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  • 1.
    Bonanni, Valentina
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Pakizeh, Tavakol
    Pirzadeh, Zhaleh
    Chen, Jianing
    Nogues, Josef
    Vavassori, Paolo
    Hillenbrand, Rainer
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Dmitriev, Alexandre
    Designer Magnetoplasmonics with Nickel Nanoferromagnets2011In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, no 12, p. 5333-5338Article in journal (Refereed)
    Abstract [en]

    We introduce a new perspective on magnetoplasmonics in nickel nanoferromagnets by exploiting the phase tunability of the optical polarizability due to localized surface plasmons and simultaneous magneto-optical activity. We demonstrate how the concerted action of nanoplasmonics and magnetization can manipulate the sign of rotation of the reflected light's polarization (i.e., to produce Kerr rotation reversal) in ferromagnetic nanomaterials and, further, how this effect can be dynamically controlled and employed to devise conceptually new schemes for biochemosensing.

  • 2.
    Bonanni, Valentina
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Dumas, Randy K.
    Department of Physics, University of Gothenburg, Sweden.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    First-order reversal curve analysis of graded anisotropy FePtCu films2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 20, p. 202501-Article in journal (Refereed)
    Abstract [en]

    The reversal mechanisms of graded anisotropy FePtCu films have been investigated by alternating gradient magnetometer (AGM) and magneto-optical Kerr effect (MOKE) measurements with first-order reversal curve (FORC) techniques. The AGM-FORC analysis, which clearly shows the presence of soft and hard components, is unable to resolve how these phases are distributed throughout the film thickness. MOKE-FORC measurements, which preferentially probe the surface of the film, reveal that the soft components are indeed located toward the top surface. Combining AGM-FORC with the inherent surface sensitivity of MOKE-FORC analysis allows for a comprehensive analysis of heterogeneous systems such as graded materials.

  • 3.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Magnetization Dynamics in Nano-Contact Spin Torque Oscillators: Solitonic bullets and propagating spin waves2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Magnetization dynamics in nano-contact spin torque oscillators (STOs) is investigated from an experimental and theoretical point of view. The fundamentals of magnetization dynamics due to spin transfer torque are given.

    A custom-made high frequency (up to 46 GHz) in large magnetic fields (up to 2.2 T) microwave characterization setup has been built for the purpose and described in this thesis. A unique feature of this setup is the capability of applying magnetic fields at any direction θe out of the sample plane, and with high precision.

    This is particularly important, because the (average) out-of-plane angle of the STO free magnetic layer has fundamental impact on spin wave generation and STO operation.

    By observing the spin wave spectral emission as a function of θe, we find that at angles θe below a certain critical angle θcr, two distinct spin wave modes can be excited: a propagating mode, and a localized mode of solitonic character (so called spin wave bullet). The experimental frequency, current threshold and frequency tuneability with current of the two modes can be described qualitatively by analytical models and quantitatively by numerical simulations. We are also able to understand the importance, so far underestimated, of the Oersted field in the dynamics of nano-contact STOs. In particular, we show that the Oersted field strongly affects the current tuneability of the propagating mode at subcritical angles, and it is also the fundamental cause of the mode hopping observed in the time-domain. This mode hopping has been observed both experimentally using a state-of-the-art real-time oscilloscope and corroborated by micromagnetic simulations. Micromagnetic simulations also reveal details of the spatial distribution of the spin wave excitations.

    By investigating the emitted power as a function of θe, we observed two characteristic behaviors for the two spin wave modes: a monotonic increase of the power for increasing out-of-plane angles in the case of the propagating mode; an increase towards a maximum power followed by a drop of it at the critical angle for the localized mode. Both behaviors are reproduced by micromagnetic simulations. The agreement with the simulations offers also a way to better understand the precession dynamics, since the emitted power is strongly connected to the angular variation of the giant magnetoresistance signal.

    We also find that the injection locking of spin wave modes with a microwave source has a strong dependence on θe, and reaches a maximum locking strength at perpendicular angles. We are able to describe these results in the theoretical framework of non-linear spin wave dynamics.

    Download full text (pdf)
    Stefano_Bonetti_PhD_Thesis
  • 4.
    Bonetti, Stefano
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    de Vreede, Niels
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Conte, Gianluca
    Finocchio, Giovanni
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Phase locking of nanocontact spin torque osillators as a function of applied field angleManuscript (preprint) (Other academic)
  • 5.
    Bonetti, Stefano
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Kim, Jang-Yong
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Khartsev, Sergiy
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Grishin, Alexander
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Buried tantalate-niobate microwave varactors2007In: 2006 15TH IEEE INTERNATIONAL SYMPOSIUM ON APPLICATIONS OF FERROELECTRICS, 2007, p. 347-350Conference paper (Refereed)
    Abstract [en]

    We present characteristics of microwave variable capacitors (varactors) buried in 2.5 mu m thick AgTa0.5Nb0.5O3 (ATN) film pulsed laser deposited on sapphire single crystal. 2 gm gap interdigital capacitors (IDC) were fabricated by photolithographic, dry etching and lift-off processes. For comparison, similar IDCs were also defined on top of ATN film. Capacitance and loss tangent have been determined using a modified de-embedding technique in the microwave range 25 MHz - 40 GHz. Buried structures show higher values of capacitance and tunability, keeping the same level of losses compared to standard topped devices and resulting in an increased K-factor = tunability/tan delta. Experimental results are explained within equivalent circuit model. Besides the increased performance, the new design avoids the need of a successive planarization step, which could be required in an integration process.

  • 6.
    Bonetti, Stefano
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Muduli, Pranaba
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Spin torque oscillator frequency versus magnetic field angle: The prospect of operation beyond 65 GHz2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 10Article in journal (Refereed)
    Abstract [en]

    We study the impact of the magnetic field angle on the oscillation frequency of a nanocontact spin torque oscillator (STO) in magnetic fields up to 2.1 T. A model based on a single nonlinear, nonpropagating spin wave mode is found to explain the experimental data. We observe oscillation frequencies as high as 46 GHz in high magnetic fields applied normal to the film plane, and we are able to extrapolate the maximum expected operating frequency to beyond 65 GHz for in-plane magnetic fields. The STO signal remains surprisingly strong at these conditions, which opens up for possible millimeter-wave applications.

  • 7.
    Bonetti, Stefano
    et al.
    Stanford University.
    Pulisgiyo, Vito
    Consolo, Giancarlo
    Mancoff, Fred
    Tiberkevich, Vasyl S.
    Slavin, Andrei N.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Power and linewidth of propagating and localized modes in nanocontact spin-torque oscillators2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 17, p. 174427-Article in journal (Refereed)
    Abstract [en]

    The integrated power and linewidth of a propagating and a self-localized spin-wave mode excited by spin-polarized current in an obliquely magnetized magnetic nanocontact are studied experimentally as functions of the angle theta(e) between the external bias magnetic field and the nanocontact plane. It is found that the power of the propagating mode increases monotonically with theta(e), while the power of the self-localized mode has a broad maximum near theta(e) = 40 degrees and exponentially vanishes near the critical angle theta(e) = 58 degrees, at which the localized mode disappears. The linewidth of the propagating mode in the interval of angles 58 degrees < theta(e) < 90 degrees, where only this mode is excited, is adequately described by the existing theory, while in the angular interval where both modes can exist the observed linewidth of both modes is substantially broadened due to the telegraph switching between the modes. Numerical simulations and an approximate analytical model give a good semiquantitative description of the observed results.

  • 8.
    Bonetti, Stefano
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Tiberkevich, Vasil
    Consolo, Giancarlo
    Finocchio, Giovanni
    Muduli, Pranaba
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven magnetic nanocontacts2010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 21, p. 217204-Article in journal (Refereed)
    Abstract [en]

    Through detailed experimental studies of the angular dependence of spin wave excitations in nanocontact-based spin-torque oscillators, we demonstrate that two distinct spin wave modes can be excited, with different frequency, threshold currents, and frequency tunability. Using analytical theory and micromagnetic simulations we identify one mode as an exchange-dominated propagating spin wave, and the other as a self-localized nonlinear spin wave bullet. Wavelet-based analysis of the simulations indicates that the apparent simultaneous excitation of both modes results from rapid mode hopping induced by the Oersted field.

  • 9.
    Bonetti, Stefano
    et al.
    Stanford University, USA.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Nano-Contact Spin-Torque Oscillators as Magnonic Building Blocks2013In: Magnonics: From Fundamentals to Applications, Springer Berlin/Heidelberg, 2013, p. 177-187Chapter in book (Refereed)
    Abstract [en]

    We describe the possibility of using nano-contact spin-torque oscillators (NC-STOs) as fundamental magnonic building blocks. NC-STOs can act as spin wave generators, manipulators, and detectors, and can hence realize all the fundamental functions necessary for fully integrated magnonic devices, which can be fabricated using available CMOS compatible large-scale spin-torque device production processes. We show in particular how a 200 nm sized nano-contact located on an out-of-plane magnetized permalloy "free" magnetic layer can generate spin waves at f approximate to 15 GHz that propagate up to 4 mu m away from the nano-contact with wavelength lambda = 200-300 nm, decay length lambda(r) approximate to 2 mu m and group velocities v(g) approximate to 3 mu m/ns. We propose that the same type of NC-STOs can be used as spin wave manipulators, via control of the local Gilbert damping, and as spin wave detector using the spin torque diode effect.

  • 10. Chen, Jianing
    et al.
    Albella, Pablo
    Pirzadeh, Zhaleh
    Alonso-Gonzalez, Pablo
    Huth, Florian
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Nogues, Josep
    Vavassori, Paolo
    Dmitriev, Alexandre
    Aizpurua, Javier
    Hillenbrand, Rainer
    Plasmonic Nickel Nanoantennas2011In: SMALL, ISSN 1613-6810, Vol. 7, no 16, p. 2341-2347Article in journal (Refereed)
    Abstract [en]

    The fundamental optical properties of pure nickel nanostructures are studied by far-field extinction spectroscopy and optical near-field microscopy, providing direct experimental evidence of the existence of particle plasmon resonances predicted by theory. Experimental and calculated near-field maps allow for unambiguous identification of dipolar plasmon modes. By comparing calculated near-field and far-field spectra, dramatic shifts are found between the near-field and far-field plasmon resonances, which are much stronger than in gold nanoantennas. Based on a simple damped harmonic oscillator model to describe plasmonic resonances, it is possible to explain these shifts as due to plasmon damping.

  • 11.
    Eklund, Anders
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Sani, Sohrab R.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Persson, Johan
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Banuazizi, S. Amir Hossein
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Iacocca, Ezio
    Östling, Mikael
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Malm, B. Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Dependence of the colored frequency noise in spin torque oscillators on current and magnetic field2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, no 9, p. 092405-Article in journal (Refereed)
    Abstract [en]

    The nano-scale spin torque oscillator (STO) is a compelling device for on-chip, highly tunable microwave frequency signal generation. Currently, one of the most important challenges for the STO is to increase its longer-time frequency stability by decreasing the 1/f frequency noise, but its high level makes even its measurement impossible using the phase noise mode of spectrum analyzers. Here, we present a custom made time-domain measurement system with 150MHz measurement bandwidth making possible the investigation of the variation of the 1/f as well as the white frequency noise in a STO over a large set of operating points covering 18-25GHz. The 1/f level is found to be highly dependent on the oscillation amplitude-frequency non-linearity and the vicinity of unexcited oscillation modes. These findings elucidate the need for a quantitative theoretical treatment of the low-frequency, colored frequency noise in STOs. Based on the results, we suggest that the 1/f frequency noise possibly can be decreased by improving the microstructural quality of the metallic thin films.

  • 12.
    Eklund, Anders
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Sani, Sohrab R.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Malm, Gunnar
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    1/f and white frequency noise in a synchronized spin torque oscillator pair2011In: 56th Annual Conference on Magnetism and Magnetic Materials, 2011, p. 504-504Conference paper (Refereed)
  • 13.
    Fang, Yeyu
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    FORC studies of exchange biased NiFe in L1(0)(111) FePt-based spin valve2010In: INTERNATIONAL CONFERENCE ON MAGNETISM (ICM 2009) / [ed] Goll, G; Lohneysen, HV; Loidl, A; Pruschke, T; Richter, M; Schultz, L; Surgers, C; Wosnitza, J, BRISTOL: IOP PUBLISHING LTD , 2010, Vol. 200, p. 072002-Conference paper (Refereed)
    Abstract [en]

    We use First-Order Reversal Curves (FORC) to study the switching distribution and exchange bias in L1(0) (111) FePt-based spin valves with a layer structure of Ta (6)/Pt (3)/L1(0) (111) Fe(53)Pt(47) (20)/Co(50)Fe(50) (1.5)/Cu (4.5)/Co(50)Fe(50) (2)/Ni(80)Fe(20)(3) (in nm). We find that the switching of the free layer magnetization is strongly influence by the magnetic state of the FePt/CoFe fixed layer, as evidenced by tunable coercivity and exchange bias fields.

  • 14.
    Mohseni, Seyed Majid
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Redjai Sani, Sohrab
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Persson, J.
    Nguyen, T. N. Anh
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Chung, Sunjae
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Pogoryelov, Ye
    Muduli, P. K.
    Iacocca, E.
    Eklund, R. K.
    Dumas, R. K.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Deac, A.
    Hoefer, M.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Spin Torque-Generated Magnetic Droplet Solitons2013In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 339, no 6125, p. 1295-1298Article in journal (Refereed)
    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.

  • 15.
    Muduli, Pranaba
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Pogoryelov, Yevgen
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Consolo, G.
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Nonlinear frequency and amplitude modulation of a nanocontact-based spin-torque oscillator2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 14Article in journal (Refereed)
    Abstract [en]

    We study the current-controlled modulation of a nanocontact spin-torque oscillator. Three principally different cases of frequency nonlinearity (d(2)f/dI(dc)(2) being zero, positive, and negative) are investigated. Standard nonlinear frequency-modulation theory is able to accurately describe the frequency shifts during modulation. However, the power of the modulated sidebands only agrees with calculations based on a recent theory of combined nonlinear frequency and amplitude modulation.

  • 16.
    Pogoryelov, Yevgen
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Muduli, P. K.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Iacocca, E.
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Frequency modulation of spin torque oscillator pairs2011In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 19, p. 192501-Article in journal (Refereed)
    Abstract [en]

    The current controlled modulation of nanocontact based spin torque oscillator (STO) pairs is studied in both the synchronized and nonsynchronized states. The synchronized state shows a well behaved modulation and demonstrates robust mutual locking even under strong modulation. The power distribution of the modulation sidebands can be quantitatively described by assuming a single oscillator model. However, in the nonsynchronized state, the modulation sidebands are not well described by the model, indicating interactions between the two individual nanocontact STOs. These findings are promising for potential applications requiring the modulation of large synchronized STO arrays.

  • 17.
    Pogoryelov, Yevgen
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Muduli, P. K.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Mancoff, Fred
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Spin-torque oscillator linewidth narrowing under current modulation2011In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 19, p. 192506-Article in journal (Refereed)
    Abstract [en]

    We study the behavior of the linewidth of a nanocontact based spin torque oscillator (STO) under application of a radio frequency (100 MHz) modulating current. We achieve a significant (up to 85% ) reduction in the STO linewidth when it is modulated across a region of high nonlinearity. The mechanism responsible for the linewidth reduction is the nonlinear frequency shift under the influence of current modulation, which reduces the nonlinear amplification of the linewidth. The reduction in the linewidth during modulation can be quantitatively calculated from the free-running behavior of the STO.

  • 18.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zhou, Yan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Pseudo-spin-valve with L1(0) (111)-oriented FePt fixed layer2009In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 105, no 7, p. 07E910-Article in journal (Refereed)
    Abstract [en]

    Recently we proposed a spin torque oscillator where the fixed layer has its magnetization easy-axis tilted with respect to the film plane to simultaneously achieve zero-field operation and high output power [Y. Zhou, C. L. Zha, S. Bonetti, J. Persson, and J. angstrom kerman, Appl. Phys. Lett. 92, 262508 (2008)]. Here we take the first step toward the realization of this device and fabricate successfully a pseudo-spin-valve using an L1(0) (111)-oriented FePt fixed layer with tilted magnetocrystalline anisotropy. A total magnetoresistance (MR) of 0.86% is experimentally observed for the standard pseudo-spin-valve of the L1(0) FePt/Cu/NiFe structure in applied fields up to 1.5 T. While part of the MR may originate from the FePt film alone, the dominating part of MR correlates with switching of the NiFe free layer.

  • 19.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Pseudo spin valves based on L10 (111)-oriented FePt fixed layers with tilted anisotropy2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 16, p. 163108-Article in journal (Refereed)
    Abstract [en]

    We demonstrate magnetoresistance (MR) in excess of 4% in FePt/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111)-oriented FePt fixed layers with a 36 degrees out-of-plane tilted magnetization. The high MR is achieved by increasing the spin polarization at the Cu interfaces, using thin CoFe, and optimizing the FePt growth and Cu interface quality using Ta and Ta/Pt underlayers. We observe well-separated switching of the FePt/CoFe fixed layer and the CoFe/NiFe free layer, suggesting that CoFe is rigidly exchange coupled to FePt and NiFe in the respective layers.

  • 20.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Capacitance Enhanced Synchronization of Pairs of Spin-Transfer Oscillators2009In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 45, no 6, p. 2421-2423Article in journal (Refereed)
    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.

  • 21.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Zero field precession and hysteretic critical currents in spin torque oscillators with tilted polarizerIn: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630Article in journal (Other academic)
  • 22.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zero-field precession and hysteretic threshold currents in a spin torque nano device with tilted polarizer2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11Article in journal (Refereed)
    Abstract [en]

    Using nonlinear system theory and numerical simulations, we map out the static and dynamic phase diagrams in the zero applied field of a spin torque nano device with a tilted polarizer (TP). We find that for sufficiently large currents, even very small tilt angles (beta > 1 degrees) will lead to steady free layer precession in zero field. Within a rather large range of tilt angles, 1 degrees < beta < 19 degrees, we find coexisting static states and hysteretic switching between these using only current. In a more narrow window (1 degrees < beta < 5 degrees) one of the static states turns into a limit cycle (precession). The coexistence of current-driven static and dynamic states in the zero magnetic field is unique to the TP device and leads to large hysteresis in the upper and lower threshold currents for its operation. The nano device with TP can facilitate the generation of large amplitude mode of spin torque signals without the need for cumbersome magnetic field sources and thus should be very important for future telecommunication applications based on spin transfer torque effects.

  • 23.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Tunable intrinsic phase of a spin torque oscillator2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 9, p. 092505-Article in journal (Refereed)
    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.

  • 24.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Microwave generation of tilted-polarizer spin torque oscillator2009In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 105, no 7, p. 07D116-Article in journal (Refereed)
    Abstract [en]

    Microwave frequency generation in a spin torque oscillator (STO) with a tilted fixed layer magnetization is studied using numerical simulation of the Landau-Lifshitz-Gilbert-Slonczewski equation. The dependence of the STO free layer precession frequency on drive current is determined as a function of fixed layer tilt angle. We find that zero-field STO operation is possible for almost all tilt angles, which allow for great freedom in choosing the detailed layer structure of the STO.

  • 25.
    Zhou, Yan
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonetti, Stefano
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Spin-torque oscillator with tilted fixed layer magnetization2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 26, p. 262508-Article in journal (Refereed)
    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.

1 - 25 of 25
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