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  • 1.
    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.

  • 2. Dahlberg, D
    et al.
    Miller, B
    Hill, B
    Jonsson, J
    Ström, Valter
    KTH, Superseded Departments, Physics.
    Rao, K Venkat
    KTH, Superseded Departments, Physics.
    Nogués, Josep
    Schuller, K
    Measurements of the ferromagnetic/antiferromagnetic interfacial exchange energy in CO/CoO and Fe/FeF2 layers1998In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 83, no 11, p. 6893-6895Article in journal (Refereed)
    Abstract [en]

    Two measurement techniques, both relying on reversible rotations of the magnetization, have been used to determine the magnitude of the interfacial exchange energy (IEE) between ferromagnetic and antiferromagnetic (F/AF) layers. One technique is to use the anisotropic magnetoresistance to determine rotations of the magnetization away from the unidirectional easy axis, where the rotation is accomplished by applying external magnetic fields less than the effective F/AF exchange field. The second technique uses measurements of the ac susceptibility as a function of the angle between the ac field and the unidirectional exchange field. Both of the reversible process techniques result in values of the IEE larger (by as much as a factor of 10 in Co/CoO bilayers) than the traditional irreversible technique of measuring a shift in the hysteresis loop. The ac susceptibility technique was also used to measure one Fe/FeF2 bilayer. For this sample, the IEE values obtained by reversible and irreversible methods are equivalent.

  • 3. Dumas, Randy K.
    et al.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Kirby, B. J.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Nogues, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Probing vertically graded anisotropy in FePtCu films2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 5, p. 054434-Article in journal (Refereed)
    Abstract [en]

    Field-dependent polarized neutron reflectivity (PNR) and magnetometry are employed to study the magnetic properties of compositionally uniform and graded FePtCu films as a function of annealing temperature (T(A)). The PNR results are able to directly probe the compositional and anisotropy variations through the film thickness. Further details about how the reversal mechanisms evolve are then elucidated by using a first-order reversal curve technique. The reversal of the graded sample annealed at 300 degrees C occurs by an initial rapid switching of the dominant soft A1 phase toward the surface of the film, followed by the gradual reversal of the residual hard phase components toward the bottom. This indicates that the anisotropy gradient is not well established at this low T(A). A fundamentally different mechanism is found after annealing at 400 degrees C, where the rapid switching of the entire film is preceded by a gradual reversal of the soft layers. This suggests that the anisotropy gradient has become better established through the film thickness. The field-dependent PNR measurements confirm the existence of an anisotropy gradient, where the lower (higher) anisotropy portions are now toward the bottom (top) of the film because of the Cu compositional gradient. However, after annealing at 500 degrees C, a single rapid reversal is found, indicating the formation of a uniform hard film. In this case, PNR demonstrates a more uniform magnetic depth profile that is consistent with a uniform reference sample, suggesting significant interdiffusion of the Cu is degrading the compositional and induced anisotropy gradient at this elevated T(A).

  • 4. Dumas, Randy K.
    et al.
    Zha, Chaolin
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Bonanni, Valentina
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Lau, June W.
    Nogués, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Graded Anisotropy FePtCu Films2011In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 47, no 6, p. 1580-1586Article in journal (Refereed)
    Abstract [en]

    The fabrication and subsequent analysis of continuously graded anisotropy films are discussed. During deposition, a compositional gradient is first achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after thermal post-annealing, and by utilizing the strong composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The magnetic properties are investigated by surface sensitive magneto-optical Kerr effect and alternating gradient magnetometer (AGM) measurements. AGM first-order reversal curve (FORC) measurements are employed in order to provide a detailed analysis of the reversal mechanisms, and therefore the induced anisotropy gradient. At low annealing temperatures, the FORC measurements clearly indicate the highly coupled reversal of soft and hard phases. However, significant interdiffusion results in virtually uniform films at elevated annealing temperatures. Additionally, the A1 to L1(0) ordering process is found to depend on the film thickness.

  • 5.
    Olsson, Richard T.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Salazar-Alvarez, German
    Institut Català de Nanotechnologia, Facultat de Ciencies, Barcelona.
    Said Azizi Sami, My Ahmed
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Nogués, Josep
    Institució Catalana de Recerca i Estudis Avancats (ICREA) and Departement de Física, Universitat Autònoma de Barcelona.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Nanoparticle decoration of bacterial cellulose fibres: a method to obtain porous high-surface area ultra-flexible ferromagnertic materialsManuscript (preprint) (Other academic)
  • 6.
    Salazar-Alvarez, German
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Sort, Jordi
    Institució Catalana de Recerca i Estudis Avançats (ICREA).
    Macedo, Waldemar A. A.
    Laboratório de Física Aplicada, Centro de Desenvolvimento da Tecnologia Nuclear, Minais Gerais.
    Ardisson, Jose D.
    Laboratório de Física Aplicada, Centro de Desenvolvimento da Tecnologia Nuclear, Minais Gerais.
    Baro, Maria Dolors
    Departament de Física, Universitat Autònoma de Barcelona, Bellaterra.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Nogués, Josep
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Enhanced coercivity in Co-rich near-stoichiometric CoFe3-xO4+delta nanoparticles prepared in large batches2007In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 19, no 20, p. 4957-4963Article in journal (Refereed)
    Abstract [en]

    The relationship between the structure and composition with the magnetic properties of near stoichiometric cobalt ferrite nanoparticles CoxFe3-xO4+delta (0.85 < x < 1.1) prepared in large batches with average sizes in the range 60-210 nm has been investigated. Chemical analysis and Rietveld refinement of the X-ray diffraction data in conjunction with Mossbauer spectroscopy allowed us to identify an interplay between particle size, microstructure (concentration of interstitial ions, microstrain, cation arrangement in octahedral and tetrahedral sites), and composition, which sensitively controls the magnetic properties such as coercivity and saturation magnetization. In all cases, cobalt-rich compositions resulted in a higher coercivity, whereas lower degrees of inversion and higher iron contents led to slightly higher saturation magnetization values.

  • 7.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Dumas, R. K.
    Department of Physics, University of Gothenburg, Sweden.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Bonanni, Valentina
    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.
    Continuously graded anisotropy in single (Fe53Pt47)(100-x)Cu-x films2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 18, p. 182504-Article in journal (Refereed)
    Abstract [en]

    We report on continuously graded anisotropy. During deposition, a compositional gradient is achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after annealing using the composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The critical role of the annealing temperature on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both a reduced coercivity and moderate thermal stability. These results demonstrate that an anisotropy gradient can be realized, and tailored, in single continuous films without the need for multilayers.

  • 8.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Dumas, R. K.
    Lau, J. W.
    Mohseni, Seyed Majid
    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.
    Golosovsky, I. V.
    Monsen, Á. F.
    Nogués, Josep
    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.
    Nanostructured MnGa films on Si/SiO(2) with 20.5 kOe room temperature coercivity2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 9, p. 093902-Article in journal (Refereed)
    Abstract [en]

    Nanostructured Mn(67)Ga(33) films exhibiting high room temperature coercivity (H(C) = 20.5 kOe) have been prepared by sputtering onto thermally oxidized Si substrates. Both the morphology and the coercivity of the films can be tuned by varying the growth parameters. The low deposition rate film, sputtered at a reduced power and working pressure, demonstrates a discontinuous island-like growth and the highest H(C). The large H(C) is linked to the presence of the high anisotropy DO(22) Mn(3)Ga phase and the single domain character of the exchange isolated, dipolar interacting, single crystal islands.

  • 9.
    Zha, Chaolin
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101).
    Dumas, Randy K.
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101).
    Persson, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101).
    Mohseni, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101).
    Nogues, Josep
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101). Universitat Autònoma de Barcelona, Spain .
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Material Physics (Closed 20120101). University of Gothenburg, Sweden .
    Pseudo spin valves using a (112)-Textured D022 Mn2.3-2.4Ga fixed layer2010In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 1, p. 2500104-Article in journal (Refereed)
    Abstract [en]

    We demonstrate pseudo spin valves (PSVs) with a (112)-textured D022 Mn2.3-2.4Ga (MnGa) tilted magnetization fixed layer and an in-plane CoFe free layer. Single D022 MnGa films exhibit a small magnetoresistance (MR) typically observed in metals. In MnGa/Cu/CoFe PSVs, a transition from a negative (-0.08%) to positive (3.88%) MR is realized by introducing a thin spin polarizing CoFe insertion layer at the MnGa/Cu interface and tailoring the MnGa thickness. Finally, the exchange coupling between the MnGa and CoFe insertion layer is studied using a first-order reversal curve technique.

  • 10.
    Zha, Chaolin
    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.
    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.
    Improved magnetoresistance through spacer thickness optimization in tilted pseudo spin valves based on L10 (111)-oriented FePtCu fixed layers2009In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 106, no 5Article in journal (Refereed)
    Abstract [en]

    Through optimization of the Cu spacer thickness, we demonstrate magnetoresistance (MR) up to 5% in FePtCu/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111) FePtCu fixed layers with a tilted magnetization. We find an optimum spacer thickness of about 2.4 nm which correlates with a clear onset of strong interlayer exchange coupling below 2.4 nm and spin-independent current shunting in the spacer above 2.4 nm. We argue that yet higher MR should be possible through further reduction in the interlayer exchange coupling.

  • 11.
    Zha, Chaolin
    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.
    Nogues, 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.
    Exchange-bias-like effect in L1(0) (111) FePt based pseudo spin valves2010In: 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, DIRAC HOUSE , 2010, Vol. 200, p. UNSP 072110-Conference paper (Refereed)
    Abstract [en]

    The coupling between hard FePt/CoFe and soft CoFe/NiFe layers through a Cu spacer has been studied in L1(0) (111) FePt(20)/CoFe(1.5)/Cu(3.5or4.5)/CoFe(2)/NiFe(3) (in nm) pseudo spin valves. The soft layer hysteresis loops exhibit clear shifts in the field axis (exchange bias-like) and a marked coercivity enhancement. The thickness of the Cu layer or the interface roughness influence the exchange bias properties of the systems. This interlayer coupling arises from the competition between dipolar and RKKY interactions between the layers.

  • 12.
    Zha, Chaolin
    et al.
    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.
    Exchange Bias in L1(0) (111)-Oriented FePt-Based Pseudo Spin Valves2009In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 45, no 10, p. 3881-3884Article in journal (Refereed)
    Abstract [en]

    The coupling between hard FePt and soft NiFe layers through a Cu interlayer has been studied in L1(0) (111) FePt(20)/CoFe(1.5)/Cu(3.5 or 4.5)/CoFe(2)/NiFe(3) (in nanometers) pseudo spin valves. The soft-layer hysteresis loops exhibit clear shifts in the field axis (exchange-bias-like) and a marked coercivity enhancement. It is found that several parameters such as the thickness of the Cu layer or the interface roughness influence the exchange bias properties of the systems, while others (e. g., temperature) are less important. This interlayer coupling arises from the competition between dipolar and Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions between the layers.

1 - 12 of 12
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