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Spin momentum transfer effects for spintronic device applications
KTH, School of Information and Communication Technology (ICT), Material Physics. (Applied Spintronics)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The recent discovery that a spin-polarized current can exert a large torque on a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of electrical current controlled manipulation of magnetic moment in nanoscale magnetic device structures. This so-called spin torque effect holds great promise for two applications, namely, spin torque oscillators (STOs) for wireless communication and radar communication, and spin transfer torque RAM (STT-RAM) for data/information storage.

 

The STO is a nanosized spintronic device capable of microwave generation at frequencies in the 1-65 GHz range with high quality factors. Although the STO is very promising for future telecommunication, two major shortcomings have to be addressed before it can truly find practical use as a radio-frequency device. Firstly, its very limited output power has to be significantly improved. One possibility is the synchronization of two or more STOs to both increase the microwave power and further increase the signal quality. Synchronization of serially connected STOs has been suggested in this thesis. In this configuration, synchronization relies on phase locking between the STOs and their self-generated alternating current. While this locking mechanism is intrinsically quite weak, we find that the locking range of two serially connected spin-valve STOs can be enhanced by over two orders of magnitude by adjusting the circuit I-V phase to that of an intrinsic preferred phase shift between the STO and an alternating current. More recently, we have also studied the phase-locking of STOs based on magnetic tunnel junctions (MTJ-STO) to meet the power specifications of actual application where the rf output levels should be above 0 dBm (1 mW). In addition to the spin torque terms present in GMR spin valves, MTJs also exhibit a significant perpendicular spin torque component with a quite complex dependence on both material choices and applied junction bias. We find that the perpendicular torque component modifies the intrinsic preferred I-V phase shift in single MTJ-STOs in such a way that serially connected STOs synchronize much more readily without the need for additional circuitry to change the I-V phase.

 

Secondly, equal attention has been focused on removing the applied magnetic field for STO operation, which requires bulky components and will limit the miniaturization of STO-based devices. Various attempts have been made to realize STOs operating in zero magnetic field. By using a tilted (oblique angle) polarizer (fixed layer) instead of an in-plane polarizer (standard STO), we show zero field operation over a very wide polarizer angle range without sacrificing output signal. In addition, the polarizer angle introduces an entirely new degree of freedom to any spin torque device and opens up for a wide range of additional phenomena.

 

The STT-RAM has advantages over other types of memories including conventional MRAM in terms of power consumption, speed, and scalability. We use a set of simulation tools to carry out a systematic study on the subject of micromagnetic switching processes of a device for STT-RAM application. We find that the non-zero k spin wave modes play an important role in the experimentally measured switching phase boundary. These may result in telegraph transitions among different spin-wave states, and be related to the back-hopping phenomena where the switching probability will decrease with increasing bias in tunnel junctions.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , xi, 79 p.
Series
Trita-ICT/MAP, 2009:04
Keyword [en]
Spin Torque Oscillator, Giant Magnetoresistance, Tunneling Magnetoresistance
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-10605ISRN: KTH/ICT-MAP/AVH-2009:04-SEISBN: 978-91-7415-285-2 (print)OAI: oai:DiVA.org:kth-10605DiVA: diva2:220709
Public defence
2009-06-08, C1, KTH-Electrum, Isafjordsgatan 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20100819Available from: 2009-06-02 Created: 2009-06-02 Last updated: 2010-08-19Bibliographically approved
List of papers
1. Intrinsic phase shift between a spin torque oscillator and an alternating current
Open this publication in new window or tab >>Intrinsic phase shift between a spin torque oscillator and an alternating current
2007 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, no 9, 09A510- p.Article in journal (Refereed) Published
Abstract [en]

We report on a preferred phase shift Delta phi(0) between a spin torque oscillator (STO) and an alternating current (I-ac) injected at the intrinsic frequency (f(STO)) of the STO. In the in-plane (IP) precession mode the STO adjusts to a state where its resistance (or voltage) lags I-ac about a quarter of a wavelength [Delta phi(0)=(87-94)degrees]. In the IP mode Delta phi(0) increases somewhat with the direct current. As the precession changes into the out-of-plane (OOP) mode, Delta phi(0) jumps by about 180 degrees, i.e., the STO resistance now precedes I-ac about a quarter of a wavelength (Delta phi(0)=-86 degrees). At the IP/OOP boundary, the alternating current mixes the two oscillation modes and both periodic and chaotic oscillations are observed. As a consequence of mixing, subharmonic terms appear in the STO signal. The intrinsic Delta phi(0) will impact any circuit design based on STO technology and will, e.g., have direct consequences for phase locking in networks of serially connected STOs.

Keyword
dynamics, driven
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-16641 (URN)10.1063/1.2710740 (DOI)000246567900121 ()2-s2.0-34248588611 (Scopus ID)
Note
Conference: 10th Joint Magnetism and Magnetic Materials Conference/International Magnetics Conference. Baltimore, MD. JAN 07-11, 2007 Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Phase-locked spin torque oscillators: Impact of device variability and time delay
Open this publication in new window or tab >>Phase-locked spin torque oscillators: Impact of device variability and time delay
2007 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, no 9, 09A503- p.Article in journal (Refereed) Published
Abstract [en]

We have carried out detailed Landau-Lifshitz-Gilbert simulations of pairs of serially connected spin torque oscillators (STOs) in parallel with a resistive load. To study the impact of realistic process variations on STO synchronization we let the two STOs have different in-plane anisotropy fields (H-k). The simulation also provides for a time delay (tau). We construct a phase diagram of the STO synchronization as a function of H-k and direct current (I-dc) at different tau. The phase diagram turns out to be quite rich with different types of synchronized precession modes. While the synchronized state is originally very sensitive to STO process variations and can only sustain up to 4% H-k variation, the addition of a small time delay dramatically improves its robustness and allows as much as 145% H-k variation in the entire out-of-plane precession regime.

Keyword
polarized current, co/cu/co pillars, driven, dynamics, locking
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-16640 (URN)10.1063/1.2670045 (DOI)000246567900114 ()2-s2.0-34248585293 (Scopus ID)
Note
Conference: 10th Joint Magnetism and Magnetic Materials Conference/International Magnetics Conference. Baltimore, MD. JAN 07-11, 2007 Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
3. Tunable intrinsic phase of a spin torque oscillator
Open this publication in new window or tab >>Tunable intrinsic phase of a spin torque oscillator
2008 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 9, 092505- p.Article in journal (Refereed) Published
Abstract [en]

We show analytically why a spin torque oscillator (STO) locks to an alternating current (I-ac), having the same frequency as the STO, at a nontrivial, intrinsic, phase value Delta phi(0). We determine both the stability and the exact phase of the locked state and explain the recently observed phase jump by about 180 degrees at the in-plane-to-out-of-plane precession boundary. Delta phi(0) depends on the STO material parameters and is tunable through both the applied field and the dc current, which provides an additional control of the exact I-V phase in an STO driven radio frequency circuit.

Keyword
magnetic tunnel-junctions, co/cu/co pillars, nanopillars, reversal, locking
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-17354 (URN)10.1063/1.2891058 (DOI)000253761500069 ()2-s2.0-40549102449 (Scopus ID)
Note
QC 20100525 QC 20120321Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
4. Spin-torque oscillator with tilted fixed layer magnetization
Open this publication in new window or tab >>Spin-torque oscillator with tilted fixed layer magnetization
Show others...
2008 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 26, 262508- p.Article in journal (Refereed) Published
Abstract [en]

A spin-torque oscillator with a fixed layer magnetization tilted out of the film plane is capable of strong microwave signal generation in zero magnetic field. Through numerical simulations, we study the microwave signal generation as a function of drive current for two realistic tilt angles. The tilted magnetization of the fixed layer can be achieved by using a material with high out-of-plane magnetocrystalline anisotropy, such as L1(0) FePt.

Keyword
polarized current, wave excitations, driven, field
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-17673 (URN)10.1063/1.2955831 (DOI)000257424500049 ()2-s2.0-46649106924 (Scopus ID)
Note
QC 20100525. Tidigare titel: Spin-torque oscillator based on tilted magnetization of the fixed layer Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
5. Perpendicular spin torque promotes synchronization of magnetic tunnel junction based spin torque oscillators
Open this publication in new window or tab >>Perpendicular spin torque promotes synchronization of magnetic tunnel junction based spin torque oscillators
2009 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 11, 112503- p.Article in journal (Refereed) Published
Abstract [en]

We study how the perpendicular spin transfer torque term (b(j)), present in magnetic tunneling junctions (MTJs), affects the synchronization of serially connected MTJ-based spin torque oscillators (MTJ-STOs). We find that b(j) modifies the intrinsic preferred I-V phase shift in single MTJ-STO in such a way that serially connected MTJ-STOs synchronize much more readily without the need for additional circuitry to change the I-V phase. This result strongly suggests that synchronization of serially connected MTJ-STOs should be significantly more straightforward than the previously attempted serial synchronization of giant magnetoresistance based STOs (GMR-STOs).

Keyword
boron alloys, cobalt alloys, iron alloys, magnetic microwave devices, magnetic thin films, microwave oscillators, tunnelling magnetoresistance, phase-locking, driven, dependence, emission, devices
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-18260 (URN)10.1063/1.3100299 (DOI)000264380300047 ()2-s2.0-63049098851 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
6. Capacitance Enhanced Synchronization of Pairs of Spin-Transfer Oscillators
Open this publication in new window or tab >>Capacitance Enhanced Synchronization of Pairs of Spin-Transfer Oscillators
2009 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 45, no 6, 2421-2423 p.Article in journal (Refereed) Published
Abstract [en]

Macro-spin simulations have been performed to study the synchronization of pairs of serially connected spin torque oscillators in parallel with a capacitor. The sensitivity to the STO process variation, which is modeled by the anisotropy field distribution for different STO samples, can be dramatically decreased by the coupling of a capacitor in the circuit. There exists an optimum capacitance where the multi-STO synchronized state develops the highest robustness.

Keyword
Preferred phase shift, spin torque oscillator, synchronization
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-24171 (URN)10.1109/TMAG.2009.2018595 (DOI)000266329800019 ()2-s2.0-66549103953 (Scopus ID)
Note
QC 20100819Available from: 2010-08-19 Created: 2010-08-19 Last updated: 2017-12-12Bibliographically approved
7. Zero field precession and hysteretic critical currents in spin torque oscillators with tilted polarizer
Open this publication in new window or tab >>Zero field precession and hysteretic critical currents in spin torque oscillators with tilted polarizer
(English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630Article in journal (Other academic) Submitted
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-24172 (URN)
Note
QS 20120328Available from: 2010-08-19 Created: 2010-08-19 Last updated: 2017-12-12Bibliographically approved
8. Micromagnetic study of switching boundary of a ferromagnetic nanodevice
Open this publication in new window or tab >>Micromagnetic study of switching boundary of a ferromagnetic nanodevice
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-24173 (URN)
Note
QC 20100819Available from: 2010-08-19 Created: 2010-08-19 Last updated: 2010-08-19Bibliographically approved

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