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Publications (10 of 37) Show all publications
Wei, Y., Akansel, S., Thersleff, T., Harward, I., Brucas, R., Ranjbar, M., . . . Svedlindh, P. (2015). Exponentially decaying magnetic coupling in sputtered thin film FeNi/Cu/FeCo trilayers. Applied Physics Letters, 106(4), Article ID 042405.
Open this publication in new window or tab >>Exponentially decaying magnetic coupling in sputtered thin film FeNi/Cu/FeCo trilayers
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2015 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 106, no 4, article id 042405Article in journal (Refereed) Published
Abstract [en]

Magnetic coupling in trilayer films of FeNi/Cu/FeCo deposited on Si/SiO2 substrates have been studied. While the thicknesses of the FeNi and FeCo layers were kept constant at 100 angstrom, the thickness of the Cu spacer was varied from 5 to 50 angstrom. Both hysteresis loop and ferromagnetic resonance results indicate that all films are ferromagnetically coupled. Micromagnetic simulations well reproduce the ferromagnetic resonance mode positions measured by experiments, enabling the extraction of the coupling constants. Films with a thin Cu spacer are found to be strongly coupled, with an effective coupling constant of 3 erg/cm(2) for the sample with a 5 angstrom Cu spacer. The strong coupling strength is qualitatively understood within the framework of a combined effect of Ruderman-Kittel-Kasuya-Yosida and pinhole coupling, which is evidenced by transmission electron microscopy analysis. The magnetic coupling constant surprisingly decreases exponentially with increasing Cu spacer thickness, without showing an oscillatory thickness dependence. This is partially connected to the substantial interfacial roughness that washes away the oscillation. The results have implications on the design of multilayers for spintronic applications.

National Category
Physical Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics; Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-247149 (URN)10.1063/1.4906591 (DOI)000348996200043 ()
Available from: 2015-03-17 Created: 2015-03-13 Last updated: 2017-12-04Bibliographically approved
Puliafito, V., Pogoryelov, Y., Azzerboni, B., Akerman, J. & Finocchio, G. (2014). Hysteretic Synchronization in Spin-Torque Nanocontact Oscillators: A Micromagnetic Study. IEEE transactions on nanotechnology, 13(3), 532-536
Open this publication in new window or tab >>Hysteretic Synchronization in Spin-Torque Nanocontact Oscillators: A Micromagnetic Study
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2014 (English)In: IEEE transactions on nanotechnology, ISSN 1536-125X, E-ISSN 1941-0085, Vol. 13, no 3, p. 532-536Article in journal (Refereed) Published
Abstract [en]

Several experiments report the presence of finite jumps in the properties of spin-torque oscillators at room temperature, such as oscillation frequency and power as functions of current or field. On the basis of micromagnetic simulations, this paper links those experimental discontinuities to the changes in the curve slope numerically observed in the absence of thermal effects. Our numerical results show the key ingredient triggering this behavior is the presence of abrupt changes in the oscillation axis of the magnetization precession. We also predict that by fixing the bias point of the oscillator near those critical regions, it is possible to observe hysteretic synchronization. This result should be a key point in the design of nanoscale on-chip phase-locked loop receivers with improved sensitivity.

Keywords
Hysteretic synchronization; micromagnetism; nanooscillators; spin-transfer torque
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-239337 (URN)10.1109/TNANO.2014.2308474 (DOI)
Available from: 2014-12-22 Created: 2014-12-22 Last updated: 2017-12-05
Wei, Y., Jana, S., Brucas, R., Pogoryelov, Y., Ranjbar, M., Arena, D. A., . . . Svedlindh, P. (2014). Magnetic coupling in asymmetric FeCoV / Ru / FeNi trilayers. Journal of Applied Physics, 115(17), 17D129
Open this publication in new window or tab >>Magnetic coupling in asymmetric FeCoV / Ru / FeNi trilayers
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2014 (English)In: Journal of Applied Physics, Vol. 115, no 17, p. 17D129-Article in journal (Refereed) Published
Abstract [en]

We have investigated the magnetic anisotropy and interlayer coupling in trilayer films of permendur(100 \AA)/Ru/permalloy(100 \AA), with the thickness of the Ru spacer varying from 0 to 200 \AA. While the permendur/permalloy sample exhibits a small in-plane uniaxial magnetic anisotropy with Hu = 27 Oe, all trilayers are magnetically isotropic in-plane. Results from hysteresis loop and ferromagnetic resonance measurements were fitted to a micromagnetic model, with the results indicating that all the films are ferromagnetically coupled except the one with 10 \AA Ru spacer, which shows antiferromagnetic coupling. The trilayers with Ru spacer layer thickness larger than 20 \AA exhibit only very weak ferromagnetic coupling.

National Category
Condensed Matter Physics Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-218308 (URN)10.1063/1.4864743 (DOI)000335643700407 ()
Available from: 2014-02-10 Created: 2014-02-10 Last updated: 2014-07-18Bibliographically approved
Mohseni, S. M., Sani, S. R., Dumas, R. K., Persson, J., Anh Nguyen, T. N., Chung, S., . . . \AA kerman, J. (2014). Magnetic droplet solitons in orthogonal nano-contact spin torque oscillators. Paper presented at 9th International Symposium on Hysteresis Modeling and Micromagnetics (HMM 2013). Physica. B, Condensed matter, 435, 84-87
Open this publication in new window or tab >>Magnetic droplet solitons in orthogonal nano-contact spin torque oscillators
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2014 (English)In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 435, p. 84-87Article in journal (Refereed) Published
Abstract [en]

We study microwave signal generation as a function of drive current and applied perpendicular magnetic field in nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has strong easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The orthogonal NC-STOs exhibit a dramatic transition from the conventional ferromagnetic resonance-like spinwave mode to a magnetic droplet soliton mode. In particular, the field and current dependence of the droplet soliton near threshold are discussed. Near threshold the droplet soliton undergoes complex dynamics that include mode hopping, as evident in the experimental frequency domain and magnetoresistance response.

Keywords
Magnetoresistance, Spin wave, magnetic droplet soliton, perpendicular magnetic anisotropy, spin torque oscillators
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-217992 (URN)10.1016/j.physb.2013.10.023 (DOI)
Conference
9th International Symposium on Hysteresis Modeling and Micromagnetics (HMM 2013)
Available from: 2014-02-06 Created: 2014-02-06 Last updated: 2017-12-06Bibliographically approved
Filatov, A., Pogorelov, A. & Pogoryelov, Y. (2014). Negative differential resistance in magnetic tunnel junction systems. Physica status solidi. B, Basic research, 251(1), 172-177
Open this publication in new window or tab >>Negative differential resistance in magnetic tunnel junction systems
2014 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 251, no 1, p. 172-177Article in journal (Refereed) Published
Abstract [en]

We study the electrophysical properties of the Fe/MgO/Femagnetic tunnel junctions (MTJ) with impurities. Samplestructures are fabricated on top of fine-crystalline glass–ceramicsubstrates by e-beam evaporation in a relatively low vacuum(~10-4 Torr). The influence of the first magnetic layerfabrication conditions on the degradation of the MTJ isexplained by the interlayer diffusion. Various models ofelectrophysical processes in MTJ on polycrystalline substratesare discussed. The current–voltage (I–V) characteristics of thefabricated structures are found to exhibit a region with negativedifferential resistance, similar to the one in tunneling diodes.We explain this phenomenon by the formation of excitons inthe MgO layer modified by the conductive impurity atoms andtheir diffusion. The obtained results will be useful in thedevelopment of MRAM devices containing MTJs andtunneling diodes.

Keywords
magnetic properties, magnetic tunnel junctions, negative differential resistance, polycrystalline materials, thin films
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-215352 (URN)10.1002/pssb.201349258 (DOI)
Available from: 2014-01-13 Created: 2014-01-13 Last updated: 2017-12-06Bibliographically approved
Dumas, R. K., Sani, S. R., Mohseni, S. M., Iacocca, E., Pogoryelov, Y., Muduli, P. K., . . . Akerman, J. (2014). Recent Advances in Nanocontact Spin-Torque Oscillators. Paper presented at 1st International Symposium on Frontiers in Materials Science (IS FMS), NOV 17-19, 2013, Hanoi, VIETNAM. IEEE transactions on magnetics, 50(6), 4100107
Open this publication in new window or tab >>Recent Advances in Nanocontact Spin-Torque Oscillators
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2014 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 6, p. 4100107-Article in journal (Refereed) Published
Abstract [en]

We present a comprehensive review of the most recent advances in nanocontact spin torque oscillators (NC-STOs). NC-STOs are highly tunable, with both applied magnetic field and dc, broadband microwave signal generators. As opposed to the nanopillar geometry, where the lateral cross section of the entire device has been confined to a typically <100 nm diameter, in NC-STOs, it is only the current injection site that has been laterally confined on top of an extended magnetic film stack. Three distinct material combinations will be discussed: 1) a Co/Cu/NiFe pseudospin valve (PSV) where both the Co and NiFe have a dominant in-plane anisotropy; 2) a Co/Cu/[Co/Ni](4) orthogonal PSV where the Co/Ni multilayer has a strong perpendicular anisotropy; and 3) a single NiFe layer with asymmetric non-magnetic Cu leads. We explore the rich and diverse magnetodynamic modes that can be generated in these three distinct sample geometries.

Keywords
Magnetodynamics, spin waves (SWs), spin-torque oscillator (STO), spin-transfer torque (STT)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-237595 (URN)10.1109/TMAG.2014.2305762 (DOI)000343033900036 ()
Conference
1st International Symposium on Frontiers in Materials Science (IS FMS), NOV 17-19, 2013, Hanoi, VIETNAM
Available from: 2014-12-04 Created: 2014-12-03 Last updated: 2017-12-05Bibliographically approved
Chung, S., Mohseni, S. M., Sani, S. R., Iacocca, E., Dumas, R. K., Nguyen, T. N., . . . Akerman, J. (2014). Spin transfer torque generated magnetic droplet solitons (invited). Journal of Applied Physics, 115(17)
Open this publication in new window or tab >>Spin transfer torque generated magnetic droplet solitons (invited)
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2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, no 17Article in journal (Refereed) Published
Abstract [en]

We present recent experimental and numerical advancements in the understanding of spin transfer torque generated magnetic droplet solitons. The experimental work focuses on nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has an easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The NC-STO resistance and microwave signal generation are measured simultaneously as a function of drive current and applied perpendicular magnetic field. Both exhibit dramatic transitions at a certain current dependent critical field value, where the microwave frequency drops 10 GHz, modulation sidebands appear, and the resistance exhibits a jump, while the magnetoresistance changes sign. We interpret these observations as the nucleation of a magnetic droplet soliton with a large fraction of its magnetization processing with an angle greater than 90 degrees, i.e., around a direction opposite that of the applied field. This interpretation is corroborated by numerical simulations. When the field is further increased, we find that the droplet eventually collapses under the pressure from the Zeeman energy. (C) 2014 AIP Publishing LLC.

National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-239338 (URN)10.1063/1.4870696 (DOI)
Available from: 2014-12-22 Created: 2014-12-22 Last updated: 2017-12-05
Sani, S., Persson, J., Mohseni, S., Pogoryelov, Y., Muduli, P., Eklund, A., . . . Åkerman, J. (2013). Mutually synchronized bottom-up multi-nanocontact spin–torque oscillators. Nature Communications, 4
Open this publication in new window or tab >>Mutually synchronized bottom-up multi-nanocontact spin–torque oscillators
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2013 (English)In: Nature Communications, Vol. 4Article in journal (Refereed) Published
Abstract [en]

Spin–torque oscillators offer a unique combination of nanosize, ultrafast modulation rates and ultrawide band signal generation from 100MHz to close to 100 GHz. However, their low output power and large phase noise still limit their applicability to fundamental studies of spin-transfer torque and magnetodynamic phenomena. A possible solution to both problems is the spin-wave-mediated mutual synchronization of multiple spin–torque oscillators through a shared excited ferromagnetic layer. To date, synchronization of high-frequency spin–torque oscillators has only been achieved for two nanocontacts. As fabrication using expensive top–down lithography processes is not readily available to many groups, attempts to syn- chronize a large number of nanocontacts have been all but abandoned. Here we present an alternative, simple and cost-effective bottom-up method to realize large ensembles of synchronized nanocontact spin–torque oscillators. We demonstrate mutual synchronization of three high-frequency nanocontact spin–torque oscillators and pairwise synchronization in devices with four and five nanocontacts.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-215351 (URN)10.1038/ncomms3731 (DOI)
Available from: 2014-01-13 Created: 2014-01-13 Last updated: 2014-01-13
Mohseni, S. M., Sani, S. R., Persson, J., Nguyen, T. N., Chung, S., Pogoryelov, Y., . . . Akerman, J. (2013). Spin Torque-Generated Magnetic Droplet Solitons. Science, 339(6125), 1295-1298
Open this publication in new window or tab >>Spin Torque-Generated Magnetic Droplet Solitons
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2013 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 339, no 6125, p. 1295-1298Article in journal (Refereed) Published
Abstract [en]

Dissipative solitons have been reported in a wide range of nonlinear systems, but the observation of their magnetic analog has been experimentally challenging. Using spin transfer torque underneath a nanocontact on a magnetic thin film with perpendicular magnetic anisotropy (PMA), we have observed the generation of dissipative magnetic droplet solitons and report on their rich dynamical properties. Micromagnetic simulations identify a wide range of automodulation frequencies, including droplet oscillatory motion, droplet “spinning,” and droplet “breather” states. The droplet can be controlled by using both current and magnetic fields and is expected to have applications in spintronics, magnonics, and PMA-based domain-wall devices.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-217990 (URN)10.1126/science.1230155 (DOI)
Available from: 2014-02-06 Created: 2014-02-06 Last updated: 2017-12-06
Pogoryelov, Y., Muduli, P. K. & Akerman, J. (2012). Combined Wide-Narrow Double Modulation of Spin-Torque Oscillators for Improved Linewidth During Communication. IEEE Transactions on Magnetics, 48(11), 4077-4080
Open this publication in new window or tab >>Combined Wide-Narrow Double Modulation of Spin-Torque Oscillators for Improved Linewidth During Communication
2012 (English)In: IEEE Transactions on Magnetics, Vol. 48, no 11, p. 4077-4080Article in journal (Refereed) Published
Abstract [en]

Spin-torque oscillators (STOs) offer the advantage of high modulation rates. However the existence of regions with high-frequency nonlinearity, accompanied by increased signal linewidth, limits both the usable STO frequency range and the minimum frequency of modulation. On the other hand, use of a large modulation frequency increases the frequency modulation (FM) bandwidth. In order to overcome these problems, we propose to simultaneously modulate the STO with two signals having different frequencies. We use a high-frequency, “wide” $(f_{rm W}=500 {rm MHz})$ , modulation signal to improve the STO linewidth. At the same time we use a low-frequency, “narrow” $(f_{rm N}=40 {rm MHz})$, modulation signal to show successful narrow bandwidth modulation of the improved STO signal. The proposed method of combined wide-narrow double modulation can significantly improve the usable operating frequency range of the STOs for communication applications.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-215353 (URN)10.1109/TMAG.2012.2200465 (DOI)
Available from: 2014-01-13 Created: 2014-01-13 Last updated: 2014-01-13
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-5867-425X

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