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An In Situ anneal study of graded anisotropy FePtCu films
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
Department of Physics, University of Gothenburg, Sweden.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
2011 (English)In: IEEE Magnetics Letters, ISSN 1949-307X, E-ISSN 1949-3088, Vol. 2Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
2011. Vol. 2
Keyword [en]
hard magnetic materials, FePtCu films, graded anisotropy, in situ magnetic characterization, magnetic recording media
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-34094DOI: 10.1109/LMAG.2011.2114332ISI: 000208910300012Scopus ID: 2-s2.0-79958715767OAI: oai:DiVA.org:kth-34094DiVA: diva2:419113
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20110525

Available from: 2011-05-25 Created: 2011-05-25 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Titled and graded anisotropy FePt and FePtCu thin films for the application of hard disk drive and spin torque oscillators
Open this publication in new window or tab >>Titled and graded anisotropy FePt and FePtCu thin films for the application of hard disk drive and spin torque oscillators
2011 (English)Licentiate 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.

Place, publisher, year, edition, pages
Kista: KTH Royal Institute of Technology, 2011. v, 65 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2011:08
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-34013 (URN)978-91-7415-996-7 (ISBN)
Presentation
2011-05-31, C1, Isafjordsgatan 22, Kista, 10:00 (English)
Opponent
Supervisors
Note
QC 20110525Available from: 2011-05-25 Created: 2011-05-23 Last updated: 2011-05-25Bibliographically approved

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