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  • 1.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Spin-diode effect and thermally controlled switching in magnetic spin-valves2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis demonstrates two new device concepts that are based on the tunneling and giant magnetoresistance effects. The first is a semiconductor-free asymmetric magnetic double tunnel junction that is shown to work as a diode, while at the same time exhibiting a record high magnetoresistance. It is experimentally verified that a diode effect, with a rectification ratio of at least 100, can be obtained in this type of system, and that a negative magnetoresistance of nearly 4000% can be measured at low temperature. The large magnetoresistance is attributed to spin resonant tunneling, where the parallel and antiparallel orientation of the magnetic moments shifts the energy levels in the middle electrode, thereby changing their alignment with the conduction band in the outer electrodes. This resonant tunneling can be useful when scaling down magnetic random access memory; eliminating the need to use external diodes or transistors in series with each bit.

    The second device concept is a thermally controlled spin-switch; a novel way to control the free-layer switching and magnetoresistance in spin-valves. By exchange coupling two ferromagnetic films through a weakly ferromagnetic Ni-Cu alloy, the coupling is controlled by changes in temperature. At room temperature, the alloy is weakly ferromagnetic and the two films are exchange coupled through the alloy. At a temperature higher than the Curie point, the alloy is paramagnetic and the two strongly ferromagnetic films decouple. Using this technique, the read out signal from a giant magnetoresistance element is controlled using both external heating and internal Joule heating. No degradation of device performance upon thermal cycling is observed. The change in temperature for a full free-layer reversal is shown to be 35 degrees Celsius for the present Ni-Cu alloy. It is predicted that this type of switching theoretically can lead to high frequency oscillations in current, voltage, and temperature, where the frequency is controlled by an external inductor or capacitor. This can prove to be useful for applications such as voltage controlled oscillators in, for example, frequency synthesizers and function generators. Several ways to optimize the thermally controlled spin switch are discussed and conceptually demonstrated with experiments.

  • 2.
    Andersson, Sebastian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Exchange coupling and magnetoresistance in CoFe/NiCu/CoFe spin valves near the Curie point of the spacer2010In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 9, p. 09D711-Article in journal (Refereed)
    Abstract [en]

    Thermal control of exchange coupling between two strongly ferromagnetic layers through a weakly ferromagnetic Ni-Cu spacer and the associated magnetoresistance is investigated. The spacer, having a Curie point slightly above room temperature, can be cycled between its paramagnetic and ferromagnetic states by varying the temperature externally or using joule heating. It is shown that the giant magnetoresistance vanishes due to a strong reduction in the mean free path in the spacer at above similar to 30% Ni concentration-before the onset of ferromagnetism. Finally, a device is proposed which combines thermally controlled exchange coupling and large magnetoresistance by separating the switching and the readout elements.

  • 3.
    Andersson, Sebastian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Thermoelectrically Controlled Spin-Switch2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2140-2143Article in journal (Refereed)
    Abstract [en]

    The search for novel spintronic devices brings about new ways to control switching in magnetic thin-films. In this work we experimentally demonstrate a device based on thermoelectrically controlled exchange coupling. The read out signal from a giant magnetoresistance element is controlled by exchange coupling through a weakly ferromagnetic Ni-Cu alloy. This exchange coupling is shown to vary strongly with changes in temperature, and both internal Joule heating and external heating is used to demonstrate magnetic switching. The device shows no degradation upon thermal cycling. Ways to further optimize the device performance are discussed. Our experimental results show a new way to thermoelectrically control magnetic switching in multilayers.

  • 4. Balkashin, O. P.
    et al.
    Fisun, V. V.
    Triputen, L. Yu
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Naidyuk, Yu G.
    Spin-valve effects in point contacts to exchange biased Co40Fe40B20 films2014In: Low temperature physics (Woodbury, N.Y., Print), ISSN 1063-777X, E-ISSN 1090-6517, Vol. 40, no 10, p. 915-918Article in journal (Refereed)
    Abstract [en]

    Nonlinear current-voltage characteristics and magnetoresistance of point contacts between a normal metal (N) and films of amorphous ferromagnet (F) Co40Fe40B20 of different thickness, exchange-biased by antiferromagnetic Mn80Ir20 are studied. A surface spin valve effect in the conductance of such F-N contacts is observed. The effect of exchange bias is found to be inversely proportional to the Co40Fe40B20 film thickness. This behavior as well as other magneto-transport effects we observe on single exchange-pinned ferromagnetic films are similar in nature to those found in conventional three-layer spin-valves.

  • 5.
    Iovan, Adrian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Naidyuk, Yu. G.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Vedyaev, A.
    Dieny, B.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Spin diode based on Fe/MgO double tunnel junction2008In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 8, no 3, p. 805-809Article in journal (Refereed)
    Abstract [en]

    We demonstrate a spin diode consisting of a semiconductor-free nanoscale Fe/MgO-based double tunnel junction. The device exhibits a near perfect spin-valve effect combined with a strong diode effect. The mechanism consistent with our data is resonant tunneling through discrete states in the middle ferromagnetic layer sandwiched by tunnel barriers of different spin-dependent transparency. The observed magnetoresistance is a record high > 1000%, essentially making the structure an on/off spin switch. This, combined with the strong diode effect, similar to 100, demonstrates a new device principle, promising for memory and reprogrammable logic applications.

  • 6.
    Iovan, Adrian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Lam, Kanber
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Cherepov, Sergiy
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Haviland, David B.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Tunneling spectroscopy of magnetic double barrier junctions2007In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 43, no 6, p. 2818-2820Article in journal (Refereed)
    Abstract [en]

    Scanning tunneling microscopy (STM) is used to study transport in magnetic double tunnel junctions (DTJs) formed using a fixed transparency barrier of a patterned tunnel junction (TJ), and a variable tunnel barrier between the top electrode of the patterned junction and the STM tip. A sufficiently thin top electrode has been predicted to result in a rectification of charge current through a DTJ when the two barriers have different transparency. Our measurements indeed show a high current rectification ratio for 3-nm-thick, continuous film top electrodes, which is observed for junctions with asymmetric tunnel barriers.

  • 7.
    Kadigrobov, A.
    et al.
    Department of Physics, University of Gothenburg.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Park, Hee Chul
    Department of Physics, University of Gothenburg.
    Radic, D.
    Department of Physics, University of Gothenburg.
    Shekhter, R. I.
    Department of Physics, University of Gothenburg.
    Jonson, M.
    Department of Physics, University of Gothenburg.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Thermal-magnetic-electric oscillator based on spin-valve effect2012In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 111, p. 044315-Article in journal (Refereed)
    Abstract [en]

    A thermal-magnetic-electric valve with the free layer of exchange-spring type and inverse magnetoresistance is investigated. The structure has S-shaped current-voltage characteristics and can exhibit spontaneous oscillations when integrated with a conventional capacitor within a resonator circuit. The frequency of the oscillations can be controlled from essentially dc to the GHz range by the circuit capacitance.

  • 8. Kadigrobov, A. M.
    et al.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Radic, D.
    Shekhter, R. I.
    Jonson, M.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Thermoelectrical manipulation of nanomagnets2010In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 12, p. 123706-Article in journal (Refereed)
    Abstract [en]

    We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic device based on a magnetic multilayer (F/f/F), in which two strongly ferromagnetic layers (F) are exchange-coupled through a weakly ferromagnetic spacer (f) with the Curie temperature in the vicinity of room temperature. We show theoretically that the Joule heating produced by the spin-dependent current allows a spin-thermoelectronic control of the ferromagnetic-to-paramagnetic (f/N) transition in the spacer and, thereby, of the relative orientation of the outer F-layers in the device (spin-thermoelectric manipulation of nanomagnets). Supporting experimental evidence of such thermally-controlled switching from parallel to antiparallel magnetization orientations in F/f(N)/F sandwiches is presented. Furthermore, we show theoretically that local Joule heating due to a high concentration of current in a magnetic point contact or a nanopillar can be used to reversibly drive the weakly ferromagnetic spacer through its Curie point and thereby exchange couple and decouple the two strongly ferromagnetic F-layers. For the devices designed to have an antiparallel ground state above the Curie point of the spacer, the associated spin-thermionic parallel to antiparallel switching causes magnetoresistance oscillations whose frequency can be controlled by proper biasing from essentially dc to GHz. We discuss in detail an experimental realization of a device that can operate as a thermomagnetoresistive switch or oscillator.

  • 9. Kadigrobov, A. M.
    et al.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Radic, D.
    Shekhter, R. I.
    Jonson, M.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Thermoelectrical manipulation of nano-magnets: A spin-thermionic oscillator2010In: Proceedings of SPIE - The International Society for Optical Engineering, San Diego, CA, 2010, Vol. 7760, p. 77600R-Conference paper (Refereed)
    Abstract [en]

    We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic magnetic multilayer, in which two strongly ferromagnetic layers are exchange-coupled through a weakly ferromagnetic spacer. We show theoretically that the system allows a spin-thermoelectronic control of the relative orientation of the outer layers. Supporting experimental evidence of thermally controlled switching from parallel to anti-parallel magnetization orientations in the sandwich is presented. We show magneto-resistance oscillations may take place with frequencies up to GHz. We discuss in detail an experimental realization of a device that can operate as a thermo-magneto-resistive switch or oscillator.

  • 10. Kadigrobov, A. M.
    et al.
    Shekhter, R. I.
    Kulinich, S. I.
    Jonson, M.
    Balkashin, O. P.
    Fisun, V. V.
    Naidyuk, Yu G.
    Yanson, I. K.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Hot electrons in magnetic point contacts as a photon source2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, p. 023007-Article in journal (Refereed)
    Abstract [en]

    We propose to use a point contact between a ferromagnetic and a normal metal in the presence of a magnetic field for creating a large inverted spin population of hot electrons in the contact core. The key point of the proposal is that when these hot electrons relax by flipping their spin, microwave photons are emitted, with a frequency tunable by the applied magnetic field. While point contacts are an established technology, their use as a photon source is a new and potentially very useful application. We show that this photon emission process can be detected by means of transport spectroscopy and demonstrate stimulated emission of radiation in the 10-100 GHz range for a model point contact system using a minority-spin ferromagnetic injector. These results can potentially lead to new types of lasers based on spin injection in metals.

  • 11.
    Kravets, A. F.
    et al.
    Institute of Magnetism, National Academy of Sciences of Ukraine.
    Timoshevskii, A. N.
    Institute of Magnetism, National Academy of Sciences of Ukraine.
    Yanchitsky, B. Z.
    Institute of Magnetism, National Academy of Sciences of Ukraine.
    Salyuk, O. Y.
    Institute of Magnetism, National Academy of Sciences of Ukraine.
    Yablonovskii, S. O.
    Institute of Magnetism, National Academy of Sciences of Ukraine.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Exchange-induced phase separation in Ni-Cu films2012In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 324, no 13, p. 2131-2135Article in journal (Refereed)
    Abstract [en]

    Magneto-structural properties of films of diluted ferromagnetic alloys NixCu1-x in the concentration range 0.7 < x < 1.0 are studied experimentally. Films deposited by magnetron sputtering show partial phase separation, as evidenced by structural analysis and ferromagnetic resonance measurements. The phase diagram of the NixCu1-x bulk system is obtained using numerical theoretical analysis of the electronic structure, taking into account the interatomic exchange interactions. The results confirm the experimentally found partial phase separation, explain it as magnetic in origin, and indicate an additional metastable region connected with the ferromagnetic transition in the system.

  • 12.
    Kravets, Anatolii F.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Timoshevskii, A. N.
    Yanchitsky, B. Z.
    Bergmann, Michael A.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Buhler, Johannes
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Temperature-controlled interlayer exchange coupling in strong/weak ferromagnetic multilayers: A thermomagnetic Curie switch2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 21, p. 214413-Article in journal (Refereed)
    Abstract [en]

    We investigate interlayer exchange coupling based on driving a strong/weak/strong ferromagnetic trilayer through the Curie point of the weakly ferromagnetic spacer, with exchange coupling between the strongly ferromagnetic outer layers that can be switched on and off, or varied continuously in magnitude by controlling the temperature of the material. We use Ni-Cu alloys of varied composition as the spacer material and model the effects of proximity-induced magnetism and the interlayer exchange coupling through the spacer from first principles, taking into account not only thermal spin disorder but also the dependence of the atomic moment of Ni on the nearest-neighbor concentration of the nonmagnetic Cu. We propose and demonstrate a gradient-composition spacer, with a lower Ni concentration at the interfaces, for greatly improved effective-exchange uniformity and significantly improved thermomagnetic switching in the structure. The reported multilayer materials can form the base for a variety of magnetic devices, such as sensors, oscillators, and memory elements based on thermomagnetic Curie switching.

  • 13. Naidyuk, Yu G.
    et al.
    Balkashin, O. P.
    Fisun, V. V.
    Yanson, I. K.
    Kadigrobov, A.
    Shekhter, R. I.
    Jonson, M.
    Neu, V.
    Seifert, M.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Stimulated emission and absorption of photons in magnetic point contacts2012In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 14, p. 093021-Article in journal (Refereed)
    Abstract [en]

    Point contacts between high anisotropy ferromagnetic SmCo5 and normal metal Cu are used to achieve a strong spin-population inversion in the contact core. Subjected to microwave irradiation in resonance with the Zeeman splitting in Cu, the inverted spin population relaxes through stimulated spin-flip photon emission, detected as peaks in the point-contact resistance. Resonant spin-flip photon absorption is detected as resistance minima, corresponding to sourcing the photon field energy into the electrical circuit. These results demonstrate fundamental mechanisms that are potentially useful in designing metallic spin-based lasers.

  • 14. Yanson, I. K.
    et al.
    Naidyuk, Yu. G.
    Balkashin, O. P.
    Fisun, V. V.
    Triputen, L. Yu.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Yanson, Yu. I.
    Zabel, H.
    Spin Torques in Point Contacts to Exchange-Biased Ferromagnetic Films2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, p. 2094-2096Article in journal (Refereed)
    Abstract [en]

    Hysteretic magneto-resistance of point contacts formed between non-magnetic tips and single ferromagnetic films exchange-pinned by antiferromagnetic films is investigated. The analysis of the measured current driven and field driven hysteresis agrees with the recently proposed model of the surface spin-valve, where the spin orientation at the interface can be different from that in the bulk of the film. The switching in magneto-resistance at low fields is observed to depend significantly on the direction of the exchange pinning, which allows identifying this transition as a reversal of interior spins of the pinned ferromagnetic films. The switching at higher fields is thus due to a spin reversal in the point contact core, at the top surface of the ferromagnet, and does not exhibit any clear field offset when the exchange-pinning direction or the magnetic field direction is varied. This magnitude of the switching field of the surface spins varies substantially from contact to contact and sometimes from sweep to sweep, which suggests that the surface coercivity can change under very high current densities and/or due to the particular microstructure of the point contact. In contrast, no changes in the effect of the exchange biasing on the interior spins are observed at high currents, possibly due to the rapid drop in the current density away from nanometer sized point contact cores.

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