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Burgos-Parra, E., Keatley, P. S., Redjai Sani, S., Durrenfeld, P., Åkerman, J. & Hicken, R. J. (2019). Time-resolved imaging of magnetization dynamics in double nanocontact spin torque vortex oscillator devices. Physical Review B, 100(13), Article ID 134439.
Open this publication in new window or tab >>Time-resolved imaging of magnetization dynamics in double nanocontact spin torque vortex oscillator devices
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 13, article id 134439Article in journal (Refereed) Published
Abstract [en]

Double nanocontact (NC) spin transfer vortex oscillator devices, in which NCs of 100-nm diameter have center-to-center separation ranging from 200 to 1100 nm, have been studied by means of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM). The NCs were positioned close to the edge of the top electrical contact so that the magnetization dynamics of the adjacent region could be probed optically. The electrical measurements showed different ranges of frequency operation for devices with different NC separations. For 900-nm NC separation, TRSKM showed magnetic contrast consistent with the formation of a magnetic vortex at each NC, while for 200-nm NC separation a lack of magnetic contrast near the NC region suggests that the magnetization dynamics occur closer to the NC and underneath the top contact. TRSKM also reveals the presence of additional localized dynamical features far from the NCs, which are not seen by electrical measurements; has not been reported previously for double NCs with different separations; and provides insight into how the dynamic state of the phase-locked oscillators is established and stabilized.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-264157 (URN)10.1103/PhysRevB.100.134439 (DOI)000493513500002 ()2-s2.0-85074947136 (Scopus ID)
Note

QC 20191210

Available from: 2019-12-10 Created: 2019-12-10 Last updated: 2019-12-10Bibliographically approved
Burgos-Parra, E., Bukin, N., Redjai Sani, S., Figueroa, A. I., Beutier, G., Dupraz, M., . . . Ogrin, F. Y. (2018). Investigation of magnetic droplet solitons using x-ray holography with extended references. Scientific Reports, 8, Article ID 11533.
Open this publication in new window or tab >>Investigation of magnetic droplet solitons using x-ray holography with extended references
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2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 11533Article in journal (Refereed) Published
Abstract [en]

A dissipative magnetic soliton, or magnetic droplet, is a structure that has been predicted to exist within a thin magnetic layer when non-linearity is balanced by dispersion, and a driving force counteracts the inherent damping of the spin precession. Such a soliton can be formed beneath a nano-contact (NC) that delivers a large spin-polarized current density into a magnetic layer with perpendicular magnetic anisotropy. Although the existence of droplets has been confirmed from electrical measurements and by micromagnetic simulations, only a few attempts have been made to directly observe the magnetic landscape that sustains these structures, and then only for a restricted set of experimental parameter values. In this work we use and x-ray holography technique HERALDO, to image the magnetic structure of the [ Co/ Ni] x4 multilayer within a NC orthogonal pseudo spin-valve, for different range of magnetic fields and injected electric currents. The magnetic configuration imaged at -33 mA and 0.3 T for devices with 90 nm NC diameter reveals a structure that is within the range of current where the droplet soliton exist based on our electrical measurements and have it is consistent with the expected size of the droplet (similar to 100 nm diameter) and its spatial position within the sample. We also report the magnetisation configurations observed at lower DC currents in the presence of fields (0-50 mT), where it is expected to observe regimes of the unstable droplet formation.

Place, publisher, year, edition, pages
Nature Publishing Group, 2018
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-233289 (URN)10.1038/s41598-018-29856-y (DOI)000440411300018 ()30069062 (PubMedID)2-s2.0-85050962853 (Scopus ID)
Funder
Swedish Foundation for Strategic Research EU, European Research Council, 307144Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20180816

Available from: 2018-08-16 Created: 2018-08-16 Last updated: 2018-08-16Bibliographically approved
Mohseni, S. M., Hamdi, M., Yazdi, H. F., Banuazizi, S. A., Redjai Sani, S., Chung, S., . . . Mohseni, S. M. (2018). Magnetic droplet soliton nucleation in oblique fields. Physical Review B Condensed Matter, 97(184402)
Open this publication in new window or tab >>Magnetic droplet soliton nucleation in oblique fields
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2018 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 97, no 184402Article in journal (Refereed) Published
Abstract [en]

We study the auto-oscillating magnetodynamics in orthogonal spin-torque nano-oscillators (STNOs) as a function of the out-of-plane (OOP) magnetic-field angle. In perpendicular fields and at OOP field angles down to approximately 50°, we observe the nucleation of a droplet. However, for field angles below 50°, experiments indicate that the droplet gives way to propagating spin waves, in agreement with our micromagnetic simulations. Theoretical calculations show that the physical mechanism behind these observations is the sign changing of spin-wave nonlinearity (SWN) by angle. In addition, we show that the presence of a strong perpendicular magnetic anisotropy free layer in the system reverses the angular dependence of the SWN and dynamics in STNOs with respect to the known behavior determined for the in-plane magnetic anisotropy free layer. Our results are of fundamental interest in understanding the rich dynamics of nanoscale solitons and spin-wave dynamics in STNOs.

Place, publisher, year, edition, pages
American Physical Society, 2018
Keywords
nanocontact, spin torque nano-oscillator, droplet, nucleation
National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-228245 (URN)10.1103/PhysRevB.97.184402 (DOI)000431986600004 ()2-s2.0-85047128171 (Scopus ID)
Note

QC 20180524

Available from: 2018-05-21 Created: 2018-05-21 Last updated: 2018-06-05Bibliographically approved
Keatley, P. S., Redjai Sani, S., Hrkac, G., Mohseni, S. M., Durrenfeld, P., Åkerman, J. & Hicken, R. J. (2017). Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting. Journal of Physics D: Applied Physics, 50(16), Article ID 164003.
Open this publication in new window or tab >>Imaging magnetisation dynamics in nano-contact spin-torque vortex oscillators exhibiting gyrotropic mode splitting
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2017 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 50, no 16, article id 164003Article in journal (Refereed) Published
Abstract [en]

Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by the injected RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2017
Keywords
nano-contact, spin-torque vortex oscillator, time-resolved scanning Kerr microscopy, vortex gyration, magnetisation dynamics, mode splitting, injection locking
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-206681 (URN)10.1088/1361-6463/aa628a (DOI)000399122800001 ()2-s2.0-85017691624 (Scopus ID)
Note

QC 20170510

Available from: 2017-05-10 Created: 2017-05-10 Last updated: 2017-05-10Bibliographically approved
Qejvanaj, F., Mazraati, H., Jiang, S., Persson, A., Redjai Sani, S., Chung, S., . . . Åkerman, J. (2015). Planar Hall Effect Bridge sensor with NiFeX (X = Cu, Ag and Au) sensing layer.. In: : . Paper presented at IEEE International Magnetics Conference (Intermag), MAY 11-15, 2015, Beijing, PEOPLES R CHINA. Institute of Electrical and Electronics Engineers (IEEE), Article ID 7156561.
Open this publication in new window or tab >>Planar Hall Effect Bridge sensor with NiFeX (X = Cu, Ag and Au) sensing layer.
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2015 (English)Conference paper, Published paper (Other academic)
Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2015
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-199028 (URN)10.1109/INTMAG.2015.7156561 (DOI)000381606000071 ()2-s2.0-84942474795 (Scopus ID)978-1-4799-7321-7 (ISBN)
Conference
IEEE International Magnetics Conference (Intermag), MAY 11-15, 2015, Beijing, PEOPLES R CHINA
Note

QC 20170113

Available from: 2017-01-13 Created: 2016-12-22 Last updated: 2017-04-04Bibliographically approved
Redjai Sani, S. (2013). Fabrication and Characterization of Nanocontact Spin-Torque Oscillators. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
Open this publication in new window or tab >>Fabrication and Characterization of Nanocontact Spin-Torque Oscillators
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The manufacturing of nanocontact-based spin-torque oscillators (NC-STOs)has opened the door for spintronic devices to play a part as active microwaveelements. The NC-STO has the capability of converting a direct current intoa microwave signal, and vice versa, by utilizing the spin transfer torque (STT)in ferromagnetic multilayer systems. However, the high-frequency operation ofNC-STOs typically requires high magnetic fields and the microwave power theygenerate is rather limited. As a result, NC-STOs are not yet commercially used,and they require improvements in both material systems and device geometriesbefore they can find actual use in microwave applications.

In order to improve and advance this technology, NC-STOs are requiredwith both different nanocontact (NC) sizes and geometries, and using differ- ent stacks of magnetic materials. This dissertation presents experimental in- vestigations into the manufacturing of such devices using different fabrication techniques and a number of different magnetic material stacks. Currently, the fabrication of NC-STOs is limited to advanced laboratories, because NC fabri- cation requires high-resolution lithography tools. In the present work, we have developed an alternative method of fabrication, which does not require such tools and has the capability of fabricating NC-STOs having one to hundreds of NCs in a variety of sizes, possibly  down to 20 nm. Devices fabricated with this method have shown mutual synchronization of three parallel-connected NCs, and pairwise synchronization in devices with four and five NCs.

Furthermore, the present work demonstrates low-field operation (down to0.02 Tesla) of NC-STOs at a record high frequency of 12 GHz. This wasachieved by implementing multilayers with a perpendicular magnetic anisotropy(PMA) material in the free layer of the NC-STO. In addition, the fabricateddevices revealed an unexpected dynamic regime under large external appliedfield (above 0.4 Tesla). The new dynamic regime was found to be due to anentirely novel nanomagnetic dynamic object â a so-called magnetic droplet soliton,predicted theoretically in 1977 but not experimentally observed until now.Detailed experiments and micromagnetic simulations show that the droplet hasvery rich dynamics.

Finally,  spin-torque-induced  transverse spin wave instabilities have beenstudied.  A NC-STO with  a material stack consisting of a single ferromag- netic metal sandwiched between two non-ferromagnetic metals was fabricated. Prior to this work, evidence of spin wave instabilities was reported as resis- tance switching in nanopillar- and mechanical point contact based STOs. In the present  work, the fabricated NC-STOs showed actual microwave  signals up to 3 GHz under zero applied field with strong current hysteresis. All  the fabricated NC-STOs open up new means of studying STT in different environ- ments, in order to resolve their current drawbacks for industrial applications.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. p. xvi, 65
Series
TRITA-ICT/MAP AVH, ISSN 1653-7610 ; 2013:04
Keywords
Spin-torque oscillators, phase locking, spin wave, giant magnetoresistance, spin transfer torque, thin films.
National Category
Physical Sciences Nano Technology
Identifiers
urn:nbn:se:kth:diva-122292 (URN)978-91-7501-760-0 (ISBN)
Public defence
2013-06-14, Sal/Hall E, Foru KTH-ITC, Isafjordsgatan 39, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 20130527

Available from: 2013-05-27 Created: 2013-05-17 Last updated: 2014-01-14Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-3726-9738

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