

• Bubbles • Static, Non-linear, Topological

• NC-STO modes (propagating mode, bullet) • Dynamic, Non-linear, Non-topological

• Droplets • Dynamic, Non-linear, Non-topological


Summary

First observation of a Magnetic Droplet

- Large frequency drops of 10 GHz

- Associated jump in MR and sign change in MR vs H

- Auto-modulation related to intrinsic droplet modes (periodic motion, spinning, breathing, death-rebirth, …)

The droplet joins the domain wall and the vortex as distinct nano-magnetic objects with potential for rich science


2 Postdoc Positions open at University of Gothenburg ERC Grant, Project ”MUSTANG”

Funding and open positions

Magnetic droplets in nano-contact spin-torque oscillators with

Documents

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