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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. 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).

  • 3. 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.

  • 4.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Titled and graded anisotropy FePt and FePtCu thin films for the application of hard disk drive and spin torque oscillators2011Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

     The FePt and FePtCu thin films with graded anisotropy and titled anisotropy are utilized to solve both the magnetic recording ‘‘trilemma’’ of the hard disk drives (HDDs) and the large field operation problem of spin torque oscillators (STOs). We have successfully realized the FePtCu thin films with graded anisotropy. During deposition a compositional gradient is achieved by continuously varying the Cu content from the top to bottom. After annealing at proper temperatures, the top Cu-poor regions remain at soft A1 phase, while the bottom Cu-rich regions transform into hard L1 0phase. Hence the gradient anisotropy is established through the film thickness. The critical role of the annealing temperatures (TA) on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both the reduced coercivity and moderate thermal stability. In conjunction of the fabrication, the subsequent analysis of the graded material is not trivial. The reversal mechanism of graded anisotropy have been investigated by alternation gradient magnetometer (AGM) and magneto optical Kerr effect (MOKE) measurements with first order reversal curves (FORC) technique. The AGM-FORC analysis, which clearly shows the soft and hard phases, is not able to resolve how these phases are distributed through the film thickness. MOKE-FORC measurement which preferentially probes the surface of the film, reveal that the soft components are indeed located toward the top surface. The TA plays a critical role in the induced anisotropy gradient. We provide a detailed study of the how the anisotropy gradient in a compositional graded FePtCu film gradually develops as a function of the TA. By utilizing the in-situ annealing and magnetic characterization capability of a physical property measurement system, the evolution of the induced anisotropy gradient is elucidated. These results are important and useful for the application of HDDs. In order to achieve the zero-field operation for STOs, we have successfully fabricated pseudo spinvalves based on L1 0(111) textured FePt or FePtCu. 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 ° 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. Futuremore, through optimization of the Cu spacer thickness, we demonstrate 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. These results are an important milestone for future applications of tilted spin polarizers in STOs.

  • 5.
    Fang, Yeyu
    et al.
    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.
    Åkerman, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    An In Situ anneal study of graded anisotropy FePtCu films2011In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 2Article in journal (Refereed)
    Abstract [en]

    We provide a detailed study of how the anisotropy (Ku) gradient in a compositionally graded FePtCu film gradually develops as a function of the postannealing temperature (TA). By utilizing the in situ annealing and magnetic characterization capabilities of a physical property measurement system, the evolution of the induced Ku gradient is elucidated. For low TA, the sample primarily remains in the low-Ku A1 phase. At intermediate TA, the gradual development of an A1 to L10 anisotropy gradient occurs. As TA is further increased, a well-developed L10 gradient is realized. Finally, annealing temperatures greater than 475 ◦C reduce the gradient until the film is effectively uniform for TA ≥ 525 ◦C and higher, presumably due to interdiffusion of the Cu through the film thickness. The resulting coercivity shows a nonmonotonic dependence on TA with an initial steep increase as the L10 fraction of the sample increases, a local minimum at TA = 525 ◦C where the gradient vanishes, and a final increase as the uniform L10 film orders completely.

  • 6.
    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.

  • 7. Nguyen, T. N. Anh
    et al.
    Fang, Yeyu
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Fallahi, Vahid
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Benatmane, Nadjib
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Mohseni Armaki, Seyed Majid
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Dumas, R. K.
    Åkerman, Johan
    [Co/Pd]-NiFe exchange springs with tunable magnetization tilt angle2011In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 17, p. 172502-Article in journal (Refereed)
    Abstract [en]

    We investigate exchange coupled [Co/Pd](5)-NiFe thin films. Due to competition between the in-plane shape anisotropy of the NiFe and strong perpendicular magnetic anisotropy of the [Co/Pd](5) multilayer, unique magnetic configurations are achievable. In particular, we explore the out-of-plane magnetization tilt angle of the NiFe layer. Experimental results, based on conventional magnetometry, ferromagnetic resonance, and magnetic force microscopy, agree well with one-dimensional micromagnetic simulations. We find that the tilt angle is highly tunable [0 degrees-60 degrees] over a small range of NiFe thicknesses (4.8-8nm). Potential applications for high power, zero-field operation spin torque oscillators are discussed.

  • 8.
    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.

  • 9.
    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.

  • 10.
    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.

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