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Spin transfer torques and spin dynamics in point contacts and spin-flop tunnel junctions
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0002-2725-0558
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The first part of this thesis is an experimental study of the spin-dependent transport in magnetic point contacts. Nano-contacts are produced micromechanically, by bringing a sharpened non-magnetic (N) tip into contact with a ferromagnetic (F) film. The magnetic and magneto-transport properties of such N/F nanocontacts are studied using transport spectroscopy, spanning the ballistic, diffusive, and thermal transport regimes.

Single N/F interfaces can exhibit current driven magnetic excitations, which are often manifest as peaks in the differential resistance of a point contact defining the N/F interface. Our experiments show that such surface magnetization excitations, and thus the single-interface spin torques, are observed for diffusive and thermal transport regimes where the conduction electrons experience strong scattering near the N/F interface, and are absent for purely ballistic contacts. We conclude that the single-interface spin torque effect is due to impurity scattering at N/F interfaces.

Single N/F interfaces can also exhibit hysteretic conductivity, which is qualitatively similar to the spin-valve effect found in F/N/F trilayers. Based on our measurements of N/F point contacts in the size range of 1-30 nm, we propose two mechanisms of the observed hysteresis. The first mechanism relies on a non-uniform spin distribution near the contact core and is magnetoelastic in origin. This interpretation is in good agreement with some of our experiments on larger point contacts as well as with a numerical micromagnetic model we have developed, where a stress-induced anisotropy creates a non-uniform, domain-wall-like spin distribution in the contact core. The second mechanism we propose is a surface effect which relies on a difference between the surface and interior spins in the ferromagnet in terms of their exchange and anisotropy properties. The surface spin-valve mechanism is in good agreement with the hysteretic magnetoresistance observed for our smallest contacts (~1 nm) and for contacts to nanometer thin ferromagnetic films. This interpretation means that the surface magnetization can be reduced and weakly coupled to the interior spins in the ferromagnet. We find that this surface spin layer can be affected by both external fields and the spin torque of a transport current. The surface magnetization can even form nano-sized spin vorticies at the interface.

The nature of the magnetic excitations induced by by nominally unpolarized currents through single N/F interfaces was probed directly using microwave irradiation. We observed two characteristic high-frequency effects: a resonant stimulation of spin-wave modes by microwaves, and a rectification of off-resonant microwave currents by spin-wave nonlinearities in the point contact conductance. These experiments demonstrate that the effects observed are spin-dynamic in nature.

In the second part of the thesis we study the spin-dynamics in spin-flop tunnel junctions used in toggle magnetic random access memory. Current pulses in the range of 100 ps used to excite the magnetic moments of the two coupled Py free layers into an oscillatory state, in both the antiparallel and scissor states of the cell. These oscillations are detected directly by measuring the junction resistance in real time with a 6 GHz measurement bandwidth. The junctions had the shape of an ellipse, with lateral size ranging from 350x420 to 400x560 nm. The optical and acoustical precession modes of the the spin-flop trilayer are observed in experiment, as expected from single-domain model. The experimental spectra contain additional features, which are explained using numerical micromagnetic simulations, as originating from magnetic state transitions between different magnetization states with non-uniform spin distributions.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , v, 86 p.
Series
Trita-FYS, ISSN 0280-316X ; 2008-26
Keyword [en]
spin dynamics, spin transfer torques, point contacts, magnetization dynamics
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-4805ISBN: 978-91-7415-031-5 (print)OAI: oai:DiVA.org:kth-4805DiVA: diva2:14066
Public defence
2008-06-13, FB52, Albanova Univ. Center, Roslagstullbacken 21, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100818Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2010-08-18Bibliographically approved
List of papers
1. Spectroscopy of phonons and spin torques in magnetic point contacts
Open this publication in new window or tab >>Spectroscopy of phonons and spin torques in magnetic point contacts
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2005 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 95, no 18, 186602-1-186602-4 p.Article in journal (Refereed) Published
Abstract [en]

Phonon spectroscopy is used to investigate the mechanism of current-induced spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization excitations observed in the magneto-conductance of the point contacts are pronounced for diffusive and thermal contacts, where the electrons experience significant scattering in the contact region. We find no magnetic excitations in highly ballistic contacts. Our results show that impurity scattering at the N/F interface is the origin of the new single-interface spin torque effect.

Keyword
electric-current, multilayer, driven, excitation, waves
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-15142 (URN)10.1103/PhysRevLett.95.186602 (DOI)000232887400055 ()2-s2.0-28844464487 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Spin-torque driven excitations and hysteresis in magnetic point contacts
Open this publication in new window or tab >>Spin-torque driven excitations and hysteresis in magnetic point contacts
2006 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 99, no 8, 08G503-1-08G503-3 p.Article in journal (Refereed) Published
Abstract [en]

Magnetic multilayers are known to exhibit magnetization excitations, which are due to exchange fields produced by densely distributed spin-polarized electron currents. We report an observation of such excitations as well as hysteretic spin states produced by unpolarized currents in point contacts to single ferromagnetic layers. By measuring diffusive and ballistic contacts for various material combinations, we investigate the microscopic mechanism of the single interface spin-torque effect and discuss the possible origin of the observed hysteresis.

Keyword
Electric excitation; Electrons; Ferromagnetism; Magnetization; Microscopes; Multilayers; Ballistic contacts; Ferromagnetic layers; Magnetic multilayers; Magnetization excitations
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-8677 (URN)10.1063/1.2159410 (DOI)000237404200368 ()2-s2.0-33646747195 (Scopus ID)
Note
QC 20100818Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2010-08-18Bibliographically approved
3. Surface Spin-Valve Effect
Open this publication in new window or tab >>Surface Spin-Valve Effect
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2007 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 7, no 4, 927-931 p.Article in journal (Refereed) Published
Abstract [en]

We report an observation of spin-valve-like hysteresis within a few atomic layers at a ferromagnetic interface. We use phonon spectroscopy of nanometer-sized point contacts as an in situ probe to study the mechanism of the effect. Distinctive energy phonon peaks for contacts with dissimilar nonmagnetic outer electrodes allow localizing the observed spin switching to the top or bottom interfaces for nanometer thin ferromagnetic layers. The mechanism consistent with our data is energetically distinct atomically thin surface spin layers that can form current- or field-driven surface spin-valves within a single ferromagnetic film.

Keyword
Interfaces (materials); Nanostructured materials; Phonons; Spectroscopic analysis; Surface treatment; Thin films; Phonon spectroscopy; Surface spin-valve effects; Thin surface spin layers
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-8678 (URN)10.1021/nl0628192 (DOI)000245600500014 ()2-s2.0-34248187016 (Scopus ID)
Note

QC 20100818

Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2016-12-19Bibliographically approved
4. On the mechanism of hysteresis in conductance of point contacts to single ferromagnetic films
Open this publication in new window or tab >>On the mechanism of hysteresis in conductance of point contacts to single ferromagnetic films
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2007 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, no 9, 09A513-1-09A513-3 p.Article in journal (Refereed) Published
Abstract [en]

Single nonmagnetic/ferromagnetic interfaces can exhibit magnetic excitations and hysteretic switching, provided that the current density traversing the interface is sufficiently high (greater than or similar to 10(8) A/cm(2)) and the flow regime is diffusive. We measure hysteretic switching in conductance induced by nominally unpolarized electron currents in nanocontacts to thin Co films and successfully model the effect for similar to 20 nm scale point contacts using micromagnetic simulations, which take into account an out of plane stress-induced magnetic anisotropy in the point contact region.

Keyword
Computer simulation; Electric conductance; Hysteresis; Magnetic films; Magnetization; Switching; Ferromagnetic films; Ferromagnetic interfaces; Micromagnetic simulations; Unpolarized electron currents
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-8679 (URN)10.1063/1.2696709 (DOI)000246567900124 ()2-s2.0-34248523423 (Scopus ID)
Note

Conference: 10th Joint Magnetism and Magnetic Materials Conference/International Magnetics Conference. Baltimore, MD. JAN 07-11, 2007

QC 20110911

Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2015-04-27Bibliographically approved
5. Spin dynamics in point contacts to single ferromagnetic films
Open this publication in new window or tab >>Spin dynamics in point contacts to single ferromagnetic films
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2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 9, 092419-1-092419-4 p.Article in journal (Refereed) Published
Abstract [en]

Excitation of magnons or spin waves driven by nominally unpolarized transport currents in point contacts of normal and ferromagnetic metals is probed by irradiating the contacts with microwaves. Two characteristic dynamic effects are observed: a suppression of spin-wave nonlinearities in the point contact conductance by off-resonance microwave irradiation and a resonant stimulation of spin-wave peaks in the differential resistance of the nanocontacts by the microwave field. These observations provide direct evidence that the magnetoresistance peaks observed are due to gigahertz spin dynamics at the ferromagnetic interface driven by the spin transfer torque effect of the transport current.

Keyword
cobalt; copper; ferromagnetic materials; magnetic thin films; magnetoresistance; magnons; metallic thin films; nanocontacts; point contacts; radiation effects; spin dynamics; spin polarised transport; spin waves
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-8680 (URN)10.1103/PhysRevB.79.092419 (DOI)000264768200022 ()2-s2.0-65249178343 (Scopus ID)
Note
QC 20100818. Uppdaterad från manuskript till artikel (20100818).Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2010-08-18Bibliographically approved
6. Spin dynamics of two-coupled nanomagnets in spin-flop tunnel junctions
Open this publication in new window or tab >>Spin dynamics of two-coupled nanomagnets in spin-flop tunnel junctions
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2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 14, 144425-1-144425-6 p.Article in journal (Refereed) Published
Abstract [en]

Collective spin dynamics of two dipole-coupled nanomagnets in spin-flop tunnel junctions are studied experimentally and theoretically. The measured GHz magnetization oscillations reveal several collective spin-precessional modes. Analytical macrospin and numerical micromagnetic models of the spin-flop dynamics are developed, which provide a detailed explanation of the observed frequency spectra in terms of optical, acoustical, and micromagnetic modes in the antiparallel, parallel, and scissor magnetization states of the junctions.

Keyword
RANDOM-ACCESS MEMORY; MAGNETIC THIN-FILMS; TOGGLE MRAM; REVERSAL
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-8681 (URN)10.1103/PhysRevB.80.144425 (DOI)000271351500082 ()2-s2.0-71449109792 (Scopus ID)
Note
QC 20100818. Uppdaterad från manuskript till artikel (20100818). Tidigare titel: Spin dynamics of two coupled nanomagnets in spin-flop tunnelAvailable from: 2008-06-04 Created: 2008-06-04 Last updated: 2010-12-09Bibliographically approved

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